Aluminium phytotoxicity is becoming an increasingly widespread problem around the world, particularly in the tropics, causing decreased plant growth and crop productivity (Foy 1984; Delhaize and Ryan, 1995).
This study looked at the effect of aluminium on hydroponically grown Brachiaria decumbens (signal grass) and Brachiaria ruziziensis (ruzi grass), two very aluminium tolerant sub tropical grass species widely grown on sub tropical acids as cattle pasture and fodder. The study also looked at Triticum aestivum cv atlas, a tolerant wheat cultivar, for comparison.
Measurements of root growth inhibition and callose induction in response to increasing aluminium treatment were used to determine relative aluminium tolerance of the three species. Brachiaria decumbens root growth was found to be unaffected by treatment with 200 μM aluminium, whereas in Brachiaria ruziziensis 200 μM aluminium treatment caused 40% root growth inhibition and in Triticum aestivum it caused 100% inhibition.
Examination of the effect of aluminium treatment on nutrient uptake in Brachiaria decumbens and Brachiaria ruziziensis found that the superior adaptation of Brachiaria decumbens to nutrient poor acid soil conditions compared to the closely related Brachiaria ruziziensis was probably due in part to a better nutrient uptake system, particularly iron, and lower optimum nutrient requirements. It is also probably due in part to better 'maintenance of nutrient uptake and root and shoot nutrient concentrations when exposed to aluminium. Induction of increased organic acid accumulation and exudation from roots is widely accepted to be a major aluminium tolerance mechanism (Kochian, 1995). Organic acid accumulation was not triggered by phosphate starvation, although organic acid tissue accumulation was increased in response to general nutrient deficiency in root tips of Brachiaria decumbens and Brachiaria ruziziensis. Brachiaria decumbens and Brachiaria ruziziensis were found to employ two separate organic acid aluminium tolerance mechanisms. The first primarily involved a large induction of citrate in root tissue for internal detoxification and sequestering of aluminium. The second mechanism involved exudation of malate and oxalate for chelating and excluding aluminium from root tip cells.
The greater aluminium tolerance of Brachiaria decumbens and Brachiaria ruziziensis than Tricium aestivum cv Atlas was found to probably be due to higher organic tissue concentrations, particularly citrate, and exudation of relatively high concentrations of oxalate and malate. Organic acid accumulation in root tip tissues did not account for the difference in aluminium tolerance between Brachiaria ruziziensis and Brachiaria decumbens.
Recent studies have implicated the oxidative stress system in aluminium tolerance (Cakmak and Horst, 1991; Yamamoto et al. 1997; Ono et al.1995; Richards et al, 1998). However aluminium was found to cause no increase in either superoxide dismutase, peroxidase or catalase in either Brachiaria species. Aluminium also had no significant effect on ascorbate and dehydroascorbate concentration, indicating that the oxidative stress system was not involved in aluminium tolerance in either Brachiaria decumbens or Brachiaria ruziziensis.