High Salinity Tolerance in the stl2 Mutation of Ceratopteris richardii is Associated with Enhanced K+ Influx and Loss

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 Independent 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

Are preventive and generative causal reasoning symmetrical? Extinction and competition

We tested whether preventive and generative reasoning processes are symmetrical by keeping the training and testing of preventive (inhibitory) and generative (excitatory) causal cues as similar as possible. In Experiment 1, we extinguished excitors and inhibitors in a blocking design, in which each extinguished cause was presented in compound with a novel cause, with the same outcome occurring following the compound and following the novel cause alone. With this novel extinction procedure, the inhibitory cues seemed more likely to lose their properties than the excitatory cues. In Experiment 2, we investigated blocking of excitatory and inhibitory causes and found similar blocking effects. Taken together, these results suggest that acquisition of excitation and inhibition is similar, but that inhibition is more liable to extinguish with our extinction procedure. In addition, we used a variable outcome, and this enabled us to test the predictions of an inferential reasoning account about what happens when the outcome level is at its minimum or maximum (De Houwer, Beckers, & Glautier, 2002). We discuss the predictions of this inferential account, Rescorla and Wagner’s (1972) model, and a connectionist model—the auto-associator.Irina Baetu & A. G. Bake