The apical plasma membrane of young Arabidopsis root hairs has recently been found to contain a depolarisation-activated Ca(2+) channel, in addition to one activated by hyperpolarisation. The depolarisation-activated Ca(2+) channel may function in signalling but the possibility that the root hair apical plasma membrane voltage may oscillate between a hyperpolarized and depolarized state suggests a role in growth control. Plant NADPH oxidase activity has yet to be considered in models of oscillatory voltage or ionic flux despite its predicted electrogenicity and voltage dependence. Activity of root NADPH oxidase was found to be stimulated by restricting Ca(2+) influx, suggesting that these enzymes are involved in sensing Ca(2+) entry into cells.