Increasing Efficiency of Tomato (Lycopersicon esculentum Mill.) Resistance to Drought Stress with Conventional and Molecular Genetic Methods

Reliable and sustainable production of food crops is highly related to genetic diversity and continuous germplasm improvement, particularly, when the desirable traits show high heritability. Drought tolerance of tomato (Lycopersicon esculentum Mill.) is a trait to be urgently improved due to recent climate changes and limited water availability. Therefore, a greenhouse screening experiment was carried out at King Abdulaziz University (KAU, Jeddah, Saudi Arabia) in 2014 and 2015 under three levels of drought stress (600, 400 and 200 mL water on 3 days per week) to identify tomato cultivars having improved drought tolerant. Several sensitivity and tolerance indices were determined based on morphological markers. Aiming at establishing a correlation to these markers, a total of 16 Inter-Simple Sequence Repeat (ISSR) primers was used, additionally elucidating the genetic diversity among cultivars and clustering the cultivars into groups based on their molecular profiles. The results indicated that selection indices such as geometric mean productivity (GMP), mean productivity (MP), tolerance index (TOL), and stress tolerance index (STI) represented suitable indices for screening the drought tolerance of tomato cultivars. The cultivars C9 followed by C15 and C11 were identified as the most drought tolerant genotypes, while cultivars C1, C2, C6, C7 and C13 were classified as cultivars being sensitive to drought stress. An interesting correlation of the ISSR analyses to these morphological findings was established according to 83 detectable fragments derived from 10 primers. Among these 83 fragments, 35 were polymorphic across the cultivars. Specific fragments were proposed to be used for future drought tolerance screenings of larger cultivar germplasm. The highest value of the effective multiplex ratio (EMR) and marker index (MI) was detected for primer INC7 followed by INC1. Genetic relationships among the cultivars were evaluated by generating a similarity matrix based on the Jaccard's coefficient and the Unweighted Pair Group Method with Arithmetic Average (UPGMA) dendrogram. Based on Jaccard's similarity coefficients, the genetic distance of the genotypes varied from 0.702 to 0.942 with a mean value of 0.882. The results showed a clear-cut separation of the 15 tomato cultivars due to their genetic variability as compared to local tomato accessions, making them a valuable genetic source for their incorporation into potential breeding programs. Molecular data were in good agreement with the results considering selection indices, and both of them will be useful tools for the future preservation and improvement of the tomato germplasm.