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When sprayed on plant leaves, several compounds, notably phenylmercuric acetate (PMA), can induce stomatal closure (16, 17). Most of these investigations with PMA were carried out on detached leaves with petioles dipping in water; it was found that the stomatal closing markedly reduced transpiration, whereas photosynthesis was reduced to a lesser extent. In these experiments it was demonstrated that the relations between both transpiration and photosynthesis and stomatal opening agreed with the equations relating diffusion of water vapor and CO2 to resistance calculated from stomatal dimensions (8, 17). The possibility of reducing plant transpiration by chemical treatment, without materially reducing photosynthesis, is of practical importance in arid regions. In addition, this method of transpiration control offered a promising tool for the study of soilwater-plant interrelations. To serve both ends, we sprayed various plants with PMA, and observed stomatal opening, transpiration, and photosynthesis or growth. This was performed on young maize growing in soil in a controlled-environment chamber, as well as on tobacco growing in a greenhouse and sunflowers outdoors. These experiments, performed in the controlled-environment chamber are reported here; the other experiments are reported in the accompanying paper. It was realized that entire plants, drawing their water from the soil, might respond differently to a spray of PMA than did detached leaves with petioles dipping in water. The prevailing opinion concerning transpiration (13) is that, within the entire range of available soil moisture, the main resistance to transpiration is in the gaseous phase, from the evaporating surfaces of the mesophyll cells, through the stomata to the ambient air; therefore, at a given diffusion pressure gradient from the soil to the air, the most decisive factor in controlling transpiration is the stomatal opening. If this be so, stomatal closing by chemical treatment should affect transpiration in the same way and magnitude observed in detached leaves. Further, this effect should be nearly independent of soil moisture per se. This similarity of response might apply to photosynthesis as well. These similarities, however, would not occur if changes in the soil moisture brought about changes in the resistances of the soil and the plant that would be of a comparable magnitude to the diffusion resistance in the gaseous phase. Under these latter circumstances, stomatal opening might not be the only major factor controlling transpiration. The analysis of the interaction of stomatal opening and soil moisture in their effect on transpiration and photosynthesis is usually complicated by the fact that low soil moisture is generally associated with a closing of stomata; thus it is difficult to evaluate these factors independently. The control of stomatal opening by chemical treatment and the measurement of stomata provided a possibility of isolating the stomatal resistance at various levels of soil moisture.

Effect of Soil Moisture and Phenylmercuric Acetate upon Stomatal Aperture, Transpiration, and Photosynthesis