Maize canopies under two soil water regimes: II. Seasonal trends of evapotranspiration, carbon dioxide assimilation and canopy conductance, and as related to leaf area index

Abstract

Seasonal course of evapotranspiration and of canopy CO 2 assimilation are determined by interactions among the plant, aerial, and soil factors over time and culminate in the total water requirement and primary productivity of a crop. In this study, maize was grown for two seasons in large fields under two contrasting soil water regimes (WET and DRY), and monitored for canopy water vapor and CO 2 fluxes with the Bowen-ratio/energy-balance/CO 2 gradient technique (BREB+). Soil CO 2 efflux was quantified and added to the canopy CO 2 flux to obtain canopy net assimilation rate ( A n). Trends in daily integrals of evapotranspiration ( E) and daily averages of midday A n, spanning the period from 10 days before tasseling to nearly full senescence, are presented along with data on trends in soil water status, leaf water potential, and green leaf area index (LAI). Also presented are data on midday photosynthesis of the upper exposed leaves. Aerodynamic conductance for the crop was calculated from wind data. Canopy conductance was calculated using the Penman–Monteith big leaf model. Over the season but before senescence became substantial, the mean midday conductance of the aerodynamic segment of the plant-atmosphere transport path was slightly higher than that of the canopy segment of the transport path. The wind pattern on most days were such, however, that E may be said to be more under the influence of air motion in the morning, but more under the influence of the canopy in the afternoon [Steduto, P., Hsiao, T.C., 1998a. Maize canopies under two soil water regimes: I. Diurnal patterns of energy balance, carbon dioxide flux and canopy conductance. Agric. For. Meteorol. 89, 173–188]. One view of the soil–plant–atmosphere continuum was obtained by examining the links among soil and plant water status, LAI, canopy conductance ( g cw), E and A n over the season. E and A n declined only when g cw fell below a threshold range of 10 to 20 mm s −1, for the WET as well as the DRY treatment. The midvalue of the threshold was 15 mm s −1, a value in agreement with the very limited data in the literature. Significance of this threshold is discussed in terms of crop coefficients in the estimation of crop E and in the use of the Penman Monteith equation for the estimation of reference E. The fall in g cw over time was largely due to the decline in LAI as the crop matured or as the result of early senescence induced by water deficit. A plot of g cw vs. LAI revealed that during the senescence phase g cw was lower for the DRY than the WET treatment at any given LAI, a difference that may be attributed to lower stomatal conductance under water stress. Overall, however, canopy processes appeared to be largely dictated by LAI for our conditions where water stress developed gradually over a long period.