Coevolution Of Root Characteristics And Yield In Dryland Spring Wheat

Crop production is a population process.The improvement of individual plant fitness influences the performance of population yield in densely planted crop ecosystems.Characteristics and behaviors related to individual fitness are favored by natural selection.According to recent theories of crop evolution,the improvement of population yield in crop production requires the weakening of"selfish"traits and behaviors of individual crop plant.According to these theories,increased yield is achieved by reducing some aspects of plant fitness.Decreasing the allocation of dry matter to organs,traits and behaviors associated with competition,while increasing reproductive output and tolerance of higher planting density are the most effective ways to improve population yield during cultivar evolution.In dryland regions,plant roots show many characteristics and behaviors that increase individual fitness through exploration of the soil and absorption of water and nutrient in competition with neighbors.In this research I test the hypothesis that the root system of dryland spring wheat exhibits convergence characteristics of architecture and morphology over the course of cultivar evolution that are consistent with selection at the population level,reducing competitiveness and phenotypic plasticity.To test the hypothesis,I conducted 4 studies using a historical sequence of 8 wheat(Triticum aestivum)cultivars that have been widely grown in north-western China.In Experiment 1,root architectural traits are investigated.Plants of 8 wheat cultivars were grown in gel-filled chambers to examine growth angles,numbers,and lengths of seminal roots,and in soil-filled chambers under eight soil resource levels(2 water,2Nitrogen and 2 Phosphorus treatments)for fractal analysis of root system architecture.Yield was measured under field conditions of standard density(256 plants/m~2)and low density(128 plants/m~2).In Study 2,root system morphological characteristics were investigated under field conditions.Biomass characteristics,root characteristics such as root length and density,specific root length and root diameter within 1 m of depth,and morphological characteristics of aboveground parts including plant height,flag leaf area and peduncle length were measured.In addition,an experiment restricting root lateral growth was performed.In Study 3,the competitiveness of two cultivars,which are typical representatives of new and old varieties,was determined in monoculture and mixtures in the field,and intra-and inter-plant root competition was investigated at 4densities and 2 water treatments in a pot experiment.Study 4 focused on phenotypic plasticity experiment.In this experiment,I applied 2 water treatments,2 nutrient heterogeneities and 3 planting patterns to analyze phenotypic plasticity of new and old cultivars.The main results are as follows:1.In the field yield test,the yield of wheat increased significantly with cultivar substitution at the current standard density,but there was no significant difference at lower planting densities for 6 of the 8 cultivars.2.In gel-filled chambers,seminal root number and growth angles were negatively correlated with yield,while primary seminal root length was positively correlated with yield.Thus,the roots of modern cultivars are narrower,deeper and spread less laterally than old cultivars.In soil-filled chamber,the fractal dimension of root branching was negatively correlated with yield at all resource levels.Root:shoot ratio was negatively correlated with yield under high soil resource levels,and the root system of modern cultivars are simpler and more efficient.3.The relationship between root morphological characteristics and yield is influenced by planting density.Morphological characteristics such as plant height and root length density and the above-and below-ground biomass characteristics continued to decrease with cultivar evolution at lower planting density,but the relationships were unimodal at a higher density.The root length density of old cultivars increased at low planting density,while the root length density of recent and modern cultivars decreased.4.Planting density influences the effects of morphological and biomass characteristics on yield.Shoot and root biomass,plant height and root length density had positive effects on yield at lower density.Shoot biomass,harvest index and root tissue density had positive effects on yield at standard planting density.However,root biomass,plant height,flag leaf area and root length density were positive effects when the yield was lower than 260 g/m~2,and were negative effects when the yield was higher than 260g/m~2.5.Landraces(very old cultivars)were more competitive than modern cultivars.The existence of neighboring roots resulted in significant root proliferation of target plants after 60 days after sowing,and the above-ground biomass decreased after 90days after sowing.The response of the modern cultivar to roots of neighboring plants is less than that of the old cultivar.6.In the phenotypic plasticity experiment,water treatments,nutrient heterogeneity and planting patterns all affected the phenotypic and biomass characteristics of the landrace and modern cultivar differently and to different degrees.The landrace with strong competitiveness exhibited more adaptive phenotypic plasticity than the modern cultivar.Landrace was sensitive to nutrient information,while modern cultivar was sensitive to neighbors of different cultivars.7.Restriction of root lateral growth in the old and new cultivars increased grain yields at the standard density and lower planting density conditions,respectively.When the lateral extension of the root system was restricted,the root length density increased significantly,especially in deep soil.Specific root length did not change,root tissue density increased,and root diameter decreased.These results of this research indicate that the changes in root architectural and morphological characteristics during the cultivar evolution of dryland spring wheat in China has resulted in narrower,deeper rooted,less laterally distributed,convergent root systems,consistent with"group selection".The weakening of selfish,competitive traits and phenotypic plasticity has reduced the impact on neighboring plants and allowed for higher planting densities,which produce high yields.We must consider soil resources to understand yield increases and cultivar evolution.Competition for resources is important in agricultural fields as well as in nature.Natural selection maximizes individual fitness,but agriculture is about population performance.Therefore,the optimal strategy for crop production will be very different from that which natural selection produces.Wheat plants change their biomass allocation in response to resource levels and the presence of neighboring roots,and some of these responses reduce population performance.Therefore,in future breeding,it is more effective to reduce the expression of selfishness and competitive behavior of roots and select“communal”plants that are more in line with the interests of the population rather than those of natural selection.