The Change Of Leaf Water Content And Its Effects On Photosynthesis In Summer Maize

Leaves are the main organs of photosynthesis,and the changes in leaf water content will affect photosynthesis.What's more,the leaf water content is the embodiment of the comprehensive effects of air dryness,soil water supply capacity and drought tolerance of crops.Study on the changes of leaf water content and its effect on photosynthesis under different soil water conditions may provide a reference for improving the accuracy of photosynthesis simulation and contribute to objective identification of the occurrence and development of drought events in summer maize.In this study,summer maize from north China,which suffers from frequent drought,was used to determine the relationship of leaf water content with net photosynthetic rate(9))and soil water content under different soil water stress intensities,lasting duration and different growth stages,by water gradient manipulation experiments beginning from 3-leaf stage in the ecological and agricultural meteorology field station of the Chinese Academy of Meteorological Sciences.On the basis of average July natural precipitation during the period of 1981-2010 in Baoding,Hebei Province,six watering treatments(7%,20%,40%,60%,80% and 100%)were used,with three replicates per treatment from June to October,2014.The watering treatments with disposable irrigation were used to simulate the effects of consecutive droughts of different intensities on the photosynthetic characteristics of summer maize.The plants were not watered after July 2.The soil water content,fresh and dry weight of leaves,and net photosynthetic rate(on the same leaf)were measured under sunny day every one to two weeks.The growth period was recorded every day.The different watering treatments strongly affected the crop index of summer maize.The main conclusions are listed as follows:(1)There was a threshold value of leaf water content response to soil moisture.Leaf water content decreased significantly only after a certain degree of water stress.The intensity of soil water stress affected the lowing speed of leaf water content,the higher the degree of water stress,the faster the rate of decrease.The lasting duration of soil water stress decreased leaf water content and the difference between treatments.The decreasing rate of leaf water content responding to the decrease of relative soil water content in summer maize varied from growth stages,and was ranged as jointing stage > 3-leaf stage > 7-leaf stage.Moreover,leaf water content showed a non-rectangular hyperbolic relationship with relative soil water content,which differed from the linear relationship reported in the previous studies due to the narrow water gradient.The leaf water content increased with increasing relative soil water content and then tended to the leaf water holding capacity.According to the Michaelis-Menten equation,the maximum leaf water content was estimated to be 85.14%,using the double reciprocal Lineweaver-Burk plot.(2)The leaf water content of summer maize significantly affected net photosynthetic rate,and the two factors showed a quadratic curve relation within limited range of leaf water content.Leaf water content was saturated under high soil moisture conditions,and leaf water gradient shrank under severe water stress,which caused the law insignificance.(3)The critical leaf water content when 9)reduced to zero were different under different conditions.It decreased overall with the aggravation of soil water stress,and it decreased first and then increased during the period of soil water stress duration.Moreover,the critical leaf water content varied from growth stages,was ranged as 3-leaf stage > jointing stage > 7-leaf stage,which meant the endurance to low soil moisture was 7-leaf stage > jointing stage >3-leaf stage.On an average,9)reduced to zero when the leaf water content reached approximately 70.30%,this was considered as an important threshold in the photosynthetic physiological activity of leaves.It could provide the basis for dynamic monitoring of large-scale drought development and accurate assessment of drought degree in crops.(4)A quantitative expression equation based on the leaf water content was established for the biochemical model of Rubisco-limitation in C4 plant.