Exogenous Application of Silicon and Sulfate Improved Drought Tolerance in Sunflowers Through Modulation of Morpho-physiological and Antioxidant Defense Mechanisms

Sunflower ( Helianthus annuus L.) is one of the main oilseed crops used in essential components of human and animal nutrition. Drought stress due to climate change severely affects the sunflower's growth and productivity. Therefore, the current study investigated the potential of sole and combined application of silicon (Si) and sulfate (SO 4 ) in improving morpho-physiological and yield-related traits of sunflower under drought stress. Experimental treatments consisted of two drought stress levels, i.e., 80% water holding capacity (WHC) (no drought stress) and 40% WHC (drought stress at 50% heading) and five foliar treatments of Si and SO 4 , i.e., control (Ck), water spray, 4.0 mM Si, 4.0 mM SO 4 and Si + SO 4 (4.0 mM + 4.0 mM). The experiment was performed in a complete randomized design (CRD) with two factor factorial arrangements having three replications. Drought stress significantly reduced leaf chlorophyll pigments and relative water contents. Similarly, plant height, stem girth, head diameter, leaf area index, numbers of achene per head, 100-achene weight, and biological and achene yields per plant were notably reduced by drought stress. Results revealed that chlorophyll and relative water contents significantly increased with the sole and combined application of Si and SO 4 under drought stress. Moreover, combined application of Si and SO 4 significantly improved superoxide dismutase (25.51%), peroxidase (33.88%), catalase (24.83%) and ascorbate peroxidase (40.75%) under drought stress. Likewise, higher accumulations of soluble proline and protein were noted with the combined application of Si and SO 4 under drought stress. The combined application of Si and SO 4 enhanced plant height, stem girth, head diameter and leaf area index which consequently increased the 19.46% number of achene per head, 10.70% 100-achene weight, 17.12% biological yield and 17.21% achene yield per plant under drought stress as compared to the control treatment. The combined application of Si and SO 4 could alleviate the adverse impact of drought stress on the growth and development of sunflowers. Overall, the application of Si and SO 4 showed significant potential for increasing sunflower productivity in drought-affected regions. Furthermore, Si and SO 4 applications could regulate multiple antioxidant defence systems at the transcriptional level in response to drought stress.