The relation between tannin, phytochelatins, glutathione and peroxidation in two cultvars of Sorghum (Sorghum bicolor) exposed to aluminium

The objective of the present investigation was to study the role of tannin, glutathione and nonprotein thiols, and related enzymes in sorghum grown at aluminium (A1) stressed levels. Two sorghum cultivars 132 (high tannin) and 552 low tannin) were selected. Plants were cultivated in four concentrations (0. 10, 20 and 30 mg 1-¹) of AICI₃ in a mixture of river sandand peat. The exposure to A1 increased allocation to tannins, glutathione and nonprotein thiols (NP-SH), which are potential defensive compounds. In roots of both cultivars the production of NP-SH was coupled with a decrease in total glutathione. The concentration of glutathione in leaves of cultivar 132 tended to be lower, but in cultivar 552, it tended to be higher. Aluminium enhanced the level of lipid peroxidation in the leaves of cultivar 552 higher than in cultivar 132. Glutathione is a substrate for NP- SH synthase so that the depletion of glutathione could be ascribed to the A-induced NP-SH synthesis in the roots of both cultivars. Although the synthesis of NP-SH, glutathione and lipid peroxidation could be sensitive to changes in tannins, the amount of glutathione, lipid peroxidation and NP-SH are useful for Al resistance. Studies have shown that phenylalanine-ammonia lyase (PAL) activity is stimulated by Al which is the key enzyme in the general phenylpropanoide pathway. The capacity of the two cultivars in antioxidant and NP-SH n new and old leaves was different: in cultivar 552. the antioxidant compounds such as glutathione with a rapid turnover and high cumulative could be cost effective than tannin for the leaves as tannin was low and stable. In cultivar 132, the amount of tannin was high: therefore high cumulative of glutathione in leaves is not necessary. The surface of roots was darker and seemed to be from A1 stimulated efflux of organic acids. The threshold concentration level of Al for acute toxicity in cultivar 132 was higher than cultivar 552 suggesting that tannins are normally involved in Al sequestration under conditions of subtoxic exposure. The localization of NP-SH in the roots could provide an effective means of restricting A1 to this organ by chelations in cultivar 132 seedlings. A1 treatment increased total tannin content mediated by enhanced activities of PAL. In cultivar 552 glutathione contents may have been consumed for two strategies: the maintenance of regular redox potential, and the precursor for NP-SH. This was a predictable result because it is known that A1 acts as a strong sink for thiols, which increase the demand for sulfate absorption. The transformation of glutathione in cultivar 552 was much parallel to the rate of synthesis. However in cultivar 132 the rate of transformation was higher than the rate of synthesis. Thus, the imbalance between MDA and PAL generation, and the scavenging of compound tannin n leaves may reflect the different defense strategy in the two cultivars of sorghum studied.