Nitrate Uptake, Root and Shoot Growth, and Ion Balance of Soybean Plants during Acclimation to Root-Zone Acidity

Abstract

The effects of acidity on NO3- absorption by nonnodulated, vegetative soybean (Glycine max [L.] Merrill) plants were examined during a 30-day growth period in flowing solution culture. The acidities of nutrient solutions were maintained at pH 6.1, 5.1, or 4.1 ± 0.1. Root growth rates were reduced for about 20 days by increasing acidity but then recovered to rates which exceeded those at low acidity so that root mass in all treatments was similar after 30 days. In contrast, the uptake rate of NO3- per unit of root mass was enhanced by increased acidity within the first day of exposure to rates that remained constant throughout the treatment period. Total nitrate uptake and shoot growth rates were restricted at higher acidities until about day 20 and thereafter recovered to rates which exceeded those at low acidity in parallel with the recovery in root growth rates. Alterations in total NO3- uptake in response to acidity also were evaluated in relation to changes in uptake of other ions and ionic balance in the plant tissue. As acidity increased, anion uptake was increased relative to cation uptake. Though greater reduction of NO3-, and thus greater generation of internal OH- ions, occurred in plants at higher acidities, tissues contained smaller amounts of organic anions. Acidity of the root-zone, therefore, influenced partitioning of internal OH- generated during NO3- reduction between synthesis of organic anions in the tissue and efflux to the external solution in association with excess anion uptake.