Response of Photosynthesis, Stomatal Conductance, Water Use Efficiency and Production to Long-Term Elevated CO2 in Winter Wheat

Responses of photosynthesis, stomatal conductance, water use efficiency (at the beginning of flowering) and production allocation (at full ear/grain ripening) to long-term elevated CO2 were assessed in winter wheat (Triticum aestivum L. cv. MV16). Plants were grown in open top chambers under a temperate-continental climate from germination at ambient (350 μmol mol-1) and elevated (700 μmol mol-1) CO2 concentrations. High CO2 plants displayed a decreased initial slope of the A/Ci response curve, with the assimilation rate (A) continuing to increase above 400 μmol mol-1 internal CO2 concentration (C). A in the ambient plants showed P regeneration limitation while RuBP regeneration appeared to be limiting A in the high CO2 treatment. Variable fluorescence ratios (Rfd 690) were lower in the high CO2 plants indicating a lower potential photochemical activity. The increase in the values for the chlorophyll fluorescence ratio F690/F735 in the high CO2 plants was in agreement with the lower chlorophyll a+b concentrations. The high CO2 plants had higher concentrations of starch in their leaves and roots that the ambient plants. Stomatal conductance (gs) was lower in the high CO2 plants at every CO2 concentration (Ca) and Ci and the Ci-dependent gs response had a large influence on the A/gs function. The higher water use efficiency (WUE) values (at Ca's>350 μmol mol-1) in the high CO2 wheat plants were the result of a larger decrease in transpiration rate (E) in the high CO2 plants than in the ambient plants, and of a simultaneous larger increase in A in the range of Ca above 350 μmol mol-1 CO2. The integrated and combined effect of the photosynthetic and stomatal acclimation to elevated CO2 produced a higher C-assimilation in high CO2 plants at elevated CO2 than in the ambient plants, however, this was not followed by an acclimation in C-allocation. These were reflected in a slightly increased (6.7 %) overall dry matter production and lower reproductive allocation (RA).