The influence of leaf photosynthetic efficiency and stomatal closure on canopy carbon uptake and evapotranspiration – a model study in wheat and sugar beet

Abstract. In this study two crop species, winter wheat (Triticum aestivum) and sugar beet (Beta vulgaris), were monitored over the course of five days during the entire season. We investigated the link of the main physiological leaf-level mechanisms, stomatal conductance and efficiency of photosynthetic energy conversion on canopy transpiration and photosynthetic CO₂ uptake. The physiological status of 900 leaves of different plants in a natural canopy was characterized on the leaf level using chlorophyll fluorescence. Gas exchange measurements were performed at leaves of 12 individual plants of each species. Eddy covariance flux measurements provided information on CO₂, water and energy fluxes on the field scale. The diurnal pattern of stomatal resistance on the leaf level was especially for sugar beet similar to the canopy resistance, which indicates that stomatal resistance may have a large impact on the bulk canopy resistance. The diurnal changes in canopy resistance appeared to have less effect on the evapotranspiration, which was mainly dependent on the amount of incoming radiation. The similar diurnal pattern of water use efficiency on the leaf level and on the canopy level during the day, underline the influence of physiological mechanisms of leaves on the canopy. The greatest difference between water use efficiency on leaf and canopy occurred in the morning, mainly due to an increase of stomatal resistance. Limitation of CO₂ uptake occurred in the afternoon when water vapor pressure deficit increased. Maxima of net ecosystem productivity corresponded to the highest values of photosynthetic capacity of single leaves, which occurred before solar noon. Within the course of a few hours, photosynthetic efficiency and stomatal resistance of leaves varied and these variations were the reason for diurnal variations in the carbon fluxes of the whole field. During the seasonal development, the leaf area index was the main factor driving carbon and water exchange, when both crops were still growing. During senescence of winter wheat these structural parameters did not account for changes in canopy fluxes and remaining high green leaf material of sugar beet did not present the reduction in canopy fluxes due to beginning dormancy. We thus hypothesize that the functional status of plants is also important to correctly predict carbon and water fluxes throughout the season. We propose to additionally include the physiological status of plants in carbon flux models in order to improve the quality of the simulation of diurnal patterns of carbon fluxes.