Green light promotes healing and root regeneration in double-root-cutting grafted tomato seedlings

Abstract

Double-root-cutting (DRC) grafting is a new grafting method that makes it possible to ship grafted seedlings without using plug trays, through cutting off the roots of both scion and rootstock. However, graft healing and root regeneration are the two important factors determining the survival of the DRC grafted plants. Recent studies have shown that green light plays a crucial role in plant growth and development. We hypothesized that green light could stimulate the graft healing and root regeneration of the rootstock in DRC grafted seedlings. To test this hypothesis, we used tomato seedlings (Solanum lycopersicum cv. Beaded Curtain grafted on cv. Rootstock No. 1) exposed to white LED lamp (W) as a control (CK). In the treatment group, green light (10, 20 and 30%) was added to the red and blue spectrum without changing the intensity of red and blue fractions. The effects of different light qualities on graft union formation, survival rate, rooting of the rootstock hypocotyls, activities of antioxidant enzymes, chlorophyll content and grafted seedling quality of grafted tomato seedlings were then studied. Compared with DRC grafted seedlings exposed to the same light intensity, addition of 30% green light to red and blue lights increased the activities of antioxidant enzymes, chlorophyll content, plant height and index of seedling strength of grafted seedlings, showed a more positive effect on the development and performance of grafted seedlings, and promoted graft union, rootstock rooting and grafted seedling growth. Increasing the proportion of green light brought about better graft union and rootstock rooting. Our results showed that green light may reduce the degree of stomatal opening and transpiration rate during the early stages of grafting. Green light increased the activity of antioxidant enzymes, which was enhanced tissue lignification. Photosynthesis was driven by green light, which improved the development of aboveground parts, enhanced the transport of auxin from leaves to rooting zone, and promoted hypocotyl rooting of DRC grafted tomato seedlings.