Application Methods of Zinc Sulphate Increased Safflower Seed Yield and Quality under End-Season Drought Stress

Zinc deficiency is one of the most widespread nutritional problems, affecting nearly one-third of the world population. In addition, it is known that zinc deficiency not only reduces crop yield but also its quality. The effect of different methods of zinc application on the growth, yield, and quality of safflower seeds under regular irrigation and interruption of irrigation from flowering to harvest (82 and 80 DAS in the first and second years, respectively) was evaluated. Zinc sulfate was applied in both soil and foliar methods. The zinc sulfate treatments include no zinc sulfate, soil application of 20, 40, and 60 kg ha−1 at the planting stage; spraying 2.5, 5, and 7.5 g L−1 in the rosette stage; and spraying 2.5, 5, and 7.5 g L−1 in the flowering stage. The end-season drought caused a decrease in the chlorophyll index, leaf area index, relative water content, plant height, yield components, biological yield, seed yield, harvest index, seed oil content, oil harvest index, and seed element content compared to regular irrigation. The decrease in yield occurred with a decrease in the capitol number and diameter, seed number per capitol, and 1000-seed weight. The severity of the damage of the end-season drought stress in the second year was higher than in the first year due to the higher temperatures and the decrease in the rainfall. In both years, the application of zinc sulfate in different ways had an increasing effect on the studied traits in both normal and stress conditions. The application of zinc sulfate reduced the negative effects of unfavorable environmental conditions and improved the yield and nitrogen, phosphorus, potassium, zinc, and iron element content in the seed. In both application methods of zinc sulfate, the increment in the zinc sulfate concentration decreased the seed phosphorus content. However, the phosphorous content was more than that of the treatment of non-zinc application. The application of zinc increased the biological, seed, and oil yield of the treated plants, but the seed and oil yield were more affected. This effect was shown in the seed and oil harvest index increment. Under regular irrigation, higher concentrations of zinc sulfate enhanced plant performance, but under stress conditions, medium and lower concentrations were more effective. The highest 1000-seed weight and potassium and zinc content were obtained by spraying zinc sulfate at 5 g L−1 in the flowering stage under normal irrigation conditions. A comparison of the two methods of applying zinc sulfate showed that foliar spraying was more effective than soil application in improving the seed yield. The soil application is more effective on biological yield than seed yield.