Study On The Optimized Root-Cut Grafting Technique Of Watermelon Seedling Nursery

Watermelon (Citrullus lanatus (Thunb.) Matsum. and Nakai) is one of the most important crops, the planting area and total yield of watermelon in China is more than 50% of the world. Grafting is an effective measure to improve the resistance of watermelon, and it has been applied in watermelon production. Compared with Japan and Korea, the technique of grafting in the watermelon seedling nursery starts much later in China, it is also comparatively less developed, thus restricting the popularization of grafting. Our research group has established a root-cut grafting technique system of industrialization. However, the system is mainly dependent on the experience. Thus, the grafting technique needs to be optimized further. To increase the quality, and to decrease the cost of seedling nursery, as well as to promote the use of root-cut grafted watermelon seedlings, the present study investigated the effects of grafting methods on the grafting efficiency, growth and fruit quality, and also investigated the effects of rootstock sowing density, night temperature, day temperature, light intensity, and seedling age on the growth and physiological characteristics of root-cut grafted watermelon seedlings. The results were as follows:1. Currently, the insert grafting is the most popular grafting method for watermelon in China. However, the root-cut grafting method is expanding rapidly in seedlings nursery companies in recent years. In the present study, the effects of insert and root-cut grafting methods on grafting working efficiency, seedlings growth and fruit quality of watermelon were investigated. The results showed that the grafting speed, survival rate and grafting working efficiency of root-cut grafting method was significantly higher than those of insert grafting method. Compared with insert grafted watermelon seedlings, root-cut grafted seedlings have higher seedling quality as well as higher root/shoot dry weight ratio and root activity. However, there were no significant effect on plant growth and fruit quality of watermelon between the two grafting methods. Thus, the root-cut grafting is an effective method for the production of grafted watermelon seedlings.2. To find out the optimal sowing density of rootstock for root-cut grafted watermelon seedlings, the growth changes of rootstock seedlings and grafted seedlings under different sowing density of rootstock were investigated. The results showed that rootstock seedlings and grafted seedlings grow best when the sowing density of rootstock is 1 seed/cell, increase sowing density will lead to excessive growth of rootstock seedlings and decline of grafted watermelon seedlings quality. Considering both seedling quality and production cost, the most economical and effective sowing density of rootstock for root-cutgrafted watermelon seedlings is 2 seeds/cell in the low temperature period and 1 seed/cell in the other period.3. To find out the optimal night temperature for root-cut grafted watermelon seedlings, the growth and physiological changes of grafted seedlings under different night temperatures were investigated. The results showed that the growth of grafted watermelon seedlings change slightly when the night temperature decreased from 18℃to 15℃, only leaf area reduced by 26.5% and the MDA content of leaves increased by 34.9%, but the root activity nearly increased by 50%, heating costs reduced by about 10%. The growth of grafted watermelon seedling was significantly inhibited and the quality of grafted seedlings was declined when night temperature is 12℃. Considering both seedling quality and production cost, the optimum night temperature for root-cut grafted watermelon seedlings is 15℃in the low temperature period.4. To find out the optimal daytime temperature and light intensity forroot-cut grafted watermelon seedlings, the growth and physiological changes of grafted seedlings under different daytime temperature and light intensity were investigated The results showed that the quality of grafted watermelon seedlings is significantly higher under 180μmol m-2s-1 light intensity than those under 100μmol m-2s-1 light intensity. It demonstrated that the quality of grafted watermelon seedlings can be improved when the light intensity was increased in a certain range. Although the growth of grafted watermelon seedlings under 24℃daytime temperature was slightly lower than those under 28℃and 32℃daytime temperature, but the seedlings under 24℃daytime temperature had more dry matter with higher root-shoot ratio and seedling index, which were accord with the standard of strong seedling.5. To find out the optimal transplanting stage for root-cut grafted watermelon seedlings, the growth and physiological changes of grafted seedlings under different physiological ages were investigated. The results showed that the optimum transplanting stage for the grafted watermelon seedlings is that the second or the third true leave of watermelon seedlings is fully expanded, about 21-24 days after grafting, at this period the seedlings had less space limitation to the root size. As the seedling age was older three true leaves, the seedlings would grow excessively, resulting in lower leaves yellowed, weakened mineral absorption capacity of root.