EFFECTS OF WATER-STRESS ON CELLULAR ULTRASTRUCTURE AND ON CONCENTRATIONS OF ENDOGENOUS ABSCISIC-ACID AND INDOLE-3-ACETIC-ACID IN FATSIA-JAPONICA LEAVES

Ultrastructural alterations in mesophyll cells as well as variations in bulk leaf endogenous ABA and IAA concentrations were studied in water-stressed field-grown plants of Fatsia japonica. Under water deficit cellular membranes were modified and an increase in vesicles was observed. The main damage to the chloroplasts included thylakoid swelling and disruption of the chloroplast envelope. Concomitant variations in abscisic acid and indole-3-acetic acid were observed. Despite the expected increased in endogenous ABA concentration in relation to water stress, after the highest concentration of ABA, observed at predawn in severely stressed plants (psi(predawn) = -0.681 MPa), there was a sharp decline from 2768 pmol g fw(-1) to 145 pmol g fw(-1); thus in severely stressed plants ABA levels were not related to changes in bulk leaf ABA contents. Water stress did not influence the concentrations of indole-3-acetic acid, although the increase in the endogenous abscisic acid concentration could be related with the ultrastructural changes.