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 (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).
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.
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).
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 DefencesMore 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).
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).
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
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).