Interactive effects of elevated carbon dioxide and drought on wheat

Atmospheric CO₂ concentration (C_a) continues to rise. An imperative exists, therefore, to elucidate the interactive effects of elevated C_a and drought on plant water relations of wheat (Triticum aestivum L.). A spring wheat (cv. Yecora Rojo) crop was exposed to ambient (Control: 370 μmol mol^-1) and free-air CO₂ enrichment (FACE: ambient + 180 μmol mol^-1) under ample (Wet), and reduced (Dry), water supplies (100 and 50% replacement of evapotranspiration, respectively) over a 2-yr study. Our objective was to characterize and quantify the responses of 26 edaphic, gas exchange, water relations, carbohydrate pool dynamics, growth, and development parameters to rising C_a and drought. Increasing C_a minimized the deleterious effects of soil-water depletion by increasing drought avoidance (i.e., lower stomatal conductance and transpiration rate, and growth and development of a more robust root system) and drought tolerance (i.e., enhanced osmoregulation and adaptation of tissue) mechanisms, resulting in a 30% reduction in water stress-induced midafternoon depressions in net assimilation rate. An elevated C _a-based increase in daily and seasonal carbon gain resulted in a positive feedback between source capacity (shoots) and sink demand (roots). Devoid of a concomitant rise in global temperature resulting from the rise in C_a, improved water relations for a herbaceous, cool-season, annual, C₃ cereal monocot grass (i.e., wheat) are anticipated in a future high-CO₂ world. These findings are applicable to other graminaceous species of a similar function-type as wheat common to temperate zone grassland prairies and savannas, especially under dryland conditions.