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

Abstract

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 M HgCl₂ decreased by 80–90% in 10 min at 25˚C. Reversal of this inhibition was obtained using a sulfhydryl [β-mercaptoethanol (β-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. β-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.