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Characterization of water channels in wheat root membrane vesicles
- Niemietz, C.M., Tyerman, S.D.
- Plant physiology 1997 v.115 no.2 pp. 561-567
- Triticum aestivum, roots, cell membranes, water uptake, measurement, osmotic pressure, membrane permeability, diffusion, mercury compounds, energy balance
- The functional significance of water channels in wheat (Triticum aestivum L.) root membranes was assessed using light scattering to measure vesicle shrinking in response to osmotic gradients rapidly imposed in a stopped flow apparatus. Vesicles were obtained from both a plasma membrane fraction and a plasma membrane-depleted endomembrane fraction including tonoplast vesicles. Osmotic water permeability (P(os)) in the endomembrane fraction was high (P(os) = 86.0 micrometers s-1) with a low activation energy (E(A) = 23.32 kJ mol-1 +/- 3.88 SE), and was inhibited by mercurials (K1 = 40 micromolar HgCl2, where K1 is the inhibition constant for half-maximal inhibition), suggesting participation of water channels. A high ratio of osmotic to diffusional permeability (P(d)) (using D2O as a tracer, P(os)/P(d) = 7 +/- 0.5 SE) also supported this view. For the endomembrane fraction there was a marked decrease in P(os) with increasing osmotic gradient that was not observed in the plasma membrane fraction. Osmotic water permeability in the plasma membrane fraction was lower (P(os) = 12.5 micrometer s-1) with a high activation energy (E(A)= 48.07 kJ mol-1 +/- 3.63 SE) and no mercury inhibition. Nevertheless, P(os)/P(d) was found to be substantially higher than one (P(os) = 3 +/- 0.2 SE), indicating that water channels mediated water flow in this fraction, too. Possible distortion of the P(os)/P(d) value by unstirred layer effects was shown to be unlikely.