Response of Photosynthesis, Stomatal Conductance, Water Use Efficiency and Production to Long-Term Elevated CO 2 in Winter Wheat

Abstract

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 CO 2 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 ) CO 2 concentrations. High CO 2 plants displayed a decreased initial slope of the A/C i response curve, with the assimilation rate (A) continuing to increase above 400 μmol mol -1 internal CO 2 concentration (C). A in the ambient plants showed P regeneration limitation while RuBP regeneration appeared to be limiting A in the high CO 2 treatment. Variable fluorescence ratios (Rfd 690) were lower in the high CO 2 plants indicating a lower potential photochemical activity. The increase in the values for the chlorophyll fluorescence ratio F690/F735 in the high CO 2 plants was in agreement with the lower chlorophyll a+b concentrations. The high CO 2 plants had higher concentrations of starch in their leaves and roots that the ambient plants. Stomatal conductance (g s ) was lower in the high CO 2 plants at every CO 2 concentration (C a ) and C i and the C i -dependent g s response had a large influence on the A/g s function. The higher water use efficiency (WUE) values (at C a 's>350 μmol mol -1 ) in the high CO 2 wheat plants were the result of a larger decrease in transpiration rate (E) in the high CO 2 plants than in the ambient plants, and of a simultaneous larger increase in A in the range of C a above 350 μmol mol -1 CO 2 . The integrated and combined effect of the photosynthetic and stomatal acclimation to elevated CO 2 produced a higher C-assimilation in high CO 2 plants at elevated CO 2 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).