CALCIUM AMELIORATION OF ALUMINUM TOXICITY EFFECTS ON ROOT HAIR DEVELOPMENT IN SOYBEAN [GLYCINE-MAX (L) MERR]

Low activities of the monomeric aluminium (Al) species, Al3+, Al(OH)2 and Al(OH)2+ in solution reduce root growth and root hair development in soybean [Glycine max (L.) Merr.]. Calcium (Ca) ameliorates to a ce rtain extent the toxic effects of Al on root growth, but the interacti ve effects of Ca and Al on root hair development are not known. In the absence of added Al in solution culture, Ca concentration over the ra nge 500-2000 mum had little effect on root growth or root hair develop ment of soybean cv. Fitzroy. Where the sum of the activities of the mo nomeric Al species was 2 mum, taproot elongation and lateral root deve lopment of soybean was suppressed in solution with 500 mum Ca. The len gth of the root hair zone was only 1 0 % of that in plants not exposed to Al, and scanning electron microscopy revealed a low density of roo t hairs. Increasing the Ca concentration to 2000 mum largely overcame these deleterious effects. The results are discussed in relation to th e role of root hairs as infection sites for Bradyrhizobium, and the kn own effects of Al in suppressing nodulation in soybean.