HEAVY METAL-HORMONE INTERACTIONS IN RICE PLANTS - EFFECTS ON GROWTH, NET PHOTOSYNTHESIS, AND CARBOHYDRATE DISTRIBUTION

The effect of external applications of gibberellins (GA(3)) and abscis ic acid (ABA) on the growth, carbohydrate content, and net photosynthe sis of heavy metal-stressed rice plants (Oryza sativa cv. Bahia) was i nvestigated. Treatment with cadmium (0.1 mM) and nickel (0.5 mM) inhib ited rice growth and stimulated carbohydrate accumulation, especially in seeds from which seedlings were developing, stems, and first leaves . The addition of GA(3) (14 mu M) to the rice culture solution togethe r with Cd or Ni partially reversed the effects of heavy metals, stimul ating growth as well as mobilization of carbohydrate reserves in seeds from which seedlings had developed. GA(3) increased the sugar content in roots and second and third leaves and also modified the carbohydra te distribution pattern compared with heavy metal-treated plants. In c ontrast to GA(3), ABA (19 mu M) supplied to rice cultures potentiated the effect of heavy metals, inhibiting the growth of young leaves and the translocation of storage products from source to sink organs. In a ddition, sugars were accumulated in roots and second leaf but not in t he third leaf, the extension in length of which was also inhibited by the treatment. Net photosynthesis rates recovered transitorily in Cd-t reated plants after the addition of hormones. The possible relationshi p between growth and carbohydrate distribution, as well as the involve ment of hormones, in the response of plant to heavy metal stress is di scussed.