Canopy conductance, carbon assimilation and water use in wheat

Abstract

Chambers were used to investigate changes in assimilation ( A) and evaporation ( E) rates in the field on a diurnal and a daily basis in rainfed and irrigated crops of wheat. Measurements were made in crops at growth stages between ear emergence and physiological maturity. Leaf area index ranged from a maximum of ∼8 to a minimum of ∼0.2. Assimilation and canopy conductance, g C, decreased rapidly in rainfed treatments during grain filling, with midday values of g C becoming progressively smaller than morning values as stress progressed. Hysteresis was also evident in well-watered crops, indicating that E was adversely affected by the increase in evaporative demand during the day. Evaporative fluxes were analysed in terms of the sensitivity (α) of leaf conductance to solar radiation. Changes in α implied both long-term and diurnal effects of stress on E. The estimate of α in the middle of the day was ∼0.02 mm s −1 (W m −2) −1 in well-watered crops and so corresponded with the upper limit of the slope, leaf conductance vs. net radiation, reported elsewhere for wheat. Estimates of α were generally larger in the morning than at noon. Diurnal changes in g C and A indicated that the diffusive component in the assimilation pathway was the more sensitive to stress. The mean rate of assimilation during the day, A , was curvilinearly related to g C such that the A: g C ratio decreased with an increase in g C . These changes suggested that stress increased relative stomatal control over CO 2 assimilation and increased water use efficiency. Daily carbon assimilation was poorly associated with daily light interception in these data. However, the strength of the relationship between A and g C was sufficient to postulate that estimates of light use efficiency, derived from measurements of A and light interception or, less directly, growth and light interception, may be used to infer effects of stress on canopy conductance to CO 2 and H 2O under conditions of water stress in wheat.