Gradual Drought Under Field Conditions Influences the Glutathione Metabolism,Redox Balance and Energy Supply in Spring Wheat

Glutathione (GSH) metabolism, redox balance and energy supply in spring wheat (Triticum aestivum L.) during gradual drought stress under field conditions were investigated. Although levels of total and reduced GSH were decreased, the ratio of GSH/GSSG (glutathione disulfide) was markedly increased by drought. Levels of GSH biosynthetic precursors, cysteine (Cys) and -glutamylcysteine (-GC), and the activities of their biosynthetic enzymes, -glutamylcysteine synthetase (-GCS) and glutathione synthetase (GSHS) were also significantly increased in stressed plants. Glutathione reductase (GR) activity, which is responsible for the conversion of GSSG to GSH, was also increased under this field stress. However, two other important enzymes in GSH metabolism, glutathione peroxidase (GP) and glutathione S-transferase (GST), showed decreased activity in the droughted plants. These results suggest that the higher ratio of GSH/GSSG, the rate of GSH biosynthesis and the capacity of its redox cycling rather than GSH accumulation might be essential for drought resistance of plants. Activities of the two key Calvin-cycle enzymes possessing exposed sulfhydryl groups, NADP⁺-dependent glyceraldehydes-3-phosphate dehydrogenase (G3PD) and fructose-1,6-bisphosphatase (FBPase) were not affected by drought stress, whereas, activity of the key enzyme in the pentose-phosphate pathway (PPP), 6-phosphogluconate dehydrogenase (6-PGD), increased in the droughted plants. The ratios of NADPH/NADP⁺, NADH/NAD⁺ and ATP/ADP increased in the droughted plants, indicating that an up-regulation of the reduced redox state and the energy supply in the plant cells might be an important physiological strategy for plants responding to drought stress. A simple correlation between the high ratio of GSH/GSSG, the rate of GSH biosynthesis and the redox cycle and the high reduction states of redox status in the plant cells was also observed under field drought.