Effect of Plant Growth Regulators on the Nitrogen Fixing (Acetylene Reduction) Activity of Soybean Plants

Abstract

Two plant growth regulators, kinetin and 2, 4-D, were tested on soybean plants to find their relative effects on nitrogen fixation. 2, 4-D gave much more severe effects on both morphologically and physiologically than did the kinetin. As a result of the treatments with exogenous kinetin and 2, 4-D, the leaf-area, leaf-number, and the height of the plants was reduced. Leaves showed chlorosis, and the chlorophylls in the leaves (mostly the chlorophyll b) was markedly decreased, the accumulation of the dry matter in the tissue fractions diminished, and other apparent changes involved the shoot-root ratio and the chlorophyll-a to chlorophyll-b ratio. The nitrogen fixing (acetylene reduction) activity of intact plants or of detached nodules was suppressed by kinetin and 2, 4-D. One of the detrimental effects of 2, 4-D or kinetin on nitrogen fixation was a decrease in the root-nodule-number. It is presumed that the nodule-initiation or nodule-degeneration is inhibited or promoted by the stimulation with kinetin or 2, 4-D.