The Poisoning of Roots of Zea Ma Ys by Nickel Ions, and the Protection Afforded by Magnesium and Calcium

Summary The growth of seedlings oiZea mays L. has been studied in water cultures containing Ni2+, Mg2+ and Caa+ singly and in certain combinations. Root growth was reduced to half that of the controls (RG50) by as little as 01 mg I−1 Ni2+. This poisoning was prevented by 4 mg I”1 Mg2+ and by 32 mg T1 Ca2+. RG50 for Mg2+ alone was 64 mg T1 and for Ca2+ more than 1000 mg I−1. Root growth could be stopped within 8 h of exposure to 1 mg I−1 Ni2+. In the absence of Ni2+, Ca2+ protected against Mg2+ toxicity but Mg2+ did not reduce the effects when CaCl2 created osmotic loss at very high concentration. In combination, a ratio of 6:1 excess of Mg: Ca was necessary for Mg2+ to begin to reduce growth. At higher concentrations Mg2+ exerted its own toxicity despite the presence of Ca2+. When Ni2+ would otherwise have poisoned growth, Ca2+ added only to the relief of poisoning by Mg2+ if the Ca2+ was in excess by more than 4:1 over Mg2+. Thus Mg2+ appeared to be more fitted to prevent Ni2+ poisoning, yet at higher concentrations it was toxic itself, while Ca2+ could at still higher concentrations protect against both Ni2+ and Mg2+. Ni2+ poisoned growth by stiffening the expansion zone tissues and by destroying the integrity of root meristems. This latter was the first and most important toxic effect, and was achieved by arresting mitosis.