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

Abstract

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 μg/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 (O 3). Cress plants which received Cd wilted faster during O 3 exposure and became a gray-green color by the end of a 6-hr O 3 exposure. Those receiving O 3 alone also wilted but were normal in color during wilting. Leaf water content (percentage) significantly declined in both O 3 + Cd- and O 3-treated plants. However, leaves after Cd + O 3 exposure were severely dessicated and necrotic, whereas O 3-treated plants recovered their water content completely but had some injury. Increased stomatal aperture in cress but not tomato before O 3 exposure and significantly lower water content at 1 and 24 hr after the end of O 3 exposure were associated with the higher Cd content of leaves before and subsequent to O 3 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 measureable 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.