MAIZE CANOPIES UNDER 2 SOIL-WATER REGIMES - I - DIURNAL PATTERNS OF ENERGY-BALANCE, CARBON-DIOXIDE FLUX, AND CANOPY CONDUCTANCE

Photosynthesis of individual leaves and canopies in relation to transp iration has been studied fairly extensively with chambers, which, unfo rtunately, alter the environment of the plant. Scaling up remains a pr oblem and there are only limited data on simultaneously measured water vapor and CO2 fluxes of clop canopies, especially comparing treatment !, grown side-by-side. In the present study, diurnal patterns of fluxe s of CO2, latent heat, and other components of energy over adjacent fi elds of maize, one well watered and one growing only on water stored i n the soil, were determined under clear and variable cloud conditions with the Bowen-ratio/energy balance/CO2 gradient (BREB+) technique. To follow the dynamic changes caused by weather conditions and to link t hem to canopy processes, averaging time of the signals was set for 5 m in. Several lines of evidence validating this short averaging time are presented in the fourth paper of this series. Our data confirmed the known pivotal role of radiation in both latent heat (lambda E) and CO2 flux (A(cf)) of the canopy. Further, it was shown that the canopy res ponded to the movement of clouds without measurable lag, with A(cf), l ambda E and sensible heat (H) fluctuating in unison as the sun disappe ared and reappeared. A(cf), reaching peak values of about 30-35 mu mol m(-2) s(-1), was higher in the morning than in the afternoon for the same level of Q(p) (photosynthetic photon flux density), and dropped t o zero late in the afternoon when Q(p) was still substantial (e.g., 0. 3 mmol m(-2) s(-1)). This was apparently the result of increases in re spiration as the temperature rose from the morning to the afternoon. S enescence of the canopy, whether normal as maturity was approached or accelerated by water stress, was associated with a general reduction i n A(cf) for a given green leaf area index. Late in the life cycle, A(c f) was apparently light saturated at 1.3 to 1.6 mmol m(-2) s(-1). At t hat time, abnormally high radiation due to focusing by clouds caused a n increase in lambda E and H, but not in A(cf). Canopy conductances we re calculated using the Penman-Monteith big leaf model after aerodynam ic conductances were derived from wind data. The diurnal patterns show ed that canopy conductance was strongly influenced by radiation, and w as independent from and usually lower than aerodynamic conductances. C anopy conductance for water vapor reached noon-time values up to 40 mm s(-1) under the most favorable water conditions. Water stress and sen escence reduced canopy conductance. Aerodynamic conductance for warts vapor ranged between 15 and 90 mm s(-1) for wind velocity between 1 an d 5 m s(-1). respectively. (C) 1998 Elsevier Science B.V.