Interactive effects of elevated atmospheric CO 2 concentrations and plant available soil water content on canopy evapotranspiration and conductance of spring wheat

Abstract

The present study was conducted to investigate the possible interactive effects of rising atmospheric CO 2 concentration [CO 2] and drought stress on water use of wheat. Spring wheat ( Triticum aestivum cv. “Minaret”) was grown either in 1 m diameter lysimeters with 0.4 m soil depth (1998) or in the field (1999) in open-top chambers under two CO 2-concentrations (ambient, ambient + 280 ppm) and two watering regimes (well-watered = WW with a plant available water content PAW > 40 mm and drought stressed = DS, 10 mm < PAW < 30 mm) beginning after first node stage. Canopy evapotranspiration ( E C) was measured continuously from first the node stage until the beginning of flag leaf senescence using four open-system canopy chambers (0.78 m 3). Seasonal changes of the absorption of photosynthetically active radiation (APAR) of the canopy and root growth (1999) were also measured. In both growing seasons leaf area index increased in response to elevated [CO 2] in both water treatments. The related effects of [CO 2] on canopy radiation absorption (APAR) were, however, smaller. E C was linearily related to APAR in both growing seasons. While elevated [CO 2] reduced the slope of this relation under WW conditions by ca. 20% in both growing seasons, it was not reduced (1998) and even increased (1999) under drought. Canopy conductance ( G C) calculated as E C divided by vapour pressure deficit of air, showed a non-linear relationship to APAR that was best explained by saturation curves. Under WW conditions, elevated [CO 2] reduced the initial slope of G C versus APAR as well as G C at saturating light conditions (ca. −30%), while under DS conditions no effect of elevated [CO 2] could be detected. Under high light conditions (PAR > 400 μmol m −2 s −1) a critical “threshold value” of PAW ( T PAW, ca. 40 mm) could be identified above which G C did not respond to PAW. While in 1998 G C did not respond to elevated [CO 2] at PAW < T PAW, it was slightly increased at low PAW values in the field experiments of 1999. The reduction of T PAW by elevated [CO 2] may be explained by enhanced root growth (1999) that would have given the plants better access to soil water resources. The present results suggest that below a critical soil water content elevated [CO 2] will not reduce canopy water loss of wheat or may even enhance it.