Effect Of CO₂Enrichment On Plant Growth And Nitrogen Use Of Winter Wheat

The research was carried out with winter wheat（Triticum aestivum L.） as materialsduring the three growth seasons from2007to2010. Some pot and water culture experimentswere launched in four open top chambers （OTCs）. The main aim of research was to assess theinfluence of CO₂concentration elevation on the photosynthesis, chlorophyll fluorescenceparameters, source components, root morphology and organic carbon secretion, yieldformation, nitrogen use efficency of winter wheat. The main results were as followings：（1） CO₂enrichment reduced chlorophyll a, chlorophyll b and total chlorophyll contents,increased soluble sugar content in wheat flag leaf, and led to photosynthetic down-regulationunder no nitrogen application or low nitrogen level, but under ample nitrogen application, iteased off or removed photosythesitic down-regulation mainly by increasing chlorophyllcontent, especially chlorophyll a content, and limitedly by chlorophyll b and soluble sugarcontents. This showed nitrogen nutrition improvement could regulate leaf photosythesiticacclimation to some extent.（2） When air CO₂concentration was elevated, the average value of initialfluorescence（Fo） in wheat flag leaf increased1.4%¹5.2%, and the average value ofmaximumu fluorescence（Fm）, variable fluorescence（Fv）, maximal photochemicalefficiency（Fv/Fm） and potential photochemical efficiency（Fv/Fo） reduced0.3%⁹.3%,2.4%¹2.0%,0.9%⁴.7%and5.9%²2.7%, respectively. The ranges of Fo increase, and Fm,Fv, Fv/Fm and Fv/Fo decrease fell with the increase of nitrogen level. It showed elevated CO₂gave biger photo-damage to wheat PSII, and restrained its electron transport, potential activityand original light conversion ability. Nitrogen addication could weaken or eliminate thenegative effects.（3） Compared of ambient CO₂concentration, CO₂enrichment increased thephotosynthesis areas of leaf, stem&sheath and whole stem by11.5%³7.3%,14.5%¹8.1%and7.3%³8.3%, respectively. The expansion of photosynthesis area should mainly be due tothe increase of leaf and node lengths. The dry matter accumulation after anthesis, the ratios ofgrain number and weight to leaf area responded weakly or negatively to elevated CO₂under no nitrogen application, but significantly increased in the very great majority of cases undernitrogen addication. It showed that the positive effects of CO₂enrichment on dry matterproduction after anthesis and sink bearing capacity per source area could be enhanced byimproving nitrogen nutrition.（4） The hydroponics results showed that compared with the background CO₂concentration, elevated CO₂increased root and shoot biomass, root length, root area, rootorganic carbon excretion rate and content, and leaf NR activity in wheat seedling by1.9%⁴2.8%,5.5%²7.7%,14.9%⁹2.9%,5.9%⁷1.4%,8.7%¹1.8%,13.3%and14.8%³3.5%, but its effect on the root/shoot ratio lacked regularity. Elevated CO₂influnenced root length and area, organic carbon excretion more significantly under lownitrogen nutrition than under apmle nitrogen. It showed that elevated CO₂led to the moreassimilate product distributed to root when lacking nitrogen nutrition.（5） CO₂enrichment significantly increased wheat grain weight per pot by48.3%⁷0.0%,mainly due to the improvement of spike number per pot and grain number per spike. ElevatedCO₂promoted the accumulation of dry matter in different organs before anthesis. The amountand rate of pre-anthesis dry matter in shoot transferring to grain after anthesis increased andthe contribution rate of pre-anthesis dry matter translocation rose with CO₂elevation. Theeffects of elevated CO₂on grain weight per pot, dry matter accumulation and translocationwere weak or not significant, but were mostly significant under nitrogen addication, whichindicated reasonable nitrogen application could improve the positive effects of elevated CO₂.（6） Elevated CO₂increased plant nitrogen accumulation at anthesis and mature stages,the amount of pre-anthesis nitrogen in shoot transferring to grain after anthesis by15.0%²7.9%,13.9%²6.1%and13.8%⁷7.4%, respectively, but it had different effect onthe contribution rate of pre-anthesis nitrogen in shoot of two varieties. Compared with thebackground CO₂concentration, the high CO₂concentration treatments increased nitrogenefficiencies for grain and dry matter production, and nitrogen harvest index. The effects ofelevated CO₂were not significant under no nitrogen application, and all sigficant undernitrogen application.（7） The effects of CO₂enrichment on wheat growth could be increased by applying morenitrogen at the middle or later stages. Elevated CO₂resulted in photosynthetic acclimation ofwheat flag leaf along with the decease of SPAD and soluble protein content under nonitrogen application, but the appearances were found under nitrogen addication; thephotosynthetic rate at grain-filling stage was enhanced by elevated CO₂under nitrogenapplications with ratios of5:5:0and5:3:2at before sowing, regreening and booting stage.Additionly, the length and area of the top three leaves at heading stage, photosynthesis after anthesis, grain yield, pre-anthesis dry matter accumulatio and translocation, and nitrogenusage were more easyly promoted by CO₂elevation under nitrogen spilt application thanunder nitrogen all-based application..Main innovations:（1） CO₂elevation would down-regulate winter wheat leaf photosythesis by inceasing leafsoluble sugar content and PSII light suppression, and reducing chlorophyll content, PSII lightchemistry efficiency and activity. By improvementing nitrogen nutrition, especiallyapplying more nitrogen at the middle and later stages, the leaf photosynthetic acclimationwould be relieved, and the positive effects of CO₂elevation on plant growth and development,yield formation and nitrogen usage would be strengthened.（2） CO₂enrichment would promote the growth and development of source organs androot, pre-antheis dry matter and nitrogen accumulation in shoot, and translocation to grainafter anthesis, increase grain yield and nitrigen accumulation in grain, and improve thenitrogen production effciency.3) The synthetical analyses showed that when nitrogen in medium was ample, thepositively effect of CO₂elevation on winter yield was mainly due to its accelerating thenon-leaf source areas and the translocation of dry matter restored in vegetative organs beforeanthesis to grain after anthesis.