Drought stress is one of the most important limiting factors, decreasing the production of vegetable crops in the global scale. The aim of this study was initially to examine two extreme seeded watermelon genotypes (C6 and C12) using physiological and biochemical characteristics in response to drought stress. The expression of key genes against drought stress (CLsHSP18.1A and CcNAC2) were also investigated using RT-PCR in the same condition. The highest concentration of proline content occurred in C6 (tolerant genotype) with 9.93 μmol/g fresh leaf after 5 days water stress. Malondialdehyde (MDA) and electrolyte leakage were significantly higher in C6 during drought stress. The result of gene expression levels for CLsHSP18.1A and CcNAC2 indicated that there was a significant increase of expression in both genes in response to drought stress. In C6, drought stress after four days caused 2.5 fold up-regulation of CLsHSP18.1A compared with the control (p≤0.05), but in C12 (susceptible genotype), the expression decreased 1.4 fold compared to the relevant control. Semi-quantitative measurement of CcNAC2 expression in both genotypes showed that maximum up-regulations occurred after two days of stress with the highest level in C6. Furthermore, four days after stress, a significant decrease occurred in the expression of this gene in both genotypes, but there was not a significant expression difference between susceptible C12 and its relative control. Overall, the expression levels of both targeted genes were higher in tolerant (C6) than that of susceptible genotype (C12) in all time points of drought stress. It seems that these genes may play a role in mechanisms involving in the drought tolerance in watermelon which could be important in molecular breeding of drought tolerant watermelons.

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