A COMPARATIVE-STUDY OF MASS AND ENERGY-EXCHANGE RATES OVER A CLOSED C-3 (WHEAT) AND AN OPEN C-4 (CORN) CROP .2. CO2 EXCHANGE AND WATER-USE EFFICIENCY

Major differences exist between the photosynthetic and transpiration rates Of C3 and C4 leaves as a result of biochemical and physiological factors. Whether or not differences between CO2 and water vapor exchange rates Of C3 and C4 species scale from leaf to field dimensions is poorly known. The aim of this work is to improve our understanding on how environmental, architectural and physiological variables affect the flux densities Of CO2 and water vapor over C3 and C4 crop stands during day and night periods. Experimental data were obtained over a closed wheat and an open com stand using the eddy correlation method. Interpretation of the field measurements is aided by the use of a canopy photosynthesis/evaporation model. The flux density of absorbed photosynthetically active radiation (Q(a)) had a disproportionate influence on CO2 flux densities measured over a closed C3 and an open C4 crop. Variations in Q(a) explained over 88% of the variance in daytime CO2 flux densities, F(c). At night, canopy radiative temperature was the main environmental factor controlling the respiratory CO2 efflux by the two crops. Leaf area index and growth stage were the plant variables that affected F(c) most. Incremental increases in leaf area index enhanced the com crop's ability to absorb incident solar radiation and enlarged the com's sink strength for CO2. Heading by the wheat caused rates of daytime CO2 gains to decrease and rates of night-time CO2 losses to increase. Water use efficiency of the wheat crop improved as the absolute humidity deficit of the atmosphere decreased. Water use efficiency of the com, on the other hand, was relatively insensitive to humidity deficits. With regard to canopy CO2 exchange and water use efficiency, differences in canopy structure between the wheat and com overwhelmed physiological differences. The closed C3 wheat crop assimilated CO2 at a higher rate than the sparse C4 com canopy, even though com uses a more efficient photosynthetic pathway. Consequently, water use efficiency of the com was not greater than values measured over the wheat, Instead, water use efficiencies of the two crops were similar. The com crop assimilated CO2 at a lower rate than wheat because the com's canopy quantum yield was lower and because its sparse canopy absorbed less photosynthetically active radiation than the closed wheat stand.