Solutes in the Free Space of Growing Stem Tissues

Abstract

The concentration of osmotically active solutes in the cell wall free space of young stem tissues was studied using a variety of extraction methods. When the intercellular air spaces of etiolated pea (Pisum sativum L.) internodes were perfused with distilled H2O, the resulting solution contained a solute concentration of about 70 milliosmoles/kg. A 2nd procedure involving vacuum infiltration of segments followed by centrifugation to collect the free space solution gave similar results. Apical stem segments yielded free space extracts about twice as concentrated as those from basal portions of the stem. After correcting for dilution of the free space solution by the infiltrated water, the osmotic pressure of the undiluted free space in pea stem tissue was 2.9 bars for apical segments, 1.8 bars for basal regions. These values may be somewhat overestimated due to solute efflux from intracellular pools during the extraction procedure. Similar results were obtained for stem regions of etiolated soybean (Glycine max [L.] Merr.) and cucumber (Cucumis sativus L.) seedlings. Measurements of the electrical conductivity and refractive index of free space extracts before and after ashing indicate that 25% of the solutes are inorganic electrolytes and 75% are organic nonelectrolytes with an average size similar to that of glucose. A significant osmotic pressure in the wall space offers an explanation for the frequent observation that nontranspiring plants have negative water potentials. Calculations of hydraulic resistance from water potential data must take into account solutes in the free space, else apparent, but unreal, changes in resistance may be calculated.