Controls on na influx in corn roots

We have investigated the effects of hyperpolarization and depolarization, and the presence of K⁺ and/or Ca²⁺, on ²²Na⁺ influx into corn (Zea mays L.) root segments. 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⁺. However, 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. Calcium is equally inhibitory to Na⁺ influx in washed as in fresh tissue. Other divalent cations are only slightly less effective. Net Na⁺ uptake was about 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⁺. The data suggest that an inwardly directed Na⁺ current is largely offset by an efflux current, giving both a small net uptake and isotopic exchange.