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Abstract. Modifications of the design and calibration procedure of a diffusion porometer permit determinations of stomatal resistance which agree well with results obtained bv leaf energy balance. The energy balance and the diffusion porometer measurements indicate that the boundary layer resistances of leaves in the field are substantially less than those predicted from hcat transport formulas based on wind flow and leaf size.

The vertical energy balance yields reliable estimates of evapotranspiration on an hourly (or halfhourly) basis under large variations of thermal stratification provided the measurements are made close to a reasonably homo geneous surface and provided that suitable spacial and timesampling procedures are followed. The energy bal ance method is relatively insensitive to incorrect assump tions concerning the eddy coefficients and to estimates of the Bowen ratio (ratio of sensible to latent heat) except during the unusual, and most often uninteresting, condi tion where the evaporation approaches the sensible heat flux to the surface (E = -A). The energy balance meth od, which measures the radiation exchange at the surface, places reasonable limits on evaporation estimates and thus is a promising method for daily estimates provided either periods of positive and negative net radiation are con sidered separately or that a reasonable estimate of the 24hour Bowen ratio can be developed. In humid regions there is little vertical transfer of sensible heat to the sur face, so that when "potential evapotranspiration" condi tions are obtained, the evapotranspiration will approximate the daily net radiation. In arid regions the evapotranspira tion from a wellwatered field may be, under extreme con ditions, almost twice the net radiation.Though the vertical energy balance is not adequate for estimating evapotranspiration from small plots, considera tion of the complete energy balance indicates serious limitations of smallplot experiments for evaluation of field practice. Energy balance measurements on corn, including the measurement of the radiation transmitted to the soil surface, indicates that the heat exchange at the soil surface is an appreciable fraction of the total heat exchange even in mature corn at high populations. Because of the rela tively high heat exchange at the soil surface, an under 'Contribution from the Department of Soils, University of Wisconsin, Madison. Published with the permission of the

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Spectral distributions of shade light between 400 and 740 mm were measured under corn, sugar maple, oaks, pines, and spruce with a portable recording spectrophotometer. Differences were found between hardwoods and softwoods and between clear cloudy days. An energy maximum at 550 nm, a minimum at 670 to 680 nm, and a very high maximum in the near infrared occurred under all species. Four components of light within a plant canopy can be distinguished: both beam solar radiation and diffused sky radiation are transmitted both directly and indirectly (reflected and scattered). Separate consideration of each of these components leads to great understanding of similarities and differences between light regimes in different stands.

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Individual leaves of potato (Solanum tuberosum L. W729R), a C3 plant, were subjected to various irradiances (400-700 nm), CO2 levels, and temperatures in a controUled-environment chamber. As irradiance increased, stomatal and mesophyH resistance exerted a strong and somewhat paraUeled regulation of photosynthesis as both showed a similar decrease reaching a minimum at about 85 neinsteins cm-2 sec-' (about '/2 of fuU sunlight). Also, there was a proportional hyperbolic increase in transpiration and photosynthesis with increasing irradiance up to 85 neinsteins* cm-2 sec-'. These results contrast with many C3 plants that have a near ful opening of stomata at much less light than is required for saturation of photosynthesis.Inhibition of photosynthesis by 21% 02 was nearly overcome by a 2-fold increase in atmospheric levels of CO2 (about 1,200 ng cm-3). Photosynthesis at 25 C, high irradiance, 2.5% 02 and atmospheric levels of CO2 was about 80% of the C02-saturated rate, suggesting that CO2 can be rte-limiting even without 02 inhibition of photosynthesis. With increasing CO2 concentration, mesophyll resistance decreased slightly while stomatal resistance increased markedly above 550 ng cm-3 which resulted in a significant reduction in transpiration.Although potato is a very productive C3 crop, there is substantial 02 inhibition of photosynthesis. The level of 02 inhibition was maximum around 25 C but the percentage inhibition of photosynthesis by 02 increased steadily from 38% at 16 C to 56% at 36 C. Photosynthesis and transpiration showed broad temperature optima (16-25 C). At higher temperatures, both the increased percentage inhibition of photosynthesis by 02 and the increased stomatal resistance limit photosynthesis, while increased stomatal resistance limits transpiration. Water use efficiency, when considered at a constant vapor pressure gradient, increased with increasing irradiance, CO2 concentration, and temperature.In spite of the importance of potato as a staple food crop, only limited studies (9, 24) have been done on its carbon balance and water use efficiency relative to other crops with respect to environmental factors. Potato is a C3 plant with photorespiration based on its high CO2 compensation point; yet its productivity is high (average market yield in U.S.: 4.66 metric tons dry weight of tuber/ha4) (1) and even comparable to that of some C4 crops (30). The productivity of a C3 crop depends on the carbon I

Concurrent measurements of abaxial and adaxial stomatal resistance and leaf-water potentials of snap beans (Phaseolus vulgaris L.) in the field and growth chamber show that the stomata on the 2 surfaces of the leaflet react differently to water deficit. The stomata on the abaxial surface, which are about 7 times more numerous than on the adaxial surface, are not significantly affected at leaf-water potentials greater than -11 bars, but with further decrease in leaf-water potential, the resistance rapidly increases. On the other hand, the resistance of the adaxial stomata increases sharply at a leaf-water potential of about -8 bars and is constant at higher water potentials. The average stomatal resistance for both surfaces of the leaf, which is the major diffusive resistance to water vapor, to a first approximation acts as an on-off switch and helps prevent further decline in leaf-water potential. The relation between the leaf-water potential and the stomatal resistance links the soil-water potential to the transpiration stream as needed for soil-plant-atmosphere models.