EFFECT OF VARYING SOLUTION CALCIUM OR MAGNESIUM CONCENTRATIONS IN THEPRESENCE OR ABSENCE OF ALUMINUM ON YIELD AND PLANT CALCIUM OR MAGNESIUM CONCENTRATIONS IN WHEAT

The effect of varying solution calcium (Ca) and magnesium (Mg) concent rations in the absence or presence of 10 mu M aluminum (Al) was invest igated in several experiments using a low ionic strength (2.7 x 10(-3) M) solution culture technique. Aluminium-tolerant and Al-sensitive li nes of wheat (Triticum aestivum L.) were grown. In the absence of Al, top yields decreased when solution Ca concentrations were <50 mu M or plant Ca concentrations were <2.0 mg/g. Top and root yields decreased when solution Mg concentrations were <50 mu M or plant Mg concentratio ns were <1.5 mg/g. There were no differences between the lines in solu tion or plant concentrations at which yield declined. Increasing solut ion Ca concentrations decreased plant Mg concentrations in the tops (c ompetitive ion effect) but increased plant Mg concentrations in the ro ots of wheat. This suggests that Ca is competing with Mg when Mg is tr ansported from the roots. Increasing solution Mg concentrations decrea sed plant Ca concentrations in the tops and the roots (competitive ion effect). In the roots, increasing solution Mg concentrations decrease d plant Ca concentrations at a lower solution Ca concentration in the Al-sensitive line than the Al-tolerant line. In the presence of Al, in creasing solution Ca and Mg concentrations increased yield (Ca and Mg ameliorating Al toxicity). Yield increased until the sum of the soluti on concentrations of the divalent cations (Ca+Mg) was 2,000 mu M for t he Al-tolerant line or 4,000 mu M for the Al-sensitive line. The excep tion was that yield decreased when solution Mg concentrations were >1, 500 mu M and the solution Ca concentration was 100 mu M (Mg exacerbati ng Al toxicity). The ameliorative effects of solution Ca or Mg on Al t olerance were not related to plant Ca or Mg concentrations per se.