Effects of Ni2+ on proton extrusion, dark CO2 fixation and malate synthesis in maize roots

The presence of Ni²⁺ in the incubation medium of maize root segments ( Zen mays L. cv. Dekalb XL 640) induces an early and significant enhancement of the rate of proton extrusion both in the absence and presence of fusicoccin; with time, an inhibition of proton extrusion and a leakage of K⁺ appear. The inhibition of proton extrusion is accompanied by a decrease in the dark CO₂ fixation and by a decrease in the level of malate in the cells. Ni²⁺ inhibits in vitro phosphoenolpyruvale carboxylase activity of maize roots. The data indicate a correlation between the operativity of the proton pump and that of the malate-pH stat mechanism for the homeostasis of cytoplasmic pH.