Metabolism of 2,4-Dichlorophenoxyacetic Acid in Soybean Root Callus and Differentiated Soybean Root Cultures as a Function of Concentration and Tissue Age

Abstract

The metabolism of [1-14C]2,4-D in soybean (Glycine max [L.] Merrill var. Amsoy) root callus and in differentiated soybean root cultures was investigated as a function of pesticide concentration and age of tissue. The chronological age of the tissue correlated with the mitotic index which reached a peak at 2 wk and then declined. The metabolism of 2,4-D changed with age of the root callus tissue. The amount of free 2,4-D found in 3 wk old root callus tissue rapidly increased as the concentration of 2,4-D in the medium was increased from 10-6 to 10-5 molar, whereas the low level of aqueous (glycosides) and ether soluble metabolites (2,4-D amino acid conjugates) increased slowly. With 9 wk old root callus tissue, the amount of free 2,4-D remained at a relatively low, constant level (saturation level) as the concentration of 2,4-D in the medium increased. Under these conditions the aqueous metabolites increased only slightly but the ether fraction (2,4-D amino acid conjugates) rapidly increased. Thus, the older root callus tissue appeared to regulate the level of free 2,4-D at about 4 nmole/g by converting any excess 2,4-D into amino acid conjugates. In 3 wk old, differentiated root cultures the metabolism of 2,4-D closely paralleled the metabolism found in the older 9 wk old callus tissue. The saturation level of free 2,4-D found in this tissue was only about 1-2 nmole/g.