Exogenous Application of Silicon and Zinc Attenuates Drought Tolerance in Eruca sativa L. Through Increasing Chlorophyll Pigments, Osmoprotectants, and Modulating Defense Mechanisms

Drought stress, at different growth stages, causes substantial yield loss in major crops, thus, threatening global food security. Enhancement of drought stress tolerance in plants has been reported with the sole application of silicon (Si) and Zinc (Zn). However, the synergistic effects of their combined application and the exact mechanisms through which they enhance drought tolerance are still not fully understood. Therefore, the present study investigated the potential of Si and Zn to ameliorate drought stress in Eruca sativa (L.) and associated morphological and physio-biochemical mechanisms. The seeds were soaked in individual or combined Si (3 mM, from calcium silicate) and Zn (10 mM, from zinc sulfate) solutions for 8 h and 12 h. Water-primed and unprimed seeds were kept as control. Later, primed seeds were grown under various levels of drought viz. control (100% field capacity, FC) and 50% FC. Results revealed that drought stress significantly reduced growth traits, photosynthetic pigments, and reactive oxygen species and decreased antioxidative enzymes activities. Seed priming with Si and Zn increased seedling lengths, and their fresh and dry weights under drought stress with significantly higher values for the combined application of both. Moreover, Si and Zn-treated plants also showed a significant increase in chlorophyll pigments, i.e., chlorophyll and carotenoid contents in drought-exposed seedlings. Seed priming treatments improved drought-induced oxidative stress tolerance associated with higher activities of enzymatic and non-enzyme antioxidant activities and soluble sugar and proline contents. The results suggested that the combined application of Si and Zn serves as an appropriate treatment to increase the seedling growth, and physiological and biochemical attributes of E. sativa under drought stress conditions.