Abstract:  Drought was a major disaster that limited the growth and yield of crops worldwide. The loss of crop output caused by drought even exceeds the sum of the losses caused by all other factors, and was the most important factor threatening world food security. The influence of drought on crops was closely related to drought intensity, drought duration and the development stage of crops. It was of great significance for efficient agricultural drought prevention and drought relief to accurately assess the drought damage degree of crops and scientifically classify the drought damage levels of crops. Existing methods on crop drought assessment and grading were mostly based on yield reduction. However, yield reduction reflected the drought damage degree of the entire growth period of crop, which could not be applied to the assessment of crop drought damage degree during certain development period, restricting timely formulation and implementation of disaster prevention and mitigation measures. At present, the assessment and research on the progress of drought were generally based on one or several environmental indicators, such as precipitation, soil moisture, water deficit index, etc., or based on some single growth indicator, such as biomass. On the one hand, drought had a cumulative effect on crops, and the environmental indicators observed at that time could not necessarily reflect current growth state and damage degree of crops. On the other hand, a single growth indicator could not accurately reflect the overall growth status of crops. Since different growth indicators may have different response thresholds to drought degree, different conclusions may be drawn when grading and evaluating the drought degree of crops based on the response thresholds of different growth indicators to drought degree. Therefore, this study intended to investigate the responses of maize growth indicators to drought of different intensity and duration during its vegetative growth period (from the 3-leaf stage to jointing stage) based on a field plot experiment performed in 2014, and put forward a new way to accurately evaluating and classifying drought damage degree of maize based on response synchronicity of multiple growth indicators. In the field plot experiment, six different irrigations were performed during the three-leaf period of maize with the irrigation amounts (named treatments T1-T6) were 150, 120, 90, 60, 30, and 10mm, respectively, equivalent to 100%, 80%, 60%, 40%, 20% and 7% of the local average precipitation in July (150mm), respectively. No extra irrigation was performed thereafter. Precipitation was blocked completely by the auto-rain-shelter during the entire growth period. Then, six continuous drought processes of different initial soil moisture gradients were formed as time proceeded. Observations on soil water content, maize growth indicators were performed every 7-day after the irrigation treatments. Based on the observation data, the response regularity of maize morphological (plant height and leaf area) and biomass (stem dry mass, leaf dry mass, and total dry mass) indicators to the drought degree (D) was studied. By using of Threshold Indicator Taxa Analysis method (TITAN), the response turning points of growth indicators of maize's to drought degree were determined, and based on the response synchronicity of these growth indicators, the response turning point of maize plant level to drought degree was identified. Then the drought degree was divided into 4 levels according to these turning points. The results showed that, when 0＜D≤0.07, maize was affected by light drought, and the average decrease of maize growth indicators was only1.2%-3.0%; when 0.07＜D≤0.47, maize was affected by medium drought with an average decrease of leaf area of 15.9%, plant height of 8.6%, stem dry mass, leaf dry mass, and total dry mass of 18.8%, 15.4% and 12.4%, respectively; when 0.47＜D≤0.73, maize was affected by severe drought with an average decrease of leaf area of 37.8%, plant height of 16.9%, stem dry mass, leaf dry mass and total dry mass of 43.3%, 45.2% and 28.9%, respectively; when 0.73＜D≤1, maize was affected by extreme drought, with an average decrease of leaf area of 83.6%, plant height of 53.3%, leaf dry mass and stem dry mass above 90%, and total dry weight of 87.0%. The results would provide a method and basis for quantitative classification and evaluation of drought damage degree of crops.

Key words:  , Maize, Prolonged drought, Drought damage degree, Quantitative classification, Threshold Indicator Taxa Analysis method (TITAN)