The diffusional water permeability of Elodea leaf cells as measured by nuclear magnetic resonance

A new nuclear magnetic resonance (NMR) technique developed by Conlon and Outhred (1972. Biochim. Biophys. Acta, 288: 354–361) to measure diffusional water permeability was applied to the multicellular plant system Elodea Nuttallii (Planch) St. John leaves. This technique involves measuring a transverse relaxation time (T2) in the absence (T2 = Ta) and in the presence (T2 = Ta′) of extracellular paramagnetic cations. A valid estimate of Ta was measured for Elodea leaves. The value of Ta′ was found to decrease continuously with time. Evidence is presented that the decrease of Ta′ with time is initially related primarily to the time required for the paramagnetic ion to diffuse throughout the extracellular space and then later related to influx of the paramagnetic ion into the cells. By extrapolating to zero time to correct for paramagnetic-cation influx into the cells it was possible to estimate the value of Ta′ required to calculate the water exchange time out of the cells. It was estimated from the NMR data that Mn2+ (the paramagnetic ion used) flux into the cells occurred at a rate of 3.0 × 10−14 mol cm2 s−1. A procedure to determine whether the water-exchange time is controlled by intracellular unstirred layers or by membrane water permeability or by both is given. The water-exchange time of Elodea leaves is predominantly controlled by the intracellular unstirred layers. Thus it was only possible to set a lower limit on the diffusional water permeability coefficient (Pd) of Elodea leaf membranes of 3 × 10−2 cm s−1 at 20 °C.