The Limiting Factors Of Photosynthesis In Wheat Under Elevated CO₂ Concentration And Increased Temperature

At present,climate change has become one of the major challenges facing the world.The continuous increase of atmospheric CO₂ concentration and temperature would have a direct and important impact on plant photosynthesis,and photosynthesis is the key to affect the yield and quality formation.However,previous studies on the limiting factors of photosynthesis under elevated CO₂ concentration and increased temperature were still controversial.This experiment relies on the controlled environment chambers of Shanxi Agricultural University,with a full factorial combination of [CO₂](400 mmol·mol^-1（CK）or 600mmol·mol^-1（e C）)and temperature（normal or normal+2℃（e T））.In 2018-2020,a pot experiment was conducted with winter wheat（Triticum aestivum L.）cv.Zhengmai 9023.The experiment divided photosynthesis into three main processes: CO₂ transmission in leaves,light energy capture and transfer,and Rubisco carboxylation,to measure the photosynthetic characteristic parameters,chlorophyll fluorescence parameters,stomata and vein characteristics,the ultrastructure characteristics,photosynthetic biochemical parameters,and agronomic traits of wheat leaves.Finally,quantify the limiting factors of photosynthesis and clarify the regulation mechanism.The main results are as follows:（1）Under e T treatment,at the jointing stage,the stomatal density was increased,and the area of single stomatal of wheat leaves decreased,enhance the ability to regulate the opening and closing of the stomata,decreased the stomata conductance,and increased the water use efficiency.At the flowering stage,only the lower epidermis stomatal density was decreased,and the leaves maintain water use efficiency mainly by reducing the distance between large and small longitudinal veins,while the dependence on stomata morphology was reduced,and the stomata conductance remained unchanged.e C treatment increased stomatal density in the jointing stage,but the stomatal conductance decreased in two stages.e Ce T treatment also decreased the stomata conductance,but the stomatal density and area had little effects.（2）e T treatment decreased the liquid-phase transport distance of CO₂ diffusion,thereby significantly increased the mesophyll conductance.e C treatment also decreased the liquid-phase transport distance of CO₂ diffusion,and increased the surface area of the chloroplast facing the intercellular space,but increased the mesophyll conductance.Although e Ce T treatment decreasinged the transmission distance of the gas phase and the liquid phase,increased the thickness of the cell wall,decreased the mesophyll conductance.（3）The e C treatment could increase the maximum carboxylation rate(V_cmax)at flowering stage,and increase the net photosynthetic rate（P_n）of wheat leaves both at the two stages.The e T treatment would reduce V_cmax at jointing stage and increase the Ribulose-1,5-bisphosphate carboxylase（Rubisco）activity,but did not affect on P_n.Although V_cmax was increased at flowering stage,it decreased P_n.Under e Ce T condition,the decrease of the maximum electron transfer rate(J_max)at jointing stage,the increase of Rubisco activity at both two stage could result in the increase of P_n.（4）The e T treatment damaged the electron transporter on the acceptor side,increased the closure degree of PSⅡ centers（V_j）,and decreased the electron transport flux from QA to plastoquinone（ET_o/RC,ET_o/CSm）and quantum yield for electron transport from QA to plastoquinone（ΨEo）,thereby significantly inhibiting the PSⅡ performance(PI_ABS).The e C treatment decreased ET_o/RC,but increased the density of active PSⅡ reaction centers（RC/CSm）and decreased the reduction of QA（M_o）,thus improving the PI_ABS consequently.Under e Ce T condition,the electron transport performance of PSⅡ donor side（Wk）and acceptor side were improved（V_j,M_o）when compared with e T,so PI_ABS was unaffected when compared with CK.（5）The results of the two-year experiment showed that the e T treatment had little effect on the plant height,stem diameter and biomass accumulation at the jointing and flowering stages,and decreased the plant height,stem diameter and biomass accumulation at the mature stage.The effect of e C and e Ce T treatment on the plant height and stem diameter of wheat at the jointing and flowering stages differed depending on the year and stage.But the biomass only was decreased at e Ce T condition at the mature stage,indicating that the elevated CO₂ concentration cannot alleviate the adverse effects of elevated temperature on the biomass accumulation of winter wheat.Above all,under e T treatment,wheat photosynthesis is mainly used for biochemical limitation（lb）,accounting for 70.59%.The mesophyll limitation（lm）was the main factor of photosynthesis limitation under e C treatment,accounting for 52.31%.Under e Ce T treatment,there was no significant limitation effect among the three factors of stomatal（ls）,mesophyll and biochemical,which were 20.91%,36.81%and 42.28%,respectively.Under elevated CO₂ concentration,the increase of photosynthetic rate was conducive to the accumulation of above-ground biomass of wheat.Increased temperature and the interaction of CO₂ concentration and temperature were both not conducive to the accumulation of above-ground biomass of wheat.