INHIBITION OF NODULATION OF LUCERNE (MEDICAGO-SATIVA L) BY CALCIUM DEPLETION IN AN ACID SOIL

The effect of calcium on the nodulation of lucerne was studied using EGTA, a specific calcium-chelator. First, the effects of the chelator were tested on hydroponically grown plants at pH 7.0. Optimal numbers of nodules were obtained in nutrient solution containing 0.2 m M CaCl2. When 0.4 m M EGTA was given additionally, nodulation was completely inhibited. Nodulation was restored specifically with CaCl2, but not with MgCl2. For studies in an acid soil (pH-H2O 5.2), lucerne seedlings were grown in rhizotrons. 67% of the seedlings became nodulated when the soil around the seed was neutralized locally with 1.0 μmol of K2CO3 in drops of 12 μL volume. When native calcium was removed with 2 μmol of EGTA, nodulation was reduced to 12%. However, addition of EGTA to soil resulted in a drop of pH from 6.1 to 5.2. A phosphate buffer could also not keep soil-pH sufficiently stable. Such pH-decreases could be avoided by placing agar blocks containing 6 μmol of EGTA for three hours on freshly developed roots. This treatment reduced nodulation from 87% to 32%, with soil-pH lowering only from 6.2 to 6.0. Nodulation could be restored by adding 2 μmol of CaCl2. The depletion of soil-calcium could depress nodule formation only during the first day after inoculation. © 1990 Kluwer Academic Publishers.