The Relations Between Phenylalanine -Ammonia Lyase, Glutathione-s-Transferase Activities and the Concentrations of Total Tannins, Phytochelatins, Glutathione, and Peroxidation in two Cultivars of Sorghum (Sorghum bicolor (L.) Moench) Exposed to Aluminum

This article reports on the realtions between the activities of enzymes, such as phenylalanine -ammonia lyase (PAL) and glutathione-s-transferase (GST), and the concentrations of total tannins, total glutathione (TGSH), total phytochelatins (TPC), and lipid peroxidation (MDA) in leaves and roots of sorghum cultivars 132 and 552 that are exposed to four concentrations (0, 10, 20, and 30 mg l-1) of AlCl3 in a mixture of river sand and peat. The Al3? significantly increased the level of tannins, TGSH, and TPC in the roots and leaves of both cultivars. In the roots of both cultivars, the production of TPC was significantly coupled to decrease in the total TGSH. The concentration of TGSH in the leaves of cultivar 132 was found to be higher than in those of cultivar 552. In the plants treated with 10, 20, and 30 mg l-1 Al3?, the PAL activities in leaves of both cultivars increased (by 73, 44, and 18 %, respectively), the TGSH in the roots of cultivar 552 declined (by 50, 45, and 23 %, respectively), the GST activities in the leaves of both cultivars were higher (90, 98, and 100 %, respectively) than those of the control plants. Al3? also enhanced levels of MDA in the leaves and roots. These results suggested that the increase in PAL and GST activities might be controlled by antioxidant potentials and different routes of carbon channeling in the leaves. In cultivar 552, antioxidant compounds such as TPC and TGSH with rapid turnover and high accumulation were more effective than tannins for leaves because tannin was low. In cultivar 132, the amount of tannins was high and stable; therefore they do not need high accumulation of TPC in leaves. The depletion of TGSH can be ascribed to the Al3?-induced TPC synthesis in the leaves and roots of cultivar 552. The syntheses of TPC and MDA can be related to changes in TGSH and tannins, suggesting that TGSH and tannins are normally involved in Al3? sequestration under conditions of subtoxic exposure. The increased TPC in the roots could provide an effective means of restricting Al3? to these organs by chelating. In cultivar 552, TGSH contents may have been consumed for two strategies: the maintenance of regular redox potential, and the precursor for TPC.