Mineral Ion Contents and Cell Transmembrane Electropotentials of Pea and Oat Seedling Tissue

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Abstract
The relationships of concentration gradients to electropotential gradients resulting from passive diffusion processes, after equilibration, are described by the Nernst equation. A test was made of how closely the Nernst equation describes the electrochemical equilibrium in seedling tissues. Segments of roots and epicotyl internodes of pea (Pisum sativum, var. Alaska) and of roots and coleoptiles of oat (Avena sativa, var. Victory) seedlings were immersed and shaken in defined nutrient solutions containing 8 major nutrients (K+, Na+, Ca2+, Mg2+, Cl-, NO3-, H2PO4-, and SO4 2-) at 1-fold and 10-fold concentrations. The tissue content of each ion was assayed at 0, 8, 24, and 48 hr. Only K+ approached a reasonable fit to the Nernst equation. An efflux pump may exist for Na-, Mg2+, and Ca2+. For each of the anions the electrochemical gradient is from the tissue to the solution; thus an active influx pump for anions seems required. Differences between root and shoot tissue following excision and immersion in solution suggest that, in the intact plants, roots regulate ion uptake.