Selenium Toxicity to Plants

Selenium toxicity is influenced by plant type, form of selenium in the growth medium, and presence of competing ions such as sulfate and phosphate (9). Interestingly, there are no written reports of selenium toxicity under cultivated conditions (9,12). This result may be because most crop plants show no injury or yield suppression until they accumulate at least 300 mg Se kg^-1, which is usually more than they contain even on seleniferous soils (9,14). In nonaccumulator plants, the threshold selenium concentration in shoot tissue that resulted in a 10% restriction in yield ranged from 2 mg Se kg^-1 in rice to 330 mg Se kg^-1 in white clover (Trifolium repens L.) (10). Wild-plant species growing in areas of elevated soil selenium tend to be adapted to those regions. Indicator plants can hyperaccumulate selenium to levels above 10,000 mg Se kg^-1, but possess biochemical means to avoid toxicity.

Descriptions for toxicity symptoms come only from solution-culture experiments. Stunting of growth, slight chlorosis, decreases in protein synthesis and dry matter production, and withering and drying of leaves are most often associated with selenium toxicity (4). Toxicity of selenium appears as chlorotic spots on older leaves that also exhibit bleaching symptoms. A pinkish, translucent color appearing on roots can also occur (65). Onions grown under extremely toxic Se concentrations showed sulfur-deficiency symptoms just before plant death (D.A. Kopsell and W.M. Randle, unpublished data, 1994).

The toxic effect of selenium to plants results mainly from interferences of selenium with sulfur metabolism (10). In most plant species, selenoamino acids replace the corresponding S-amino acids and are incorporated into proteins. Nuehierl and B�ck (66) reported on a proposed mechanism of selenium tolerance in plants. In nonaccumulator plant species, Se-cys would either be incorporated into proteins or function as a substrate for downstream-sulfur pathways, which would allow selenium to interfere with sulfur metabolism. Replacing cysteine (Cys) with Se-Cys in S-proteins will alter the tertiary structure and negatively affect their catalytic activity (31). In contrast, accumulator plant species would instantly and specifically methylate Se-cys using Se-Cys methyltransferase, thereby avoiding Se-induced phytotoxicity (31). This action would remove selenium from the pool of substrates for cysteine metabolism. Thus, Se-Cys methyltransferase may be a critical enzyme conferring selenium tolerance in selenium-accumulating plants. Alternatively, tolerance may be achieved by sequestering selenium as selenate or other nonprotein Se-amino acids in the vacuole in accumulator plant cells (3).