Improving Canola (Brassica napus L.) Seed Yield through Micronutrients Sprays: Effects on Enzymatic and Non-Enzymatic Defense under Different Irrigation Regimes

Drought stress is a major limited factor that decreased seed yield and applying micronutrients such as Zinc (Zn), Manganese (Mn) and Boron (B) is considered as agricultural management practices for improving the seed yield, oil content and tolerance under different irrigation conditions. A two-year experiment was carried out as a split-plot arrangement based on a randomized complete block design with three replications during two years (2015-2017). Main plots consisted of three irrigation regimes (Optimal irrigation, cessation of irrigation from flowering, and silique setting stages) and subplots included 10 levels of foliar spray of micronutrients (2, 4, 6 g/L Zn; 2, 4, 6 g/L Mn; 2, 4, 6 g/L B, and the foliar spraying of distilled water as control treatment). Cessation of irrigation led to increase in the activity of Polyphenol oxidase (PPO), Catalase (CAT), Peroxidase (PO), Ascorbate peroxidase (APX), Superoxide dismutase (SOD), Proline (PRO) and Malondialdehyde (MDA) but seed yield (SY) and oil content (OC) decreased. Cessation of irrigation at flowering and silique stages led to a decrease in seed yield by 39.3 and 13.1%, as compared with the optimal irrigation regime, respectively. The interaction of irrigation regime x foliar spray revealed that spraying Zn (4 g/L) and B (4 g/L) maximized the SY (4311.11 kg/ha) under optimal irrigation regime. Under cessation of irrigation from flowering stage, the foliar spray of Mn (6 g/L) produced the highest SY (2862.2 kg/ha) during two tears. Cessation of irrigation at flowering and silique stages caused in decrease OC by 3.3 and 6.5%, respectively. Overall, the foliar spraying of Zn, Mn and B improved the SY and OC of canola under optimal water supply and drought stress regimes.