Abstract:【Objectives】Grafting has many advantages, such as yield increases, improvements in nutrition and water uptake, and 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 such that 95% of commercial watermelon production is grafted in some regions of the world, the higher cost of grafted watermelon transplants is one of the primary factors limiting adoption 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 pruned one-cotyledon grafting method is the most commonly used method with grafting machines. For root pruned one-cotyledon grafting method, root development is an important factor for seedling quality. Because acquiring strong roots is the main purpose of grafting, it is important to understand the root development of grafted seedlings. However, there is little information on the root growth of root pruning grafted seedlings. In order to apply this grafting technology efficiently, the influence of root-pruning 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 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 ℃ and 60–80% relative humidity. Grafting was performed after the first true leaf developed in the rootstock and scion at ten days after sowing. When grafting, the seedlings were divided into two groups. One group was grafted using root-pruning one-cotyledon splice grafting method (DRC). After grafting, the plants were transplant into 72-cell tray-plug with mixed seedling substrate. The other group was grafted using one-cotyledon splice grafting method (CK), that rootstock has intact root system. The grafted seedlings were placed under a plastic film with a day/night cycle at 28 °C/18 °C and more than 90% humidity under low light intensity (75 umol∙m−2 ∙s −1, 12/12 h photoperiod) to healing. The grafted plants were exposed to the air 1–3 h per day until the scions were alive and normally grown. After the healing process, the root growth parameters such as root morphology indexes, root activity and scattered lump rate were measured at 6-day intervals. At the same time, samples were also harvested for examine the hormone and mineral content.【Results】The root diameter of grafted watermelon seedlings with pumpkin root-pruning was thickened 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-pruning was 86.7% while the scattering rate of grafted watermelon seedlings in the control treatment was only 13.3% on the 12th day after grafting. The contents of IAA, CTK and ETH in the root system of grafted watermelon seedlings with pumpkin root-pruning were significantly increased while the contents of GA and ABA were significantly decreased. The contents of N, P, K, Mg, Fe and B in shoots and roots of grafted watermelon seedlings with pumpkin root-pruning showing a certain degree of increase. However, the contents of Ca in shoots and roots of grafted watermelon seedlings with pumpkin root-pruning were significantly decreased. 【Conclusions】A well-developed root system induces vigorous shoot growth and plant development, and root growth also influenced by shoot. The root of watermelon seedlings grafted by root-pruning grafting method was regenerate and adventitious roots. The regenerate root system has higher root activity and mineral absorption capacity. The content of N, P, K, Fe, Mg and B was increased of grafted watermelon seedlings with pumpkin root-pruning. At same time, the seedlings increase the IAA, CTK and ETH contents of root to promote the root growth. But, the seedlings using this grafting method has not enough root system at the early stage, which result the accumulation of mineral decreased. The content of Ca was reduced of grafted watermelon seedlings with pumpkin root-pruning, which may be related to the difference of root architecture, but further research is needed to understand the decrease of the absorption of Ca. The results of this study can provide theoretical guidance for the fertilizer management of watermelon grafting seedlings production, and promote the popularization and application of mechanized grafting technology.
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