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High salinity tolerance in the stl2 mutation of Ceratopteris richardii is associated with enhanced K+ influx and loss

Warne, T.R., Hickok, L.G., Kinraide, T.B., Vogelien, D.L.
Plant, cell and environment 1996 v.19 no.1 pp. 24-32
rubidium, salt tolerance, Ceratopteris, ion transport, mutation, ammonium compounds, potassium, mutants, sodium, metabolic inhibitors, membrane potential
The roles of K+ uptake and loss in the salinity response of the wild type and the salt-tolerant mutant stl2 of Ceratopteris richardii were studied by measuring Rb+ influx and loss and the effects of Na+, Mg2+, Ca2+ and K+-transport inhibitors. In addition, electrophysiological responses were measured for both K+ and Rb+ and for the effects of Na+ and NH4+ on subsequent K+-induced depolarizations. stl2 had a 26-40% higher uptake rate for Rb+ than the wild type at 0.5-10 mol m-3 RbCl. Similarly, membrane depolarizations induced by both RbCl and KCl were consistently greater in stl2. In the presence of 0-180 mol m-3 NaCl, stl2 maintained a consistently greater Rb+ influx than the wild type. stl2 retained a greater capacity for subsequent KCl-induced depolarization following exposure to NaCl. Five mol m-3 Mg2+ decreased Rb+ uptake in stl2; however, additional Mg2+ up to 40 mol m-3 did not affect Rb+ uptake further. Ca2+ supplementation resulted in a very minor decrease of Rb+ uptake that was similar in the two genotypes. Tetraethylammonium chloride and CsCl gave similar inhibition of Rb+ uptake in both genotypes, but NH4Cl gave substantially greater inhibition in the wild type than in stl2. NH4Cl resulted in a greater membrane depolarization in the wild type and the capacity for subsequent depolarization by KCl was markedly reduced. stl2 exhibited a higher K+-dependent loss of Rb+ than the wild type, but, in the absence of external K+, loss of Rb+ was equivalent in the two genotypes. Since constitutive K+ contents are nearly identical, we conclude that high K+ influx and loss exact a metabolic cost that is reflected in the inhibition of gametophytic growth. Growth inhibition can be alleviated by reduced supplemental K+ or by treatments that slightly reduce K+ influx, such as moderate concentrations of Na+ or Mg2+. We propose that high throughput of K+ allows maintenance of cytosolic K+ under salt stress and that a high uptake rate for K+ results in a reduced capacity for the entrance and accumulation of alternative cations such as Na+ in the cytosol.