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 ...

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Results of studies with polyene antibiotics have pointed to an interaction with membranes (1). These interactions were reported to occur specifically with sterols (2), and a sparing effect has been noticed upon addition of exogenous sterols (4).It has also been noted that certain phytotoxins produced by plant pathogens interact with plant cell membranes, affecting their permeability and occasionally causing them to rupture (7,12,15). Still other work has implicated sterols in membrane structure (14), and the addition of exogenous sterols was found to exhibit a sparing effect against the action of certain chemicals which induce a loss of electrolytes from plant cells (5).Since permeability may be involved in some mechanisms of disease resistance, it appeared logical to study a particular hostpathogen combination which involved a toxin (13) and to determine what differences, if any, existed in sterol content between healthy and inoculated resistant and susceptible host plants. For this purpose maize and Helminthosporium carbonum (Ullstrup) were chosen as the host-pathogen combination. Susceptible hybrids (PrxK61) and resistant hybrids (Prl X K61) of maize are referred to hereafter as Pr and Prl, respectively. The latter is resistant to both races 1 and 2 of this pathogen but Pr is susceptible to race 1 and resistant to race 2.Culture and inoculation conditions are reported fully elsewhere (6). Forty-eight hours after incubation the plants were harvested. Only infected areas of leaves or comparable areas of control leaves were used. Twenty-five grams of fresh tissue were homogenized in 100 ml of acetone in a Virtis blender, the extract was filtered, and the residue was extracted twice more with 100-ml volumes of acetone. Additional acetone extracts contained no sterols. The acetone extracts were combined and the acetone was evaporated in vacuo, as was done in all other evaporation operations. The remaining aqueous extract was then made 33 I`w ith respect to acetone and extracted four times with 100-ml portions of diethyl ether.The ether phase was washed twice with 50-ml volumes of water, dried over Na2SO4 for 30 min, and evaporated to near dryness. Saponification was accomplished by addition of 50 ml of 95% ethanol and 5 ml of 60% KOH, followed by refluxing for 1 hr. After cooling, 200 ml of water were added and the mixture was then extracted thrice with 100-ml portions of CHCl3. The combined CHCl3 extract was washed twice with 50-ml volumes of water, dried over Na2SO4 for 30 min, evaporated to dryness, and dissolved in 2 ml of CHCl3. After adsorption on 0.5 g of alumina (activity III) the CHCI3 was evaporated. The GLC3 of the eluates showed that the sterols were contained in the 4 and 40yc/ ether fractions. These were combined, evaporated to dryness, and dissolved in 2 ml of CHC13 (fraction I).The aqueous phase from the ether extract was concentrated to 75 ml, and 25 ml of concentrated HCI were added. After refluxing for 3 hr, it was cooled and extracted thrice with 100-ml volumes of benzene. The benzene was ev...

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