夏玉米叶片气体交换参数对干旱过程的响应

目前已经开展了大量的干旱对作物叶片气体交换参数影响的研究,但关于作物叶片气体交换参数对干旱过程的响应及其关键阈值的研究仍较少。基于夏玉米七叶期开始的5个初始水分梯度的长时间持续干旱模拟实验资料,分析了不同强度持续干旱过程中夏玉米叶片气体交换参数(净光合速率Pn,气孔导度Gs,蒸腾速率Tr,胞间CO_2浓度Ci和气孔限制值Ls)的变化规律及其关键阈值。结果表明,玉米的净光合速率(Pn),蒸腾速率(Tr)和气孔导度(Gs)在干旱发生初期呈大幅度下降,但随着干旱持续会出现一定的适应性。利用统计容忍限方法确定了夏玉米拔节期Pn,Tr和Gs响应干旱的临界土壤相对湿度(0—30cm)分别为53%,51%和48%,对应的临界叶含水率分别为81.8%,81.3%和81.2%。夏玉米光合作用由气孔限制向非气孔限制转换的0—30cm土壤相对湿度均为44%±2%,对应的叶含水率均为77.6%±0.3%。研究结果可为夏玉米干旱发生发展过程的监测预警提供依据。

Effect of drought on leaf gas exchange in Summer Maize

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.