SYNTHESIS, METABOLISM AND COMPARTMENTATION OF ABSCISIC-ACID IN RICCIA-FLUITANS L

Accumulation of abscisic acid (ABA) in thalli of Riccia fluitans is re duced by a factor of two after 4 days of treatment with 5 mu mol/L flu ridone. This inhibition, however, cannot be taken as a convincing argu ment against the direct biosynthetic pathway of ABA. When radioactive abscisic acid is fed to the thalli, phaseic acid (PA) and dihydrophase ic (DPA) acid are formed, similarly as in higher plants. This degradat ion step is inhibited by tetcyclacis, an inhibitor of P-450 cytochrome monooxygenases. Additionally, an unidentified less polar ABA metaboli te appears, indicating a degradation step that may be specific for Ric cia. An efflux compartmental analysis with radioactive ABA has been pe rformed with submerged and terrestrial thalli of Riccia fluitans. In b oth types of thalli ABA concentrations are markedly higher in the cyto plasm than in the vacuole. ABA fluxes across the plasmalemma are highe r in the submerged form whereas in the terrestrial form fluxes across the tonoplast are higher. Cytoplasmic ABA of the submerged form can be decreased, both by degradation and by release to the surrounding medi um. After conversion to the terrestrial form, however, ABA translocati on into the vacuole seems to become more important.