Macro nutrient cation uptake by plants. I. Rate-determining steps in net inflow of cations into intact and decapitated sunflower plants and intensity factors of cations in soil solution

Seedlings of sunflower plants, Helianthus annuus were grown under controlled conditions in a growth chamber in a complete nutrient solution using a method allowing variations in ion activities of K, Ca, Mg and NO₃ and at the same time maintaining the activity ratios, ${a_K}/\sqrt {{a_{Ca}} + {a_{{M_g}}} $ and aCa/aMg constant. Under these conditions increases in activities of K-, Ca- and Mg-ions at the root surface did not affect net flow of these ions into intact plants but were positively correlated with corresponding net inflow into decapitated plants. This difference between intact and decapitated plants confirms the validity of the proposition that growth rates of sinks and/or sorption rates of these cations into new tissues constitute the rate-determining step in net inflow of cations under conditions of sufficient nutrient supply to insure maximum growth. Net inflow of K was more depressed than that of Ca and Mg by the aerial part of the plant. Based on the above findings it is suggested that plants may behave as a cation exchanger in equilibrium with surrounding solution. The positive correlation between net inflow of cations and activities of corresponding cations at the root surface of decapitated plants supports the idea that the relative partial molar free energy or activity of a cation are expressions of the intensity by which soil offers cations to plants.