CANOPY CONDUCTANCE, CARBON ASSIMILATION AND WATER-USE IN WHEAT

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, gC, decreased rapidly in rainfed treatments during grain filling, with midday values of gC 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 gC 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 gC such that the A:gC ratio decreased with an increase in gC. These changes suggested that stress increased relative stomatal control over CO2 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 gC 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 CO2 and H2O under conditions of water stress in wheat. © 1990.