Ab8tractChloride influx measured with 36Cl over a 10 min loading period followed by a 5 min wash in inactive solution is used as an estimate of influx across the plasmalemma in barley roots. Chloride influx measured over a 60 min loading period followed by a 30 min wash is used as an estimate of the quasi-steady influx from the external solution to the vacuole.The 10 min load, 5 min wash flux estimate is shown not to be in error due to inclusion of extracellular contents in the root cortex or in the stele. It therefore appears to be a good estimate of the influx across the plasmalemma of the cortical cells.At external chloride concentrations below about 5 mM the plasmalemma influx and the influx to the vacuole are approximately equal, and therefore the influx across the plasmalemma appears to be the rate-limiting step in the influx of chloride to the vacuole. At external chloride concentrations above about 10 mM the plasmalemma influx increases nearly linearly with external chloride concentration to very high values at 80 mM, whereas the influx to the vacuole does not increase above 20-30 mM. The "saturation" in influx to the vacuole at high external chloride concentrations therefore appears to be determined by the saturation of transport across the tonoplast or of a transport process directly through the cytoplasm to the vacuole, in confirmation of the conclusion of Torii and Laties (1966).Some observations suggest that part of the fluxes across the plasmalemma are mediated by exchange diffusion.In salt-loaded tissue the plasmalemma influx and the influx to the vacuole are both reduced. This apparent negative feedback relationship between chloride content and influx of chloride is briefly discussed.
Estimates of plasmalemma influx and steady-state vacuolar influx of potassium in low-salt barley roots have been obtained in the concentration range 10-80 mM by the use of controlled loading and washing times. Both fluxes are reduced by preloading the tissue in solutions containing potassium. When the experimental temperature is increased from 20 to 30°C, an apparent increase in the steady-state vacuolar influx occurs; separation of this flux from the apparent plasmalemma influx is not possible. The data support the hypothesis that the kinetics of potassium exchange in barley root tissue may be confounded by both loading time and temperature, and thus interpretation of the influx isotherm in the high concentration range is difficult. Where experiments are carried out at 20-25°C the steady- state vacuolar influx is measured unless closely defined experimental conditions obtain. At higher temperatures the plasmalemma influx may be measured.
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