The general thumb rules dictating the use of LED growing lights are being overturned by increasing research in specific vegetable and flower species, as 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.
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 investigates the 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
|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 (7, 9).
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 variety, for just 5 days toward the end of production to change from an all green plant (10). In the case of kale, it 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).
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). However, far-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 a blue and red
combination is good for vitamins (6, 8, 11).
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).
Since the recommendations for post-harvest storage is generalized, at present red light is recommended for both the crops, since it closes the stomata in the leaves, and helps in water retention. This is useful in maintaining form and freshness in all leafy vegetables. Moreover, it 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 oleracea, is the common kale, and Brassica napus the Russian Red kale, which also have important differences (1, 14). Then there are seven kinds of lettuce of Lactuca sativa, depending on compactness and color; depending on the characteristics desired, light rendering can be fine-tuned for each type.