Effects of double-root-cutting grafing on root growth，endogenous hormone content and nutrient uptake in watermelon seedlings

【Objective】Grafting has many advantages, such as increasing yield, improving nutrition and water uptake, and raising resistance to soil-borne diseases and damage caused by biotic or abiotic stressors. It has become a common practice in watermelon. Although the utilization of watermelon grafting has been steadily increasing over time and 95% of commercial watermelon productions actually rely on it in some regions of the world, the higher cost of grafted watermelon transplants is still one of the primary factors limiting application in regions with medium and large-scale production. The labor-efficient automation of grafting has been recognized as a key factor in the wider adoption of grafting. In growing cucurbits, the root-one-cotyledon- removing grafting method is the most commonly used with grafting machines. For root-cutting-one-cotyledon grafting method, root development is an important factor for seedling quality. Because acquiring strong roots is the main purpose for grafting, it is important to un-derstand the root development of grafted seedlings. However, there is little information on the root growth of root-cutting grafted seedlings. In order to apply this grafting technology efficiently, the influence of root- cutting grafting on the root growth and mineral nutrient absorption of watermelon seedlings was studied.【Methods】The watermelon variety Zaojia 84- 24 was used as the scion, and the pumpkin variety Zhenzhuang was used as the rootstock. The scion and rootstock seeds were respectively sown into 98- and 72-cell trays with one seed in one cell filled with mixed seedling substrate (peat moss and pearlite at a volume ratio of 3∶1). The seeds of rootstock and scion were sown on the same day to meet the requirements for grafting experiments. The plants were cultivated in an artificial chamber with a day/night (12 h/12 h) cycle at 28 ℃/18 ℃ with 60%-80% relative humidity. Grafting was performed after the first true leaf developed on the rootstock and scion at 10th day after sowing. When grafting, the seedlings were divided into two groups. One group was grafted using root-cutting-one-cotyledon grafting method (DRC). After grafting, the plants were transplanted into 72-cell tray-plug with mixed seedling substrate. The other group was grafted using one-cotyledon splice grafting method (CK), in that rootstock had intact root system. The grafted seedlings were placed under a plastic film with a day/night cycle at 28 ℃/18 ℃ with more than 90% humidity under low light intensity (75 μmol·m-2 ·s -1 , 12 h/12 h photoperiod) for healing. The grafted plants were exposed to the air 1-3 h per day until the scions were alive and grew. After the healing process, the root growth parameters such as root morphology indexes, root activity and scattered lump rate were measured at 6-day interval. At the same time, samples were also harvested for examining the hormone and mineral contents.【Results】The root diameter of grafted watermelon seedlings with pumpkin root-cutting was thick while the root length, root surface area, root volume, root tip number, root fresh weight and root dry weight were significantly reduced. The scattering lump rate of grafted watermelon seedlings with pumpkin root-cutting was 86.7% while the scattering rate of grafted watermelon seedlings with the control was only 13.3% on the 12th day after grafting. The contents of IAA, CTK and ETH in the root of grafted watermelon seedlings with pumpkin root-cutting significantly increased while the contents of GA and ABA significantly decreased. The contents of N, P, K, Mg, Fe and B in stems and roots of grafted watermelon seedlings with pumpkin root-cutting showing a certain degree of increase. However, the contents of Ca in stems and roots of grafted watermelon seedlings with pumpkin root-cutting decreased significantly.【Conclusion】A welldeveloped root system induced vigorous stem growth and plant development, and root growth was also influenced by stem. The root of watermelon seedlings grafted by root-cutting grafting method was regenerated with adventitious roots. The regenerated root system had higher root activity and mineral absorption capacity. The contents of N, P, K, Fe, Mg and B increased in grafted watermelon seedlings with pumpkin root-cutting. At the same time, the seedlings increased the IAA, CTK and ETH contents of root to promote the root growth. However, the seedlings by using this grafting method had not enough root system at the early stage, which decreased the accumulation of mineral elements. The content of Ca was reduced in grafted watermelon seedlings with pumpkin root-cutting, which may be related to the difference in root architecture, but further research is needed to understand the decrease of Ca absorption. The results of this study can provide theoretical guidance for the fertilizer management of watermelon grafting seedlings, and promote the popularization and application of mechanized grafting technology.