Effects of heavy metals on stomatal movements in broad bean leaves

How do heavy metals affect stomatal movements and whether water channels are involved in stomatal movements was investigated in broad bean (Vicia faba L.) leaves. Three-week old fully expanded leaves were harvested. Leaf epidermis was peeled off and soaked in the Mes-KOH buffer containing the salts of heavy metals. Stomatal aperture was measured under the microscope. The tested heavy metal ions, such as Hg2+, Pb2+, Zn2+, and La3+, partly inhibited stomatal opening in light or closing in darkness at submillimolar concentrations, while K+, Na+ and Mg2+ had no visible effects on stomatal movements. As compared to La3+, Hg2+ affected stomatal movements more significantly. Stomatal movements were almost completely inhibited under a combined Hg2+ and La3+ treatment. Apparently, La3+, a Ca2+ channel blocker, inhibits the changes in the cytosolic Ca2+ concentration in guard cells, thus affecting stomatal movements. The inhibitory effect of Hg2+ on stomatal movements may be explained by the inhibition of water channels. Like Hg2+, Zn interfered with stomatal movements. It is concluded that heavy metals at submillimolar concentrations inhibit stomatal movements. They may affect water fluxes through guard cell membranes in different ways, i.e., Hg2+, Pb2+ Zn2+ inhibit water channels, whereas La3+ block ion channels. Water channels may be involved in stomatal movements by regulating water fluxes and play a dominant and primary role in stomatal movements.