Magnesium requirements of subterranean clover (Trifolium subterraneum L.) either reliant on symbiotic nitrogen fixation or supplied with nitrate

Abstract

SUMMARYSubterranean clover vt*as grown in solution culture with Mg supplied at 0, 10, 20, 30, 40, or HO//M in factoria) combination with nitrogen (N), supplied either symbiotically or as calcium nitrate (500 /(M). External requirements for Mg, as assessed by tbe concentration in tbe medium required for maximum growth, were similar for plants irrespective of N source and were in the range 21 to 24 /iM. Similarly, internal requirement was assessed by ibe critical concentration of Mg in whole shoots at ] 100 /Ag g"', and tbere was no difference between plants reliant on symbiotic X^-fixation and tbose supplied with CaCNOg)^. However, because of the effects of added Ca(NO^)2 on the distribution of Mg witbin plants, a higher minimum concentration of Mg in the youngest open leaf (YOL) was required for uninhibited growtb by plants reliant on N\-fixation.The concentration of N in sboots 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 Nj-fixation or supplied witb nitrate. Thus Mg appeared to be required in greater amounts for protein synthesis tban for eitber nodule formation or N.,-lixation.