Use of an in vitro assay to investigate the antioxidative defence potential of wheat genotypes under drought stress as influenced by nitrogen nutrition

A rapid 'in vitro'-test was used to study the impact of mineral nutrition on the antioxidative defence potential of wheat genotypes under drought stress. Segments of the third leaf of hydroponically grown wheat seedlings were subjected to a 8 hour treatment by floating segments on destilled water or a solution containing PEG 6000 under high light intensity, simulating drought stress on a cellular level. After the treatment, the leaf segments were evaluated for their extent of pigment breaching. Plasma membrane damage was estimated by measuring the efflux of K(+) into the incubation solution. Comparing six genotypes, the extent of leaf damage was significantly correlated to the activity of glutathione reductase (r(2) = 0.909; P = 0.009). Two genotypes, differing in tolerance to drought stress under field conditions, were grown at two levels (1 or 4 mM) of nitrate, ammonium or a combination of both, and were compared using the described assay. Photooxidative damage was less severe at low N-level and under NH(4)(+) supply. At 4 mM N tocopherol contents decreased relatively to the chlorophyll levels, irrespectively of the form of N supply, whereas xanthophyll/chrorophyll-ratios were not affected, indicating a similar capacity for harmless dissipation of excitation energy at both N levels. At 4 mM NH(4)(+) the leaf concentrations of K(+) and Mg(2+) were markedly lower. Therefore, during desiccation harmful tissue concentrations of this ions might have been avoided. Besides, NH(4)(+) grown plants contained higher amounts of reduced N compounds, especially of putrescine which could have exerted a protecting effect on the membrane integrity during the photooxidative stress treatment.