Effects Of Free Air CO₂Enrichment On Wheat Growth And Yield:the Physiological Basis

Fossil fuel combustion and deforestation have resulted in a rapid increase in atmospheric carbon dioxideconcentration ([CO₂]) since the1950's. It is predicted that atmospheric CO₂concentration will reachabout550mol mol–1in2050. Globally, wheat is a major staple crop. Field experiments were based onthe soybean and wheat rotation Free Air CO₂Enrichment system at Beijing, China, two high glutencultivars of winter wheat (Triticum aestivum L. cv CA0493and Zhongmai175) were test for threegrowing seasons from2007to2010. Wheat was grown to maturity under elevatedCO₂(550±17μmol·mol^-1) and ambient CO₂(415±16μmol·mol^-1) rings, with normal nitrogen supply (NN,basal dressing before sowing118kg N·hm^-2+side dressing at jointing stage70kg N·hm^-2)and lownitrogen supply(LN,basal dressing before sowing66kg N·hm^-2+side dressing at jointing stage17kgN·hm^-2). Each treatment had three replicates. This study focus on the yield response of winter wheat toelevated CO₂, and get more detailed understanding of crop processes under high CO₂, such asphotosynthetic carbon assimilation, biomass production and distribution among wheat tissues, nitrogenuptake and utilization, grain filling rate and duration. Some main findings are listed as follows:â‘´Elevated CO₂increased grain yield by10.71%across the three years. CO₂-induced yieldenhancement under low nitrogen and normal nitrogen supply were10%and12%, respectively. Theyield enhancement under elevated CO₂attributed to the increase of square meter panicle number (6.16%)and grain number per panicle (4.08%),and the reduction of infertile spikelet (^-21.68%).⑵Elevated CO₂increased the photosynthetic rate and daily integrated carbon assimilation by23.51%and11.91%under elevated CO₂before flowering, thus increase the dry matter accumulation beforeflowering, but shorten the grain filling duration after flowering. The carbon assimilation of CO₂-inducedenhancement reduced gradually after flowering. It might due to the imbalance of carbon and nitrogenbetween source and sink at high CO₂. Wheat plants under elevated CO₂required more carbon andnitrogen after anthesis than ambient CO₂, which may increase the nitrogen transfer from flag leaf.Therefore, leaf chlorophyll concentration and Rubisco content decreased under elevated CO₂,photosynthetic acclimation occurs.⑶Elevated CO₂increased the aboveground biomass at jointing, flowering and ripening stage across thethree years by10.25%,17.65%and10.88%, respectively. The study revealed that CO₂-inducedaboveground biomass enhancement was higher at normal nitrogen than low nitrogen, especially on thestem biomass accumulation. Elevated CO₂increased the harvest index by0.97%under low nitrogeninput, but decreased by5.48%under normal nitrogen input.â‘·Elevated CO₂increased the nitrogen accumulation by5.63%,12.34%and6.12%under elevated CO₂at jointing, flowering and ripening stage across three years, respectively. Elevated CO₂increased theaboveground biomass per nitrogen uptake on an area basis and grain yield per nitrogen uptake on anarea basis at ripening stage by3.56%and3.90%. It indicated that winter wheat had larger nitrogen demand under elevated CO₂than ambient CO₂. To establish the new balance of nitrogen demand andsupply, this study estimated the recommended N application as145kg N hm^-2at550μmol mol^-1[CO₂]in the future. Fertilizer application could be adjusted based on the plant N demand. This strategy couldimprove the nitrogen recovery and reduce nitrogen loss in the soil, thus environment and grain yieldachieve a win-win objective. Grain yield and N uptake after flowering had a significant positivecorrelation at high CO₂, which indicated that regulate on the sidedressing date at jointing stage, couldbecome an effective way to increase the nitrogen uptake after flowering to improve the grain yield.Elevated CO₂decreased plant nitrogen concentration at jointing, flowering and ripening stage by3.55%,4.73%and2.94%across the three years, respectively. The enhancement of aboveground biomass wasmore than nitrogen accumulation under elevated CO₂, which may dilute the nitrogen concentration inplant tissues. On the other hand, elevated CO₂decreased the activity of nitrate reductase of flag leaf by22.24%across the flowering stage and milk stage. It indicated that elevated CO₂inhibited NO₃ˉassimilation, which caused the reduction of N concentration under elevated CO₂.⑸Elevated CO₂promote the grain filling rate of ZM175, but decreased grain filling rate of CA0493.The flag leaf of ZM175had longer photosynthesis function duration than CA0493, which wasbeneficial to accumulation more photoassimilation and transfer from leaf to grain to improve grainfilling rate for ZM175. Individual panicle grain weight for ZM175was45.26%higher than CA0493.From the source-sink balance point of view, the ability of photoassimilation supply from source andestablishment of new carbon sink for ZM175was comparatively favourable with CA0493underelevated CO₂. Therefore, ZM175was a stable and high-yield cultivar at high CO₂.