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

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Abstract
The effect of phenylmercuric acetate (PMA) spray (90 [mu][image]) on stomatal opening, transpiration and photosynthesis in young corn plants was studied at 4 soil moisture levels. The plants were grown in containers and were placed in a controlled-environment chamber during the experiment. Stomatal aperture was measured by means of silicone rubber impressions, transpiration by weighing and photosynthesis by means of labeled CO2 introduced into the chamber. Stomatal closure induced by PMA, caused a marked reduction of transpiration, especially at high soil moisture contents, whereas the reduction of photosynthesis was less marked. In addition, decreased soil moisture, even within available range, caused a further reduction in both transpiration and photosynthesis, which could not be attributed to stomatal closure. From the analysis of the resistance to water movement it seems that as the soil dries, a considerable resistance to water movement develops at the evaporating surfaces of the mesophyll cells. The water tension at these surfaces may reach 80 bars without the plants wilting. Similarly, the mesophyll resistance to CO2 movement apparently increases considerably as soil moisture decreases.