Evidence on the mechanism of enhanced sucrose uptake at low cell turgor in leaf discs of Phaseolus coccinius

Abstract

Plants often tolerate water deficits by lowering the osmotic potential of their cell sap. This may be achieved by accumulation of solutes which results in the maintenance of a positive turgor potential. In this study, the effect of water deficit on sugar uptake was investigated in leaf discs of Phaseolus coccinius L. (cv. Scarlet). Evidence is presented that cell turgor affects the kinetics of sugar transport at the membrane level. Uptake kinetics of sucrose, glucose and 3‐O‐methyl glucose by tissues equilibrated in solutions of relatively high (200–400 mOsm) osmotic concentration consisted of a sat‐urable and a linear component. Low external osmotic concentration i.e., high cellular turgor inhibited the saturating component of sucrose uptake, resulting in a linear uptake profile. However, high cell turgor had no effect on glucose or 3‐O‐methyl glucose uptake kinetics. The effect of turgor versus osmotic component of water potential was differentiated by comparing responses to non‐penetrating (manmtol) or polyethylene glycol, (3350) and penetrating (ethylene glycal) osmotica. Changes in sucrose uptake rates and kinetics were due to changes in cellular turgor and not osmotic potential. Furthermore, at low cellular turgor, a net increase in sucrose uptake occurred as a consequence of enhanced influx rates and not as a result of reduced efflux rates. The data are consistent with previous findings that sugar uptake rates are enhanced under low turgor. We present first evidence indicating that the mechanism by which higher rates of sucrose uptake are maintained underwater deficit conditions is by the activation of the saturable transport system. This mechanism supports previous suggestions that changes in cell turgor are sensed and manifested at the membrane level.