Yields of wheat in semiarid and arid zones are limited by drought, and water condition is very important at each stage of development. Studies carried out at Loess Plateau in the northwestern part of China indicated that yield of spring wheat (Triticum aestivum L.) cv. Dingxi 81-392 was reduced by 41% when subjected to water stress. The effects of two water regimes on net photosynthetic rate (PN), stomatal conductance (gs), and intercellular CO2 concentration (Ci) were investigated at the jointing, booting, anthesis, and grain filling stages. Low soil moisture in comparison to adequate one had invariably reduced PN during the diurnal variations at the four growth stages. PN and gs in both soil moisture regimes was maximally reduced at midday. Ci and the stomatal limitation fluctuated remarkably during photosynthesis midday depression processes, especially at the grain filling stage. Hence atmospheric drought at midday was one of the direct causes inducing stomata closure and the gs depression, but it was beneficial for maintaining stable intrinsic water use efficiency. Fluctuation in Ci implicated that non-stomatal limitation also plays an important role during the period of photosynthesis midday depression. Consequently stomatal and/or non-stomatal limitation are the possible cause of the midday photosynthesis decline. and Xi-ping Deng ... [et al.].