Rootstock Effect of Auto- and Allotetraploid Citron (Citrullus lanatus var. citroides) on Hydroponically Grown Cucumber Under Salt Stress

A hydroponic experiment was conducted to assess whether grafting with citroides rootstocks could improve the salt tolerance of cucumber. One cucumber cultivar (Mercur F1) was grafted onto six diploid and tetraploid (auto and allo) citroides genotypes and the commercial rootstocks Argentario and RS841. Plants were grown in hydroponic culture at two electrical conductivity (EC) levels (control at 1.5 dS m−1 and salt at 6.0 dS m−1). Hydroponic salt stress led to a significant reduction in biomass growth of both grafted and nongrafted cucumbers. However, the plants least affected by salt stress were those grafted onto tetraploid citroides rootstocks. The leaf nutrient uptake of cucumber plants was significantly (p < 0.001) affected by salt, graft combination, and the salt × graft interaction. Ion leakage was significantly increased by salt application, and rootstock genotypic variation was significant. While the highest amount of proline was measured in plants grafted onto RS841 and N7T, the lowest amount of proline was determined in nongrafted control plants. Antioxidative enzyme activities were significantly affected by rootstocks under both control and salt-stress conditions. In this study, all graft combinations showed increased superoxide dismutase, catalase, and peroxidase activities with salt application, which differed according to rootstock genotypes. Tetraploid citroides cultivars have high rootstock potential for cucumber and their significant contribution to salt tolerance was closely associated with inducing physiological and biochemical responses of scions. These traits could be useful for the selection and breeding of salt-tolerant rootstocks for sustainable agriculture in the future.