Diurnal oscillation in the intercellular CO2 concentration of spring wheatunder the semiarid conditions

Yields of wheat in semiarid and arid zones are limited by drought, and wate r condition is very important at each stage of development. Studies carried out at Loess Plateau in the northwestern part of China indicated that yiel d of spring wheat (Triticum aestivum L.) cv. Dingxi 81-392 was reduced by 4 1 % when subjected to water stress. The effects of two water regimes on net photosynthetic rate (P-N), stomatal conductance (g(s)), and intercellular CO2 concentration (C-i) were investigated at the jointing, booting, anthesi s, and grain filling stages. Low soil moisture in comparison to adequate on e had invariably reduced P-N during the diurnal variations at the four grow th stages. P-N and g(s) in both soil moisture regimes was maximally reduced at midday. C-1 and the stomatal limitation fluctuated remarkably during ph otosynthesis midday depression processes, especially at the grain filling s tage. Hence atmospheric drought at midday was one of the direct causes indu cing stomata closure and the g(s) depression, but it was beneficial for mai ntaining stable intrinsic water use efficiency. Fluctuation in C-i implicat ed that non-stomatal limitation also plays an important role during the per iod of photosynthesis midday depression. Consequently stomatal and/or non-s tomatal limitation are the possible cause of the midday photosynthesis decl ine.