LED light quality has a significant impact on the growth, development, and photosynthesis of plant seedlings. Red light is beneficial to stem elongation and dry matter accumulation of vegetable seedlings, while blue light is beneficial to protein accumulation and promotes the activity of antioxidant enzymes. The combined light is more beneficial to the photosynthesis and growth of vegetable seedlings than single light quality. Ultraviolet radiation can reduce the leaf area per plant, inhibit hypocotyl elongation, reduce photosynthesis and productivity, and make plants vulnerable to pathogens, but can induce flavonoid synthesis and defense mechanisms; it can also significantly reduce soybean plant height, dryness the damage to the photosynthetic pigments of the seedlings is more serious. Blue light can inhibit the elongation of the hypocotyl of red bean sprouts and the elongation of the stems of tobacco seedlings, and reduce the relative growth rate; it has an extremely important effect on the growth of plant leaves and roots and can reduce the leaf area and the number of leaves of lettuce seedlings. It is beneficial to promote the synthesis of nutrients related to flower bud differentiation and flower formation. Greenlight is not an efficient absorption spectrum for photosynthesis, but supplementing green light can synergize with red and blue light to increase the synthesis of pigments, which can significantly increase the plant height and stem thickness of tomato seedlings, and promote the growth of pea sprouts. Red-orange light is beneficial to stem growth and promotes plant flowering and chlorophyll formation shortens the growth cycle and increases soluble sugar content and yield. Far-red light can increase plant dry and fresh weight, stem length, leaf length, and leaf width; however, in many cases, it can counteract the red light effect and reduce anthocyanins, carotenoids, and chlorophyll content. The cucumber seedlings were supplemented with low-intensity blue and red light for two hours in the morning, and it was found that the supplemental light increased the fresh weight, leaf area, and stem thickness of the seedlings. The use of red LED for night-time delayed supplemental light can promote the early growth of cucumber seedlings, and the nighttime delayed supplemental light of red and blue mixed light can promote the late growth of cucumber seedlings and improve the seedling strength index. Using LED red and blue light as the light source can effectively promote the morphogenesis of cowpea, bitter gourd, lettuce, and pepper seedlings, and with the enhancement of LED red and blue light, the morphological indicators of seedlings are gradually improved, the synthesis of chlorophyll is gradually increased, and the root vitality is gradually enhanced. Different LED light quality has significant and different effects on the growth of seedlings of different varieties such as cucumber, pepper, and tomato. Supplementing red light or red and blue light at the seedling stage can promote the growth of seedlings and is conducive to cultivating strong seedlings. Supplemental light can increase the content of flavonoids and total phenols in tomatoes and peppers, and enhance the activities of antioxidant enzyme systems cAt and soD, which is beneficial to improving the stress resistance and adaptability of plants to the environment. After being transplanted into the field, the greenhouse tobacco seedlings that have been supplemented with light are better than the conventional greenhouse seedlings in the growth stage, growth rate, and maturity stage, and the differences in the number of leaves and stem thickness have reached extremely significant levels; The ratio, single leaf weight, thickness, and specific leaf weight are far better than the control.
As an important characteristic of the light environment, light quality directly or indirectly affects the synthesis and transport of plant hormones. Irradiation of red and blue light at the seedling stage can significantly promote the growth of vegetable seedlings and improve the seedling strength index. Different wavelengths of light can regulate the growth of plant internodes by affecting hormone levels in plants, and phytochromes can regulate the elongation and growth of hypocotyls by affecting the level of endogenous GA in cowpea seedlings. Far-red light promoted marked elongation of the hypocotyls of tomato and lettuce seedlings, and the seedlings were severely leggy. Blue-violet light can increase the activity of auxin oxidase, reduce the level of auxin in the plant, weakening the apical dominance, enhance the tillering ability, and then inhibit the elongation of plant internodes. Hypocotyl elongation of seedlings was related to light quality of different wavelengths. White light and blue light could inhibit stem elongation, while green light significantly promoted internode elongation. Exogenous application of IAA or GA could restore the hypocotyl elongation of lettuce seedlings suppressed by blue light to a certain extent, indicating that blue light may inhibit the elongation of hypocotyls by reducing the level of endogenous GAs in lettuce seedlings. However, the levels of various hormones in plants decreased under different compound light quality treatments, and the hypocotyls showed a lower growth rate. In the light treatment, when the proportion of blue light was appropriately increased (R/B=7:3), the height of the seedlings decreased significantly, and the seedling strength index increased significantly.