WATER CHANNEL DOES NOT LIMIT EVAPORATION OF WATER FROM PLANT-CELLS

Evaporation of water from the cell surface of the internode of Chara c orallina was not affected by HgCl2 which is known to inhibit water cha nnels, This makes a sharp contrast to the fact that most of osmoticall y driven water transport is inhibited by HgCl2. Also in radish hypocot yls whose epidermis had been peeled off, evaporation of water was not inhibited by HgCl2, while osmotic water transport was significantly in hibited. The cell wall tube was prepared by squeezing out the content of the Chara internode, The rate of evaporation from the cell wall tub e filled with 150 mM KCI was almost equal to that from the living cell . The apparent hydraulic conductivity of the cell calculated from evap oration rate was found to be 1-2x10(-3) pm s(-1) Pa-1 which is about 1 /1000 times the hydraulic conductivity of the plasma membrane (L-p) an d 1/40 times the L-p under maximal inhibition with HgCl2.It is conclud ed that under the relative humidity of 53-70% the rate of evaporation of water from the cell surface is limited by the rate of evaporation f rom the cell wall which is so low that the loss of water can be supple mented without delay from the cell interior across the plasma membrane even when water channels are completely closed.