Transient variations of water transfer induced by HgCl2 in excised roots of young maize plants:: new data on the inhibition process

Effects of low concentrations of HgCl2 on water transport in excised maize (Zea mays L.) roots have been monitored using a reliable system that permits continuous measurement of xylem flux variations. The sap flow of exuding roots treated with 11 muM HgCl2 decreased by 80-90% in 10 min at 25 degreesC. Reversal of this inhibition was obtained using a sulfhydryl [beta -mercaptoethanol (beta -ME)], or non-sulfhydryl [ethylenediaminetetraacetate disodium salt (EDTA) and ferricyanide (FeCy)] reagents. The time course of reversal was not the same in the three cases. beta -ME reversed quickly but not sustainably, whereas EDTA or FeCy reversed slowly and sustainably. Using a cell pressure probe, turgor was measured in the epidermis, and the first layer of cells in the cortex. Turgor was considerably decreased in root epidermal cells after HgCl2 treatment, suggesting that a normal root water-transfer required an optimal turgor in these cells. Recovery of cell turgor, was obtained satisfactorily with FeCy. In parallel with flux measurements, histochemical analyses revealed a localization of Hg only in peripheral root cells, suggesting that Hg targets are localized in the first root cell layers. Involvement of water channels and/or ion transport in the regulation of root water transport is discussed in the light of our data.