The effects of aluminium on chemical defense and physical traits in two cultivars of sorghum (Sorghum bicolor)

The effect of aluminium (Al) on some defensive compounds and physical traits in two cultivars (cultivar 132, 552) of sorghum was investigated. Plants were cultivated in two A1 concentrations (0 and 30 mg L-¹) before flowering in mixture of river sand and peat. Physical (toughness, relative growth rate, leaf area) and chemical defense (total tannin, protein-bound condensed tannin (PBCT), fiber-bound condensed tannin (FBCT), total polyphenols) traits in eight harvests were considered. The traits of tannin in leaves and roots were more suitable than polyphenols with growth differentiation balance (GDB) model. The results revealed that A1 can trade off between decrease in leaf dry weight and production of tannins, which might be due to an increase in protein-bound condensed tannin (PBCT). Leaf area in cultivar 132 was higher than cultivar 552, but leaf dry weight in cultivar 132 was lower than 552. As cultivars developed, stems mass fraction in cultivar 132 and leaves mass fraction to the total plant biomass in cultivar 552 increased. Leaf toughness during various developmental stages increased in cultivar 552 but not in cultivar 132. Leaf expansion in cultivar 132 was greater than in cultivar 552. A1 steadily increased PBCT, but decreased FBCT, which was apparently involved in A1 detoxification.In both the cultivars, allocation to defense varied in both amount and type. The fitness of cultivars 132 and 552 was due to chemical resistance. Fiber-bound condensed tannins were detected in young leaves; and in cultivar 552, the levels were higher in stages 1-2 than in later stages. In cultivar 132, the FBCT decreased after five harvests. Although mechanical properties were relatively poor n cultivar 132, A1 content was high in the leaves though not significant. The transformation of tannin in cultivar 132 was much less than the rate of synthesis; however, in cultivar 552 the rate of transformation was similar to the rate of synthesis. In two cultivars 132 and 552, when the leaf growth rate was high, A1 increased PBCT and decreased FBCT, in which the rate of changes was higher in 552 than 132. When leaf growth rate was low, the amount of production of PBCT and FBCT increased in both cultivars. With increasing duration of treatment, A1 toxicity increased with subsequent decrease in the amount of FBCT in shoot and root, suggesting that section of tannins have transformed, which could be bound to A1.