The Occurrence And Development Of Drought On Summer Maize And Its Quantitative Research

Drought is the primary factor that restricts the growth,development,and yield of crops worldwide.Improving the capability of drought identification and assessment would contribute to make wise drought prevention and resistance measures and ensure the food security.Current studies were mainly focused on qualitative impacts of drought on crops,lacking of quantitative and process-based investigations into crop drought.In this paper,therefore,a 6-level progressive soil drying field experiment on summer maize since the three-leaf stage was conducted in 2014.Based on which,the soil water dynamics as well as its impacts on plant water status(leaf moisture content and stem moisture content),leaf gas exchange(net photosynthetic rate,transpiration rate,and stomatal conductance),morphological traits(leaf area and plant height),biomass(leaf,stem,root and total dry mass),dry mass partitions(root-shoot ratio,leaf-stem ratio),developmental stages,yield formation,and trade-off growth between leaf traits(leaf area,leaf water content,leaf biomass,and leaf number)were investigated.Then,the responses of different physiological and growth characteristics of summer maize to soil water were clarified,and the development process of drought in summer maize was revealed.Besides,a quantitative expression of drought degree was proposed and classified based on the responses of maize growth characteristics.The main conclusions are as follows:(1)The plant water status and leaf gas exchange of summer maize were mainly affected by drought intensity,while the morphological characteristics and biomass of summer maize were mainly affected by drought degree.(2)Normal distribution tolerance limits were adopted to identify the critical soil water content in terms of relative soil moisture of 0?3 0cm depth that led to significant variations in stem moisture content,leaf moisture content,net photosynthetic rate,transpiration rate,stomatal conductance,and leaf area of summer maize.They were 72,65,62,60,58,and 46%,respectively,indicating the response sequence of different growth characteristics of summer maize as the occurrence and progress of drought.(3)A quantitative expression of drought intensity was applied based on the crop water stress coefficient.The results showed that the drought intensity had a significant linear correlation with the plant water status and leaf gas exchange of summer maize.Leaf gas exchange were more sensitive than plant water status to drought.Stomatal conductance was more sensitive than transpiration rate and net photosynthetic rate,while stem mositure content was more sensitive than leaf mositure content.(4)A quantitative expression of drought degree was established on the basis of the cumulative effect of drought intensity as drought progressed.By use of Threshold Indicator Taxa ANalysis(TITAN),therefore,the tipping points of 5 growth characteristic of summer maize(the leaf mass,stem mass,total mass,leaf area,and plant height)were identified,and then the tipping points of the whole plant were also identified according to the synchronization of the responses of different growth characteristics to drought degree.According to these tipping points,the drought degree was divided into four levels:normal,mild drought,moderate drought and severe drought.(5)Drought affected the allometric growth of the root,leaf,and stem,as well as the trade-off growth between different leaf traits(leaf area,leaf water content,leaf dry mass,and leaf number),reflecting the adaptation mechanisms of maize to drought.As drought intensified,the maize plant gave first priority to root growth,then was the leaf,and followed by the stem.Besides,maize would tend to slow down its growth rate and conserve resource for survive by maintaining relatively lower metabolic activities and smaller sizes of leaves to minimize water loss.In general,maize would enhance its survival by reducing its growth rate and increasing resource storage as drought intensified.