Abstract:A large number of studies have been carried out to investigate how crop photosynthesis responds to drought, but few have investigated the response of crop leaf gas exchange to drought processes, and their response thresholds. Based on a prolonged drought experiment in summer maize conducted in 2013, which included five watering treatments, and began from the 7-leaf stage, we investigated how drought developed with different initial amounts of irrigation, and how leaf gas exchange parameters changed as the drought progressed. Then, we determined the thresholds of soil and leaf moisture content when leaf gas exchange parameters began to respond to drought. The results showed that treatments with different initial amounts of irrigation induced different drought processes. Drought occurred earlier, persisted longer, and was more severe with treatments receiving less irrigation. The net photosynthetic rate (Pn), transpiration rate (Tr), and stomatal conductance (Gs) of summer maize under different watering treatments decreased sharply at the initial stage of drought; however, these parameters declined slower as drought prolonged, and tended to be almost identical to those growing under normal environment conditions. This indicated the acclimation of maize photosynthesis in response to prolonged drought. The onset of agricultural drought was typically marked by a decline in soil moisture below a critical point, which significantly impact crops. As drought progressed, the plant constituents and physiological processes could be altered sequentially, while lower-level responses would lead to changes in higher-level responses, and eventually changes at an individual plant level. Plant leaf gas exchange was found to be more directly affected by leaf water status than soil moisture content. Therefore, we identified the tipping points when maize leaf gas exchange parameters started to be affected by drought based on the tolerance limits of normal distribution; these were then quantified by soil moisture and leaf moisture content, respectively. The results showed that, Pn, Tr, and Gs decreased sharply when the relative soil moisture of 0-30cm depth was lower than 53%, 51%, and 48%, respectively, and leaf moisture content was lower than 81.8%, 81.3%, and 81.2%, respectively. At the initial stage of drought, the intercellular CO2 concentration (Ci) decreased, and stomatal limitation value (Ls) increased, indicating that stomatal closure accounted for the major decline in maize photosynthesis. As drought progressed, Ci increased while Ls decreased, indicating that non-stomatal limitation, other than stomatal closure, contributes to the major decreases in maize photosynthesis. The point at which the dominant limiting factor of maize photosynthesis converted from stomatal to non-stomatal varied under different treatments. The conversion time was earlier in maize that received lower initial amounts of irrigation due to longer persistence and greater severity of drought, and later in maize that received relatively higher initial amounts of irrigation. However, the thresholds of relative soil moisture and leaf moisture content when the conversion occurred were almost identical under all five treatments. The critical relative soil moisture of 0-30cm depth was about 44%±2% and the corresponding leaf moisture content was about 77.6%±0.3%. The results could provide reference information for drought monitoring and assessment of summer maize.