USE OF AN IN-VITRO ASSAY TO INVESTIGATE THE ANTIOXIDATIVE DEFENSE 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 nutrit ion on the antioxidative defence potential of wheat genotypes under dr ought stress. Segments of the third leaf of hydroponically grown wheat seedlings were subjected to a 8 hour treatment by floating segments o n destilled water or a solution containing PEG 6000 under high light i ntensity, simulating drought stress on a cellular level. After the tre atment, the leaf segments were evaluated for their extent of pigment b reaching. Plasma membrane damage was estimated by measuring the efflux of K+ into the incubation solution. Comparing six genotypes, the exte nt of leaf damage was significantly correlated to the activity of glut athione reductase (r(2) = 0.909; P = 0.009). Two genotypes, differing in tolerance to drought stress under field conditions, were grown at t wo levels (1 or 4 mM) of nitrate, ammonium or a combination of both, a nd were compared using the described assay. Photooxidative damage was less severe at low N-level and under NH4+ supply. At 4 mM N tocopherol contents decreased relatively to the chlorophyll levels, irrespective ly of the form of N supply, whereas xanthophyll/chrorophyll-ratios wer e not affected, indicating a similar capacity for harmless dissipation of excitation energy at both N levels. At 4 mM NH4+ the leaf concentr ations of K+ and Mg2+ were markedly lower. Therefore, during desiccati on harmful tissue concentrations of this ions might have been avoided. Besides, NH4+ grown plants contained higher amounts of reduced N comp ounds, especially of putrescine which could have exerted a protecting effect on the membrane integrity during the photooxidative stress trea tment.