DIFFERENTIAL TOXICITY OF HEAVY-METALS IS PARTLY RELATED TO A LOSS OF PREFERENTIAL EXTRAPLASMIC COMPARTMENTATION - A COMPARISON OF CD-STRESS, MO-STRESS, NI-STRESS AND ZN-STRESS

Barley was grown at inhibitory concentrations of cadmium, molybdenum, nickel and zinc. Primary leaves were analyzed for cellular and subcellular compartmentation of the heavy metals. Epidermis and mesophyll protoplasts, mesophyll vacuoles and chloroplasts were isolated and apoplasmic washing fluid prepared, and the heavy metal contents of the various fractions determined. Efflux experiments showed that heavy metals were not lost from the preparations within the time span of the experiment. The different heavy metals were subjected to distinct distribution mechanisms within the leaves: (1) On a relative basis, the order of preferred epidermal accumulation was Cd = Zn > Mo > Ni (P < 0.01). (2) Within the mesophyll, Mo showed the highest degree of vacuolar compartmentation, whereas Ni was compartmentalized into the cytoplasm including chloroplasts to almost 80%. (3) The low degree of vacuolar compartmentation was correlated with the development of damage in the leaves, as visualized by chlorosis and decreased quantum yield efficiency. (4) Damage was inversely correlated with apoplasmic compartmentation. (5) Interestingly, sulfhydryl contents of stressed leaves were neither positively nor negatively correlated with toxicity of the heavy metals: maximum induction was seen in the presence of Cd, followed by Zn, and no changes under Ni and Mo-stress. Increases in leaf SH-contents were small as compared with induction in the roots.