Improvement of biochemical and antioxidant responses of borage (Borago officinalis L.) under drought stress conditions with the use of vermicompost and zinc sulfate

Drought poses a significant threat to crop production systems. Therefore, this study aimed to investigate the impact of vermicompost and foliar application of zinc sulfate under conditions of reduced irrigation on the physiological properties of Borage. A two-year experiment was conducted following a split factorial design within a randomized complete block design with three replications at Yasouj University Research Station in 2017 and 2018. The primary factor involved three levels of irrigation cut-off (I1: full irrigation, I2: irrigation cut-off at the flowering stage, and I3: irrigation cut-off during the seed-filling stage). The sub-factor included vermicompost fertilizer at three different levels (N0: control, N1: 5 ton ha−1, and N2: 10 ton ha−1), and foliar application of zinc sulfate at three levels (Z0: control, Z1: 2 and Z2: 4 mg l−1). During the flowering stage stress, foliar application of 4 mg l−1 of zinc sulfate resulted in an increased chlorophyll a + b content in plants (2.91 mg g−1 FW), while the control showed the lowest amount (2.56 mg g−1 FW). Vermicompost supplementation improved chlorophyll a + b content during the seed-filling stage under conditions of irrigation cut-off. The results indicated that an increase in vermicompost fertilizer application led to an elevation in relative water content (RWC), with the highest RWC (79.2%) achieved when 10 ton ha−1 of vermicompost was applied. Irrigation cut-off during the seed-filling stage resulted in increased electrolyte leakage and higher fertilizer usage, thus reducing cell damage. Furthermore, the findings revealed that applying 2 and 4 mg l−1 zinc sulfate reduced malondialdehyde content by 5% and 9%, respectively. The catalase, peroxidase, and superoxide dismutase activities demonstrated an increased response to stress mitigation treatments. However, their activities decreased as vermicompost and zinc sulfate levels increased. The study demonstrated that the highest biomass was obtained when 10 ton ha−1 of vermicompost and 2 mg l−1 of zinc sulfate were applied. The flowering stage of the plant exhibited the most significant negative impact under stress conditions. Nonetheless, using vermicompost and zinc sulfate, particularly during the seed-filling phase, alleviated the adverse effects of drought stress. In conclusion, our findings indicate that, although drought stress resulted in increased electrolyte leakage due to elevated free radical production, vermicompost, and zinc sulfate played a role in reducing stress.