Effect of 2,4-Dichlorophenoxyacetic Acid on Photosynthesis and Respiration

Early work with plant growth regulators offered only indirect evidence regarding their effect on photosynthesis. Mitchell et al. (15) showed that starved bean leaves exposed to light after they had been sprayed with a-naphthaleneacetic acid accumulated less starch, sugar, and dextrin than unsprayed leaves, and thus indicated the possibility of a decrease in photosynthesis. Freeland (13, 14), measuring photosynthesis directly by gas exchange, found that 2,4-dichlorophenoxyacetic acid (2,4-D) and other plant growth regulators inhibited apparent photosynthesis in Anacharis and in bean leaves. In bean, 2,4-D in a concentration of 100 ppm caused an inhibition of photosynthesis amounting to approximately 20% over the 4-day experimental period; respiration was first inhibited and then stimulated during the same period. In Anacharis, 30 and 100 ppm of 2,4-D caused a decrease in the rate of photosynthesis, the reduction being greater at the higher concentration. Respiration was at first inhibited by 2,4-D, but showed a partial or complete recovery at the end of 48 hours. Rhodes (20) showed that 2-methyl-4-chlorophenoxyacetic acid reduced apparent photosynthesis of tomato plants. Nickell (16) reported stimulation of respiration of tumor tissue of Rumex acetosa at very low concentrations of 2,4-D, with progressive inhibition of respiration at higher concentrations. Growth of the tumor tissue responded to the application of 2,4-D in much the same manner as respiration. The dependence of the growth-promoting effect of naturally occurring auxin upon the concentration of the undissociated auxin molecule was established in 1934 (6), after Dolk and Thimann (12) had demonstrated that auxin activity was highest in an acid medium. This relationship has since been verified for synthetic plant growth regulators and other weak acids and bases by observing their effect in a variety of growth and respiration responses (2, 4, 5, 21, 22, 23, 25, 26). Albaum et al. (1) showed that the relation of the pH of the medium to the effect of externally applied indoleacetic acid on Nitella was due to the penetration of the indoleacetic acid, the coincidence of the curves for penetration of indoleacetic acid and its dissociation, plotted against pH, indicating that it entered as the undissociated acid. Audus (3) found the inhibition of root growth to be proportional to the logarithm of the concentration of undissociated 2,4-D acid present, whether the range of concentrations of this molecule was achieved by varying the pH or by increasing the concentration at a constant pH.