Changes in enzymatic and nonenzymatic antioxidant defense mechanisms of canolaseedlings at different drought stress and nitrogen levels

To evaluate the responses of canola seedlings to different levels of drought stress and nitrogen at different growth stages, a factorial experiment was conducted in a completely randomized design with three replications at the experimental greenhouse of Mohaghegh Ardabili University in 2013. Treatments included three levels of drought stress (30%, 50%, and 70% of FC) and five levels of nitrogen (control, based on soil test results; 25% less than the recommended level; 50% less than the recommended level; 25% more than the recommended level; and 50% more than the recommended level). The recommended level of nitrogen, based on soil test results, was 0.09 g of nitrogen per kilogram of soil. Results showed that drought stress, nitrogen, and their interaction significantly affected the enzymatic activity of antioxidant catalase (CAT), polyphenol oxidase (PPO), and peroxidase (PO) at various stages of growth. Proline was also affected by drought stress and nitrogen. Maximum CAT activity (794.04 OD mg protein min^–1) was observed at the 4–6 leaf stage under mild drought stress conditions (50% of FC) and a nitrogen application rate 50% less than the recommended level. Increased rates of nitrogen enhanced the PPO activity at the 4–6 and 6–8 leaf stages. PO had a negative response to increased rates of nitrogen application. The highest rate of increase in proline was at the 8–10 leaf stage: 205% and 207% higher under mild drought stress (50% of FC) and severe drought stress (30% of FC) conditions, respectively, compared to favorable moisture conditions (70% of FC). Increased nitrogen application led to an increase in proline production at all stages of sampling (4–6, 6–8, and 8–10 leaf stages).