Subterranean clover was grown in solution culture with Mg supplied at 0, 10, 20, 30, 40, or 80 μM in factorial combination with nitrogen (N), supplied either symbiotically or as calcium nitrate (500 μM). External requirements for Mg, as assessed by the concentration in the medium required for maximum growth, were similar for plants irrespective of N source and were in the range 21 to 24 μM. Similarly, internal requirement was assessed by the critical concentration of Mg in whole shoots at 1100μg g−1 and there was no difference between plants reliant on symbiotic N2−fixation and those supplied with Ca(NO3)2. However, because of the effects of added Ca(NO3)2 on the distribution of Mg within plants, a higher minimum concentration of Mg in the youngest open leaf (YOL) was required for uninhibited growth by plants reliant on N2 fixation.
The concentration of N in shoots increased with increasing Mg in solution and Mg-deficient plants contained a lower proportion of total shoot N as protein. This occurred both in plants reliant on N2 fixation or supplied with nitrate. Thus Mg appeared to be required in greater amounts for protein synthesis than for either nodule formation or N2-fixation.