Transport, synthesis and catabolism of abscisic acid (ABA) in intact plants of castor bean (Ricinus communis L.) under phosphate deficiency and moderate salinity

Flows of abscisic acid (ABA) were investigated in whole plants of castor bean (Ricinus communis) grown in sand culture under either phosphate deficiency or moderate salinity, Xylem transport of ABA in P-deficient plants was stimulated by a factor of 6 whereas phloem transport was affected only very slightly. ABA deposition into leaves of P-deficient plants was not appreciably different from the controls because of strong net degradation in leaves. Since conjugation of ABA was strongly reduced in all organs of P-deficient plants ABA was presumably metabolized mainly to phaseic acid and dihydrophaseic acid, The increased import of ABA occurred predominantly into fully differentiated but not senescent leaves and showed a good correlation with the inhibition of leaf conductance under P deficiency. As with low-P-plants salt stress increased ABA synthesis in roots and associated transport in the xylem. However, salinity caused a distinctly greater accumulation of ABA in the leaves, stem segments and the apex than in P-deficient plants. As opposed to P deficiency, ABA export in the phloem from the leaves was stimulated by salinity. Modelling of ABA flows within an individual leaf over its life cycle showed that young growing leaves imported ABA from both phloem and xylem, whereas the adult non-senescent leaves were a source of ABA and thus provided a potential shoot-to-root stress signal as well as an acceptor for reciprocal signals from root to shoot. In senescing leaves ABA flows and accumulation were somewhat retarded and ABA was lost in net terms by export from the leaf.