Abscisic Acid Translocation and Metabolism in Soybeans following Depodding and Petiole Girdling Treatments

Abstract

It was found earlier that depodding and girdling treatments which obstruct translocation, result in increased leaf AbA levels and partial stomatal closure. In the present work (+/-) [2-(14)C]abscisic acid (AbA) was introduced into leaves and the mass, and radioactivity of AbA and AbA-metabolites were analyzed following translocation obstruction to determine whether the increased AbA was due to higher rates of synthesis, or lower rates of catabolism or export. The (+/-) [2-(14)C]AbA was introduced into soybean (Merr.) leaves by injection into the petiole region. AbA and AbA-metabolites (phaseic acid [PA], dihydrophaseic acid [DPA], AbA-conjugate, and an unknown metabolite) were separated with preparative high performance liquid chromatography. Methyl esters of AbA (free and that released after hydrolysis of AbA-conjugate), PA and DPA were determined with gas chromatography using electron capture detection.The level of AbA in leaf blades increased after girdling or depodding as was found previously. Accompanying this was an increase in PA in girdled leaves; but no discernible trends in the levels of DPA and AbA-conjugate were evident. The (+/-) [2-(14)C]AbA specific radioactivities declined similarly for all treatments, indicating that these treatments did not increase the rate of AbA synthesis.Export of injected (+/-)[2-(14)C]AbA from leaves was substantial. After girdling or depodding, this export was obstructed, as evidenced by the lack of decline in leaf blade radioactivity or lack of increase in pod radioactivity following these treatments. The higher AbA levels, which were observed in leaf blades of girdled and depodded plants, could be attributed largely to the translocation obstruction.