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SummaryTotal GST (glutathione S-transferase, EC 2.5.1.18) and glutathione peroxidase activity were measured spectrophotometrically in Triticum aestivum cv. MV Emese and cv. Plainsman (drought tolerant) and cv. GK Élet and Cappelle Desprez (drought sensitive) flag leaves under control and drought stress conditions during the grain filling period, in order to reveal possible roles of different GST classes in the senescence of flag leaves. Six wheat GSTs, members of 3 GST classes, were selected and their regulation by drought and senescence was investigated. High glutathione peroxidase activity (EC 1.11.1.9) was measured in wellwatered controls of the drought tolerant Plainsman cultivar. At the same time TaGSTU1B and TaGSTF6 sequences, investigated by Real-Time PCR showed high expression levels that increased with time indicating that the gene products of these genes may play important roles in monocarpic senescence of wheat. The expression levels of these genes were also induced by drought stress in all of the four investigated cultivars, but extremely high transcript amounts were detected in cv. Plainsman. Our data indicate genotypic variations of wheat GSTs; high expression levels and early induction of two senescenceassociated GSTs under drought during grain filling in flag leaves correlated with high yield stability.

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The effects of increasing osmotic stress induced by 100-400 mOsm (-0.976 MPa) polyethylene glycol (PEG 6000) were investigated in a drought-tolerant (Triticum aestivum L. cv. Mv Emese) and drought-sensitive (cv. GK É let) wheat cultivar at the three-leaf stage. During osmotic stress, the decline of the water potential (w w ) was more significant in the leaves, while the abscisic acid (ABA) levels of the roots increased earlier and remained higher in the sensitive than in the tolerant variety. There was an increasing gradient of ABA content toward the youngest leaves in the drought-sensitive GK É let, while more ABA accumulated in the fully developed, older leaves of the tolerant cultivar Mv Emese. In accordance with the rapid and significant accumulation of ABA, the stomatal conductance decreased earlier in the tolerant cultivar. The effect of water stress on the PSII photochemistry was pronounced only 1 week after the exposure to PEG, as indicated by the earlier decrease of the net CO 2 fixation, the effective quantum yield (U PSII ) and the photochemical quenching (q P ) in light-adapted samples of the tolerant variety in 400 mOsm PEG 6000. The stress treatment caused more significant reductions in these parameters toward the end of the experiment in the sensitive cultivar. In spite of small differences in the photosynthetic characteristics, the net biomass production was not significantly altered by this osmotic stress. The accumulation of ABA controlled the distribution of the biomass between the shoot and root systems under osmotic stress, and contributed to the development of stronger and deeper roots in the drought-sensitive cultivar GK É let. However, the root elongation did not correlate with the drought sensitivity of these cultivars on the basis of crop yield.