Differences And Physiological Mechanisms In Photosynthetic Efficiency And Nitrogen Absorption And Utilization Of Wheat Cultivars Released In Different Era

Since the founding of the People's Republic of China,wheat yield has been continuously increasing,among which the renewal and improvement of cultivars and the use of chemical fertilizers have played a great role.At the same time,the application of nitrogen fertilizer in order to increase the yield also leads to low nitrogen use efficiency of wheat,the production cost,and environmental pressure increased sharply.Therefore,improving nitrogen use efficiency has become the main way to achieve high-yield and high-efficiency production of wheat,ensure food security,and protect the ecological environment.To clarify the improved characteristics and physiological mechanism of photosynthetic efficiency and nitrogen uptake and utilization during wheat improvement will provide theoretical and technical support for further improving wheat yield and nitrogen use efficiency.Therefore,in this study,a large area of main wheat cultivars released in different years in the Yangtze River Basin of China were selected to study the changes and physiological characteristics of yield,nitrogen efficiency,leaf photosynthetic characteristics,and nitrogen uptake and utilization characteristics during the wheat cultivars improvement,and the physiological mechanism was analyzed from the root and leaf morphology,physiological characteristics,and molecular characteristics,as well as their relationship with photosynthetic efficiency and nitrogen uptake and utilization efficiency.The main results were summarized as follows:1 Evolution regularity of yield and nitrogen use efficiency during wheat cultivars improvement was clarified.Wheat yield and nitrogen efficiency increased significantly with the improvement of wheat cultivars.The increased yield was positively correlated with harvest index(HI),kernels per spike,1000-kernel weight,and dry matter accumulation after anthesis,while the increased nitrogen efficiency was closely correlated with the increased nitrogen accumulation pre-anthesis.The post-anthesis dry matter accumulation was positively correlated with yield,while the pre-anthesis N accumulation was positively correlated with nitrogen efficiency.Nitrogen fertilizer significantly increased wheat yield.spikes and kernels per spike,but decreased 1000-grain weight and nitrogen use efficiency.Nitrogen fertilizer application had a greater effect on wheat yield than cultivars improvement.2 The evolution characters of net photosynthetic rate and the relationship with leaf N concentration were clarified.Leaf area and photosynthetic rate increased during the cultivars improvement.The main reasons for higher photosynthetic rate of modern cultivars were higher Rubisco enzyme content and activity,maximum carboxylation rate of Rubisco(Vcmax),regeneration rate of RuBP(Jmax),and photosynthetic electron transfer rate.With the improvement of cultivars,leaf nitrogen content increased significantly,and the increase of leaf nitrogen content was higher than the increase of leaf photosynthetic rate,which led to the decrease of photosynthetic nitrogen use efficiency(PNUE).The modern cultivars have a higher distribution of nitrogen in non-photosynthetic organs,decreased Rubisco enzyme activation status,Rubisco activase content,and activity,and down-regulated TaRcas 1-?,TaRca 2a-A,TaRca 2a-B and TaRca 2a-D transcription levels after increasing nitrogen fertilizer application.Therefore,the unreasonable distribution of nitrogen in leaves and the decrease of Rubisco activase content and activity are the main reasons for the decrease of photosynthetic nitrogen efficiency in modern cultivars.3 The relationship between the improved characteristics of root morphology,physiology,and nitrogen uptake and utilization was analyzed.The root biomass,root growth rate,root length,root surface area,root weight density,and root activity of modern cultivars were significantly higher than those of early cultivars at pre-anthesis.Root length,root surface area and root volume were positively correlated with yield and nitrogen use efficiency.The expression of TaNRTs gene in pre-anthesis roots of modern cultivars was also higher than that of early cultivars,but there was no significant difference among post-anthesis.In addition,TaNRT 1.1 and TaNRT 1.2 played a greater role than TaNRT 2.1 and TaNRT 2.2.We also found that the activities of nitrate reductase(NR)and glutamine synthase(GS)of leaf and root in modern cultivar increased pre-anthesis and decreased post-anthesis.Therefore,genetic improvement of wheat cultivars improved root growth and nitrogen uptake capacity at pre-anthesis,promoted leaf nitrogen assimilation capacity,and thus improved wheat yield and nitrogen uptake efficiency.In conclusion,cultivars improvement improved wheat yield and nitrogen uptake and utilization efficiency.Cultivars improvement improves leaf photo synthetic rate and post-anthesis matter production by increasing leaf area,Rubisco enzyme content and activity,maximum carboxylation rate of Rubisco(Vcmax),regeneration rate of RuBP(Jmax)and photosynthetic electron transfer rate.Cultivars improvement improved root growth,nitrogen uptake and assimilation capacity at pre-anthesis,thus improving wheat yield and nitrogen uptake and utilization efficiency.Leaf nitrogen content increased but Rubisco activation ability decreased during cultivars improvement.The decrease of Rubisco activase content and activity,and the decrease of transcription levels of TaRcas 1?,TaRca 2a-A,TaRca 2a-B and TaRca 2a-D were the reasons for the decrease of photosynthetic nitrogen use efficiency in leaves.Therefore,improving Rubisco activation ability,increasing nitrogen distribution in photosynthetic system,increasing nitrogen assimilation,and then improving photosynthetic rate and nitrogen uptake and utilization efficiency,so as to achieve high yield and high efficiency synergy are the important objectives of future cultivars breeding and cultivation regulation.