Effect of LED supplementary light on delaying aging of plants

Chlorophyll degradation, rapid protein loss and RNA hydrolysis during plant senescence are mainly manifested in leaf senescence. Chloroplasts are very sensitive to changes in the external light environment, especially significantly affected by light quality. Red light, blue light and combination of red and blue light are beneficial to chloroplast morphogenesis, blue light is beneficial to the accumulation of starch grains in chloroplasts, and red light and far-red light have negative effects on chloroplast development. Blue light and combined red and blue light can promote the synthesis of chlorophyll in cucumber seedling leaves, and combined red and blue light can also delay the decay of chlorophyll content in leaves in the later stage. This effect is more obvious with the decrease of the proportion of red light and the increase of the proportion of blue light. The chlorophyll content of cucumber seedling leaves under the combination of red and blue LED light treatment was significantly higher than that of the fluorescent light control and monochromatic red and blue light treatment; the blue LED light could significantly increase the chlorophyll a/b value of Wutaicai and green garlic seedlings.

The contents of cytokinin (CTK), auxin (IAA), abscisic acid (ABA) and activities of various enzymes changed during leaf senescence. The content of phytohormones is easily affected by the light environment, and different light qualities have different regulatory effects on phytohormones, and the initial step of the light signal transduction pathway involves cytokinins. CTK promotes the expansion of leaf cells, enhances the photosynthesis of leaves, and at the same time inhibits the activities of ribonucleases, deoxyribonucleases and proteases, and delays the degradation of nucleic acids, proteins and chlorophyll, so it can significantly delay leaf senescence. There is an interplay between light and CTK-mediated developmental regulation, with light energy stimulating an increase in endogenous cytokinin levels. When plant tissue is in a state of senescence, its endogenous cytokinin content decreases. IAA is mainly concentrated in vigorously growing parts, and its content is very small in aging tissues or organs. Purple light can increase the activity of indoleacetic acid oxidase, and low IAA levels can inhibit plant elongation growth. ABA was mainly formed in senescent leaf tissues, mature fruits, seeds, stems, roots and other parts. The ABA content of cucumber and cabbage under combined red and blue light was lower than that under white light and blue light.

Peroxidase (POD), superoxide dismutase (SOD), ascorbate peroxidase (APX), and catalase (CAT) are relatively important protective enzymes in plants that are related to light. If plants age, The activity of these several enzymes will decrease rapidly. Different light qualities have a significant effect on the activity of plant antioxidant enzymes. After 9 days of red light treatment, the APX activity of rapeseed seedlings increased significantly, and the POD activity decreased; after 15 days of red light and blue light irradiation, the POD activity of tomato was 20.9% and 11.7% higher than that of white light, respectively. POD activity was the lowest after 20 days of green light treatment, which was only 55.4% of white light; 4h blue light supplementation could significantly increase the soluble protein content, POD, SOD, APX, and CAT enzyme activities of cucumber seedling leaves. In addition, the activities of SOD and APX gradually decreased with the prolongation of the light time. The activity of SOD and APX under blue light and red light decreased slowly but was always higher than that of white light. Red light irradiation significantly decreased the activities of tomato leaf peroxidase, IAA peroxidase and eggplant leaf IAA peroxidase, but caused the activity of eggplant leaf peroxidase to increase significantly. Therefore, adopting a reasonable LED supplementary light strategy can effectively delay the aging of facility horticultural crops and improve yield and quality.