Interactive effects of elevated atmospheric CO2 concentrations and plant available soil water content on canopy evapotranspiration and conductance of spring wheat

The present study was conducted to investigate the possible interactive effects of rising atmospheric CO₂ concentration CO₂] 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 CO₂-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 m³). 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 CO₂] in both water treatments. The related effects of CO₂] on canopy radiation absorption (APAR) were, however, smaller. E_C was linearily related to APAR in both growing seasons. While elevated CO₂] 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 CO₂] 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 CO₂] 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 CO₂] 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 CO₂] 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 CO₂] will not reduce canopy water loss of wheat or may even enhance it.