«Peristomatal Transpiration» and Stomatal Movement: A Controversial View IV. Ion Accumulation by Peristomatal Transpiration

Abstract

Summary Ions of silver, lead and mercury and potassium were fed into the transpiration stream and their accumulation in the epidermis has been demonstrated. The results were consistent with earlier experiments using thallium ( Maercker, 1965 b ). When special care was taken to provide open stomata, the deposits were found in the lumina of the guard cells. In all other instances the ions accumulated mainly in the subsidiary cells or in both the guard and the subsidiary cells. Deposits were also found to occur on the surface of these cells, in particular along the cuticular ledges of the pore. The characteristic distributional pattern reflects the pattern of peristomatal transpiration ( Maercker, 1965 , Maercker, 1965 ; Maier-Maercker, 1979a ). Transpiration rate of the guard cell, which during stomatal opening exceeds that of the subsidiary cell, explains the high accumulation rate in the guard cell. When during stomatal closure the subsidiary cell, due to its enhanced transpiration, maintains the lateral water flow, the ions are trapped before they reach the guard cell. Accumulation is seen as consequence of the water flux into the cell; the rates of accumulation are determined by the concentration gradient between a particular cell and its surrounding wall. Since the interpretation of the results apply for toxic heavy metal ions and potassium ions, it is concluded that all ions accumulate by the action of peristomatal transpiration. Shortcomings of the K+ shuttle hypothesis are discussed in the light of the findings.