Effects of double-root-cutting grafting 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 understand 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 cellfilled 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 well-developed 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

An Integrated Analysis of Anatomical and Sugar Contents Identifies How Night Temperatures Regulate the Healing Process of Oriental Melon Grafted onto Pumpkin

Graft healing is a complex process affected by environmental factors, with temperature being one of the most important influencing factors. Here, oriental melon grafted onto pumpkin was used to study changes in graft union formation and sugar contents at the graft interface under night temperatures of 18 °C and 28 °C. Histological analysis suggested that callus formation occurred 3 days after grafting with a night temperature of 28 °C, which was one day earlier than with a night temperature of 18 °C. Vascular reconnection with a night temperature of 28 °C was established 2 days earlier than with a night temperature of 18 °C. Additionally, nine sugars were significantly enriched in the graft union, with the contents of sucrose, trehalose, raffinose, D–glucose, D–fructose, D–galactose, and inositol initially increasing but then decreasing. Furthermore, we also found that exogenous glucose and fructose application promotes vascular reconnection. However, exogenous sucrose application did not promote vascular reconnection. Taken together, our results reveal that elevated temperatures improve the process of graft union formation through increasing the contents of sugars. This study provides information to develop strategies for improving grafting efficiency under low temperatures

An Experimental Study on the Effect of Cutting Angle on the Growth of Grafted Watermelon Seedlings Using the One-Cotyledon Grafting Method

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. The cutting angle, which affects the matching of the rootstock and scion, is key to the survival of the graft seedling. In the production of cucurbit graft seedlings, the cutting parameters are established based on experience, leading to low grafting success. To determine accurate cutting parameters, the watermelon cv. ‘Zaojia84-24′ was used as the scion and the pumpkin cv. ‘Zhenzhuang’ was used as rootstock, and two one-way experiments investigating the cutting angle of the watermelon scion and the rootstock as factors were conducted. The cutting angle of the rootstock and scion had no significant effect on the xylem reconnection rate or the grafting survival rate. A larger cutting angle for the rootstock and scion led to a delay in the reconnection of the phloem. Different cutting angles for the scion significantly affected the growth of the scion after grafting. Compared with a scion graft cutting angle of 10° (SL), graft cutting angles of 14° (SM) and 20° (SS) led to significantly greater scion dry weights, with increases of 16.00% and 18.61%, respectively. Different cutting angles of the rootstock significantly affected the growth of roots after grafting. Compared with a rootstock graft cutting angle of 10° (RL), graft cutting angles of 17° (RM) and 27° (RS) led to significantly greater root dry weights, with increases of 29.33% and 22.54%, respectively. The results of this study can provide a reference for the design of cutting mechanisms for cucurbit grafting robots, improving the cutting precision of grafting robots.</jats:p

An Experimental Study on the Effect of Cutting Angle on the Growth of Grafted Watermelon Seedlings Using the One-Cotyledon Grafting Method

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. The cutting angle, which affects the matching of the rootstock and scion, is key to the survival of the graft seedling. In the production of cucurbit graft seedlings, the cutting parameters are established based on experience, leading to low grafting success. To determine accurate cutting parameters, the watermelon cv. ‘Zaojia84-24′ was used as the scion and the pumpkin cv. ‘Zhenzhuang’ was used as rootstock, and two one-way experiments investigating the cutting angle of the watermelon scion and the rootstock as factors were conducted. The cutting angle of the rootstock and scion had no significant effect on the xylem reconnection rate or the grafting survival rate. A larger cutting angle for the rootstock and scion led to a delay in the reconnection of the phloem. Different cutting angles for the scion significantly affected the growth of the scion after grafting. Compared with a scion graft cutting angle of 10° (SL), graft cutting angles of 14° (SM) and 20° (SS) led to significantly greater scion dry weights, with increases of 16.00% and 18.61%, respectively. Different cutting angles of the rootstock significantly affected the growth of roots after grafting. Compared with a rootstock graft cutting angle of 10° (RL), graft cutting angles of 17° (RM) and 27° (RS) led to significantly greater root dry weights, with increases of 29.33% and 22.54%, respectively. The results of this study can provide a reference for the design of cutting mechanisms for cucurbit grafting robots, improving the cutting precision of grafting robots

A Comparative Analysis of the Grafting Efficiency of Watermelon with a Grafting Machine

The rising age of the population in rural China and the labor intensity of grafting have resulted in a decrease in the number of grafters and a subsequent increase in their wages. Manual grafting can no longer satisfy the increasing demand for watermelon-grafted transplanting; thus, machine grafting will be an effective alternative. In order to accelerate the implementation of machine grafting in China, a comparative analysis between the automatic grafting machine (model 2TJGQ-800) and traditional hand grafting was conducted. The reliability and feasibility of machine grafting were evaluated through a comprehensive evaluation of the production capacity and grafting seedling quality. This study focuses on the grafting application of watermelon plug-tray seedlings. The scion and rootstock seeds were sown on 9 November 2022. Grafting experiments using an automatic grafting machine, skilled workers, and ordinary workers were conducted with the root-pruned one-cotyledon grafting method on 24 November 2022. The results showed that the machine grafting had a high uniformity and grafting speed. The grafting speed of the grafting machine was 774 plant&middot;h&minus;1 and 1.65&ndash;2.55-fold higher than the hand grafting. With training, workers can improve their grafting speed, but it will still be slower than machine grafting. In addition, there was no significant difference in the grafting survival rate between the machine grafting and hand grafting. However, using machine grafting, the success rate decreased from 100% to 90.07% and the rootstock regrowth rate increased from 18.44% to 72.69%. Incomplete rootstock cutting, clip supply failure, and grafting drop failure are the three main factors that result in machine grafting failure. In conclusion, the grafting machine has advantages in terms of grafting speed and uniformity. Upon improving the accuracy of the cutting mechanism and grafting success rate, it will be adopted by commercial nurseries