Microclimate-plant architectural interactions: Influence of leaf width on the mass and energy exchange of a soybean canopy

Abstract

Two isolines of soybean ( Glycine max L. Merrill, cv. Clark) differing only in leaf width were studied to ascertain the effects of leaf width on the canopy microclimate and mass and energy exchanges. The study was performed during the 1981 growing season at Mead, Nebraska. Mass and energy exchanges were determined by means of micrometeorological techniques. Turbulent mixing was affected in a complex manner. Before full cover was achieved, turbulent mixing (expressed in terms of friction velocity, u ∗ ) was slightly greater over the isoline with narrower leaves (denoted CLN). After full-cover was achieved, turbulent mixing was greater over the isoline with normal leaves (CN). Analysis of data indicated that greater u ∗ values in the CN canopy were due to greater canopy density which caused greater bluff-body effects. Differences in leaf width did not affect the above-canopy radiation balance, but did affect the within-canopy profile of net radiation ( Rn). Rn was greater deep inside the CLN isoline since this isoline established a canopy with less leaf area. This difference in the vertical profile of Rn affected the canopy microclimate and the partitioning of this radiation. Higher air temperatures, lower vapor pressures, lower latent heat exchange and greater sensible and soil heat flux were observed in the CLN canopy. Greater CO 2 concentrations were also measured within the CLN canopy because warmer soil temperatures caused greater soil efflux of CO 2. Also, less leaf area of that canopy prevented a stronger drawdown in [CO 2]. Canopy CO 2 exchange was greater over the CLN isoline when expressed on a leaf area basis, whereas no differences in CO 2 exchange were observed between fluxes expressed on a ground area basis. The CO 2—water flux ratio, an index of water use efficiency, was improved in the CLN crop because of its lower evapotranspiration rates.