Blue light alleviates ‘red light syndrome’ by regulating chloroplast ultrastructure, photosynthetic traits and nutrient accumulation in cucumber plants

Abstract Plant growth and physiology are strongly influenced by blue and red lights. When exposed to prolonged red light, leaf photosynthesis is severely impaired. This ‘red light syndrome’ has been characterized by a low maximum quantum yield of chlorophyll fluorescence (Fv/Fm), a low photosynthetic capacity (Amax) and unresponsive stomatal conductance (gs). When adding blue light to red light, none of these effects occurrs. Previous studies mainly focused on photosynthetic changes, however, little information is available on how blue light alleviates ‘red light syndrome’ through comprehensively regulating photosynthesis in leaves, nutrient uptake by roots and their correlation. Four light quality treatments including white light (W), red monochromatic light (R), blue monochromatic light (B), and a combination of red and blue light (RB) were employed to investigate their effects on chloroplast ultrastructure, photosynthesis and nutrient accumulation in cucumber seedlings. The results showed that compared to W, R treatment significantly decreased RGR, Pn, Fv/Fm and ΦII, numbers and size of chloroplast and starch grain, thickness of grana and largely increased accumulation of P, K, Mn and Zn, indicating that R treatment severely decelerated plant growth, disrupted chloroplast ultrastructure, limited photosynthetic characteristics, and accumulated excess nutrients. However, these suppressions were effectively alleviated by adding blue light to red light (compare RB versus R). The B treatment plants exhibited plant growth, chloroplast ultrastructural and photosynthetic characteristics similar to those of W. It is clearly demonstrated that blue light-dependent alleviation of 'red light syndrome' is accompanied by mediating the enhancement of chloroplast ultrastructure, photosynthetic traits, and removal of red light-induced excessive nutrient accumulation in cucumber plant. We hypothesize that changes in both chloroplast ultrastructure and nutrient accumulation could mediate the alleviation.