Cultivation

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Comparison of Photomorphogenesis in Roses and Asiatic Lilies

Posted by David Jones on

Cut flower growers rely on greenhouses to provide them blooms around the year, and it is common for them to grow more than one kind of flowers. Roses and lilies are popularly grown for cut flowers as well as potted plants. Is it possible for growers to use common grow lights for these different species? 

Photomorphogenesis

All the three broad phases of plants - germination, vegetative growth and flowering, can be affected by photomorphogenesis, and LED light quality and quantity can be adjusted for an optimal harvest (1). A comparison of photomorphogenesis and its effect on the two flowers is given in Table 1, from which it is possible to find out in which phases common lighting can be used.

Germination and Sprouting 

In case of this phase, we have no specific information for either rose or lilies. So using general recommendations, blue and red can be used as they favor germination, and far-red should be avoided as it hinders breaking of seed dormancy, to germinate seeds of roses and Asiatic lilies (2). 

In case of lilies, however, it is usually bulbs that are used and not seeds in the nurseries. If bulbs are used, we can once again extrapolate from general recommendations, as we found no studies documenting any particular color constellation for bulbs sprouting. Since blue is advised for vegetative growth, this color should help in sprouting. Another reason to include blue is that it will prevent stems from getting too tall and weak in lilies. Adding red lights can make the stems strong (3, 4). So once again a combination of blue and red is good to help bulbs sprout.

Vegetative Growth

A combination of blue, red, far-red and green are recommended to promote vegetative growth for rose seedlings. Green and far-red reach the lower regions of the plant and help the overall growth of plants. Moreover, red produces thick leaves, and far-red makes leaves larger in roses (2, 3, 5). 
In the case of Asiatic lilies too, a combination of many colors gives the best results. White, red, blue, and orange help the plants to grow faster (6). However, the combination of colors that are optimal for vegetative growth differs for the two flowers.
Stem Elongation 
Stem elongation is required in case of cut flower production in roses, and this is an aspect that growers promote. Increasing far-red achieves longer internode growth. Using a high red to far-red ratio makes the stems strong as well as long (5). 
In Asiatic lilies, stems that are very long are not considered optimum as they can be weak and result when the plants receive insufficient light in terms of duration and intensity (7). This is especially true when lilies are forced to bloom and when their vegetative growth happens in winter. So to prevent long stems, adequate durations of light should be provided. It can be a combination of white, red, blue, and orange, or a combination of red with far-red; or just white can be used alone too (6, 8). 
Since stem lengths requirements are different in both cases, so is the color rendering. Here light duration is also an important consideration.
Table 1. Photomorphogenesis in Rose and Asiatic Lilies

 

Rose Asiatic Lilies
 

Germination and Sprouting

Blue and red light are necessary, and far-red should be avoided.

Blue and red light promote strong vegetative growth from bulbs.

Vegetative Growth

A combination of blue, red, far-red and green lights are recommended for overall growth.

A combination of white, red, blue, and orange are beneficial.

Stem Elongation

Green and far-red increase inter-nodal and plant height.

Lack of sufficient light makes lilies tall but weak.

Chlorophyll Formation

Blue and red light are necessary, for chlorophyll formation.

Blue and red light are necessary, for chlorophyll formation.

Photosynthesis

Blue promotes photosynthesis and green hinders it.

Blue promote photosynthesis.

Photoperiodism

Rose is a day-neutral plant.

Asiatic lilies are long-day plants and require around 14 hours of light. White or a combination of red and far-red is good.

Bud Formation

Far-red light increases the number of buds.

Bud formation and development is affected by insufficient duration of light.

Flowering

Red light increases the size of roses.

Flowering requires long duration and the correct intensity of light. Otherwise, flowering is delayed.

Plant Health

UV radiation is used to cure plants.

UV radiation can be used to build resistance to diseases.

 

Chlorophyll Formation and Photosynthesis 
Once again in the absence of specific studies on the influence of light color on chlorophyll accumulation and rate of photosynthesis we rely on the same rule of thumb, i.e., using a combination of blue and red to increase chlorophyll amounts and promote photosynthesis, in both cases (4, 3). 

So a similar combination of red and blue can be recommended at present for both roses and Asiatic lilies. 

Photoperiodism

It is in terms of photoperiodism that rose and Asiatic lilies differ most from each other. The rose is a day-neutral plant, i.e., its flowering is not affected by the number of hours of daylight (9). So there is no need to provide long hours of light, as the duration of light that it receives is not a problem. On the other hand, too many LEDs can lead to more heat, which can lower rose production (10). 

Asiatic lilies, on the other hand, are long day plants and need more hours of light than darkness to flower (7). So it is the duration and intensity of light that also become important for lilies beside color rendering. Without adequate length of daylight, bud abortion and desiccation can occur in lilies. Around 14 hours of light should be provided at the start of bud-formation and flowering in case of Asiatic lilies (6). 

Flowering
In roses, far-red which stimulates bud formation can be useful in increasing the number of flowers (5). In Asiatic lilies, the combination of white, red, blue, and orange can increase the number of flowers per stem; depending on the variety of the flower(6). 

Red light produces larger roses (11, 12). There is no information on the effect of color on the size of Asiatic lilies. It is, however, important to maintain an ideal cycle of dark and light to initiate flower opening in lilies. 

In the case of Asiatic lilies once again the duration and quality of lighting are important, and color rendering needs also differ between the two groups of flowers (7, 6). 

Plant health

Five minutes per week of UV radiation is known to control powdery mildew in roses (13). For Asiatic lilies, we again make a general recommendation of UV radiation to build resistance in plants (14). 

Roses and Asiatic Lilies have Different Light Requirements

The same color rendering can be used in germination or sprouting, to ensure plant health and for proper photosynthesis in roses and Asiatic lilies. It is important to note that these similar recommendations of color rendering are based on general effects that different colors have on plants. Where more research is available, we know roses and Asiatic lilies have different requirements in terms of color quality or rendering, as well as the quantity and duration of light for the vegetative growth and flowering.

 

Read more

Comparison of Photomorphogenesis in Roses and Asiatic Lilies

Posted by David Jones on

Cut flower growers rely on greenhouses to provide them blooms around the year, and it is common for them to grow more than one kind of flowers. Roses and lilies are popularly grown for cut flowers as well as potted plants. Is it possible for growers to use common grow lights for these different species? 

Photomorphogenesis

All the three broad phases of plants - germination, vegetative growth and flowering, can be affected by photomorphogenesis, and LED light quality and quantity can be adjusted for an optimal harvest (1). A comparison of photomorphogenesis and its effect on the two flowers is given in Table 1, from which it is possible to find out in which phases common lighting can be used.

Germination and Sprouting 

In case of this phase, we have no specific information for either rose or lilies. So using general recommendations, blue and red can be used as they favor germination, and far-red should be avoided as it hinders breaking of seed dormancy, to germinate seeds of roses and Asiatic lilies (2). 

In case of lilies, however, it is usually bulbs that are used and not seeds in the nurseries. If bulbs are used, we can once again extrapolate from general recommendations, as we found no studies documenting any particular color constellation for bulbs sprouting. Since blue is advised for vegetative growth, this color should help in sprouting. Another reason to include blue is that it will prevent stems from getting too tall and weak in lilies. Adding red lights can make the stems strong (3, 4). So once again a combination of blue and red is good to help bulbs sprout.

Vegetative Growth

A combination of blue, red, far-red and green are recommended to promote vegetative growth for rose seedlings. Green and far-red reach the lower regions of the plant and help the overall growth of plants. Moreover, red produces thick leaves, and far-red makes leaves larger in roses (2, 3, 5). 
In the case of Asiatic lilies too, a combination of many colors gives the best results. White, red, blue, and orange help the plants to grow faster (6). However, the combination of colors that are optimal for vegetative growth differs for the two flowers.
Stem Elongation 
Stem elongation is required in case of cut flower production in roses, and this is an aspect that growers promote. Increasing far-red achieves longer internode growth. Using a high red to far-red ratio makes the stems strong as well as long (5). 
In Asiatic lilies, stems that are very long are not considered optimum as they can be weak and result when the plants receive insufficient light in terms of duration and intensity (7). This is especially true when lilies are forced to bloom and when their vegetative growth happens in winter. So to prevent long stems, adequate durations of light should be provided. It can be a combination of white, red, blue, and orange, or a combination of red with far-red; or just white can be used alone too (6, 8). 
Since stem lengths requirements are different in both cases, so is the color rendering. Here light duration is also an important consideration.
Table 1. Photomorphogenesis in Rose and Asiatic Lilies

 

Rose Asiatic Lilies
 

Germination and Sprouting

Blue and red light are necessary, and far-red should be avoided.

Blue and red light promote strong vegetative growth from bulbs.

Vegetative Growth

A combination of blue, red, far-red and green lights are recommended for overall growth.

A combination of white, red, blue, and orange are beneficial.

Stem Elongation

Green and far-red increase inter-nodal and plant height.

Lack of sufficient light makes lilies tall but weak.

Chlorophyll Formation

Blue and red light are necessary, for chlorophyll formation.

Blue and red light are necessary, for chlorophyll formation.

Photosynthesis

Blue promotes photosynthesis and green hinders it.

Blue promote photosynthesis.

Photoperiodism

Rose is a day-neutral plant.

Asiatic lilies are long-day plants and require around 14 hours of light. White or a combination of red and far-red is good.

Bud Formation

Far-red light increases the number of buds.

Bud formation and development is affected by insufficient duration of light.

Flowering

Red light increases the size of roses.

Flowering requires long duration and the correct intensity of light. Otherwise, flowering is delayed.

Plant Health

UV radiation is used to cure plants.

UV radiation can be used to build resistance to diseases.

 

Chlorophyll Formation and Photosynthesis 
Once again in the absence of specific studies on the influence of light color on chlorophyll accumulation and rate of photosynthesis we rely on the same rule of thumb, i.e., using a combination of blue and red to increase chlorophyll amounts and promote photosynthesis, in both cases (4, 3). 

So a similar combination of red and blue can be recommended at present for both roses and Asiatic lilies. 

Photoperiodism

It is in terms of photoperiodism that rose and Asiatic lilies differ most from each other. The rose is a day-neutral plant, i.e., its flowering is not affected by the number of hours of daylight (9). So there is no need to provide long hours of light, as the duration of light that it receives is not a problem. On the other hand, too many LEDs can lead to more heat, which can lower rose production (10). 

Asiatic lilies, on the other hand, are long day plants and need more hours of light than darkness to flower (7). So it is the duration and intensity of light that also become important for lilies beside color rendering. Without adequate length of daylight, bud abortion and desiccation can occur in lilies. Around 14 hours of light should be provided at the start of bud-formation and flowering in case of Asiatic lilies (6). 

Flowering
In roses, far-red which stimulates bud formation can be useful in increasing the number of flowers (5). In Asiatic lilies, the combination of white, red, blue, and orange can increase the number of flowers per stem; depending on the variety of the flower(6). 

Red light produces larger roses (11, 12). There is no information on the effect of color on the size of Asiatic lilies. It is, however, important to maintain an ideal cycle of dark and light to initiate flower opening in lilies. 

In the case of Asiatic lilies once again the duration and quality of lighting are important, and color rendering needs also differ between the two groups of flowers (7, 6). 

Plant health

Five minutes per week of UV radiation is known to control powdery mildew in roses (13). For Asiatic lilies, we again make a general recommendation of UV radiation to build resistance in plants (14). 

Roses and Asiatic Lilies have Different Light Requirements

The same color rendering can be used in germination or sprouting, to ensure plant health and for proper photosynthesis in roses and Asiatic lilies. It is important to note that these similar recommendations of color rendering are based on general effects that different colors have on plants. Where more research is available, we know roses and Asiatic lilies have different requirements in terms of color quality or rendering, as well as the quantity and duration of light for the vegetative growth and flowering.

 

Read more


Tomatoes’ Growth and Yield

Posted by David Jones on

Crops are grown in greenhouses to locally produce popular vegetables like tomatoes even outside the natural growing season. In the greenhouses, there is light available through natural sources, and LEDs are needed only to supplement deficiencies. 

Tomatoes

Tomatoes (Lycopersicon esculentum) are one of the most popularly grown vegetables in greenhouse cultivation, given the year around high demand for it. Therefore, it is understandable that this plant and its light needs are well studied. Research into the use of LEDs for growing tomatoes cover, quality, intensity and different positioning of lights to enhance the final yield (1).

Germination and Seedlings’ Growth

Tomato seeds are best germinated under blue light. Once they have germinated, red light can be added to promote good vegetative growth (2). 

Tomato plants need to have an optimum stem length and a higher first flower truss; these characteristics can be controlled by using LEDs. Using more of blue in a blue and red combination was found to produce longer stemmed seedlings, which then improved and raised the position of the first inflorescence/truss. Moreover, the use of blue light also resulted in the faster onset of flowering. A light intensity or photosynthetic photon flux density (PPFD) of 75 μmol m-2 s-1 achieved the best results (3). 

Studies have shown that the use of yellow, orange, and green lights during early seedling stages can lead to a decrease in tomato yield (4, 5). Tomato seedlings on the other hand benefit from the supplementary illumination of UV lights (5).

The colour of the LEDs, which seedlings are exposed to, affect not only the initial vegetative and flowering stages, but they can also affect later processes and influence the yield of the tomato plants (4). 

Photosynthesis

Photosynthetic rates are the best in seedlings when a combination of red and blue LEDs are used in one of the three ratios 5:1, 10:1 and 19:1 (6). The rate of photosynthesis was found to increase with increasing intensity of light up to 800 μmol m-2 s-1 (6). Blue is necessary for increasing photosynthesis, so the use of only red is not advisable (3). 

Many studies have found that in tomato the levels of photosynthesis have an influence on flowering as well as the yield, so it is important to see that this process is maintained at a high rate (3, 7). 

When tomatoes are cultivated as a high-density crop, the amount of light which reaches the middle and lower canopy layers can decrease. The amount of light which reaches the middle and lower level can be as less as 33% and 18% respectively, of the light that falls on the top canopy. This can, in turn, lead to a lower rate of photosynthesis (7). It is therefore advisable to use lights at mid-canopy levels through inter-lighting LEDs as well as bottom lighting to provide more light to the lower canopy levels.

Phytochemicals

Quality, quantity and position of LED lights are all important in determining the kind of compounds that are produced in tomato fruits. 

Using red light increases the starch in chloroplasts in tomatoes, and it can also improve the fruit colour and help in post-harvest conservation (6). 

Compared to other colours, blue LEDs increase the concentration of polyphenolic compounds, which have antioxidant properties, in tomatoes (8). Seven days of exposure to yellow light at ~100 µmol m−2 s−1 can also improve production of phenolic compounds (9). 

The amount of vitamin C content in tomato fruits can be doubled by using supplementary LED lighting in addition to natural light (8). The position of lighting is crucial here, as inter-lighting can increase the level of vitamin-C in the tomatoes growing in the lower canopies (8). Moreover, nighttime LED inter-lighting increases the total soluble solids and ascorbic acid levels in tomato plants by 20 and 25% respectively in winter (7).

Flowering

Blue light promotes new flowering, by increasing photosynthesis. Studies have shown that in tomatoes flowering time and number of flowers all depend on the photosynthetic rate (3, 6). 

Plant Defences

More than any other colour, blue light was found to increase the concentration of chemicals such as proline, antioxidants and ROS (Reactive Oxygen Species). These compounds, in turn, inhibit the development of grey mould disease in tomatoes (10). In contrast, red and green LEDs damp the production of proline, leading to the development of mould symptoms (10).

Yield

Quality, quantity and position of LED lights are all important in determining the final yield of tomato fruits. 

Red and blue LEDs used in the ratio 5:1 and 19:1 produced the maximum number of fruits, as well as the largest tomatoes (6). The season of tomato cultivation also plays a major role in determining the effects of colour rendering. Thus, 100% red LED lights will produce more tomatoes in winter, whereas, tomato production is below average in summer when only red LEDs are used (6). 

Increasing PPFD from 100 μmol m-2 s-1 to 150 μmol m-2 s-1 can improve production of tomatoes by 10-16% (11). Increasing intensity in this range improved “fruit yield, mass, cluster size and per cent fruit set” giving 40% more of yield in another study (6). 
Inter-lighting can improve tomato yield by up to 50%, by increasing photosynthesis (7). Experiments with bottom lighting show that this produces shorter and bushy tomato plants. While this will also increase the biomass of the plant, the ultimate tomato yield is less (6). 

By using inter-lighting for the middle and lower canopy, photosynthesis can be improved giving 27% more yield in winter. This, however, has no effect in a summer production of tomatoes. Interestingly, use of LEDs as inter-lighting at night increases yield by 24% in winter and by 12% in summer (7). 

Since electricity is cheaper in the night, using LED inter-lighting in the night can improve growth of the plant and its yield at a lower cost in both summer and winter (7). This ultimately leads to better energy use efficiency (6). 

Environmental Effects of LEDs in Tomato Cultivation

When the growth of tomatoes under LEDs is compared to that under High Pressure Sodium (HPS) lighting, it was found that initially, the crops needed less water since LEDs do not generate too much heat. Later when the crops under LED reach the same size as those under HPS, the water requirement was more (1). It may be necessary to use energy to provide more warming so that optimum greenhouse and crop temperatures can be maintained when only LEDs are used in winter tomato crops (1). 

Position of Lighting 
One of the advantages of using LEDs over conventional systems is that during inter-lighting, they do not produce too much heat when they can be placed close to the crops. At inter -canopy height this has many benefits since plants receive a higher intensity of light from the closer placed lighting source which gives growth a boost. Conventional lighting which is used can provide optimum lighting when the plant is around 1.5 m high (6). 

Conclusion

Blue and red lights are very important in the growth and production of tomato plants in various phases of its life cycle. Blue emerges as the vital colour for improving plant growth, health and ultimately tomato yield. In terms of intensity, higher PPFD of 150 μmol m−2 s−1 has consistently been found to improve growth and yield in tomato plants. Using LEDs as inter-lighting at night can improve tomato yield and quality at lower costs.

Read more

Tomatoes’ Growth and Yield

Posted by David Jones on

Crops are grown in greenhouses to locally produce popular vegetables like tomatoes even outside the natural growing season. In the greenhouses, there is light available through natural sources, and LEDs are needed only to supplement deficiencies. 

Tomatoes

Tomatoes (Lycopersicon esculentum) are one of the most popularly grown vegetables in greenhouse cultivation, given the year around high demand for it. Therefore, it is understandable that this plant and its light needs are well studied. Research into the use of LEDs for growing tomatoes cover, quality, intensity and different positioning of lights to enhance the final yield (1).

Germination and Seedlings’ Growth

Tomato seeds are best germinated under blue light. Once they have germinated, red light can be added to promote good vegetative growth (2). 

Tomato plants need to have an optimum stem length and a higher first flower truss; these characteristics can be controlled by using LEDs. Using more of blue in a blue and red combination was found to produce longer stemmed seedlings, which then improved and raised the position of the first inflorescence/truss. Moreover, the use of blue light also resulted in the faster onset of flowering. A light intensity or photosynthetic photon flux density (PPFD) of 75 μmol m-2 s-1 achieved the best results (3). 

Studies have shown that the use of yellow, orange, and green lights during early seedling stages can lead to a decrease in tomato yield (4, 5). Tomato seedlings on the other hand benefit from the supplementary illumination of UV lights (5).

The colour of the LEDs, which seedlings are exposed to, affect not only the initial vegetative and flowering stages, but they can also affect later processes and influence the yield of the tomato plants (4). 

Photosynthesis

Photosynthetic rates are the best in seedlings when a combination of red and blue LEDs are used in one of the three ratios 5:1, 10:1 and 19:1 (6). The rate of photosynthesis was found to increase with increasing intensity of light up to 800 μmol m-2 s-1 (6). Blue is necessary for increasing photosynthesis, so the use of only red is not advisable (3). 

Many studies have found that in tomato the levels of photosynthesis have an influence on flowering as well as the yield, so it is important to see that this process is maintained at a high rate (3, 7). 

When tomatoes are cultivated as a high-density crop, the amount of light which reaches the middle and lower canopy layers can decrease. The amount of light which reaches the middle and lower level can be as less as 33% and 18% respectively, of the light that falls on the top canopy. This can, in turn, lead to a lower rate of photosynthesis (7). It is therefore advisable to use lights at mid-canopy levels through inter-lighting LEDs as well as bottom lighting to provide more light to the lower canopy levels.

Phytochemicals

Quality, quantity and position of LED lights are all important in determining the kind of compounds that are produced in tomato fruits. 

Using red light increases the starch in chloroplasts in tomatoes, and it can also improve the fruit colour and help in post-harvest conservation (6). 

Compared to other colours, blue LEDs increase the concentration of polyphenolic compounds, which have antioxidant properties, in tomatoes (8). Seven days of exposure to yellow light at ~100 µmol m−2 s−1 can also improve production of phenolic compounds (9). 

The amount of vitamin C content in tomato fruits can be doubled by using supplementary LED lighting in addition to natural light (8). The position of lighting is crucial here, as inter-lighting can increase the level of vitamin-C in the tomatoes growing in the lower canopies (8). Moreover, nighttime LED inter-lighting increases the total soluble solids and ascorbic acid levels in tomato plants by 20 and 25% respectively in winter (7).

Flowering

Blue light promotes new flowering, by increasing photosynthesis. Studies have shown that in tomatoes flowering time and number of flowers all depend on the photosynthetic rate (3, 6). 

Plant Defences

More than any other colour, blue light was found to increase the concentration of chemicals such as proline, antioxidants and ROS (Reactive Oxygen Species). These compounds, in turn, inhibit the development of grey mould disease in tomatoes (10). In contrast, red and green LEDs damp the production of proline, leading to the development of mould symptoms (10).

Yield

Quality, quantity and position of LED lights are all important in determining the final yield of tomato fruits. 

Red and blue LEDs used in the ratio 5:1 and 19:1 produced the maximum number of fruits, as well as the largest tomatoes (6). The season of tomato cultivation also plays a major role in determining the effects of colour rendering. Thus, 100% red LED lights will produce more tomatoes in winter, whereas, tomato production is below average in summer when only red LEDs are used (6). 

Increasing PPFD from 100 μmol m-2 s-1 to 150 μmol m-2 s-1 can improve production of tomatoes by 10-16% (11). Increasing intensity in this range improved “fruit yield, mass, cluster size and per cent fruit set” giving 40% more of yield in another study (6). 
Inter-lighting can improve tomato yield by up to 50%, by increasing photosynthesis (7). Experiments with bottom lighting show that this produces shorter and bushy tomato plants. While this will also increase the biomass of the plant, the ultimate tomato yield is less (6). 

By using inter-lighting for the middle and lower canopy, photosynthesis can be improved giving 27% more yield in winter. This, however, has no effect in a summer production of tomatoes. Interestingly, use of LEDs as inter-lighting at night increases yield by 24% in winter and by 12% in summer (7). 

Since electricity is cheaper in the night, using LED inter-lighting in the night can improve growth of the plant and its yield at a lower cost in both summer and winter (7). This ultimately leads to better energy use efficiency (6). 

Environmental Effects of LEDs in Tomato Cultivation

When the growth of tomatoes under LEDs is compared to that under High Pressure Sodium (HPS) lighting, it was found that initially, the crops needed less water since LEDs do not generate too much heat. Later when the crops under LED reach the same size as those under HPS, the water requirement was more (1). It may be necessary to use energy to provide more warming so that optimum greenhouse and crop temperatures can be maintained when only LEDs are used in winter tomato crops (1). 

Position of Lighting 
One of the advantages of using LEDs over conventional systems is that during inter-lighting, they do not produce too much heat when they can be placed close to the crops. At inter -canopy height this has many benefits since plants receive a higher intensity of light from the closer placed lighting source which gives growth a boost. Conventional lighting which is used can provide optimum lighting when the plant is around 1.5 m high (6). 

Conclusion

Blue and red lights are very important in the growth and production of tomato plants in various phases of its life cycle. Blue emerges as the vital colour for improving plant growth, health and ultimately tomato yield. In terms of intensity, higher PPFD of 150 μmol m−2 s−1 has consistently been found to improve growth and yield in tomato plants. Using LEDs as inter-lighting at night can improve tomato yield and quality at lower costs.

Read more


Kale: Growth and Development

Posted by David Jones on

Light is needed as a driver of growth, development, physiology, and metabolism. By providing different wavelengths at different stages of plant life, it is possible to get kales with desirable features. 

Different Kale Species

Different varieties of Brassica oleracea, give us many of the common vegetables like broccoli cauliflower, kohlrabi, and kale. However, it should be noted that not all kale varieties come from Brassica oleracea; Russian red kale is Brassica napus (1, 2). Russian red kale differs from the green of varieties in its color and also tastes sweeter. 

Kale is popular as it has various vitamins- C, A, K, magnesium, and fiber. It contains more antioxidant polyphenols, effective against effects of aging, as well as glucosinolates, ßcarotene, and carotenoids than the other Brassica vegetables (1, 3). In terms of nutrients, both red and green kales have similar properties (4). These beneficial chemicals or nutraceuticals are produced as secondary metabolites in kales (1). 

Germination

Red, blue, and white light have similar effects on seedling growth, so any one of them can be use in this stage. As with other crops it is not advisable to use far-red, as it can inhibit germination (1).

Vegetative Growth

During the seedling stage, far-red light can inhibit stem growth rate (1). While using Blue light for 21 days or more can decrease leaf length in kale (5). 

Colors of Kale 
Anthocyanins are the compounds which produce the red color in leaves and fruits and make kale more nutritious. The green is derived from chlorophyll. 

Far-red increases anthocyanins contents. The longer kale is exposed to light and absorbs it, the darker it becomes in color. Since anthocyanins affect plant pigmentation using far-red light alone should be enough to get the red colored desired in Russian kale. In case of other kale varieties which are green, therefore far-red is not suitable (1). 

Chlorophyll accumulation is good under red light with lower fluence rates in seedlings (1). However, a combination of blue and red lights which increases chlorophyll content by 50% than red light alone would be the ideal combination in case of green kale varieties in the seedling stage. Once the plants are bigger, accumulation of chlorophyll was maximum under red light (3). Far-red alone has a detrimental effect on chlorophyll accumulation (1). Far-red and red in the ratio of 3:1 produces the maximum level of anthocyanins and chlorophylls, making them suitable for green kales (1). 

Secondary Metabolite Production in Kale

The different secondary metabolites in kale show a complex pattern of response to different light wavelengths. 

Anthocyanins 
Anthocyanins accumulation was similar under white, red, and blue light. The anthocyanins concentrations can be increased further to what is obtained under far-red by adding blue light with red. When blue light was used with far-red, anthocyanin production was improved by 30% than when far-red was used alone (1). 

Antioxidants 
Similarly, far-red light helps in the accumulation of 25% more of antioxidants, than white or blue light. Red light resulted in the lowest concentration of antioxidants. White light alone can also be used to improve antioxidant levels (1). 

GLs 
Far-red light also increased the levels of total GLs by 15-42% over darkness, while red, and blue had no effect at all (1). Mixtures of light are not advisable for GLs, as adding blue light to far-red can decrease GL levels (1). 

Vitamins 
Lutein is the so-called eye vitamin because it prevents eye diseases associated with age (6). Sinigrin is a natural aliphatic glucosinolate that is anti-cancer, antibacterial, antifungal, antioxidant, and anti-inflammatory (7). Increasing levels of these two chemicals beneficial to people has been attempted with LEDs. 

Lutein and sinigrin levels responded to LED treatment, and reached maximum levels under red light, and did moderately well under blue. Lutein production was the least under far-red (3). Other secondary metabolites were not affected by irradiance, especially ß-carotene levels (3). Green light can enhance levels of vitamin C and phenolic content (9). 

So if far-red can increase levels of anthocyanins, GLs and antioxidants, they decrease some of the vitamins. 

Plant Resistance

In general, UV radiation can be used to enhance the levels of secondary metabolites which are responsible for increasing resistance to pathogens such as fungi and other external stresses (9). 

Post-Harvest

LEDs can be used not only during the cultivation but also in the post-harvest phase to keep the kales fresh, safe and nutritious.

After harvest, it is better to retain the light and dark cycles for kales, as this maintains tissue integrity, green coloration and chlorophyll content, similar to storage in cool refrigerated temperatures. By maintaining chlorophyll content, the appearance of kale is also retained. Moreover, the levels of glucosinolates in kale also remain high when the biological rhythm is maintained, instead of storing in completely dark or light conditions. A 12 hour light and 12 dark periods are recommended (8). Since LEDs produce less heat, it is advisable to use them instead of any other light source to maintain the biological rhythm to enhance preservation of the crops (9).

Shelf-life of kales during storage can be extended by using red light, as it delays senescence in kales (9). Red light also improves water retention in vegetables in general by closing stomata, but blue light will have the opposite effect (9). 

Blue light becomes important for protecting the kale from diseases in the post-harvest stage Blue light offers protection to vegetables against fungal infection and bacteria post-harvest (9). So a combination of more red and some blue lights are advisable in storage. 

It is interesting to note that it is not just the wavelength or the intensity that is important but also fluency rates, that is the amount of light the plants absorb over a period of time. By changing light regimes, different kinds of plants can be obtained from the same variety of kale, especially Russian red kale as its shows plasticity in its response (1).

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Kale: Growth and Development

Posted by David Jones on

Light is needed as a driver of growth, development, physiology, and metabolism. By providing different wavelengths at different stages of plant life, it is possible to get kales with desirable features. 

Different Kale Species

Different varieties of Brassica oleracea, give us many of the common vegetables like broccoli cauliflower, kohlrabi, and kale. However, it should be noted that not all kale varieties come from Brassica oleracea; Russian red kale is Brassica napus (1, 2). Russian red kale differs from the green of varieties in its color and also tastes sweeter. 

Kale is popular as it has various vitamins- C, A, K, magnesium, and fiber. It contains more antioxidant polyphenols, effective against effects of aging, as well as glucosinolates, ßcarotene, and carotenoids than the other Brassica vegetables (1, 3). In terms of nutrients, both red and green kales have similar properties (4). These beneficial chemicals or nutraceuticals are produced as secondary metabolites in kales (1). 

Germination

Red, blue, and white light have similar effects on seedling growth, so any one of them can be use in this stage. As with other crops it is not advisable to use far-red, as it can inhibit germination (1).

Vegetative Growth

During the seedling stage, far-red light can inhibit stem growth rate (1). While using Blue light for 21 days or more can decrease leaf length in kale (5). 

Colors of Kale 
Anthocyanins are the compounds which produce the red color in leaves and fruits and make kale more nutritious. The green is derived from chlorophyll. 

Far-red increases anthocyanins contents. The longer kale is exposed to light and absorbs it, the darker it becomes in color. Since anthocyanins affect plant pigmentation using far-red light alone should be enough to get the red colored desired in Russian kale. In case of other kale varieties which are green, therefore far-red is not suitable (1). 

Chlorophyll accumulation is good under red light with lower fluence rates in seedlings (1). However, a combination of blue and red lights which increases chlorophyll content by 50% than red light alone would be the ideal combination in case of green kale varieties in the seedling stage. Once the plants are bigger, accumulation of chlorophyll was maximum under red light (3). Far-red alone has a detrimental effect on chlorophyll accumulation (1). Far-red and red in the ratio of 3:1 produces the maximum level of anthocyanins and chlorophylls, making them suitable for green kales (1). 

Secondary Metabolite Production in Kale

The different secondary metabolites in kale show a complex pattern of response to different light wavelengths. 

Anthocyanins 
Anthocyanins accumulation was similar under white, red, and blue light. The anthocyanins concentrations can be increased further to what is obtained under far-red by adding blue light with red. When blue light was used with far-red, anthocyanin production was improved by 30% than when far-red was used alone (1). 

Antioxidants 
Similarly, far-red light helps in the accumulation of 25% more of antioxidants, than white or blue light. Red light resulted in the lowest concentration of antioxidants. White light alone can also be used to improve antioxidant levels (1). 

GLs 
Far-red light also increased the levels of total GLs by 15-42% over darkness, while red, and blue had no effect at all (1). Mixtures of light are not advisable for GLs, as adding blue light to far-red can decrease GL levels (1). 

Vitamins 
Lutein is the so-called eye vitamin because it prevents eye diseases associated with age (6). Sinigrin is a natural aliphatic glucosinolate that is anti-cancer, antibacterial, antifungal, antioxidant, and anti-inflammatory (7). Increasing levels of these two chemicals beneficial to people has been attempted with LEDs. 

Lutein and sinigrin levels responded to LED treatment, and reached maximum levels under red light, and did moderately well under blue. Lutein production was the least under far-red (3). Other secondary metabolites were not affected by irradiance, especially ß-carotene levels (3). Green light can enhance levels of vitamin C and phenolic content (9). 

So if far-red can increase levels of anthocyanins, GLs and antioxidants, they decrease some of the vitamins. 

Plant Resistance

In general, UV radiation can be used to enhance the levels of secondary metabolites which are responsible for increasing resistance to pathogens such as fungi and other external stresses (9). 

Post-Harvest

LEDs can be used not only during the cultivation but also in the post-harvest phase to keep the kales fresh, safe and nutritious.

After harvest, it is better to retain the light and dark cycles for kales, as this maintains tissue integrity, green coloration and chlorophyll content, similar to storage in cool refrigerated temperatures. By maintaining chlorophyll content, the appearance of kale is also retained. Moreover, the levels of glucosinolates in kale also remain high when the biological rhythm is maintained, instead of storing in completely dark or light conditions. A 12 hour light and 12 dark periods are recommended (8). Since LEDs produce less heat, it is advisable to use them instead of any other light source to maintain the biological rhythm to enhance preservation of the crops (9).

Shelf-life of kales during storage can be extended by using red light, as it delays senescence in kales (9). Red light also improves water retention in vegetables in general by closing stomata, but blue light will have the opposite effect (9). 

Blue light becomes important for protecting the kale from diseases in the post-harvest stage Blue light offers protection to vegetables against fungal infection and bacteria post-harvest (9). So a combination of more red and some blue lights are advisable in storage. 

It is interesting to note that it is not just the wavelength or the intensity that is important but also fluency rates, that is the amount of light the plants absorb over a period of time. By changing light regimes, different kinds of plants can be obtained from the same variety of kale, especially Russian red kale as its shows plasticity in its response (1).

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Microgreens Growth and Nutritional Value

Posted by David Jones on

Since microgreens are harvested a few days after germination, and they are consumed for their nutritional importance, the focus of photo-morphogenesis for these high-value crops is on phytochemical concentration.

Microgreens

Microgreens refer to seedlings or immature greens which are harvested two to three weeks after germination. These usually include the cotyledons, the first set of fleshy leaves, and could also include the first set of true leaves. Normal salad greens, leafy vegetables, flowering plants, and herbs, when harvested shortly after germination, can be classified as microgreens (1). 

The plants popularly grown as microgreens are members of the Brassica genus since they are easy to germinate such as broccoli, beets, mizuna, kohlrabi, radish, pak choi, tatsoi, arugula, mustard, and kale. Some other species grown as microgreens are cilantro, amaranth, etc. (2, 3). 

Microgreens are popular because they contain high quantities of phytochemicals which prevent diseases, and are good for overall health. The concentration of many essential nutrients is also higher in microgreens compared to their mature forms (2). 

It is not just the color or quality of light that is important; the intensity of light measured as photosynthetic photon flux densities (PPFD) can also be important. This latter aspect, is of course, also an important economic consideration, and the effort is to try to maximize crop yield and quality with less input of energy or light intensity (4). 

Effects of light quality and intensity have been studied for red pak choi (Brassica rapa var. chinensis ‘Rubi F1’), tatsoi (Brassica rapa var. rosularis), mustard (Brassica juncea L ) varieties, kohlrabi (Brassica oleracea var. gongylodes), and mizuna (Brassica rapa var. japonica

Vegetative growth

Light intensity has proven to be more important than the color of the LEDs in regulating the plant growth parameters to get the ideal microgreens which have short hypocotyl, large leaves, and more weight. 

Combinations of red, green, far-red and blue LEDs, where the percent of red is 74% or more are usually favorable for growing microgreens. Three different combinations red:green:blue 74:18:8 (R74:G18:B8), red:blue 87:13 (R87:B13), or red:far-red:blue 84:7:9 (R84:FR7:B9), had similar effects on the vegetative growth of kohlrabi, mizuna, and mustard microgreens (5). Broccoli microgreens’ growth and biomass were not influenced positively or negatively when grown under blue LED or a combination of red and blue LEDs (6). 

Moderate intensities of LEDs are more suitable for optimum brassica microgreen vegetative growth in red pak choi, tatsoi, mustard and kohlrabi (4). Low levels of LEDs at 110 μmol m-2 s-1 are not advisable. In kohlrabi, mizuna, and mustard microgreens, hypocotyl length decreased, and dry weight increased when light intensity was increased from 105 to 315 µmol m−2 s−1 (5). 

At higher PPFD of 545-440 μmol m-2 s-1, the length of the stem or hypocotyl of mustard, kohlrabi, and tatsoi were shorter compared to those exposed to PPFD of 220 μmol m-2 s-1. Lower PPFD levels of 110 μmol m-2 s-1 produced longer hypocotyls in red pak choi and kohlrabi (4). 

Increasing PPFD to 545 and 440 μmol m-2 s-1 increased plant weight in the case of red pak choi and tatsoi; this also increased leaf area in mustard and pak choi (4). 

Chlorophyll levels 
Leaf chlorophyll concentrations were improved by supplying levels of 545-440 μmol m-2 s-1 in red pak choi and mustard (4). This is important for photosynthesis and the appearance of the microgreens, which are green in color. Anthocyanins are important for red colored herbs.

Phytochemicals level

Low light intensities result in lower production of phytochemicals. Moderate amounts are the best, since using very high light intensity of 545 μmol m-2 s-1 does not produce any additional benefit. 

Phenols: Moderate levels of PPFD at 440 μmol m-2 s-1 increased the levels of phenols in kohlrabi, pak choi, and tatsoi (4). In mustard, light intensity produced no difference in phenol levels (4). 

Anthocyanins: The highest concentrations of anthocyanin were produced at the moderate light intensity of 330 μmol m-2 s-1 in red pak choi and tatsoi. It was only kohlrabi that needed 440 μmol m-2 s-1 for optimum production of anthocyanins (4). 

Nitrates: Low levels of light intensity result in higher concentration of nitrates in the microgreens than desired (4). 

Sucrose: Sucrose production needed varying levels of light intensity in the different species. High intensity was necessary for mustard and kohlrabi, while moderate levels of light were sufficient for red pak choi and tatsoi (4).

Vitamins and DPPHs: Lower light intensity of 110 μmol m-2 s-1 produced the highest quantities of α tocopherol (Vitamin E) in all the species. (4). Ascorbic acid and DPPH free radical activity concentrations were not affected by the light intensity in any of the species (4). 

Essential Nutrients: The concentration of different macro- and micro-nutrients such as glucoraphanin, potassium (K), magnesium (Mg), and iron (Fe) in broccoli microgreens were higher when grown under blue light than a combination of red and blue (6). 

Pre-harvest Treatment

Moreover, a short duration exposure to blue LEDs before the harvest of broccoli microgreens increased the levels of beta-carotene, glucoraphanin, glucosinolates, epiprogoitrin, and essential nutrients such as macronutrients of calcium (Ca), phosphorus (P), potassium (K), magnesium (Mg), and sulfur (S), increasing their nutritional value (6). 

Conclusion

Regardless of the colour of light of the LEDs, blue, red or far-red, or the combinations used, moderate light intensities of 320–440 μmol m-2 s-1 are more suitable to promote both growth and nutritional qualities of microgreens since this “produces larger leaves, higher total anthocyanins, total phenols, DPPH free-radical scavenging capacity and lower content of nitrates” (4).

Read more

Microgreens Growth and Nutritional Value

Posted by David Jones on

Since microgreens are harvested a few days after germination, and they are consumed for their nutritional importance, the focus of photo-morphogenesis for these high-value crops is on phytochemical concentration.

Microgreens

Microgreens refer to seedlings or immature greens which are harvested two to three weeks after germination. These usually include the cotyledons, the first set of fleshy leaves, and could also include the first set of true leaves. Normal salad greens, leafy vegetables, flowering plants, and herbs, when harvested shortly after germination, can be classified as microgreens (1). 

The plants popularly grown as microgreens are members of the Brassica genus since they are easy to germinate such as broccoli, beets, mizuna, kohlrabi, radish, pak choi, tatsoi, arugula, mustard, and kale. Some other species grown as microgreens are cilantro, amaranth, etc. (2, 3). 

Microgreens are popular because they contain high quantities of phytochemicals which prevent diseases, and are good for overall health. The concentration of many essential nutrients is also higher in microgreens compared to their mature forms (2). 

It is not just the color or quality of light that is important; the intensity of light measured as photosynthetic photon flux densities (PPFD) can also be important. This latter aspect, is of course, also an important economic consideration, and the effort is to try to maximize crop yield and quality with less input of energy or light intensity (4). 

Effects of light quality and intensity have been studied for red pak choi (Brassica rapa var. chinensis ‘Rubi F1’), tatsoi (Brassica rapa var. rosularis), mustard (Brassica juncea L ) varieties, kohlrabi (Brassica oleracea var. gongylodes), and mizuna (Brassica rapa var. japonica

Vegetative growth

Light intensity has proven to be more important than the color of the LEDs in regulating the plant growth parameters to get the ideal microgreens which have short hypocotyl, large leaves, and more weight. 

Combinations of red, green, far-red and blue LEDs, where the percent of red is 74% or more are usually favorable for growing microgreens. Three different combinations red:green:blue 74:18:8 (R74:G18:B8), red:blue 87:13 (R87:B13), or red:far-red:blue 84:7:9 (R84:FR7:B9), had similar effects on the vegetative growth of kohlrabi, mizuna, and mustard microgreens (5). Broccoli microgreens’ growth and biomass were not influenced positively or negatively when grown under blue LED or a combination of red and blue LEDs (6). 

Moderate intensities of LEDs are more suitable for optimum brassica microgreen vegetative growth in red pak choi, tatsoi, mustard and kohlrabi (4). Low levels of LEDs at 110 μmol m-2 s-1 are not advisable. In kohlrabi, mizuna, and mustard microgreens, hypocotyl length decreased, and dry weight increased when light intensity was increased from 105 to 315 µmol m−2 s−1 (5). 

At higher PPFD of 545-440 μmol m-2 s-1, the length of the stem or hypocotyl of mustard, kohlrabi, and tatsoi were shorter compared to those exposed to PPFD of 220 μmol m-2 s-1. Lower PPFD levels of 110 μmol m-2 s-1 produced longer hypocotyls in red pak choi and kohlrabi (4). 

Increasing PPFD to 545 and 440 μmol m-2 s-1 increased plant weight in the case of red pak choi and tatsoi; this also increased leaf area in mustard and pak choi (4). 

Chlorophyll levels 
Leaf chlorophyll concentrations were improved by supplying levels of 545-440 μmol m-2 s-1 in red pak choi and mustard (4). This is important for photosynthesis and the appearance of the microgreens, which are green in color. Anthocyanins are important for red colored herbs.

Phytochemicals level

Low light intensities result in lower production of phytochemicals. Moderate amounts are the best, since using very high light intensity of 545 μmol m-2 s-1 does not produce any additional benefit. 

Phenols: Moderate levels of PPFD at 440 μmol m-2 s-1 increased the levels of phenols in kohlrabi, pak choi, and tatsoi (4). In mustard, light intensity produced no difference in phenol levels (4). 

Anthocyanins: The highest concentrations of anthocyanin were produced at the moderate light intensity of 330 μmol m-2 s-1 in red pak choi and tatsoi. It was only kohlrabi that needed 440 μmol m-2 s-1 for optimum production of anthocyanins (4). 

Nitrates: Low levels of light intensity result in higher concentration of nitrates in the microgreens than desired (4). 

Sucrose: Sucrose production needed varying levels of light intensity in the different species. High intensity was necessary for mustard and kohlrabi, while moderate levels of light were sufficient for red pak choi and tatsoi (4).

Vitamins and DPPHs: Lower light intensity of 110 μmol m-2 s-1 produced the highest quantities of α tocopherol (Vitamin E) in all the species. (4). Ascorbic acid and DPPH free radical activity concentrations were not affected by the light intensity in any of the species (4). 

Essential Nutrients: The concentration of different macro- and micro-nutrients such as glucoraphanin, potassium (K), magnesium (Mg), and iron (Fe) in broccoli microgreens were higher when grown under blue light than a combination of red and blue (6). 

Pre-harvest Treatment

Moreover, a short duration exposure to blue LEDs before the harvest of broccoli microgreens increased the levels of beta-carotene, glucoraphanin, glucosinolates, epiprogoitrin, and essential nutrients such as macronutrients of calcium (Ca), phosphorus (P), potassium (K), magnesium (Mg), and sulfur (S), increasing their nutritional value (6). 

Conclusion

Regardless of the colour of light of the LEDs, blue, red or far-red, or the combinations used, moderate light intensities of 320–440 μmol m-2 s-1 are more suitable to promote both growth and nutritional qualities of microgreens since this “produces larger leaves, higher total anthocyanins, total phenols, DPPH free-radical scavenging capacity and lower content of nitrates” (4).

Read more


How do Kale and Lettuce Compare in Terms of LED Color Requirements?

Posted by David Jones on

The general thumb ruledictating the use of LED growing lights are being overturned by increasing research in specific vegetable and flower speciesas well as varieties within each species. The extent of differences in color rendering required for different phases in a plant's life can sometimes be surprising even among related vegetables.

Leafy Greens

Consider the general category of leafy greens which are favorites among indoor farmers and gardening enthusiasts. One would expect the light requirements to be similar at least in this category. A comparison is made of the two greens which are popularly grown, kale and lettuce, as shown in Table 1. There is research which investigatethe lighting requirement of different varieties within a crop, so an in-depth comparison is possible. 

Germination and Seed Dormancy

For both kale and lettuce, far-red light should be avoided. In the case of kale, red, blue or white lights can be used and give the same results. However, in lettuce, only red light is recommended (1, 2, 3).

Seedling and Plant Growth

Blue light is suitable for plant growth in both crops. However, this is best limited to the seedling stage in both the vegetables. Extending blue light can inhibit growth in kale (4). After the seedling stage, red light is the most suitable one for root and shoot growth in lettuce (5).

Leaf and Plant Size

Whereas blue light can decrease leave length in case of kale, blue light has no adverse effect in case of lettuce (4, 6). Red with some far-red is recommended for increasing leaf length and overall size of the plant in lettuce (6).

Chlorophyll and Photosynthesis

Photosynthesis is one of the most important processes in plants, as plants produce their food and grow as a result of it. This, in turn, depends on the amount of chlorophyll. So growers try to optimize both to get a better yield.

Kale varieties show variation in light needs to develop chlorophyll in various growth phases. While red light is good for red kales, a combination of blue and red lights is recommended in the seedling phase with only red light later on for green kale (1, 7). For lettuce, blue is the ideal light, as red can decrease chlorophyll levels (6, 8). 

Not surprisingly, red is good for photosynthesis in kale (7). The requirements are opposite in lettuce, where red and green lights have a detrimental effect on photosynthesis, and it is 
the blue light that results in the best production of chlorophyll (6, 3).

Table 1: Colour Rendering of LEDs for Kale and Lettuce 

 

Growth Phases

 Kale Lettuce
Germination and dormancy Red, blue or white can be used; farred is not good. Blue is not conducive; red light is good for breaking dormancy and germination. Far-red should be avoided.
Seedling growth Some blue light is good, but far-red will inhibit growth at this stage. Blue is the most suitable for seedling growth
General plant growth Though blue is good, its use should not be extended. More of red promotes root and shoot growth; another option is red and blue, with some yellow.
Leaf size Blue light decreases leaf length when used for too long. Blue light has no effect; red light and some far-red increases leaf length and plant size.
Chlorophyll levels In the seedlings stage, red is good for red kale, and a combination of blue and red is the best for green kale for chlorophyll levels; later a red light is suitable for both varieties. Chlorophyll levels increase as blue is increased, and red can decrease chlorophyll levels.
Photosynthesis A combination of red and blue are better for photosynthesis. Blue light increases photosynthesis; green and red lights reduce photosynthesis.
Color Far-red and some blue increases anthocyanins, and is best for red kale. For green kale, far-red should be less. A combination of blue and red in varying amounts can induce red color.
Weight Red light is best, as blue and farred inhibit growth. Equal amounts of blue and red, with some far-red and green, can produce bigger and heavier lettuce.
Nutraceuticals' levels Far-red was the best, followed by white to increase anti-oxidant levels. Far-red is also good for GLs. Vitamins accumulated best under red and least under far-red. Increasing blue improves levels of anti-oxidants and flavonoid levels, while mixed blue and red are good for vitamins levels.
Plant resistance UV radiation increases the level of protective chemicals. UV radiation can stimulate protective mechanisms.
Post-harvest Red light delays decay. In general, red light is better for water retention.

In general, red light is better for water retention.

Weight of the Plant

The healthiest and heaviest plants are obtained with red color in case of kale, while the biggest lettuce can be produced by using equal amounts of blue and red lights with some far-red and green (79). 

Colour

Red colored foliage which is considered attractive in lettuce can be induced by using a combination of blue and red. Various combinations with all blue, all red, or 50% of each can be used depending on the varietyfor just 5 days toward the end of production to change from an all green plant (10). In the case of kaleit is blue in combination with far-red that is suitable for producing red coloration in leaves, and only blue light to get green kale (1).

Nutraceuticals Levels

Different chemicals within a single crop need different wavelengths of light for optimum production making the requirements complicated. Far-red is the best to increase antioxidants’ and GLs’ levels in kale (1)Howeverfar-red is the least suitable for vitamin accumulation in kale, which is favored by red light (7). The picture is entirely different in case of lettuce where blue is the best for antioxidants’ and flavonoids’ levels, and blue and red 
combination is good for vitamins (6811).

Plant Resistance

In the case of plant resistance, there is a similarity in requirements by the different crops. UV radiation stimulates the production of protective chemicals in both kale and lettuce (12, 13). 

Post-harvest

Since the recommendations for post-harvest storage is generalizedat present red light is recommended for both the crops, since it closes the stomata in the leavesand helps in water retention. This is useful in maintaining form and freshness in all leafy vegetablesMoreoverit has been shown that red light delays senescence in kales (12). 

Know Your Crop

The comparison between kale and lettuce shows that there is little similarity in terms of light wavelength requirements for indoor cultivation of even related crops. It is necessary to keep in mind that there are also differences in light rendering among varieties of a vegetable as well.
Moreover, kale has varieties which belong to different species Brassica oleraceais the common kale, and Brassica napus the Russian Red kalewhich also have important difference(114)Then there are seven kinds of lettuce oLactuca sativa, depending on compactness and color; depending on the characteristics desired, light rendering can be fine-tuned for each type.

Read more

The general thumb ruledictating the use of LED growing lights are being overturned by increasing research in specific vegetable and flower speciesas well as varieties within each species. The extent of differences in color rendering required for different phases in a plant's life can sometimes be surprising even among related vegetables.

Leafy Greens

Consider the general category of leafy greens which are favorites among indoor farmers and gardening enthusiasts. One would expect the light requirements to be similar at least in this category. A comparison is made of the two greens which are popularly grown, kale and lettuce, as shown in Table 1. There is research which investigatethe lighting requirement of different varieties within a crop, so an in-depth comparison is possible. 

Germination and Seed Dormancy

For both kale and lettuce, far-red light should be avoided. In the case of kale, red, blue or white lights can be used and give the same results. However, in lettuce, only red light is recommended (1, 2, 3).

Seedling and Plant Growth

Blue light is suitable for plant growth in both crops. However, this is best limited to the seedling stage in both the vegetables. Extending blue light can inhibit growth in kale (4). After the seedling stage, red light is the most suitable one for root and shoot growth in lettuce (5).

Leaf and Plant Size

Whereas blue light can decrease leave length in case of kale, blue light has no adverse effect in case of lettuce (4, 6). Red with some far-red is recommended for increasing leaf length and overall size of the plant in lettuce (6).

Chlorophyll and Photosynthesis

Photosynthesis is one of the most important processes in plants, as plants produce their food and grow as a result of it. This, in turn, depends on the amount of chlorophyll. So growers try to optimize both to get a better yield.

Kale varieties show variation in light needs to develop chlorophyll in various growth phases. While red light is good for red kales, a combination of blue and red lights is recommended in the seedling phase with only red light later on for green kale (1, 7). For lettuce, blue is the ideal light, as red can decrease chlorophyll levels (6, 8). 

Not surprisingly, red is good for photosynthesis in kale (7). The requirements are opposite in lettuce, where red and green lights have a detrimental effect on photosynthesis, and it is 
the blue light that results in the best production of chlorophyll (6, 3).

Table 1: Colour Rendering of LEDs for Kale and Lettuce 

 

Growth Phases

 Kale Lettuce
Germination and dormancy Red, blue or white can be used; farred is not good. Blue is not conducive; red light is good for breaking dormancy and germination. Far-red should be avoided.
Seedling growth Some blue light is good, but far-red will inhibit growth at this stage. Blue is the most suitable for seedling growth
General plant growth Though blue is good, its use should not be extended. More of red promotes root and shoot growth; another option is red and blue, with some yellow.
Leaf size Blue light decreases leaf length when used for too long. Blue light has no effect; red light and some far-red increases leaf length and plant size.
Chlorophyll levels In the seedlings stage, red is good for red kale, and a combination of blue and red is the best for green kale for chlorophyll levels; later a red light is suitable for both varieties. Chlorophyll levels increase as blue is increased, and red can decrease chlorophyll levels.
Photosynthesis A combination of red and blue are better for photosynthesis. Blue light increases photosynthesis; green and red lights reduce photosynthesis.
Color Far-red and some blue increases anthocyanins, and is best for red kale. For green kale, far-red should be less. A combination of blue and red in varying amounts can induce red color.
Weight Red light is best, as blue and farred inhibit growth. Equal amounts of blue and red, with some far-red and green, can produce bigger and heavier lettuce.
Nutraceuticals' levels Far-red was the best, followed by white to increase anti-oxidant levels. Far-red is also good for GLs. Vitamins accumulated best under red and least under far-red. Increasing blue improves levels of anti-oxidants and flavonoid levels, while mixed blue and red are good for vitamins levels.
Plant resistance UV radiation increases the level of protective chemicals. UV radiation can stimulate protective mechanisms.
Post-harvest Red light delays decay. In general, red light is better for water retention.

In general, red light is better for water retention.

Weight of the Plant

The healthiest and heaviest plants are obtained with red color in case of kale, while the biggest lettuce can be produced by using equal amounts of blue and red lights with some far-red and green (79). 

Colour

Red colored foliage which is considered attractive in lettuce can be induced by using a combination of blue and red. Various combinations with all blue, all red, or 50% of each can be used depending on the varietyfor just 5 days toward the end of production to change from an all green plant (10). In the case of kaleit is blue in combination with far-red that is suitable for producing red coloration in leaves, and only blue light to get green kale (1).

Nutraceuticals Levels

Different chemicals within a single crop need different wavelengths of light for optimum production making the requirements complicated. Far-red is the best to increase antioxidants’ and GLs’ levels in kale (1)Howeverfar-red is the least suitable for vitamin accumulation in kale, which is favored by red light (7). The picture is entirely different in case of lettuce where blue is the best for antioxidants’ and flavonoids’ levels, and blue and red 
combination is good for vitamins (6811).

Plant Resistance

In the case of plant resistance, there is a similarity in requirements by the different crops. UV radiation stimulates the production of protective chemicals in both kale and lettuce (12, 13). 

Post-harvest

Since the recommendations for post-harvest storage is generalizedat present red light is recommended for both the crops, since it closes the stomata in the leavesand helps in water retention. This is useful in maintaining form and freshness in all leafy vegetablesMoreoverit has been shown that red light delays senescence in kales (12). 

Know Your Crop

The comparison between kale and lettuce shows that there is little similarity in terms of light wavelength requirements for indoor cultivation of even related crops. It is necessary to keep in mind that there are also differences in light rendering among varieties of a vegetable as well.
Moreover, kale has varieties which belong to different species Brassica oleraceais the common kale, and Brassica napus the Russian Red kalewhich also have important difference(114)Then there are seven kinds of lettuce oLactuca sativa, depending on compactness and color; depending on the characteristics desired, light rendering can be fine-tuned for each type.

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