The Allium test--an alternative in environmental studies: the relative toxicity of metal ions.

Among the test systems suitable for toxicity monitoring, the Allium test (A. cepa) is well known and commonly used in many laboratories. The onions are easy to store and to handle, and the root tip cells constitute a convenient system for macroscopic (growth, EC50 values) as well as for microscopic parameters (c-mitosis, stickiness, chromosome breaks). Since the cells possess important plant activation enzymes, the Allium test has a wide area of application. Furthermore, results from the Allium test have shown good agreement with results from other test systems, eukaryotic as well as prokaryotic. A modified version of the test method, comprising series of onions for each concentration of the test liquids, was applied to salts of eight metals: Hg (as methyl mercury chloride (MMC) and as HgCl2), Cu, Ni, Cd, Be, Al (diluted in tap water and distilled water), Mn and Li. The highest toxicity, in EC50 values, was caused by Hg (for MMC 9.0 X 10(-7) M, for HgCl2 3.3 X 10(-6) M), Cu (2.7 X 10(-6) M), Ni (1.7 X 10(-5) M) and Cd (3.1 X 10(-5) M); medium toxicity was caused by Be (4.8 X 10(-4) M) and Al (in tap water 8.0 X 10(-4) M, in distilled water 2.8 X 10(-5) M), and low toxicity by Mn (5.2 X 10(-3) M) and Li (2.0 X 10(-2) M). Some of the metals induced specific microscopic effects, requiring particular mention: thus, Ni treatment induced an unusual form of c-mitosis with the c-mitotic chromosomes remaining on the equatorial plate, Be treatment induced a type of 'banded' or 'fragmented' chromosomes. Treatment with Al led to the development, in the cytoplasm of certain root tip cells, of two oblong hyaline structures formed by material extruded from the nucleus. Chromosome breaks were mainly observed as fragments at mitotic anaphase. The metal ions tested here caused only low amounts of fragments, usually in less than 1% of the cells; only Be caused a higher frequency (4.1%). It was not possible to group the few metals tested here according to their cytological effects. The standard parameters, such as the most commonly occurring c-mitosis or stickiness, showed no correlation to atomic weight or to ion charge of the metals. Still, they gave valuable information in relation to environmental screening; thus, the finding of c-mitosis may indicate risks of aneuploidy. Generally speaking, the Allium test is a very useful tool for evaluating and ranking environmental chemicals with reference to their toxicity.