Effects of nitrogen nutrition and water deficit on net photosynthetic rate and chlorophyll fluorescence in winter wheat

The responses of photosynthetic gas exchange and Chl fluorescence to nitrogen nutrition were studied under well-watered and drought conditions in winter wheat leaves. Nitrogen deficiency and water deficit strongly reduced the photosynthetic activity at light saturation level. For the well-watered treatment, the net photosynthetic rate was stimulated in the high-N (15 mmol/L N) plants as compared with the low-N (1.5 mmol/L N) plants and leaf conductance for water vapour was lower in the high-N than in the low-N treatment. As drought progressed, the net photosynthetic rate was significantly inhibited in the high-N plants as compared with the low-N plants. However, no significant nitrogen effect was noticed for net photosynthetic rate and leaf conductance for water vapour. The quantum yield of photochemical efficiency of PS II (Fv/Fm) determined on the youngest fully expanded leaf was unaffected by water stress, but it was lower for the low-N than for the high-N treatment. Nitrogen deficiency resulted in a decrease in the total Chi content and an inaease in the Chi nib ratio; however, no difference was observed between the water treatments. During steady-scare photosynthesis, the values of photochemical quenching decreased with increasing water stress in all treatments. For plants grown at low nitrogen nutrition photochemical quenching was higher than that for plants receiving high-N nutrition. The values of non-photochemical quenching decreased with increasing water stress in all treatments. Nitrogen deficiency and water stress decreased the efficiency of the PS II (Fv/Fo) and the efficiency of potential photosynthetic quantum conversion (Fd/Fs) of leaves significantly.