Drought Decreases Optimum Population Density In Dryland Wheat:Modeling And Mechanisms

Rational planting density was crucial in agricultural production,which enabled sufficient and efficient use of space,solar energy,water and soil nutrients,and produced considerable economic yield.Estimating optimum population density in rain-fed regions relied mostly on descriptive models,which was only suitable for some special climates.However,erratic climate changes,especially the irregular precipitations and drought stress,occurred repeatedly in rain-fed regions.As one of the most important environmental stresses,drought impaired wheat plant growth,reduced population yield and lowered optimum population density.Available models for estimating optimum population density were empirical and did not provide biological or ecological significance for explaining the variations of optimum population density in response to soil water availability.In the present study,model based on the law of constant final yield and the scaling relationship of biomass allocation was built as to discover the mechanisms underlying optimum population density dynamics in response to soil water availability.Pot culture and field experiments were carried out to verify the reliability of the deduced model.In order to investigate the relationship between allometry of biomass allocation and optimum population density,dynamics of biomass accumulation and allocation during wheat plant growth under drought and crowding situations was analyzed in pot culture experiment.Furthermore,different wheat cultivars were selected to verify the above trends in a broader scope.1.The deductions of optimum population density model for dryland wheatThe yield formation of wheat was largely decided by two factors,which were biomass accumulation and biomass allocation.Two classical ecological theories,incluing the law of constant final yield and the metabolic theory of ecology,were used to build the model.1)Based on basic ecological theories,the deduced model was characterized by asymptotic biomass increasing and allometry of biomass allocation.Optimum population density was decided by two parameters,which was k(the density producing half of maximum biomass)and?(the scaling exponent between grain mass and body size).The model assumed that the optimum population density was the trade-off outcome between resources acquisition(vegetative allocation)and reproduction(reproductive allocation).2.Verification of the model in pot culture experimentTo verify the above model,two hexaploidy wheats(6n=AABBDD,Triticum aestivum L.),old landrace Monkhead and recently released cultivar Longchun8275were employed in a pot experiment in 2016 as the materials.The Monkhead was a horizontal type wheat characterized by high tillering capacity and large root system,while the Longchun8275 was a typical erect type wheat with small root system.Nine density gradients including 1?256 plants pot^-1 and three water treatments including well water(80%field water capacity),intermediate stress(55%field water capacity)and severe stress(30%field water capacity)were set in order to study the yield-density relations of wheat.2)Quadratic function simulation results indicated that there was no significant change in optimum population density under intermediate drought stress,while severe stress significantly reduced optimum population density of both cultivars.Biomass-density relation was simulated with Michaelis-Menten equation and the results showed that the k increased significantly under severe drought stress.Scaling exponent(?)of grain yield-body size allometry of both cultivars were higher than 1(p<0.01),while drought and crowding further increased the value of?.Besides,results indicated that k and?were reluctant to change in old cultivar Monkhead under intermediate stress,while they were more sensitive in Longchun8275.Increase in k was not able to catch up with that in?-1,implying a re-balance of the trade-off between vegetative and reproductive allocation.More importantly,the results indicated that the allometric relations were plastic in response to drought stress,which was responsible for the decrease of optimum population density.3.Dynamics of biomass accumulation and allocation under drought and crowding environmentThe results indicated that the plasticity of allometry was responsible for the decrease of optimum population density.However,the mechanisms underlying the plasticity of allometry remained unclear.In order to undercover the mechanisms underlying the plasticity,biomass allocation patterns were investigated.Two theories regarding to biomass allocation had emerged,which were the optimal allocation theory and the allometric allocation theory might.The biomass allocation was considered a ratio-based process in the optimal allocation theory while it was considered a size-dependent process in the allometric allocation theory.Thus,it was essential to investigate the dynamic process of biomass allocation.The hexaploid wheat cultivar Longchun8275(6n=AABBDD,Triticum aestivum L.)was selected as the material in the experiment.Two water treatments including well water and drought stress were imposed to pot grown wheat in three density gradients including 5plants,15 plants and 45 plants pot-1.The aim of the present study was to investigate the dynamic process of wheat biomass allocation and the mechanisms underlying the plasticity of allometry.3)The vegetative growth of wheat plants under drought stress was strictly limited by water.The maximal water consumption and biggest vegetative mass were concurrently recorded in the 38 days after emergency,after which the reproductive mass increased rapidly.The biomass allocation between belowground and aboveground parts during plants growth was generally in accordance with the predictions of allometric growth theory in both water treatments,characterized by the higher roots to shoots ratios in younger plants which was smaller in body size.However,the roots allocation was better explained by optimal partitioning theory at maturity.More biomass was allocated to organs that captured the limiting resources at maturity,which were water under drought stress condition.It indicated the wheat plants under drought stress took an optimal strategy to reproduce and the optimal partitioning pattern was responsible for the plasticity of allometry.4)Scaling exponent of grain yield-body size relation was significantly higher under drought or crowding situations.Well-watered condition maintained relatively stable reproductive allocation under different densities while drought stress significantly reduced the reproductive allocation in crowding group.The growth trajectory of wheat plants under drought stress could not reach the allometric trajectory of well-watered plants.In this way,drought stress lead to increased allometric exponent,which further resulted in the decrease of optimum population density.4.Lowered optimum population density under drought stress in field environmentTo verify the above trends in field condition,the hexaploid wheat cultivar Longchun29(6n=AABBDD,Triticum aestivum L.)was selected as the material.Two water treatments including rain-fed and supplementary irrigation were imposed to wheat plants grown in seven densities.The irrigation was imposed in the jointing and heading stages.5)Field experiment was conducted in wheat growth seasons of 2017 and 2018.Compared with the drier year 2017,optimum population densities under rain-fed and supplementary irrigation conditions were higher in 2018.Besides,supplementary irrigation in both years increased optimum population densities.In uncrowded wheat populations,the overall scaling exponents of grain yield-body size allometry were not significantly differed under different water treatments.The scaling exponent was significantly increased under crowding situations,which was further increased under rain-fed condition.Grain mass and leaf mass were isometry under uncrowded populations,while it was allometry under rain-fed or crowding situations.Field experiment manifested reduction of optimum population density under relatively drier environments,which was due to lowered allocation to reproduction under crowding situation.5.Lowered optimum population density under drought stress in different wheat cultivarsThe above investigations indicated that the plasticity of allometry was the reason for the decrease of optimum population density in hexaploidy wheat.However,whether the above trends held true in different ploidy levels remained unclear.To verify the above trends in different ploidy levels,two diploidy wheats(MO1 and MO4,2n=AA,Triticum monococcum L.),two tetraploidy wheats(DM22 and DM31,4n=AABB,Triticum dicoccum L.)and two hexaploidy wheats(Monkhead and Longchun8275,6n=AABBDD,Triticum aestivum L.)were selected in the present study as the materials.Three water treatments including well water,intermediate stress and severe stress were imposed to pot grown wheat plants in three densities.6)Pot culture experiment indicated that the optimum population densities of different cultivars under severe drought stress were significantly reduced.Drought or crowding significantly reduced body size of the cultivars except for diploidy wheat under severe stress.In diploidy wheat,the plants in medium density were bigger than the plants in low density,indicating a role of positive interaction in primitive wheat growing in harsh environment.Grain yield-body size scaling exponents(?)in hexaploids were not significantly differed under different water treatments,but increased significantly due to crowding.Drought and crowding both increased?in tetraploids and diploids.Additionally,tetraploid DM22 was similar with hexaploids in the scaling relations of grain yield-body size,while DM31 was similar with diploids.The present study indicated that the scaling relations of grain yield-body size had transfered from allometry to isometry during wheat domestication.Nonetheless,the plasticity of allometry was observed in all ploidy levels,i.e.,the increased allometric exponents.Besides,the optimum population density was decreased under drought stress.The observed trends were similar in different wheat cultivars despite the differences in plant type and ploidy levels.7)In this study,six wheat genotypes were classified into primitive wheats and modern wheats according to their threshing and shattering properties.In primitive wheats,scaling relations of leaf mass vs.body size changed from allometric(scaling slope>1)to isometric(slope=1)as a result of drought stress,while it remained relatively stable in modern wheats(slope<1).Specific leaf area(SLA)scaled negatively to body size in well-watered condition for both wheat types and remained stable in modern wheats,while it turns to scale positively to body size in primitive wheats in drought stressed condition.We demonstrate the modern wheats have a compact population canopy which is highly stable in different water treatments.The canopy of modern wheats improves the population productivity under drought stress by increasing the reproductive mass of the smaller plants.More importantly,modern wheats have higher water use efficiency(WUE)than primitive wheats at the same level of canopy coverage.The model deduced from basic ecological theories indicated that the optimum population density was negatively related to the scaling exponent of grain yield-body size allometric relations.Under drought stress,biomass allocation of wheat plants was in accord with the optimal allocation theory,which would result in lower reproductive allocation in crowded wheat populations.In this way,drought stress lead to plasticity of allometry in wheat,and further results in the decrease of optimum population density.Evidences from field experiment and pot culture experiments indicated that the above trends and mechanisms were general in wheat despite the ploidy levels.