Diffusive resistances at, and transpiration rates from leaves in situ within the vegetative canopy of a corn crop

At several heights and times of day within a crop of Zea mays, internal leaf diffusion resistance (r_i) and external boundary layer diffusion resistance (r_a) were evaluated by measuring the temperature of a transpiring and a non-transpiring leaf (simulated by covering both sides of a normal leaf with strips of poly-ethylene tape), and by measuring the immediate air temperature, humidity and windspeed.

Both r_a and r_i increased with depth into the crop. However, r_a generally was less than 10% of r_i.

Profiles of latent-heat flux density and source intensity of transpiration showed that transpiration corresponded roughly to foliage distribution (with an upward shift) and were not similar to the profile of radiation absorption.

The data were compared with heat budget data. The 2 approaches yielded quite similar height distributions of transpiration per unit leaf area and total transpiration resistance.

The total crop resistance to transpiration was computed as 0.027 min cm⁻¹. This compares to Monteith's values of 0.017 to 0.040 min cm⁻¹ for beans (Phaseolus vulgaris L.), and Linacre's values of 0.015 to 0.020 min cm⁻¹ for turf.