Diurnal changes and effect of elevated CO2 on gas exchange under individual and interactive salt and water stress in wheat (Triticum aestivum)

To evaluate the individual and interactive effects of drought and salt stress on diurnal stomatal behaviour and gas exchange parameters, two salt tolerant (Kharchia 65, KRL 210) and two salt sensitive (HD 2851, HD 2009) wheat (Triticum aestivum L.) varieties were subjected to drought (25% and 50% water deficit) and salt (50 mM and 100 mM NaCl) stresses, imposed separately and in combination. Diurnal changes in photosynthetic rate (Pn), stomatal conductance (gS) and transpiration rates (E) of wheat leaves were measured at 4 h intervals under field conditions, using an infrared open gas exchange system. Photosynthetic rate was also measured across a range of variable intercellular CO2 concentrations (50 - 800 ppm). Kharchia 65 (salt tolerant) and KRL 210 (moderately salt tolerant) wheat varieties showed photosynthetic rate of 19.74 and 9.19 mu mol/m(2)/s, while HD 2009 and HD 2851 (salt sensitive) showed photosynthetic rate of -27.65 and -17.65 mu mol/m(2)/s at 50 ppm intercellular CO2 concentration which increased with increasing CO2 concentration. Up to 200 ppm intercellular CO2 concentration, these sensitive varieties showed negative Pn values. Diurnal photosynthetic activities of wheat varieties significantly differed under stressful environment but these differences could not be expressed in control conditions. Pn was low in the early morning, increasing with time and reached maximum (mu mol/m(2)/s) between 10:00 AM to 2:00 PM, thereafter, Pn decreased. Similar trends of increase and decrease were observed for gS and E.