Genetic Variability Analysis for the Selection of Drought-Tolerant Tomato (Lycopersicon esculentum Mill.) Germplasm as Investigated by in Vitro Callus, Shoot, and Plantlet Cultures

In Arab Gulf countries, successful tomato production requires the identification of cultivars having an abiotic stress-tolerance, in particular, against drought. Under drought stress, tomato plants are exposed to numerous changes in their metabolism and gene expression, resulting in decreased yields and inferior quality of the obtained tomato fruits. Without including genetically variable stress-tolerant cultivars, breeding efforts remain ineffective to improve fruit yield and quality. Therefore, the present study aimed at the selection of drought-tolerant tomato cultivars. A total of 15 cultivars was exposed to drought treatments (0.0, 25, 50 mg L-1 PEG6000). For this purpose, cotyledon explants were cultured on callus induction medium, and the calli were then transferred to regeneration medium to study drought stress. The experiment was conducted following a randomized complete block design with three replications. The analysis of variance showed a highly significant two-way interaction (drought level and cultivars) (P <= 0.001) for most of the parameters. Selection indices suggested five cultivarsto be tolerant to drought stress at bothstress levelsstudied, while six cultivars were found to be sensitive to drought stress, thus being only suitable for cultivation under non-drought stress. All measured traits, except for the number of leaves, negatively correlated with SSI (stress sensitivity index) and TOL (tolerance index) at both drought levels, suggesting the suitability of the used traits for drought screening. Cluster analysis based on selection indices discriminated the cultivars into three clustersaccording to their tolerance to drought. The protein profile was analyzed by SDS-PAGE, and several new protein bands at variable molecular weights (110.115, 107, 104.321, 61.900, 54.003, 46.922, 16.456, 16.130, 15.316 and 15.263 kDa) were identified in the plants grown under stress conditions. These polypeptides may be used as future markers todiscriminate stress-tolerant and intolerant tomato plants.