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

Abstract

Abstract Flows of abscisic acid (ABA) were investigated in wholeplants of castor bean [Ricinus communis) grown insand culture under either phosphate deficiency ormoderate salinity. Xylem transport of ABA inP-deficient plants was stimulated by a factor of 6whereas phloem transport was affected only veryslightly. ABA deposition into leaves of P-deficientplants was not appreciably different from the controlsbecause of strong net degradation in leaves. Sinceconjugation of ABA was strongly reduced in all organsof P-deficient plants ABA was presumably metabolizedmainly to phaseic acid and dihydrophaseic acid. Theincreased import of ABA occurred predominantly intofully differentiated but not senescent leaves andshowed a good correlation with the inhibition of leafconductance under P deficiency.As with low-P-plants salt stress increased ABA syn-thesis in roots and associated transport in the xylem.However, salinity caused a distinctly greater accumu-lation of ABA in the leaves, stem segments and theapex than in P-deficient plants. As opposed to P defi-ciency, ABA export in the phloem from the leaves wasstimulated by salinity. Modelling of ABA flows withinan individual leaf over its life cycle showed that younggrowing leaves imported ABA from both phloem andxylem, whereas the adult non-senescent leaves werea source of ABA and thus provided a potential shoot-to-root stress signal as well as an acceptor for recip-rocal signals from root to shoot. In senescing leavesABA flows and accumulation were somewhat retardedand ABA was lost in net terms by export from the leaf.Key words: Abscisic acid, phosphorus deficiency, saltstress, phloem and xylem transport.