Quantification of stomatal uptake of ionic solutes using a new model system

Evidence for stomatal uptake of solutes by leaves without the application of surfactants or pressure has recently been provided (Eichert et al., 1998). In the present study, experimental conditions were refined in that the water potential was held at <0 on the physiologically inner side of the epidermal strips (ES) by use of a ceramic plate or the proper mesophyll of the plant. The penetrated substances were immobilized on the inner side by ion exchange membranes. The influence of humidity, light, stomatal density, and re-wetting on the uptake of anions (fluorescein) and cations (Fe(3+)) was investigated, using leaves of Allium porrum, Commelina communis and Sedum telephium. Uptake increased with humidity, stomatal aperture and stomatal density. It was restricted to stomatal areas, and was especially high below the rims of drying droplets. Again, penetration of stomatal pores was observed. Uptake was strongly correlated with the number of penetrated stomata, although usually less than 10% of the stomata contributed to uptake. The number of stomata that had been penetrated was highly variable, increasing extremely significantly with the number of repeated drying/ wetting cycles. These results indicate that stomatal uptake can be a major pathway for the foliar uptake of ionic solutes. It is a dynamic process, depending on environmental conditions and history of the residues on the leaf, aspects that had been neglected in previous concepts.