Aluminum accumulation in Amaranthus species and mechanisms of Al tolerance

The ability to accumulate and tolerate high levels of Al has been reported in relatively few plant families. Up to date there is no report on the Al tolerance or accumulation neither in members of Amaranthaceae nor in the order (Caryophyllales). In this work, we collected three Amaranthus species (A. blitoides, A. retroflexus and A. blitum ) growing as weed on acid soils of tea gardens (pH 4.4-4.7) in the north Iran and analyzed for Al. The Al accumulation in the leaves of collected plants and those grown hydroponically for four weeks with 400 mu M Al (125 mu M free Al3+ activity) exceeded the critical level of Al accumulation (1 mg g(-1) DW). A. blitoides was further studied for Al tolerance mechanisms. Shoot and root biomass and root length were stimulated by 50 mu M Al accompanied by improvement of C-4 photosynthesis and N uptake and assimilation. The growth of plants was slightly repressed under higher Al (400 mu M) exposure accompanied by activation of antioxidant defense system and lignin deposition. A high resistance to toxic Al concentration in A. blitoides was associated with elevated levels of phenolics accumulation and exudation suggesting the contribution of these compounds to both the avoidance and the internal detoxification mechanisms in this species. This is the first report on Al accumulation in Amaranthus species and the results are of interest from the botanical, physiological and ecological points of view.