1980 Volume 35 Issue 4 Pages 201-213
A semi-empirical model was proposed for the simulation of leaf temperature (Tl), and rates of net photosynthesis (Pn) and transpiration (E) of a single rice leaf in steady-states. The model consisted of two main sub-models: one is for solving the energy balance equations to give E and Tl and another for Pn. In both models the energy and mass transfer processes were expressed in terms of the diffusion resistances, namely, the boundary layer (ra), the stomatal (rs) and the overall mesophyll or residual (rM) resistances. Both the sub-models were thus connected by the resistances and Tl. The diffusion resistances in dependence of wind speed (U), shortwave radiation flux intensity (Is), the leaf temperature (Tl) and ambient humidity were formulated by means of physical and/or empirical equations. By incorporating experimentally specified parameters into the simultaneous equations thus derived, simulations were made to obtain Pn, E and Tl for various environmental conditions.
The following results were obtained from the simulations. First, with the increase in the radiation intensity (Is) up to about 0.5cal cm-2min-1, E increased very sharply due to the opening of the stomata, resulting in Tl decrease or only a slight increase, and above this Is level E and Tl both escaped from the stomatal regulations and increased proportionally to Is. The leaf air temperature difference (Tl-Ta) was larger the lower the Ta. Second, three types of curves were derived in photosynthesis (Pn)-radiation (Is) relation, depending on the environmental conditions; a non-saturation type curve at lower Ta, a saturation type curve at near optimal Ta and an optimal type curve at supra-optimal Ta with low humidity. The difference in Pn-Is curve was attributable to Is effect on Tl which in turn reflected on rs and rM. Third, as a result of the response of rs to the leaf air vapour pressure difference, Pn and E at lower humidity were suppressed considerably, resulting in Tl increase. Fourth, with increasing wind speed (U), Pn and E at optimal or above optimal Ta increased, whereas those at sub-optimal Ta decreased. These contradicting effects of U on Pn and E appeared through its effects on ra and Tl.
The simulation results were compared with measured data on rice leaves by a leaf chamber method and also with data by other workers. Except for Tl at higher Is and Ta, the model well explained the observed responses in Pn, E and Tl to the environments.
