A DYNAMIC-MODEL DESCRIBING LEAF TEMPERATURE AND TRANSPIRATION OF WHEAT PLANTS

A model of actual wheat canopy transpiration flux is presented as a fu nction of the local transpiration flux at the top of the canopy, of le af area index and of canopy nitrogen content. Three main assumptions a re stated to derive this function. Firstly, with respect to a horizont al area element, the leaf area density and local transpiration flux ar e independent of their location within a canopy layer (homogeneity in horizontal direction). Secondly, the transpiration activity of plant l eaves depends not only on driving environmental forces and available w ater but on leaf nitrogen content also. Thirdly, the profile of the ni trogen content of leaves within the canopy is divided into a ''top'' a nd into a ''bottom'' region. Both regions are vertically uniform withi n the canopy (rectangular profile). Based on these assumptions, the ca nopy transpiration flux of wheat plants grown in north Germany under f armer's usual fertilization (N4) and zero fertilization (NO) was inves tigated with the model. The influence of the normal N fertilization is twofold: transpiration increases because of the higher leaf area inde x, but also because of the higher nitrogen content of the leaves. A co mparison of measured soil water contents under the two N treatment can opies with the modelled canopy evapotranspiration shows that the prese nted approach is appropriate to capture differences in water budgets d ue to differences in plant nutrition. The transpiration model is suita ble for incorporation into complex models of plant canopies (water tra nsport, growth).