Growth and nutritional properties of lettuce affected by different alternating intervals of red and blue LED irradiation

- Intentionally designed LED panels with adjustable alternating modes of red and blue light were applied in the study.
- Effects of concurrent RB light and alternating R/B light on the growth and properties of lettuce were tested and compared.
- Four different alternating intervals of red and blue light have been involved and compared in the study.

In this study, effects of alternating red light (R) and blue light (B) provided by LEDs on the growth and nutritional properties of 'Green Oak leaf' lettuce were examined. Four alternating light treatments (R/B) had the same 8.64 μmol m−2 daily light integral (DLI) and similar R:B ratio (2:1), but different R/B alternating intervals that were respectively 8 h, 4 h, 2 h, and 1 h during a 16 h photoperiod. Two simultaneous light treatments, one of which (RB) had the same DLI and energy consumption with the alternating light treatments, while the other (RB') had the same photoperiod with the alternating light treatments were set up to compare the concurrently and alternately provided R and B. Results showed that different types of radiation led to obvious morphological changes, plants with simultaneous RB appeared the most sparse while those with RB' looked the most compact. Plant height/width and leaf length/width were all the highest under R/B (8 h) followed by R/B (1 h). Lettuce biomass under RB' was significantly higher than others, more than twice that under RB, R/B (4 h) and R/B (2 h), but less than twice that under R/B (8 h) and R/B (1 h). RB' significantly decreased the soluble sugar content by 9%-32% while increased crude fiber content by 14%-39% compared with others. Significantly higher ascorbic acid content as well as lower nitrate content were detected in lettuce under R/B (4 h) and R/B (2 h), while significantly lower ascorbic acid content as well as higher nitrate content were detected in lettuce under R/B (8 h) and R/B (1 h). In all, based on the same energy consumption, R/B (8 h) and R/B (1 h) resulted in higher yield, while R/B (4 h) and R/B (2 h) brought about higher nutritive value compared with the cocurrent light RB. Therefore, we conclude that lettuce growth and qualities can be purposely adjusted by adopting different alternating intervals of red and blue light. Meanwhile, the alternating modes will provide methods for deeply studying the relationship of red and blue light when acting on plants.