The short-term effects of O2 deficiency and sodium azide (NaN3) on the hydraulic conductivity of cortical cells in wheat roots was studied using the pressure probe. Hydraulic conductivities were obtained by measuring either turgor relaxation, or volume relaxation under pressure clamp. Both low O2 concentration and NaN3 increased the half times of pressure and volume relaxations. The increases in T½ were found to be due to a decrease in the hydraulic conductivity (Lp) of the cells. The mean values of Lp from pressure relaxation experiments were 7.75 × 10-1m s-1 Mpa-1 in the fully aerated solution and 1.15 and 2.17 x 10-1m s-1 Mpa-1 in low O2 concentration and 1 mol m-3 NaN3 solutions respectively. The pressure clamp experiments yielded similar results to pressure relaxation experiments for both low O2 concentration and 1 mol m-3 NaN3 treatments. In addition to determination of Lp, pressure and volume relaxation experiments were also used to evaluate osmotic volume of the cells. In aerated solutions the osmotic volume of the cells was about twice that of their geometric volume, but in low O2 concentration and NaN3 solutions the osmotic cell volume was reduced and approximately equal to the geometric volume. The decrease in osmotic volume and part of the reduction in Lp may be explained by the occlusion of plasmodesmata induced by low O2 concentration and NaN3.

We have tested the effects of the reported Cl- channel blockers NPPB (5-nitro-2-(3- phenylpropylamino)benzoic acid) and niflumic acid (trifluoromethyl-3-phenylamino-2-nicotinic acid) on the slow-activating K+ (IKK.out) and fast-activating Cl- (IClCl.out) outward rectifying currents across the plasma membrane of wheat root protoplasts. Both compounds display a greater potency against IKK.out(half maximal inhibition (IC50) values of 8.7 μM for NPPB and 5.3 μM for niflumic acid with [K+]o= 10 mM) than against IClCl.out(IC50 values of 150 and 100 μM respectively). Single-channel studies using detached patches show that these blockers reduce the percentage of time that IKK.out out channels are in their open state. The data show that caution is necessary when using these compounds on intact cells to ascribe physiological functions to anion channels in plant membranes.

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