Rootstock Effects On the Water Relations of Grafted Watermelons

Watermelons and other cucurbits are susceptible to several root rot, vine decline, and wilt diseases that impact productivity by reducing root capacity for water and nutrient uptake. Grafting susceptible varieties on resistant rootstocks often alleviates this problem; however, graft incompatibility and poor fruit quality have frequently been reported, hence the need to identify alternative rootstocks. The goal of this study was to characterize physiological and growth among watermelon plants grafted on rootstocks from different genera in order to identify traits that may improve drought and disease tolerance. Leaf photosynthesis (Pn) stomatal conductance (gs), transpiration (E), water potentials (Ψl), root dry matter allocation, and root hydraulic conductance (Lr) characteristics of five-week old watermelon seedlings (cv ‘Super Seedless 7167’) grafted on four hybrid squash varieties: Cucurbita maxima cv: ‘Strong Tosa’, ‘RS1330’, ‘Shintosa Camel’, and ‘Tetsukabuto’; and a Lageneria siceraria variety (cv Pelops) were studied. Leaf water potential, leaf stomatal conductance (gs), transpiration (E) and photosynthesis (Pn) values were significantly higher among grafted plants compared to non-grafted control plants.  Root:shoot allometric relationships differed between grafted plants and non-grafted plants, and also among plants grafted on different rootstocks. Non-grafted plants generally had lower root mass allocation coefficients. Plants grafted on ‘RS1330’ had the highest root dry mass allocation coefficients.  The average Lr of grafted plants was significantly greater than that of non-grafted plants. Although biomass allocation to roots was greatest in ‘RS1330’, Lr values were greatest in ‘Tetsukabuto’.  Observations of shoot physiological properties (Ψl and gs) among grafted plants suggest a strong link with the capacity for water flux through root systems of the rootstocks studied.