Calculated activity of Mn at the outer surface of the root cell plasma membrane governs Mn nutrition of cowpea seedlings

Manganese (Mn) is an essential micronutrient for plant growth but is often toxic in acid or waterlogged soils. Using cowpea (Vigna unguiculata L. Walp.) grown with 0.05–1500 mM Mn in solution, two short-term (48 h) solution culture experiments examined if the effects of cations (Ca, Mg, Na, Al, or H) on Mn nutrition are related to the root cells’ plasma membrane (PM) surface potential, c0. When grown in solutions containing levels of Mn that were toxic, both relative root elongation rate (RRER) and root tissue Mn concentration were more closely related to the activity of Mn at the outer surface of the PM, Mn2þ 0 0 (R1⁄40.812 and 0.871) than to its activity in the bulk solution, Mn2þ b (R1⁄40.673 and 0.769). This was also evident at lower levels of Mn (0.05–10 mM) relevant to studies investigating Mn as an essential micronutrient (R1⁄40.791 versus 0.590). In addition, changes in the electrical driving force for ion transport across the PM influenced both RRER and the Mn concentration in roots. The Mn2þ b causing a 50% reduction in root growth was found to be c. 500 to >1000 mM (depending upon solution composition), whilst the corresponding value was 3300 mM when related to Mn2þ 0 0 . Although specific effects such as competition are not precluded, the data emphasize the importance of non-specific electrostatic effects in the Mn nutrition of cowpea seedlings over a 1310-fold range of Mn concentration in solution.