Response of wheat canopy CO2 and water gas-exchange to soil water content under ambient and elevated CO2

The nature of the interaction between drought and elevated CO2 partial pres sure (pC(a)) is critically important for the effects of global change on cr ops. Some crop models assume that the relative responses of transpiration a nd photosynthesis to soil water deficit are unaltered by elevated pC(a), wh ile others predict decreased sensitivity to drought at elevated pCa. These assumptions were tested by measuring canopy photosynthesis and transpiratio n in spring wheat (cv. Minaret) stands grown in boxes with 100 L rooting vo lume. Plants were grown under controlled environments with constant light ( 300 mu mol m(-2) s(-1)) at ambient (36 Pa) or elevated (68 Pa) PCa and were well watered throughout growth or had a controlled decline in soil water s tarting at ear emergence. Drought decreased final aboveground biomass (-15% ) and grain yield (-19%) while elevated pC(a) increased biomass (+24%) and grain yield (+29%) and there was no significant interaction. Elevated PCa i ncreased canopy photosynthesis by 15% on average for both water regimes and increased dark respiration per unit ground area in well-watered plants, bu t not drought-grown ones. Canopy transpiration and photosynthesis were decr eased in drought-grown plants relative to well-watered plants after about 2 0-25 days from the start of the drought. Elevated pC(a) decreased transpira tion only slightly during drought, but canopy photosynthesis continued to b e stimulated so that net growth per unit water transpired increased by 21%. The effect of drought on canopy photosynthesis was not the consequence of a loss of photosynthetic capacity initially, as photosynthesis continued to be stimulated proportionately by a fixed increase in irradiance. Drought b egan to decrease canopy transpiration below a relative plant-available soil water content of 0.6 and canopy photosynthesis and growth below 0.4. The s hape of these responses were unaffected by pC(a), supporting the simple ass umption used in some models that they are independent of pC(a).