Study On Characteristics And Mechanism Of Rice Yield Response To Drought-flood Abrupt Alternation

Abnormal global climate change has led to drought-flood abrupt alternation?DFAA?occurred frequently,which seriously threaten water and food security of China,especially the grain yield in crop growing spots with relatively low standards for drought tolerance and drainage.Rice is one of the main food crops in China.The national rice yield accounts for about 42%of the total grain yield,and rice field area accounts for 29%of the total grain area.Rice growth is closely related to soil moisture content,and insufficient soil water supply?drought?or excessive moisture content?flood?will result in yield reduced.DFAA stress is not equal to the sum of drought and flood stress,and exploring the characteristics and mechanism of rice yield response to DFAA has important practical significance to establish the effective measures of disasters prevention.This paper relies on the projects from National Natural Science Foundation of China“Research on Drought-Flood Abrupt Alternation?DFAA?Occurrence Mechanism and Disaster Mitigation Method?51339004?”,and rice growing area in Huaibei Plain as the test object.The experiment was conducted at the Xin-maqiao irrigation experimental station at the Anhui and Huaihe River Institute of Hydraulic Research in China?117^o22'E,33^o09'N?in 2016 to 2018.The experiments were on three different drought and flood treatments and compared with normal irrigation conditions.Based on the experimental data,the effects of drought,flood,and DFAA stress on rice yield and yield components were analyzed.The variation characteristics of dry matter accumulation and distribution under different combination of drought and flood was explored.The rice photosynthetic production and dry matter accumulation mechanism model under DFAA stress was constructed,and the model was verified based on experimental observation data.The following research results were obtained:?1?The reduction characteristics of rice yield under DFAA conditions were researched,and the interaction law between late flooding stress and early drought stress on yield was revealed.The late flooding stress had a certain synergistic effect on rice during drought period,while the early drought stress had certain antagonism effect on rice during flooding period.On the whole,there are complex interactions and interaction mechanisms between the early drought stress and late flooding stress on rice yield.The trend graph predicts that the optimal combination,i.e.,the conditions with the lowest impact factor on yield,was not simply the combination of conditions that had the minimum impact on yield under a single factor.Compared with the normal group,drought and flood stress reduced the rice yield.The average yield reduction of the DFAA groups was 12.98%in 2016,29.94%in 2017,and 39.27%in 2018.Among them,the combination of heavy drought and heavy flood was the most unfavorable for yield.The reduction in grains per panicle and the total grain number was the main reason for yield reduction under DFAA stress.The flood stress of the DFAA groups reduced the yield under drought conditions,and the decrease in the total grain number during the flood period was the main reason.The drought stress of the DFAA groups compensated the yield under flood conditions,mainly because the grains per panicle,total grain number and seed setting rate increased during the drought period.Under long-term light drought conditions?DFAA3?,the panicles per barrel,grains per panicle,total grain number,thousand seed mass and seed setting rate increased.?2?The changes of dry matter accumulation of various organs under DFAA conditions were discussed,and the interaction effects between early drought stress and late flooding stress on dry matter distribution and harvest index were revealed.Compared with the normal group,the DFAA stress reduced the total dry matter,the average reduction rate of DFAA1⁹ was 21.88%,and the reduction range was 10.86%?DFAA2?³7.78%?DFAA7?.Among them,the reduction rate of DFAA7 exceeds 50%in re-watered period.Compared with the drought groups and flood groups,both the drought and flood stress reduced the total dry matter.The long-term heavy flood had the greatest impact on the total dry matter under drought conditions,and the early heavy drought or the early drought turning to heavy flood had the most serious impact on the total dry matter.From view point of dry matter distribution,the DFAA stress reduced the dry matter of roots,stems,leaves and panicle of plants.Compared with the normal group,the average damage rate of each part of rice was:panicle?43.58%?>stem?17.63%?>root?5.28%?>leaves?1.31%?.Compared with the drought groups,the later flooding mainly reduced the panicle weight?the average reduction rate of DFAA1⁹ was 39.58%?,and compared with the flood groups,the stem weight?the average reduction rate of DFAA1⁹ was 21.46%?and the panicle weight?the average reduction rate of DFAA1⁹ is 19.61%?under the drought condition was reduced.Under the DFAA stress,all organs competed for water,and the plants adopted different water use strategies during DFAA period.Compared with the normal group,the total dry matter of the DFAA groups was more distributed to the root system at the drought stage,promoting the growth and development of the root hair,which ensuring the mineral nutrient and water supply.At the flood stage,the leaf allocation index was slightly higher than normal,and the distribution of other organs was downward.Stems and leaves began to compensate for growth during the re-watered period to harvest;the panicle distribution index was smaller than the normal group during the whole periods.The harvest index of the DFAA groups were lower than that of the normal group,and was between the drought groups and the flood groups,indicating that drought stress can increase the efficiency of dry matter conversing to grain and reduce the redundant growth of plants during the flooding period,and obtained the possible compensatory mechanism of previous drought stress on yield during flooding period.?3?The relationship between stomatal conductance slope and CO₂ compensation point and marginal water use efficiency quotient was established?light and temperature effect coefficient k?,and the numerical relationship between relative marginal water use efficiency and drought degree and duration under pre-drought stress of DFAA was established.The stomatal behavior optimization theory was introduced into the model for the first time.Because the stomatal conductance slope g₁ reacted differently to photothermal before and after flowering,the k value was introduced to establish a quantitative characterization of its numerical relationship with physiological developmental day?PDT?.In order to simulate the influence of DFAA stress on dry matter in the model,the degree and duration of drought and that of flood are divided into four dimensions.It was considered that the degree of drought and flood had a cumulative effect on time.Attempts to establish the numerical relationship between the relative marginal water use efficiency at drought stage of DFAA stress and two-dimensional variables of drought degree and time,as well as the numerical relationship between the relative water uptake and two-dimensional variables of flood degree and time.?4?The photosynthesis production and dry matter accumulation model of rice under DFAA stress were constructed.The interaction of drought and flood stress on dry matter accumulation was analyzed based on the model and yield was predicted.Decompose the dry matter production process under DFAA stress into drought,flood and re-watered period.Dry matter production simulated by new model based on stomatal optimality theory at the drought stage,coupled with root water uptake model at the flood stage,and considered after-effect impact of drought and flood stress at the re-watered stage.Based on the model,the effects of drought and flood stress interaction on dry matter accumulation were analyzed.The dry matter and flood all reduced the dry matter accumulation,and the total dry matter at the end of flood stage?130.44g?was even lower than the end of the drought stage?137.56g?,the dry matter during the re-watering period was reduced more than 50%compared with the normal group,indicating that the interaction between pre-drought and post-flood showed a superimposed reduction in total dry matter accumulation during the flood and rewatering period.The total dry matter and panicle weight of the DFAA groups were calculated,and the panicle weight of the DFAA6?including the filled grain weight,the unfilled grain weight,and the branchlet weight of panicle?was the largest.The reduction rate of DFAA3,4,7 relative to the normal group was over 50%.Among them,DFAA7 reduced the most severely.Which was consistent with the yield?the filled grain weight?results of Chapter 2.Further analysis showed that the formation of yield was related to the interaction effects of drought and flood stress on dry matter accumulation.