Effect of Relative Humidity on Physiology and Antioxidant Metabolism of Grafted Watermelon Seedlings

This study investigated the effect of different relative humidity (RH) regimes on graft healing of grafted seedlings of watermelon (Citrullus vulgaris Schrad.). Two watermelon cultivars (‘Speed’ and ‘Sambok Honey’) were grafted onto the ‘RS-Dongjanggun’ bottle gourd rootstock (Lagenaria siceraria Stanld.) and the grafted seedlings were maintained under one of three relative humidity regimes, 95-96% [1.1-0.8 (day) or 0.8-0.6 (night) g·m -3 vapor pressure deficit (VPD)], 97-98% [ 0.7-0.4 (day) or 0.5-0.3 (night) g·m -3 (VPD)], or 99-100% [0.3-0.0 (day) or 0.2-0.0 (night) g·m -3 (VPD)] according to the Mollier diagram based on the air temperature of 25℃ day/18℃ night with 16 h photoperiod per day. Among the RH treatments, 97-98% significantly increased plant height and fresh weight of the rootstock and scion of the ‘Speed’ and it also enhanced the graft union connection of both cultivars after two days of grafting. However, plant height and thickness of the scion of ‘Sambok Honey’ was increased by the 99-100% RH treatment. Furthermore, both cultivars grown in the 95-96 and 97-98% RH treatments consisted of lower levels of endogenous H₂O₂ and less activities of antioxidant enzymes which illustrated the occurrence of less oxidative stress. Hence, the results of this study identified the optimal RH level for the graft healing of watermelon seedlings.