Effects of Monochromatic Red, Blue and Combined Red– Blue LED Light on Growth, Photosynthesis and Leaf Metabolome in Kiwifruit Seedlings

Background Light quality is a major environmental factor regulating plant growth, development, and metabolism. In protected cultivation, covering materials, seasonal variation, and geographic conditions often limit light intensity or alter its spectral composition. Light-emitting diode (LED) supplementation, particularly with red and blue light, is an effective strategy for improving photosynthetic performance and carbon metabolism. Kiwifruit seedlings were exposed to white light (W), blue light (B), red light (R), and combined red and blue light. Combined treatments used red-to-blue (R) ratios of 6:1 (R6B1), 3:1 (R3B1), and 1:1 (R1B1). Growth, photosynthetic characteristics, carbohydrate metabolism, and leaf metabolite profiles were then compared. Results R6B1 significantly promoted plant height, leaf expansion, and biomass accumulation. R3B1 significantly enhanced the net photosynthetic rate and stomatal conductance. R3B1 also increased leaf concentrations of sucrose, glucose, fructose, and starch. It enhanced the activities of sucrose phosphate synthase (SPS), sucrose synthase in the synthetic direction (SS-II), acid invertase (AI), neutral invertase (NI), soluble starch synthase (SSS), and total amylase (AMY). These results indicate that an appropriate red-to-blue ratio promotes photosynthetic carbon assimilation and sugar accumulation by coordinating carbohydrate synthesis and degradation. Metabolomic profiling of representative W, B, R, and R3B1 treatments identified 2,741 metabolites, primarily comprising terpenoids, flavonoids, phenolic acids, and lipids. Compared with W, B induced the largest number of differentially accumulated metabolites. These metabolites were predominantly enriched in flavonoid biosynthesis and flavone and flavonol biosynthesis. Differentially accumulated metabolites under R were mainly associated with isorhamnetin O-glycoside biosynthesis and unsaturated fatty acid biosynthesis. Those under R3B1 were primarily enriched in linoleic acid metabolism and flavonoid biosynthesis. Conclusions Collectively, red and blue light ratios differentially regulate growth, photosynthetic carbon metabolism, and secondary metabolism in kiwifruit seedlings. These findings provide a scientific basis for optimizing supplemental-lighting strategies in protected kiwifruit cultivation.