Re-export and metabolism of xylem-delivered ABA in attached maize leaves under different transpirational xylem ABA concentrations

By feeding radioactive ³H-ABA into attached maize leaves, the re-export 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 that was re-exported was less than 45%. When different concentrations of ³H-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 radio-activity during the first 5 h. After 5 h, very little fed radioactivity was re-exported in leaves that were fed with 100 nM ³H-ABA, while leaves that were fed with 500 nM ³H-ABA continued to re-export such that the final proportion remaining in leaves after 24 h was less as a result, suggesting a concentration-stimulated re-export. When ³H-ABA was fed at two different transpiration rates which were induced by different air humidity, a 4-fold difference in transpirational fluxes did not produce any difference in terms of re-exportation of fed radioactivity. The rate of catabolism of xylem-fed ³H-ABA in the attached leaves was much faster than that of re-export. On average fed ³H-ABA had a half-life of 2.2 h and only 8% remained unmodified after 24 h of incubation, suggesting that re-exported radioactivity might not be the intact form of ABA at all. Using the parameters obtained from the feeding experiment, we calculated that in a real soil-drying situation the possible maximum amount of xylem-delivered ABA that could accumulate in leaves during a day. It was found that the proportion of daily accumulated ABA was only 5% of the leaf ABA in well-watered plants. In soil-dried plants the maximum amount of daily accumulation by xylem ABA could reach 20% of the leaf ABA at the beginning of soil drying, but it soon declined to about 5% again. The declined contribution was mainly due to a reduced transpiration and an increased total leaf ABA as a result of aggravated leaf water deficit. A tight relationship between leaf conductance and the accumulation of xylem-delivered ABA was not found.