The role of photorespiration during drought stress: an analysis utilizing barley mutants with reduced activities of photorespiratory enzymes

The significance of photorespiration in drought-stressed plants was studied by withholding water from wild-type barley (Hordeum vulgare L.) and from h eterozygous mutants with reduced activities of chloroplastic glutamine synt hetase (GS-2), glycine decarboxylase (GDC) or serine : glyoxylate aminotran sferase (SGAT), Well-watered plants of all four genotypes had identical rat es of photosynthesis. Under moderate drought stress (leaf water potentials between -1 and -2 MPa), photosynthesis was lower in the mutants than in the wild type, indicating that photorespiration was increased under these cond itions. Analysis of chlorophyll a fluorescence revealed that, in the GDC an d SGAT mutants, the lower rates of photosynthesis coincided with a decrease d quantum efficiency of photosystem II and increased non-photochemical diss ipation of excitation energy, Correspondingly, the de-epoxidation state of xanthophyll-cycle carotenoids was increased several-fold in the drought-str essed GDC and SGAT mutants compared with the wild type, Accumulation of gly cine in the GDC mutant was further evidence for increased photorespiration in drought-stressed barley. The effect of drought on the photorespiratory e nzymes was determined by immunological detection of protein abundance. Whil e the contents of GS-2 and P- and H-protein of the GDC complex remained unc hanged as drought stress developed, the content of NADH-dependent hydroxypy ruvate reductase increased. Enzymes of the Benson-Calvin cycle, on the othe r hand, were either not affected (ribulose-1,5-bisphosphate carboxylase-oxy genase and plastidic fructose-1,6-bisphosphatase) or declined (sedoheptulos e-1,7-bisphosphatase and NADP-dependent glyceraldehyde-3-phosphate dehydrog enase). These data demonstrate that photorespiration was enhanced during dr ought stress in barley and that the control exerted by photorespiratory enz ymes on the rate of photosynthetic electron transport and CO2 fixation was increased.