Influence of Cobalt on Leaf Expansion and Oxidative Phosphorylation

Abstract

The cobaltous ion, as well as many other chemicals, particularly the purine analogs, are known to affect the growth of etiolated leaves in the presence or absence of photomorphogenically active light (11, 18, 19). Additionally, cobalt has been shown to promote growth of other etiolated tissue such as the oat (Avena) coleoptile (2, 21) and pea roots (6). There have been different explanations of the mode of action of Co+ +. Busse (2) presented evidence that Co+ + delays the deposition of secondary cell wall material, thereby prolonging the action of IAA. Galston (6) observed that the rate of peroxide formation in etiolated tissue is lowered by the addition of Co+ +, which in turn results in a decreased peroxidative destruction of IAA. Thimann (21) by using specific inhibitors came to the conclusion that Co+ + modifies some step in oxidative metabolism which makes energy available for growth by diverting it from other metabolic roles. The present investigation arose from an observation that leaf disks given Co+ + plus DNP5 responded quite differently from those given BAP plus DNP. In view of the diversity of opinion on the mechanism of Co++ action, it was of interest to obtain information about the primary site of action of Co+. The main emphasis during the present investigations was placed on the way by which Co+ + might modify oxidative phosphorylation. Evidence is presented showing that Co+ +-induced leaf expansion is not susceptible to inhibition by DNP. The amount of ATP in leaf tissue decreases sharply in the presence of DNP, but remains unchanged when Co+ + is supplied in addition to DNP. Furthermore, experiments with mitochondria isolated from sweet potato tubers show that Co+ + increases the yield of oxidative phosphorylation. Results from other experiments indicate that Co++ has an inhibiting effect on ATPase activity.