Water Use and Soil Water Depletion Patterns of Dry-Land Wheat Grown Under Elevated CO2 in a Free Air CO2 Enrichment (FACE) Facility.

Elevated [CO₂] (e[CO₂]) has profound effects on biomass, yield and water use of crops. At least in C3 crops, e[CO₂] stimulates the photosynthetic rate (A_net) and decreases stomatal conductance (g_s). Consequently the intrinsic water use efficiency (ITE) increases. Greater ITE may conserve soil water for longer, making water reserves available later in the growing season. In contrast, greater biomass can result in more water depletion under e[CO₂].

This study investigated the interactive effect of atmospheric [CO₂] and water regime (predominately rain-fed and well-watered) on water use and soil water depletion patterns of wheat in a low rainfall environment. Two cultivars, differing in their transpiration efficiency (cvs. Scout and Yitpi), were grown under ambient (a[CO₂], ∼400 µmol mol^-1) and e[CO₂] (∼550 µmol mol^-1) in the Australian Grains Free Air CO₂ Enrichment (AGFACE) facility (Horsham, Victoria, Australia). A_net, g_s, soil water content and Normalised Difference Vegetation Index (as surrogate of canopy cover) were measured throughout the growing season. Agronomic parameters were recorded at stem-elongation, anthesis and physiological maturity.

Indeed, higher A_net and lower g_s of wheat under e[CO₂] compared to a[CO₂] resulted in greater ITE. Scout had lower g_s and greater ITE compared to Yitpi. Well-watered plants had increased A_net and g_s but decreased ITE compared to rain-fed plants. e[CO₂] accelerated the canopy cover development at early stages and sped up the senescence later in the season. Well-watered plants had larger leaf area, canopy cover, biomass and yield compared to rain-fed. Despite higher ITE, wheat under e[CO₂] did not always conserve water. The effect of [CO₂] on soil water was dependent on soil depth and time of the year. The cumulative evapotranspiration under e[CO₂] was lower in well-watered conditions. Therefore, saving or depletion of water under e[CO₂] will be dependent on the timing of water input during the growing season of wheat.

Keywords: climate change, FACE, dry-land agriculture, water use, soil moisture depletion