Water relations in plants subjected to heavy metal stresses

Concentrations of heavy metals in soil seldom reach a level sufficient to cause osmotic disturbances in plants. It is likely that water entry to the roots is indirectly governed by other factors which are themselves affected by metals. Decreased elongation of the primary root, impaired secondary growth, increased root dieback, or reduced root hair caused by toxic ions all exert a deleterious effect on the root-absorbing area and water uptake. Moreover, metals are able to decelerate short-distance water transfer both in symplast and apoplast, which in turn reduce the movement of water into the vascular system and affect water supply to the shoot. Long-distance transport is limited also due to decreased hydraulic conductivity in the root, stem and leaf midrib caused by a reduction in the size of vessels and tracheids, and partial blockage of xylem elements by cellular debris or gums. Heavy metals influence water delivery to the shoot due to inhibition of transpiration as they decrease the size of the leaves and the thickness of the lamina, reduce intercellular spaces, affect the density of stomata and decrease their aperture. Stomata closure is induced by direct interaction of toxic metals with guard cells and/or as a consequence of the early effects of metal toxicity on roots and stems. In metal-stressed plants, root-derived ABA or ABA-induced signals might play a role in stomatal movement. Disturbances in water relations trigger differential regulation of aquaporin gene expression, which may contribute to further reductions in water loss.