Abstract: Deficit in soil moisture is the main cause of drought to crops. Accurate determination of crops’ responding thresholds to soil water is of great importance for identification and monitoring of the occurrence and development of crop drought. Based on the data of a progressive drought simulation experiment with six initial soil water levels on summer maize, oneway MANOVA was performed to identify maize growth indicators that respond earlier to soil water variations, and a way to identify their responding thresholds to soil water by using the statistic tolerance limits for normal populations was proposed. The results showed that the stem and leaf water content, transpiration rate, net photosynthetic rate, stomatal conductance, and leaf area of summer maize responded earlier to soil water in the seedling stage, and their corresponding critical soil moistures in terms of relative soil moisture at 0-30 cm depth were 72%, 65%, 62%, 60%, 58%, and 46%, respectively, indicating that the stem water content, leaf water content, transpiration rate, net photosynthetic rate, stomatal conductance, and leaf area would be affected successively as the soil water decreased and drought occurred. These results could provide reference to the monitoring and quantitative assessment of the development of drought to summer maize during the seedling stage, and also a way to determine responding thresholds of ecosystems.

Key words: precipitation reduction, molecular level, biomarker, soil organic matter