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Membranes and Bioenergetics

Quantification of Apoplastic Potassium Content by Elution Analysis of Leaf Lamina Tissue from Pea (Pisum sativum L. cv Argenteum) ¹

Horticulture Department, Michigan State University, East Lansing, Michigan 48824

K⁺ content and concentration within the apoplast of mesophyll tissue of pea (Pisum sativum L., cv Argenteum) leaflets were determined using an elution procedure. Following removal of the epidermis, a 1 centimeter (inside diameter) glass cylinder was attached to the exposed mesophyll tissue and filled with 5 millimolar CaCl₂ solution (1°C). From time-course curves of cumulative K⁺ diffusion from the tissue, the amount of K⁺ of extracellular origin was estimated. Apoplastic K⁺ contents for leaves from plants cultured in nutrient solution containing 2 or 10 millimolar K⁺ were found to range from 1 to 4.5 micromoles per gram fresh weight, comprising less than 3% of the total K⁺ content within the lamina tissue. Assuming an apoplastic solution volume of 0.04 to 0.1 milliliters per gram fresh weight and a Donnan cation exchange capacity of 2.63 micromoles per gram fresh weight (experimentally determined), the K⁺ concentration within apoplastic solution was estimated at 2.4 to 11.8 millimolar. Net movement of Rb⁺ label from the extracellular compartment within mesophyll tissue into the symplast was demonstrated by pulse-chase experiments. It was concluded that the mesophyll apoplast in pea has a relatively low capacitance as an ion reservoir. Apoplastic K⁺ content was found to be highly sensitive to changes in xylem solution concentration.