Effects of Red and Blue Light on Steady-State and Dynamic Photosynthesis of Cucumber Seedlings

Our previous study revealed a significant effect of red and blue light on steady-state and dynamic photosynthesis in cucumber seedlings. To further investigate the mechanism by which red and blue light affect photosynthesis, cucumber (Cucumis sativus L. cv. Jinyou 365) seedlings were exposed to white light (W), monochromatic red light (R), monochromatic blue light (B), and mixed red–blue light with red:blue photon flux ratios of 9:1 (9R1B), 7:3 (7R3B), 5:5 (5R5B), 3:7 (3R7B), and 1:9 (1R9B). We measured photosynthetic pigment contents, induction curves of photosynthetic gas exchange and chlorophyll fluorescence parameters during low-to-high-light transition, as well as the activities and gene expression of key Calvin cycle enzymes. The results showed that B yielded higher photosynthetic pigment contents than R, although both remained lower than W. Mixed red–blue light with a blue-light ratio above 50% produced significantly higher pigment contents than W. Under low light, blue light exhibited negligible effects on photosynthesis. In contrast, under high light, 7R3B, 5R5B, 3R7B and 1R9B elevated net photosynthetic rate (A) and stomatal conductance (Gs). Among these, 7R3B, 5R5B and 3R7B also accelerated photoinduction, as reflected by a shorter time to reach 90% steady-state net photosynthetic rate (T90%A). Chlorophyll fluorescence parameters changed rapidly and attained a steady state within 5 min during light induction. During photoinduction, biochemical limitation was severe in the early stage but declined rapidly, while stomatal limitation appeared in mid-induction and persisted into the late stage; photochemical limitation remained minor, and the severity of these limitations was modified by light quality. Red and blue light exerted negligible effects on chlorophyll fluorescence parameters under low light. However, blue light enhanced the relative electron transport rate of PSII under high light by elevating the fraction of open PSII centers. Furthermore, blue light significantly increased the activities and gene expression of key Calvin cycle enzymes. Correlation analysis revealed that higher chlorophyll contents were positively associated with A and Gs under high light but showed no significant correlation with photoinduction. Higher A and Gs under both low- and high-light intensities were associated with faster photoinduction.