THE BINDING OF ZINC, BUT NOT CADMIUM, BY PHYTIC ACID IN ROOTS OF CROPPLANTS

Plant species adapted to soils enriched with heavy metals often accumu late these metals in their above or below ground organs. In this study , electron probe microanalysis of fractured, quench-frozen root specim ens of common crop species shows that an appreciable quantity of Zn ca n be bound as Zn phytate (myo-inositol kis-hexaphosphate) within small vacuoles of cells in the root elongation zone of lucerne, soybean, lu pins, tomato, rapeseed, cabbage, radish, maize and wheat exposed to hi gh levels of Zn (80-300 mu M). Globular deposits of Zn phytate are mos t frequently observed in the endodermis of dicotyledonous species and in the pericycle of monocotyledonous species, but may also occur in th e stele and inner cortex after prolonged exposure to toxic levels of Z n. The deposits could not be found in Zn-treated sunflower, field peas and Italian ryegrass. In three crop species, lucerne, soybean and mai ze, Zn-induced phytate globules were frequent, but exposure of roots t o 30 mu M Cd did not induce the formation of Cd-containing globular de posits as observed in Lemna minor (Van Steveninck et al., 1990a, 1992) . Simultaneous Zn and Cd treatment induced the formation of Zn phytate globules as effectively as Zn alone, and Cd was not detected in the d eposits.