ESTIMATION OF MAIZE (ZEA-MAYS-L) CANOPY CONDUCTANCE BY SCALING UP LEAF STOMATAL CONDUCTANCE

Transpiration is partially controlled by the plant at leaf level through the degree of aperture of the stomata. Mathematical models estimating the transpiration of plant stands using a conductance network approach to water vapor transfer thus need a plant surface control term (g(s)). Techniques involving different degrees of simplification of canopy structure have been proposed to estimate g(s) from measurements or estimates of leaf stomatal conductance (g(s)). This study compares the performance of some of these techniques by examining the patterns of durnal and seasonal variation for a maize crop grown at Ottawa, Canada. Measurements of g(s) were made for sunlit and shaded leaves at three levels in the plant canopy and the response of g(s) to photosynthetic photon flux density (Q(p)) has been parameterized. Values of g(s) obtained by different scaling-up methods were compared among themselves and with those obtained from the Penman-Monteith equation (g(c)). The main results and conclusions were daily maximum g(s) values of sunlit leaves generally occurred on or slightly before maximum radiation; the response of g(s) to Q(p) was a function of leaf levels in the canopy and growth stage; the response of g(s) to low Q(p) increased with leaf area index for shaded leaves; modelling of g(s) based on Q(p), leaf temperature and leaf water potential failed to give good estimates under overcast afternoon conditions; measurement of g(s) on horizontal portions of leaves led to an overestimation of g(s); spherical leaf angle distribution assumption gave the best estimates of g(s); the shelter factor (ratio of scaled-up g(s) over g(c) tended to increase as the ratio of the canopy aerodynamic conductance to top leaf stomatal conductance increased.