AMYLASE ACTIVITY IN TAPROOTS OF MEDICAGO-SATIVA L AND LOTUS-CORNICULATUS L FOLLOWING DEFOLIATION

Although the patterns of starch metabolism in taproots of alfalfa (Medicago sativa L.) and birdsfoot trefoil (Lotus corniculatus L.) have been characterized, little is known regarding the activities of starch-degrading enzymes in taproots of these species. Our objective was to determine how defoliation influences starch degradation and activities of amylases in taproots of alfalfa and birdsfoot trefoil. In Exp. 1, amylolytic activities and starch concentrations in taproots of defoliated and undefoliated plants were compared on days 0, 3, 7, 10, and 14 after defoliation. Taproot starch concentrations declined in defoliated plants, while increasing in taproots of undefoliated plants. Exoamylase activities in taproots of defoliated plants did not change with defoliation, while endoamylase activities increased 2-fold in defoliated alfalfa and 50% in defoliated birdsfoot trefoil plants when compared to undefoliated plants. In Exp. 2, activity and isoform complement of amylases were monitored during seedling development. High endoamylase activity was found in taproots of both species at all samplings. In contrast, exoamylase accumulated in taproots of alfalfa, but not birdsfoot trefoil, in a pattern similar to starch accumulation. As in Exp. 1, defoliation increased endoamylase, but not exoamylase activity in taproots of both species. Taproots of both species contained one major and two minor endoamylase isoforms, but the electrophoretic mobility of these isoforms differed between species. Activities of all isoforms, as indicated on starch-gel blots, increased in response to defoliation. These results indicate that defoliation increases activity of taproot endoamylases, whose activity is associated with taproot starch degradation.