Measuring Transpiration Resistance of Leaves

The transpiration process can be described physically, following Raschke (10), in terms of a resistance to diffusive and turbulent vapor flux in the external air, a similar diffusive resistance which results from the internal leaf geometry, inclusive of the stomata, and parallel to the latter, a resistance to vapor diffusion through the cuticle. In contrast, the last 2 resistances do not exist in the evaporation from an open water surface or a moist blotter paper. The resistance in these cases can be described using only external paramneters. In applying the resistance concept, the internal vapor concentration is generally computed from the leaf temperature, assuming that the state of the leaf water is such that its relative vapor pressure is substantially equal to one. This view is open to some objections; see, for example, Bange (2) and Heath (5). Nevertheless, it is applied by most plant physiologists as it will be in this paper. The leaf resistance must be known when computing leaf or canopy evaporation from measured environmental parameters, although it may not always be of significant magnitude compared to the external vapor transport resistance. Thus, Bange (2) developed a procedure for calculating the resistance from measurements of stomatal dimensions and numbers, and substomatal cavity dimensions. This procedure does not allow for the contributioln of cuticular transpiration to the total water loss, which still must be measured directly. Using an experimental correction for cuticular loss, Bange's work gave close agreement between measured and computed transpiration from leaf disks of Zebrina pendula, a hypostomatous plant. Earlier, Penman and Schofield (9) suggested a method similar to that of Bange for estimating the stomatal resistance, the values to be applied in calculating canopy evaporation. For the latter, they proposed a combination metlhod devised by Penman see Pelton and Tanner (8) for a critical discussion in which the energy balance and aerodynamics of the vegetative surface are simultaneously taken into account. Penman and Schofie&d did not give such (letailed consideration to substomatal cavity dimensions and other geometrical details as did Bange, nor did they verify their results by direct experimentation. Raschke (10) gave a method for computing the leaf resistance indirectly from simultaneous measurements of leaf transpiration, leaf temperature, leaf radia' ion balance, and ambient temperature and vapor pressure. Criginally, he expressed the result by means of a dimensionless factor a, the wetness factor, being the ratio between the actual transpiration rate and the calculated evaporation rate of a hypothetical free water surface having the same dimensions, temperatture, and exposure as the leaf. Raschke showed in a later publication (11) that a was identical with the ratio of the combined turbulent and laminar vapor flow resistance in the air outside the leaf (the external resistance, RA) to the sum of RA and the leaf resistance (RL): a RA/(RA + RL)I RA can be approximated from observing the evaporation from properly exposed wet filter paper of leaf dim1ensions. RA then equals the vapor concentration gradient from paper to bulk air divided by the evaporation rate per unit area. Raschke's method is sound, but obviously not simple, nor always practical. On the other hand, the approaches taken by Bange, and by Penman and Schofield, do not account for cuticular water loss, and require a precise knowledge of stomatal aperture and internal leaf geometry. Whereas Bange showed agreement between theory and experiment, Pelton and Tanner (8) report unsatisfactory results from tests of the Penman-Schofield procedure. These tests were not stringent, and the disagreement may have originated in Penman and Schofield's proposal to use a generalized value for the stomatal resistanice and apply it to a canopy as one would to a single leaf. The stomatal aperture itself can be measured by various methods, such as visual observation of leaves or their replicas, measurements with pressure poroleters, anld infiltration techniques. Recent. reviews of suichl teclhniques are found in Heath (5) and Eckhardt (3). Regardless of technical feasibility, none permits an exact calculation of the leaf resistance factor mentioned above. Until now, the only available field method for measuring transpiration resistance directly was the classical cobalt chloride method, described in many textbooks. It is slow and subjective, and also likely to influence both stomatal aperture and leaf temperaI Received November 2, 1964.