Evaluating the effect of plastic mulch and some plant growth regulators on changes in some leaf elements, enzymes and nitrate accumulation in onion (Allium cepa) at water deficit conditions

The effect of plant growth regulators (PGRs) and plastic mulch on the amount of some leaf elements, enzymes and nitrate accumulation in onions under water deficit stress was investigated. The main plots consisted of water deficit stress treatment at three levels (100% water requirement, 80% and 60% water requirement), and subplots including mulch (in two levels of application and no application of dark plastic) and PGRs factor in Six levels (zero, salicylic acid (0.5 mM), methyl jasmonate (5 and 7.5 µM), 24-Epibrassinolide (0.5 and 1 µM). Significant interaction was observed between low water stress and PGRs in the amount of nitrogen, catalase, superoxide dismutase and malondialdehyde enzymes, methionine, isoleucine and proline amino acids, ion leakage, chlorophyll and carotenoids pigmentation. The effect of the PGRs was mainly effective in water deficit conditions. Also, the interaction of water deficit and plastic mulch in addition to these traits was significant on the amino acid valine and tyrosine traits and phosphorus and potassium of leaf and this effect was significant under water deficit condition. Brassinolide growth regulator treatment had the most effect on reducing nitrate in onion bulb. Under non stress, mild stress and severe stress condition, 1 micromolar brassinosteroid reduced 26, 23 and 31 percent of onion nitrate, respectively. Also, plastic mulch under non stress conditions have no effect on reducing the accumulation of nitrate, but under severe stress conditions (60% water requirement), use of plastic mulch detergent reduced the 13% accumulation of nitrate compared with the control