Growth, Quality, and Nitrogen Metabolism of Medicago sativa Under Continuous Light from Red–Blue–Green LEDs Responded Better to High Nitrogen Concentrations than Under Red–Blue LEDs

Alfalfa is a widely grown forage with a high crude protein content. Clarifying the interactions between light quality and nitrogen level on yield and nitrogen metabolism can purposely improve alfalfa productivity in plant factories with artificial light (PFAL). In this study, the growth, quality, and nitrogen metabolism of alfalfa grown in PFAL were investigated using three nitrate-nitrogen concentrations (10, 15, and 20 mM, labeled as N₁₀, N₁₅, and N₂₀) and continuous light (CL) with two light qualities (red-blue and red-blue-green light, labeled as RB-C and RBG-C). The results showed that the adaptation performance of alfalfa to nitrogen concentrations differed under red-blue and red-blue-green CL. Plant height, stem diameter, leaf area, yield, Chl a + b, Chl a, Chl b, crude protein contents, and NiR activity under the RB-CN₁₅ treatment were significantly higher than RB-CN₁₀ and RB-CN₂₀ treatments. The RB-CN₂₀ treatment showed morphological damage, such as plant dwarfing and leaf chlorosis, and physiological damage, including the accumulation of proline, H₂O₂, and MDA. However, the difference was that under red-blue-green CL, the leaf area, yield, and Chl a + b, carotenoid, nitrate, and glutamate contents under RBG-CN₂₀ treatment were significantly higher than in the RBG-CN₁₀ and RBG-CN₁₅ treatments. Meanwhile, the contents of soluble sugar, starch, and cysteine were significantly lower. However, the crude protein content reached 21.15 mg·g^-1. The fresh yield, dry yield, stomatal conductance, leaf area, plant height, stem diameter, crude protein, GS, and free amino acids of alfalfa were positively correlated with increased green light. In addition, with the increase in nitrogen concentration, photosynthetic capacity, NiR, and GOGAT activities increased, promoting growth and improving feeding value. The growth, yield, photosynthetic pigments, carbon, nitrogen substances, and enzyme activities of alfalfa were significantly affected by the interaction between nitrogen concentration and light quality, whereas leaf/stem ratio and DPPH had no effect. In conclusion, RB-CN15 and RBG-CN₂₀ are suitable for the production of alfalfa in PFAL, and green light can increase the threshold for the nitrogen concentration adaptation of alfalfa.