Background: Light quality significantly affects plant growth and development, photosynthesis, and carbon and nitrogen metabolism. Apple (Malus domestica Borkh.) is a widely cultivated and economically important fruit crop worldwide. However, there are still few studies on the effects of different light qualities on the growth and development of apple seedlings. Results: In this study, we explored the effects of blue and red light treatments on the growth and development, photosynthetic characteristics, leaf chloroplast ultrastructure, and carbon and nitrogen metabolism of apple seedlings. Blue light significantly inhibited apple plant growth and leaf extension, but it promoted the development of leaf tissue structures and chloroplasts and positively affected leaf stomatal conductance, the transpiration rate, and photosynthetic efficiency. The red light treatment promoted apple plant growth and root development, but it resulted in loosely organized leaf palisade tissues and low chlorophyll contents. The blue and red light treatments enhanced the accumulation of ammonium nitrogen in apple seedlings. Moreover, the blue light treatment significantly promoted nitrogen metabolism. Additionally, an RNA-seq analysis revealed that both blue light and red light can significantly up-regulate the expression of genes related to carbon and nitrogen metabolism. Blue light can also promote amino acid synthesis and flavonoid metabolism, whereas red light can induce plant hormone signal transduction. The expression of a gene encoding a bHLH transcription factor (MYC2-like) was significantly up-regulated in response to blue light, implying it may be important for blue light-mediated plant development. Conclusions: Considered together, blue and red light have important effects on apple growth, carbon and nitrogen metabolism. These findings may be useful for determining the ideal light conditions for apple cultivation to maximize fruit yield and quality.

The active regulation of the plant growth environment is a common method for optimizing plant yield and quality. In horticulture today, light quality control is carried out using photo-selective nets or membranes to improve the yield and quality of cultivated plants. In the present study, with natural light as the control (CK), we tested different photo-selective nets (white, WN; blue, BN; green, GN; yellow, YN; and red, RN) with 30% shade for characteristics of growth, development, quality, yield, photosynthesis, and chlorophyll fluorescence, considering the antioxidant system, as well as the influence of element absorption and transformation of green onion (Allium fistulosumL.) plants at different growth stages. We found that plants under BN and WN have greater height and fresh weight than those of plants under the other nets. Plants under the BN treatment had the highest quality, yield, photosynthetic pigment content, net photosynthetic rate, transpiration rate, and stomatal conductance, whereas the intercellular CO₂concentration was the highest in plants in the YN treatment. The photosynthesis noon break phenomenon was significantly lower in plants with covered photo-selective nets than in CK plants. NPQ was the highest in the YN treatment, and Fv/Fm, ΦPSII, and qP among the plants in the other treatments were different; from highest to lowest, they were as follows: BN > WN > CK > RN > GN > YN. The active oxygen content of green onion leaves in the BN treatment was significantly lower than that in the other treatments, and their key enzyme activity was significantly increased. BN also improved the absorption and transformation of elements in various organs of green onion.