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 heterozygous mutants with reduced activities of chloroplastic glutamine synthetase (GS-2), glycine decarboxylase (GDC) or serine : glyoxylate aminotransferase (SGAT). Well-watered plants of all four genotypes had identical rates 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 conditions. Analysis of chlorophyll a fluorescence revealed that, in the GDC and SGAT mutants, the lower rates of photosynthesis coincided with a decreased quantum efficiency of photosystem II and increased non-photochemical dissipation of excitation energy. Correspondingly, the de-epoxidation state of xanthophyll-cycle carotenoids was increased several-fold in the drought-stressed GDC and SGAT mutants compared with the wild type. Accumulation of glycine in the GDC mutant was further evidence for increased photorespiration in drought-stressed barley. The effect of drought on the photorespiratory enzymes was determined by immunological detection of protein abundance. While the contents of GS-2 and P- and H-protein of the GDC complex remained unchanged as drought stress developed, the content of NADH-dependent hydroxypyruvate reductase increased. Enzymes of the Benson–Calvin cycle, on the other hand, were either not affected (ribulose-1,5-bisphosphate carboxylase-oxygenase and plastidic fructose-1,6-bisphosphatase) or declined (sedoheptulose- 1,7-bisphosphatase and NADP-dependent glyceraldehyde-3-phosphate dehydrogenase). These data demonstrate that photorespiration was enhanced during drought stress in barley and that the control exerted by photorespiratory enzymes on the rate of photosynthetic electron transport and CO2 fixation was increased

Bread wheat varieties as influenced by different nitrogen levels

Experiment was conducted to determine the effect of different nitrogen levels on four bread wheat varieties (Triticum aestivum L.) viz. Inqilab-91, Daman-98, Dera-98 and Punjab-96 at Gomal University, Dera Ismail Khan (NWFP), Pakistan during 2000~2001. The experiment was laid out in split plot design having four replications using a net plot size of 2 m×5 m. Nitrogen doses used were 0, 50, 100, 150 and 200 kg/ha. The results showed that different nitrogen levels had significant effects on plant height, total number of plants/m(2), number of grains/spike, number of spike/m(2), spike weight, biological yield, grain yield and grain protein content. Maximum plant height, total number of plants/m(2), number of spikes/m(2), spike weight, biological yield and grain protein content were observed at 200 kg N/ha. Among wheat varieties Daman-98 had maximum plant height, spike weight, grains/spike, 1000-grain weight, biological yield and grain yield. Inqilab-91 had heavier grains and the most grain protein content, while Dera-98 had the maximum plant population and spikes/m(2). Grain yield and biological yield were statistically similar at doses of 150 kg N/ha and 200 kg N/ha. However, dose of 200 kg N/ha, compared to dose of 150 Kg N/ha, significantly increased the protein content