Growth, photosynthesis and adaptive responses of wild and domesticated watermelon genotypes to drought stress and subsequent re-watering

The growth and morphophysiological responses of wild watermelon (C. lanatus var. citroide) M20 and Chinese domesticated watermelon (C. lanatus var. lanatus) Y34 to drought stress and subsequent re-watering were compared. Wild watermelon is drought-tolerant, whereas the domesticated watermelon is susceptible. Irrigation was withheld from seedlings for 10 days and the seedlings were then allowed to recover for 1 day. Drought treatment resulted in the wilting and yellowing of leaves in both genotypes, but symptoms occurred earlier and more visibly in Y34. Drought stress inhibited the growth of both genotypes but increased the root/shoot ratio more pronouncedly in M20 than in Y34. Under drought conditions, M20 maintained a higher leaf water status than Y34 due to its denser trichomes and more sensitive stomatal control, which minimized the transpiration rate. Y34 was more vulnerable to drought, resulting in larger decreases in photosystem II efficiency, initial Rubisco activity and chlorophyll concentration. H2O2, O2−, and MDA contents were significantly increased in both genotypes; however, these increases were smaller in M20, possibly due to a greater enhancement of antioxidant enzyme activities and related-gene expression levels. Moreover, M20 accumulated soluble sugars and proline to greater levels to counter reduced soil moisture. These adaptive mechanisms enabled M20 to recover more rapidly after re-watering. Our findings provide guidance for improving the drought tolerance of Chinese watermelon cultivars.