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A method for recovering 2,4-D from citrus fruit is described. The 2,4-D was made volatile by esterification with diazomethane and was detected as chloride by a microcoulometric gas chromatograph. Analyses of oranges picked from trees sprayed with 20 p.p.m. of 2,4-D showed an average of 0.1 p.p.m. of 2,4-D residue 1 day after spraying and lemons dipped in wax emulsion containing 500 p.p.m. of 2,4-D showed an average of 1.1 p.p.m. of 2,4-D residue 2 days after treatment.

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. 64The widespread use of 2,4-D in citrus culture for such purposes as increasing fruit size, delaying leaf and fruit abscission, and prolonging storage life of harvested fruit (24) has resulted in a situation in which a great deal of practical knowledge concerning the effects of the growth regulator on citrus is available, whereas little is known regarding the means by which 2,4-D alters the functioning of the plant to produce the observed responses. The present investigation is concerned with a quantitative study of the effect of 2,4-D on photosynthesis and respiration of citrus leaves and, for comparative purposes, its effect on the same processes in the unicellular green alga Chlorella pyrenoidosa. MATERIALS AND METHODSEXPERIMENTS WITH CITRUS LEAVES: To ...