INVOLVEMENT OF ABSCISIC-ACID AND ETHYLENE IN THE RESPONSES OF CITRUS SEEDLINGS TO SALT SHOCK

The responses of salt-sensitive citrus rootstocks to 200 mM NaCl were periodically determined on seedlings of citrange Carrizo (Citrus sinen sis [L.] Osbeck x Poncirus trifoliata [L.] Raf) during 30 days. The st ressed seedlings adjusted osmotically, reduced stomatal conductance, i ncreased proline content and ethylene production, and showed massive l eaf abscission (92%). The salt shock also increased abscisic acid (ABA ) and aminocyclopropane-1-carboxylic acid (ACC) in roots, xylem fluid and leaves, and in addition promoted Cl- accumulation. The pattern of change of ABA, ACC and proline followed a two-phase response: an initi al transient increase (10-12 days) overlapping with a gradual and cont inuous accumulation. This biphasic response appears to be compatible w ith the proposal that the transitory hormonal rises are induced by the osmotic component of salinity, whereas the Cl- increase determines th e subsequent accumulations. During the second phase, Cl- levels correl ated with abscission in leaves. Production of leaf ethylene was also c oncomitant with the increase in the abscission rate. Salt-induced absc ission was either reduced with CoCl2 (52%) or inhibited with silver th iosulphate (14%). The results suggest that in salt-stressed citrus, le af abscission is induced by the chloride build-up through a mechanism that stimulates leaf ACC synthesis and further conversion to ethylene.