Scaling photosynthesis and water use from leaves to paddocks

Abstract

Plant breeders can select cultivars with physiological traits that confer a growth or yield advantage to individual plants. The extent to which single plant characters influence canopy performance depends on interactions between vegetation and the atmosphere and the non-linear response of physiological processes to the environment. Better understanding of the scaling of photosynthesis and water use will allow the assessment of changes to leaf scale physiological traits at the canopy scale and prediction of the response of vegetation to climate change. This thesis examines the relationship between reduced stomatal conductance and canopy scale water-use efficiency (ratio of instantaneous net canopy photosynthesis to total canopy evaporation). A multi-disciplinary research project was established with two large paddocks of wheat with cultivars of contrasting leaf-scale water-use efficiency, due to inherent differences in stomatal conductance. Intensive measurements were made of C02 and H20 fluxes at leaf and canopy scales. Different stomatal conductances at the leaf scale were reflected at the. canopy scale, although their effects on transpiration were reduced due to canopy boundary layers and soil evaporation. Comparison of scaling from leaf to canopy in the two crops was complicated by their different leaf area indices. To facilitate scaling from leaves to canopies, models of stomatal conductance, leaf photosynthesis and radiation penetration in canopies were used. A comparison of several models of conductance with field data found that using the correlation of conductance with photosynthesis was the best approach. The same model was found to work equally well at the canopy scale, using parameters derived from leaf scale data. Canopy photosynthesis was modelled with a biochemical model of leaf photosynthesis incorporated into different integration schemes. A canopy model which divided the canopy into a single layer of sunlit and shaded leaves was found to be as accurate as a multi-layer model, but simpler and allowed incorporation of within-canopy profiles of photosynthetic capacity. A big-leaf model of canopy photosynthesis was found acceptable if tuned, but the uncertainties increased when it was used to predict responses of canopies with different properties. Photosynthetic capacity, the main parameter of the canopy photosynthesis model, was found to decrease during the day under conditions of mild water stress at both the leaf and canopy scale. Combined models of photosynthesis, conductance and energy balance accurately described diurnal variation of canopy gas exchange. The model predicted that a 40% reduction in stomatal conductance would result in 36% greater leaf transpiration efficiency and 19% greater canopy transpiration efficiency (ratio of gross canopy photosynthesis to canopy transpiration) which compared favourably with field measurements, but depended on the magnitude of the conductance and wind speed. Measurements of air temperature, humidity and surface temperature along a transect across the interface between the two crops with different evaporation rates, showed that advection did occur, but that it had minimal impact on canopy fluxes. It was concluded that reduced stomatal conductance does result in reduced transpiration and better transpiration efficiency at the canopy scale, but that canopy boundary layers and greater soil evaporation reduce the benefit. In this case reduced conductance was also accompanied by greater yield, although this result depends on the availability of soil water. The models presented were an effective tool for scaling nonlinear physiological processes from leaves to canopies and provide a useful framework for assessing the impact of climate change on vegetation.