Effect of water stress on photosystem 2 in two wheat cultivars

The effects of osmotic dehydration on the photochemical activity, gene transcription, and protein content of photosystem 2 (PS 2) in leaves of two wheat (Triticum aestivum L.) cultivars Miannnong No. 4 and No. 5 were studied. Roots of both cultivars were submerged into polyethylene glycol (PEG) solutions with an osmotic potential of -0.5 MPa for 0, 24, 48, and 72 h. Relative water content (RWC) decreased markedly after 48 and 72 h. Simultaneously, marked increase in electrolyte leakage, decrease in contents of chlorophylls (Chl) a and b, and inhibition in PS 2 activity were observed. Northern hybridization indicated that progressive water stress remarkably reduced contents of the chloroplast gene psbA and psbD and nuclear gene cab transcripts. Urea-SDS-PAGE and Western blotting analysis showed that the contents of major PS 2 proteins, including the D1 and D2 proteins in the PS 2 reaction centre (RC) and the light-harvesting Chl a/b-protein complex (LHC 2) in periphery, declined with increasing water stress. Miannong No. 5 had less destroyed plasma membranes and higher RWC, Chl contents, and PS 2 activity during water stress than Miannong No. 4, which suggested its better drought resistance. The significant difference in steady state contents of LHC 2 proteins of two cultivars can be mainly attributed to the marked difference in transcript level of cab gene, which indicated that LHC 2 proteins protect PS 2 RC.