Effects of monochromatic light on growth and development of Phoebe bournei seedlings tend to be "extreme"

Abstract

Light quality is a key factor affecting seedling regeneration in natural Phoebe bournei forests. We used the current-year P. bournei seedlings as experimental materials and cultured them under five LED spectral conditions for 60 days to explore the response mechanism of P. bournei seedlings to different light qualities (blue, red, green, blue-red (1:1) and white (control)). We analyzed the impact of monochromatic lights and blue-red mixed light on leaf structure, starch, soluble sugar, soluble protein, root development, and seedling quality of P. bournei seedlings. The effects of monochromatic light on the development of P. bournei seedlings tended to be "extreme". Red and green light increased starch grain size by 2.12 and 2.03 times compared to white light. The excessive accumulation of starch grains disrupted the ultrastructure of chloroplasts, inhibited the synthesis and utilization of carbohydrates, and thus limited the development of seedlings. Blue light accelerated the transport and utilization of carbohydrates, significantly improved the development of chloroplast ultrastructure, and activated the rapid growth of seedlings. Especially, blue light promoted the formation of lateral roots, and significantly increased leaf area, leaf thickness, and stomatal density. However, the allocation of stem biomass was reduced under blue light, resulting in imbalanced inter-organ development and decreased seedling resistance. In contrast, under the blue-red light, the chloroplast development state, the relationship between storage and utilization of starch and other substances, as well as carbon and nitrogen metabolic activities in seedlings were more coordinated, indicating that multispectral light has the potential to cultivate P. bournei seedlings of superior quality. Although the seedling quality index (SQI) of P. bournei under the blue-red light in this experiment did not reach its best, our findings provide a valuable theoretical basis of high-quality P. bournei seedlings and for regulating the light environment when planting P. bournei under the forest canopy.