Changes in Transpiration, Net Carbon Dioxide Assimilation and Leaf Water Potential Resulting from Application of Hydrostatic Pressure to Roots of Intact Pepper Plants

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AbstractHydrostatic pressures varying from 0 to 6.0 bar were applied to roots of intact Capsicum annuum L. cv. California Wonder plants growing in nutrient solution and the rates of transpiration, and net CO2 assimilation, apparent compensation point and leaf water potential measured.Increasing the pressure on the roots of plants with roots in solution with either ‐0.5 or ‐5.0 bar osmotic potential with 1 bar increments resulted in a decrease in transpiration. With the application of 1 or 2 bar pressure the rate of transpiration returned to near or above the original rate. An application of 3 or 4 bar pressure reduced the rate of transpiration of all plants. The transpiration of plants with roots in solution with ‐0.5 bar osmotic potential remained at the reduced rate for as long as these pressures were maintained. The transpiration of plants with roots in solution with ‐5.0 bar was only temporarily suppressed at these pressures. Changing the applied pressure from 3 or 4 bar to 0 resulted in a rapid increase in transpiration which lasted approximately 15 minutes. This was followed by a decrease in transpiration to a rate lower than before the pressure was applied. The pattern of response was similar for plants at low or high light intensity or at normal or low CO2 concentrations. When leaf diffusive resistance was 6.0 s cm−1 or greater, changes in net CO2 assimilation were similar to those of transpiration. The apparent CO2 compensation point increased as pressure was applied and decreased with a release in pressure. Leaf water potential increased with an increase in pressure and decreased with a decrease in pressure. The changes in leaf water potential were frequently but not always proportional to changes in pressure.It is postulated that the respouses noted were due to changes in resistance to flow of water from xylem terminals through the mesophyll cells and stomatal cavities to the atmosphere.