Abstract: By using light emitting diode (LED) to accurately modulate spectral energy distribution, and with fluorescent light as the control, this paper studied the effects of light quality on the growth and chloroplast ultrastructure of tomato and lettuce seedlings. Under red LED, the soluble sugar, starch, and carbohydrate contents of tomato and lettuce seedlings were significantly higher than those under fluorescent light, and the starch grains in chloroplast swelled obviously. Under blue LED, the hypocotyl length of tomato seedlings were restrained remarkably, but the leaf chlorophyll a and carotenoid contents of tomato and lettuce seedlings were increased significantly. Under red + blue LED, the soluble sugar, starch, carbohydrate, and sucrose contents and C/N ratio of lettuce seedlings reached the maximal and were significantly higher than those under red LED, the main root length of the seedlings increased significantly, their leaf chloroplast morphology was normal, granum number increased, stroma lamellae was clear, but the starch grain size was smaller than that under red LED. The results suggested that light quality had remarkable effects on the photomorphogenesis, growth, carbon and nitrogen metabolism, and chloroplast development of plant seedlings. Red LED promoted the accumulation of photosynthates but hindered their transport, while adding appropriate amount of blue LED to red LED could more benefit the accumulation of photosynthates in lettuce seedlings, and promote the seedlings root growth and the export of assimilation products.

Key words: light quality, tomato, lettuce, growth, chloroplast ultrastructure, Chrysanthemum morifolium Ramat, crop rotation, continuous cropping obstacle, soil microbial biomass carbon and nitrogen, the amount of microorganisms.