Controls on 22Na+ Influx in Corn Roots

Abstract

The effects of hyperpolarization and depolarization, and the presence of K+ and/or Ca2+, on 22Na+ influx into corn root segments was investigated. In freshly excised root tissue which is injured, Na+ influx is unaffected by hyperpolarization with fusicoccin, or depolarization with uncoupler (protonophore), or by addition of K+. Added Ca2+ suppresses Na+ influx by 60%. In washed tissue which has recovered, Na+ influx is doubled over that of freshly excised tissue, and the influx is increased by fusicoccin and suppressed by uncoupler. This energy-linked component of Na+ influx is completely eliminated by low concentrations of K+, leaving the same level and kind of Na+ influx seen in freshly excised roots. The K+-sensitive energy linkage appears to be by the carrier for active K+ influx. Ca is equally inhibitory to Na+ influx in washed as in fresh tissue. Other divalent cations are only slightly less effective. Net Na+ uptake was .apprx. 25% of 22Na+ influx, but proportionately the response to K+ and Ca2+ was about the same. The constancy of K+-insensitive Na+ influx under conditions known to hyperpolarize and depolarize suggests that if Na+ transport is by means of a voltage-sensitive channel, the rise or fall of channel resistance must be proportional to the rise or fall in potential difference. The alternative is a passive electroneutral exchange of 22Na+ for endogenous Na+. An inwardly directed Na+ current is largely offset by an efflux current, giving both a small net uptake and isotopic exchange.