Use of the pressure vessel to measure concentrations of solutes in apoplastic and membrane-filtered symplastic sap in sunflower leaves.

A simple, repeatable, and accurate method is described for the collection of apoplastic and membrane-filtered symplastic sap fractions, and for the determination of the origin of these fractions within the leaf. The apoplastic distribution patterns of the naturally occurring apoplastic leaf solutes, and the apoplastic dye PTS (trisodium 3-hydroxy-5, 8, 10-pyrenetrisulfonate) were compared. Aliquots of sap were expressed from detached sunflower leaves in a pressure chamber over intervals of 0.02 to 0.04 megapascal. Three distinct fractions were detected in the expressed sap volume. These were successively released and identified as a petiole-midrib fraction, a minor vein-cell wall fraction, and a mixed fraction consisting of a contribution from the minor vein-cell wall with an increasing proportion of membrane-filtered cell sap.