RE-EXPORT AND METABOLISM OF XYLEM-DELIVERED ABA IN ATTACHED MAIZE LEAVES UNDER DIFFERENT TRANSPIRATIONAL FLUXES AND XYLEM ABA CONCENTRATIONS

By feeding radioactive H-3-ABA into attached maize leaves, the re-expo rt and metabolism of xylem-delivered ABA and their relationships with xylem ABA transpirational fluxes and concentrations were investigated, ABA entering leaves in the transpirational stream was re-exported out of leaves slowly, Within 24 h the proportion of fed radioactivity tha t was re-exported was less than 45%. When different concentrations of H-3-ABA (100 nM versus 500 nM) was fed, no difference between the two concentrations was found in their rates of re-export of the fed radioa ctivity during the first 5 h, After 5 h, very little fed radioactivity was re-exported in leaves that were fed with 100 nM H-3-ABA, while le aves that were fed with 500 nM H-3-ABA continued to re-export such tha t the final proportion remaining in leaves after 24 h was less as a re sult, suggesting a concentration-stimulated re-export, When H-3-ABA wa s fed at two different transpiration rates which were induced by diffe rent air humidity, a 4-fold difference in transpirational fluxes did n ot produce any difference in terms of re-exportation of fed radioactiv ity, The rate of catabolism of xylem-fed H-3-ABA in the attached leave s was much faster than that of re-export, On average fed H-3-ABA had a half-life of 2.2 h and only 8% remained unmodified after 24 h of incu bation, suggesting that re-exported radioactivity might not be the int act form of ABA at all. Using the parameters obtained from the feeding experiment, we calculated that in a real soil-drying situation the po ssible maximum amount of xylem-delivered ABA that could accumulate in leaves during a day, It was found that the proportion of daily accumul ated ABA was only 5% of the leaf ABA in well-watered plants, In soil-d ried plants the maximum amount of daily accumulation by xylem ABA coul d reach 20% of the leaf ABA at the beginning of soil drying, but it so on declined to about 5% again, The declined contribution was mainly du e to a reduced transpiration and an increased total leaf ABA as a resu lt of aggravated leaf water deficit, A tight relationship between leaf conductance and the accumulation of xylem-delivered ABA was not found .