Some Physiological Responses ofChlorella pyrenoidosato 2,4-Dichlorophenoxyacetic Acid

An 18-h treatment of synchronously-grown Chlorella pyrenoidosa with 2,4-D did not significantly alter the size, dry weight, degree of synchrony, or pigment content of the cells, nor were detectable quantities of ethylene produced. When Chlorella pyrenoidosa was treated with 5×10−4 M 2,4-D, there was a statistically significant stimulation of both net oxygen uptake and production while 5×10 M 2,4-D inhibited both processes. When Chlorella pyrenoidosa was treated with 5×10−4 M and 5×10−3 M 2,4-D, significantly greater amounts of glycollate were present in the culture medium, even though an assay for glycollate dehydrogenase showed that the activity of this enzyme from 2,4-D-treated Chlorella pyrenoidosa was three times greater than in control cells. Loosely bound 2,4-D was partitioned from a nonaqueously isolated chloroplast fraction, while other cell fractions failed to show detectable quantities of 2,4-D. It is postulated that in Chlorella pyrenoidosa the chloroplast is a target for 2,4-D action and that interference in photorespiratory processes may underlie the observed responses.