Effect of aluminum on soybean nodulation and nodule activity in a vertical split‐root system

Abstract

Soybean plants (Glycine max L. cv Santa Rosa) grown hydroponically in nutrient solutions had reduced nodule mass and numbers in the presence of aluminum (Al). Reduced nodule number was attributed mainly to hydrogen (H) ion toxicity, whereas Al had a stronger effect on nodule growth. Using a vertical split‐root system with Al exclusively in the lower (hydroponic) layer also resulted in a significant reduction of nodulation and nodule growth in the surface compartment (vermiculite). This indirect effect could be attributed mainly to Al rather than H. Subsurface Al had no apparent effect on shoot growth or root growth of the upper compartment, but significantly limited root growth in the lower compartment where it was applied. The indirect effect of Al on nodulation could be a reflection of the abnormal root growth in the lower compartment. Split‐root experiments with a high Al soil, however, produced different effects. High Al in soil used exclusively in the lower compartment did not reduce nodule numbers or mass in the upper compartment despite being more harmful than the Al solutions to nodulation and growth of plants when used in a single compartment. Growth of roots in the subsurface compartment was also much less affected by the high soil Al compared with the Al‐containing nutrient solutions. Nodule activity, as estimated by xylem sap ureide levels, was only reduced after direct exposure of nodules to Al. A pronounced increase in the ratio of asparagine/glutamine occurred in all Al treatments where nodulation was reduced, and in some cases, there was an increase in total amino acid concentration of the xylem sap.