Effect of apoplastic solutes on water potential in elongating sugarcane leaves

Solute concentration in the apoplast of growing sugarcane (Saccharum spp. hybrid) leaves was measured using one direct and several indirect methods. The osmotic potential of apoplast solution collected directly by centrifugation of noninfiltrated tissue segments ranged from -0.25 megapascal in mature tissue to -0.35 megapascal in tissue just outside the elongation zone. The presence of these solutes in the apoplast manifested itself as a tissue water potential equal to the apoplast osmotic potential. Since the tissue was not elongating, the measurements were not influenced by growth-induced water uptake and no significant tension was detected with the pressure chamber. Further evidence for a significant apoplast solute concentration was obtained from pressure exudation experiments and comparison of methods for estimating tissue apoplast water fraction. For elongating leaf tissue the centrifugation method could not be used to obtain direct measurements of apoplast solute concentration. However, several other observations suggested that the apoplast water potential of -0.35 to -0.45 megapascal in elongating tissue had a significant osmotic component and small, but significant tension component. Results of experiments in which exudate was collected from pressurized tissue segments of different ages suggested that a tissue age-dependent dynamic equilibrium existed between intra- and extracellular solutes.