Blue LED light irradiation enhances yield in green beans

Green beans (Phaseolus vulgaris L.) is one of the most interesting vegetable crops for extra-seasonal cultivation, as it may achieve high prices on the market, making its cultivation more remunerative when done in greenhouse. However, indoor cultivation may be subjected to disadvantageous conditions in terms of light quantity, especially in autumn and winter, and this condition may be limited for green bean indoor cultivation. Light-emitting diode (LED) technology is gaining popularity, as it allows using specific spectra to increase light quantity and modulate the quality of the spectrum, while its cost is decreasing. In this study, we applied three light spectra (blue (450 nm), red (660 nm), and blue plus red light, during four hours before sun rise, with an intensity of 500 μmol m-2 s-1) supported by LED fixtures to enhance yield of green beans in a cold greenhouse. The LED treatments received also sunlight, and the control treatment was the sunlight without LED. Plants ('Maestrale') were transplanted on August 22 and harvested from November 4 to December 18, 2019. Colour (CIELab colour scale) of pods, SPAD of leaves and yield were determined. Based on our results, with the addition of supplemental blue light, the plants produced more, especially compared to sunlight (on average, 54.7 vs. 47.1 pods plant-1 and 99.8 vs. 78.0 g plant-1). With blue light, the leaves showed SPAD values lower than red light but 5.4% higher than in sunlight, while the green colour (a*) of the pods was less intense with blue light (with or without red light) compared to red light and sunlight (-15.7 vs. -16.6)