A detailed examination was conducted on the linear, or first-order kinetic component for K'("Rb) influx into root segments of both lowand high-salt grown corn seedlings (Zea mays 1A632 x Oh 431). In tissue from both low-and high-salt grown. roots, replacement of Cl-in the uptake solution by either SO3-, H2PO0, or NO! caused a significant (50-60%) and specific inhibition of the linear component of K' influx. The anion transport inhibitor, 4,4'-diisothiocyano-2,2'-disulfonic acid, was found to abolish saturable Cl-influx in corn roots while causing a significant (50-60%) and specific inhibition of the linear K' uptake system; this inhibition was identical to that observed when Cl was replaced by other anions in the K' uptake solution. Additionally, the quaternary ammonium cation, tetraethylammonium, which has been shown to block K' channels in nerve axons, also caused a dramatic (70%) and specific inhibition of the linear component of K influx, but this was obtained only in high-salt roots. The reasons for this difference are discussed with respect to the differing abilities of low-and high-salt roots to absorb tetraethylammonium.Our present results indicate that the linear component of K' influx may occur by a passive process involving transmembrane K' channels. Fluxes through these K' channels may be partly coupled to a saturating C1-influx mechanism.In studies involving the kinetics of uptake for various plant tissues, the majority of the work has focused on the generation of discontinuous isotherms, via either the operation of multiple Michaelis-Menten transport systems, or by complex, multisite carriers. In many of these studies, the presence of a linear component was either ignored or considered to reflect diffusion across the plasma membrane (13 and references therein).Recently there has been a growing interest in nonsaturating transport kinetics for organic solutes and ions. In contrast to earlier investigations, the more recent literature suggests that the nonsaturating or linear transport component may be a relatively complex process (2, 6, 7, 13-15, 20, 21, 31 KI and ClO Influx Experiments. Short-term (10 min) 86Rb+ and 3Cl-influx experiments were performed using2-cm root segments as previously described (13). Briefly, experiments were performed with 2-cm root segments cut from the 1st through 8th cm of the primary root. Root segments were washed (4 h) in solutions of identical composition to their growth solutions to allow for recovery from excision. Uptake was initiated by the addition of 86Rb+ (as RbCl, New England Nuclear, Boston) or 3Cl-(as NaCl, ICN Biochemicals, Irvine, CA) and terminated by the vacuum withdrawal of radioisotope solution. Free space radiolabel was removed by either two 8-min washes for 'Rb+ or two 5-min washes for 'Cl-in ice-cold 0.5 mM CaSO4 + 1 mM KCI. We have found that these desorption regimes removed approximately 95% ofthe free space radiolabel while exchanging only about 5% of the intracellular label. Following desorption, root segments were centrifuged (300g) for...