Figure 8.

Schematic overview of K⁺ uptake in plant roots under steady-state conditions (i.e. in the presence of high [millimolar] [NH₄⁺]_ext). Under low (0.0225 mm) [K⁺]_ext, K⁺ uptake is predominantly mediated by K⁺ channels (AtAKT1) in Arabidopsis (Col-0 wild type; A), whereas in barley (B), K⁺ channels do not operate and uptake is likely mediated by high-affinity transporters (HvHAK1), albeit at a residual capacity due to NH₄⁺-induced inhibition. Above intermediate K⁺ levels ([K⁺]_ext = 0.1125 mm), K⁺ channels do operate in barley, with further Cl⁻- and NO₃⁻-induced stimulations of K⁺ uptake observed at high K⁺ ([K⁺]_ext = 5 mm). In barley, K⁺ efflux is likely channel mediated at low and intermediate K⁺, although with varying sensitivities to Cs⁺, whereas K⁺ efflux is likely inoperative at high K⁺. K⁺ efflux in Arabidopsis is also likely channel mediated (Maathuis and Sanders, 1993). [K⁺]_cyt and resting ΔΨ_m values, when measured, are also listed. Asterisks refer to references as follows: *see Maathuis and Sanders (1993) and Halperin and Lynch (2003); **see Leigh and Wyn-Jones (1984), Walker et al. (1996), and Kronzucker et al. (2006). [See online article for color version of this figure.]