A cool, juicy vegetable and a staple for greenhouse growers, cucumber (Cucumis sativus) is being cultivated successfully with LEDs. In fact, more and more farmers are choosing them over other traditional lights.
Cucumber seeds can be germinated in the dark, and don’t need any light. Once they have germinated and the hypocotyl has emerged, LEDs can be used.
For greenhouses, different research studies have come to the same conclusion, namely that both blue and red are necessary for the initial growth. Growing under only blue or only red is not advisable (1)
If only red is used, photosynthesis will be dysfunctional, and little carbon dioxide (CO2) is fixed. This would, in turn, affect the proper growth of the plant, since photosynthesis is the process by which plants, produce their food. Adding just 7% of blue light can double photosynthesis (1). Moreover, applying blue light decreased the length of hypocotyl, which is what growers want. However, if only the blue light is used, the hypocotyl becomes too long (2).
The best combination that this gives a good growth rate, but keeps the plant compact is 50% blue and 50% red lights. This combination also ensures increased leaf growth and biomass, and higher nitrogen and chlorophyll content. The blue component of the combination should not decrease to less than 15% (1). Pulsing LED light is not recommended at this stage as it will produce seedlings which are taller (3).
For indoor grow chambers, where there is no daylight, the light requirements are different. Here the main combination of LEDs of red, blue, and far-red, is used along with green LEDs of 12 µmol.m-2.s-1 and orange LEDS of 30 µmol.m-2.s-1, as supplements. The intensity of all lights should add up to a total of 200 µmol.m-2.s-1. These five colors together can increase the growth of cucumber transplants (4).
Later Vegetative growth
Supplementary green LED light of 15 µmol.m-2.s-1 increases leaf area, the weight of the plant and production of photosynthetic pigments in cucumber transplants. This can be used in greenhouse conditions to supplement natural light, which has an intensity of 120 µmol.m-2.s-1. Green LEDs can also be used in grow chambers where lighting with intensities of 90 µmol.m-2.s-1 are used (4).
In cucumber, the flowers are unisex, and a plant will bear both male and female flowers. The commercial varieties of cucumber have been manipulated to produce more female flowers to boost yield (5).
However, female flower numbers can be manipulated further by growers by using LEDs. The maximum number of flowers are produced by using 80% red LED, 10% blue LED and 10% yellow LED (RBY) for eight hours. This also produces shorter internodes which are more desirable. The amount of sucrose/sugar, produced in the plant increases under this LED combination. This is advantageous, as the number of female flowers that are produced will increase as sucrose levels rise. An important factor here is also the intensity of LEDs, where PAR of 120 µmol.m-2.s-1 gives more flowers (6).
Some studies indicate that light intensity as high as 290 µmol.m-2.s-1 can be used to increase flowering. Though the sources of light used in these cases were HPS, this intensity can also be reached using LEDs (7).
Photoperiodism is also important in controlling flowering in cucumbers, and when day length is extended to 16 hours, more sugar and ethylene production occur, both of which in turn produce more flowers (8).
Like any other crop that is high, cucumber also benefits from inter-lighting. Cucumbers are trailed on high wires and can reach a height of three to four meters. So, only top lighting is not enough to ensure that all parts of the plants, especially near the ground, get sufficient light. Farmers have found that using LED as inter-lighting produces more robust plants (9).
Inter-lighting increases yield during the early production stage, but the effect decreases in the later stages when compared to plants grown without inter-lighting. Using LED inter- lighting system also improves the visual quality of cucumber fruit (4).
The light recipes recommended to improve yield are blue:green:red:far-red in the ratio 14:16:53:17 respectively, at intensities of 160 µmol.m-2.s-1 for top lighting and 125 µmol.m-2.s-1 for inter-lighting (4).
The increase in disease resistance in cucumbers grown with LEDs has been noted by more than one study. Red LEDs make cucumbers resistant to powdery mildew. As a result, farmers can avoid the use of chemical pesticides, and lower costs. This removes occupational risks associated with pesticide use for the farmers and also keeps the produce healthy for consumers. In addition, this is the more environmentally friendly way to control pests, without harming the ecosystem (4)
Other Benefits of LEDs
There are many other associated benefits of using LEDs to grow cucumbers other than photomorphogenesis.
The heights to which high-wire cultivated cucumber climbers grow can cause a lot of challenges in terms of lighting. The top of the cucumber can reach close to the roof of the greenhouse. In these cases, experienced large-scale cucumber farmers prefer LEDs over other lights such as HID lights, which generate too much heat. This is especially true in greenhouses, which are not very high. Placing traditional lights at close proximity to plants is not possible without damaging them (9). Since the LEDs do not generate too much heat, it is possible to keep them closer to the plant, which in addition, allows for better utilisation of light from the source (10).
LEDs are used to extend the length of day when cucumbers are grown in greenhouses in winter (9).
Research findings suggest that the heat produced by HPS lamps improve yield LEDs (7). On the other hand, large-scale farms have found this heat is excessive and LEDs were more efficient (10).
Reduce Energy and Water Bills
Cucumber growers, who want to produce cucumbers the year-round, have been able to reduce their energy bills by 40% by switching to LEDs (10). When only LEDs are used, water consumption is 14% lower than in situations where HPS or a combination of LED and HPS lights are used, as LEDs produce less heat and therefore result in less evaporation (10).
All Aspects of Light Are Important
So we see quality or color, intensity, and duration of light are all important during the vegetative and flowering stages, while color and intensity are important to improve the yield of cucumber. The ability to manipulate plant growth by narrow bandwidths lamps and the beneficial interaction with other environmental factors, are making LEDs popular among cucumber indoor farmers.
1) Hogewoning SW, Trouwborst G, Maljaars H, Poorter H, van Ieperen W, and J Harbinson. 2010. Blue light dose-responses of leaf photosynthesis, morphology, and chemical composition of Cucumis sativus grown under different combinations of red and blue light. J Exp Bot. 61:3107-17. doi: 10.1093/jxb/erq132.
2) Hernández R, and C Kubota. 2014. Blue Light Dose-response of Growth and Morphology of Cucumber Seedlings Under Different Blue and Red Photon Flux Ratios Using LEDs. https://www.researchgate.net/publication/267353682_Blue_Light_Dose-response_of_Growth_and_Morphology_of_Cucumber_Seedlings_Under_Different_Blue_and_Red_Photon_Flux_Ratios_Using_LEDs
3) Hernández R, Dragotakes A, and C Kubota. 2014. Pulsing effects of supplemental LED lighting on cucumber seedlings growth and morphology in greenhouse. Acta Hortic. 1037, 875-880
4) Olle M and A. Virsile. 2013. Agricultural and Food Science. 22: 223-234
6) Riadi MF, Esyanti R, and A Faizal. 2015. The effect of LED light on production of female flowers in cucumber (Cucumis sativus L.). Conference: International Congress on Science and Engineering Research. DOI: 10.13140/RG.2.1.1083.8160
7) Särkkä L, Jokinen K, Ottosen C, and T Kaukoranta. 2017. Effects of HPS and LED lighting on cucumber leaf photosynthesis, light quality penetration and temperature in the canopy, plant morphology and yield. Agricultural and Food Science. 26: 101-109. 10.23986/afsci.60293
8) Ikram MMM, Esyanti R, and A Faizal. 2015. The effect of photoperiodism with LED light on productivity of female flower in cucumber (Cucumis sativus L.). Conference: International Congress on Science and Engineering Research. DOI: 10.13140/RG.2.1.4032.9360