Different Traits Affect Salinity and Drought Tolerance during Germination of Citrullus colocynthis, a Potential Cash Crop in Arid Lands

Citrullus colocynthis, a native plant with potential uses as a feedstock for edible oil, biodiesel, and animal feed make it a potential cash crop. The importance of propagating this species under saline arid habitats necessitates understanding environmental factors affecting salinity and drought tolerance during the germination stage. Here, we assessed the impacts of seed collection time, the temperature of incubation on salinity, and drought tolerance during the seed germination stage of different accessions growing in the botanical garden of the University of Sharjah. No previous study assessed the interactive effects of these factors on the drought and salinity tolerance of this species. Three accessions (9, 10, and 13) differed in fruit and seed size and color, and germination behavior, and were selected from an earlier study. Seeds that matured in summer and winter on these accessions were treated with three salinities (0, 50, and 100 mM NaCl), and PEG levels (0, −0.3, and −0.6 MPa) and incubated at two temperatures (20/30 and 25/35 °C). The results showed significant effects of all factors (collection time, temperature, drought, and salinity) and their interactions on germination percentage. Seeds of C. colocynthis were very sensitive to salinity and drought stress, and the sensitivity depended on the time of seed collection, accession, and incubation temperature. The overall germination and tolerance to salinity and drought were significantly greater in seeds of accession 10, seeds that matured in summer, and seeds incubated at 25/35 °C. The germination in NaCl solutions was greater than in PEG solutions, indicating that seed germination was more sensitive to osmotic stress created by PEG than NaCl. Moreover, when transferred from NaCl, the recovery of ungerminated seeds was greater than in PEG solutions. This result indicates that the detrimental effect of salinity in C. colocynthis could be mainly attributed to osmotic rather than ion-toxicity effects. To adopt C. colocynthis as a cash crop or to restore degraded desert habitats, it is recommended to use seeds of drought- and salt-tolerant accessions (e.g., 10), especially those that mature in summer.