THE POISONING OF ROOTS OF ZEA MA YS BY NICKEL IONS, AND THE PROTECTION AFFORDED BY MAGNESIUM AND CALCIUM

S ummary The growth of seedlings oiZea mays L. has been studied in water cultures containing Ni 2+ , Mg 2+ and Ca a+ singly and in certain combinations. Root growth was reduced to half that of the controls (RG 50 ) by as little as 01 mg I −1 Ni 2+ . This poisoning was prevented by 4 mg I” 1 Mg 2+ and by 32 mg T 1 Ca 2+ . RG 50 for Mg 2+ alone was 64 mg T 1 and for Ca 2+ more than 1000 mg I −1 . Root growth could be stopped within 8 h of exposure to 1 mg I −1 Ni 2+ . In the absence of Ni 2+ , Ca 2+ protected against Mg 2+ toxicity but Mg 2+ did not reduce the effects when CaCl 2 created osmotic loss at very high concentration. In combination, a ratio of 6:1 excess of Mg: Ca was necessary for Mg 2+ to begin to reduce growth. At higher concentrations Mg 2+ exerted its own toxicity despite the presence of Ca 2+ . When Ni 2+ would otherwise have poisoned growth, Ca 2+ added only to the relief of poisoning by Mg 2+ if the Ca 2+ was in excess by more than 4:1 over Mg 2+ . Thus Mg 2+ appeared to be more fitted to prevent Ni 2+ poisoning, yet at higher concentrations it was toxic itself, while Ca 2+ could at still higher concentrations protect against both Ni 2+ and Mg 2+ . Ni 2+ 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.