Influence of Osmotic Stress on Nitrate Reductase Activity in Wheat (Triticum aestivumL.) and the Role of Abscisic Acid

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Abstract
Wheat seedlings (Triticum aestivum L. cv. Timmo) were treated with up to 20% (w/v) polyethylene glycol (PEG, mol. wt. 3350) in the nutrient medium for 6 d. Shoot growth and nitrate transport and metabolism were substantially affected by PEG treatment. At 20% PEG (corresponding to a water potential of approximately −1.6 MPa), which caused plants to wilt within 1–2 h, activity of nitrate reductase (NR) declined with a half-life of approximately 5 h in both roots and shoots. The decline was considerably slower at lower PEG concentrations. Significant increases in levels of abscisic acid (ABA) only occurred in shoots. Application of ABA to intact plants or excised shoots did not induce or accelerate decline in shoot NR activity. The rapid decline in NR activity during wilting appears unrelated to both nitrate flux and ABA. At lower PEG concentrations and in the long-term, however, NR activity corroborates rates of both transport and growth-related utilization of nitrate. The role of ABA in this context appears to be indirect through its action on stomatal function which reduces water flux and gas exchange.