Long-term cadmium exposure accelerates oxidant injury: significance of bound/free water states during long-term metal stress.

Lepidium sativum (cress) and Lycopersicon esculentum (tomato) plants were grown in peatlite in controlled environments with or without long-term (4 weeks) cadmium stress (Cd) (100 micrograms/ml every fourth day) and with a single exposure (6 hr at 35 parts per hundred million (pphm)) or no exposure to the oxidant ozone (O3). Cress plants which received Cd wilted faster during O3 exposure and became a gray-green color by the end of a 6-hr O3 exposure. Those receiving O3 alone also wilted but were normal in color during wilting. Leaf water content (percentage) significantly declined in both O3 + Cd- and O3-treated plants. However, leaves after Cd + O3 exposure were severely dessicated and necrotic, whereas O3-treated plants recovered their water content completely but had some injury. Increased stomatal aperture in cress but not tomato before O3 exposure and significantly lower water content at 1 and 24 hr after the end of O3 exposure were associated with the higher Cd content of leaves before and subsequent to O3 exposure. These factors contributed to a greater injury and cell death observed in the leaves of combined cadmium-oxidant stress. Dielectric properties of Thlaspi arvense (field penny cress) leaves grown at continuous exposure to Cd and/or nickel (Ni) indicated that there were measurable differences between metal-containing vs control leaves with regard to bound/unbound water status. This indicated that there was more free water under metal stress, and that the bound water content significantly declined in the leaves of these plants.