Involvement of abscisic acid and ethylene in the responses of citrus seedlings to salt shock

The responses of salt‐sensitive citrus rootstocks to 200 m M NaCl were periodically determined on seedlings of citrange Carrizo ( Citrus sinensis [L.] Osbeck × Poncirus trifoliata [L.] Raf) during 30 days. The stressed seedlings adjusted osmotically, reduced stomatal conductance, increased proline content and ethylene production, and showed massive leaf 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 initial transient increase (10‐12 days) overlapping with a gradual and continuous accumulation. This biphasic response appears to be compatible with the proposal that the transitory hormonal rises are induced by the osmotic component of salinity, whereas the Cl⁻ increase determines the subsequent accumulations. During the second phase, Cl⁻ levels correlated with abscission in leaves. Production of leaf ethylene was also concomitant with the increase in the abscission rate. Salt‐induced abscission was either reduced with CoCl₂ (52%) or inhibited with silver thiosulphate (14%). The results suggest that in salt‐stressed citrus, leaf abscission is induced by the chloride build‐up through a mechanism that stimulates leaf ACC synthesis and further conversion to ethylene.