The Mechanisms of Different Light Supply Patterns in the Nutrient Uptake and Chlorophyll Fluorescence of Hydroponic Lettuce

Vertical agriculture has developed rapidly in recent years, pushing artificial light planting to new heights. Under indoor artificial light, the light supply mode has practical significance in studying the impact of lighting conditions on plants. This experiment involved five intermittent light supply modes (with a light period of 16 h and a dark period of 8 h (16/8) as the control group, with a light period of 8 h and a dark period of 4 h repeated twice a day (8/4), a light period of 4 h and a dark period of 2 h repeated four times a day (4/2), a light period of 2 h and a dark period of 1 h repeated eight times a day (2/1), and a light period of 1 h and a dark period of 0.5 h repeated 16 times a day (1/0.5) (as the experimental groups). A total of ten treatments were combined before the continuous light supply mode (B) and after the continuous light supply mode (A). Through experimental settings, we explored the response mechanisms to intermittent and continuous light supply modes as they pertain to lettuce growth, nutrient content, photosynthetic parameters, and light stress. Through research, it was found that continuous light supply significantly increased plant height, root length, aboveground dry and fresh weight, and the underground dry and fresh weight of lettuce. The treatments with a light period 4 h/dark period 2 h (4/2) and a light period 8 h/dark period 4 h (8/4) significantly increased the N, P, K, and Cu contents. Additionally, continuous light supply helped stabilize the Mg, Ca, and Mn contents across all treatment groups. Stomatal closure has been found to cause a decrease in the rate of net photosynthesis, transpiration, and intercellular CO2 concentration. The energy absorbed by antenna pigments is significantly increased when participating in photochemical reactions; however, continuous light supply has been observed to reduce the absorption flux per reaction center (ABS/RC), trapped energy flux per reaction center (TR0/RC), electron transport flux per reaction center (ET0/RC), and the probability that a trapped exciton moves an electron into the electron transport chain beyond QA− (at t = 0) (ET0/TR0). Conversely, the electron transport flux per cross section (ET0/CS) has been found to increase significantly. In summary, among the intermittent light supply treatment groups, the 2/1 treatment group showed the best response to growth indicators, nutrient absorption, and photosynthesis, and could improve the quality of lettuce without adding additional light energy. Continuous light supply in the short term can improve the growth and nutrient absorption of lettuce; both of the two light supply modes produced light stress on lettuce, and this light stress caused by non-circadian rhythm forced the lettuce to increase its photochemical quenching (qI) and electro transport flux crossover (ET0/CS). This paper may provide a theoretical reference for the use of light supply modes in plant factories to improve vegetable yield, and for the study of the response mechanism of light stress under non-circadian rhythm.