Red and Blue LED Light Supplementation in the Morning Pre-activates the Photosynthetic System of Tomato (Solanum lycopersicum L.) Leaves and Promotes Plant Growth

Supplementary light exposure using light-emitting diodes (LEDs) promotes the growth of tomato plants in greenhouses. Owing to the biological clock in plants, determining the period during which they must be exposed to supplementary light is essential to enhance growth. In this study, we used red and blue LEDs (red:blue = 7:2) as the supplementary light source, to determine the effects of different light supplemental periods on the growth and photosynthetic characteristics of tomato seedlings. Light supplementation in the morning and evening promoted the growth of tomato plants to varying degrees, including the accumulation of photosynthetic products in the leaves. Light supplementation in the morning enhanced dry matter accumulation, root growth, and the contents of chlorophyll and carotenoids in the leaves. Although both morning and evening light supplementation increased the levels of gas exchange parameters and Rubisco activity in tomato leaves, these effects were more prominent after morning light supplementation. Furthermore, red and blue light supplementation in the morning pre-activated the key photosynthetic enzymes, promoted the synthesis and accumulation of photosynthetic pigments, increased the photosynthetic capacity of, and photosynthate production in, tomato leaves. These findings suggest that light supplementation in the morning is more effective in promoting the growth and development of tomato plants cultivated in greenhouses.