Abscisic Acid Catabolism in Maize Kernels in Response to Water Deficit at Early Endosperm Development

To further our understanding of the greater susceptibility of apical kernels in maize inflorescences to water stress, abscisic acid (ABA) catabolism activity was evaluated in developing kernels with chirally separated (+)-[(3)H]ABA. The predominant pathway of ABA catabolism was via 8'-hydroxylase to form phaseic acid, while conjugation to glucose was minor. In response to water deficit imposed on whole plants during kernel development, ABA accumulated to higher concentrations in apical than basal kernels, while both returned to control levels after rewatering. ABA catabolism activity per gram fresh weight increased about three-fold in response to water stress, but was about the same in apical and basal kernels on a fresh weight basis. ABA catabolism activity was three to four-fold higher in placenta than endosperm, and activity was higher in apical than basal kernels. In vitro incubation tests indicated that glucose did not affect ABA catabolism. We conclude that placenta tissue plays an important role in ABA catabolism, and together with ABA influx and compartmentation, determine the rate of ABA transport into endosperms.