Uptake of 2,4-Dichlorophenoxyacetic Acid by Pseudomonas fluorescens

Factors influencing the uptake of the sodium salt of 2,4-dichlorophenoxyacetic acid (2,4-D), under conditions in which no net metabolism occurred, were investigated in an effort to determine both the significance of "non-metabolic" uptake as a potential agent in reducing pesticide levels and the mechanisms involved. Uptake of 2,4-D was affected by pH, temperature, and the presence of other organic and inorganic compounds. Uptake was more pronounced at pH values less than 6, which implies that there may be some interaction between charged groups on the cell and the ionized carboxyl group of 2,4-D. Active transport, carrier-mediated diffusion, passive diffusion, and adsorption were considered as possible mechanisms. Though uptake was inhibited by glucose, sodium azide, and fluorodinitrobenzene (but not by uranyl ion), 2,4-D was not accumulated against a concentration gradient, a necessary consequence of an active transport system, nor was isotope counterflow found to occur. Thus, carrier-mediated diffusion was finally precluded, implying that uptake probably occurs by a two-step process: sorption onto the cell wall followed by passive diffusion into the cytoplasm.     

Mineralization of 2,4-Dichlorophenoxyacetic Acid (2,4-D) and Mixtures of 2,4-D and 2,4,5-Trichlorophenoxyacetic Acid by Phanerochaete chrysosporium

Evidence is presented for mineralization of 2,4-dichlorophenoxyacetic acid (2,4-D) in nutrient-rich media (high-nitrogen and malt extract media) by wild-type Phanerochaete chrysosporium and by a peroxidase-negative mutant of this organism. Mass balance analysis of [U-ring-¹⁴C]2,4-D mineralization in malt extract cultures showed 82.7% recovery of radioactivity. Of this, 38.6% was released as ¹⁴CO₂ and 27.0, 11.2, and 5.9% were present in the aqueous, methylene chloride, and mycelial fractions, respectively. 2,4-D and 2,4,5-trichlorophenoxyacetic acid (2,4,5-T) were simultaneously mineralized when presented as a mixture, and mutual inhibition of degradation was not observed. In contrast, a relatively higher rate of mineralization of 2,4-D and 2,4,5-T was observed when these compounds were tested as mixtures than when they were tested alone.     

Growth Rates of a Pseudomonad on 2,4-Dichlorophenoxyacetic Acid and 2,4-Dichlorophenol

The growth of a pseudomonad on 2,4-D (2,4-dichlorophenoxyacetic acid) and 2,4-DCP (2,4-dichlorophenol) was studied in batch and continuous culture. The optimum growth rate using 2,4-D was 0.14/h at 25 C in a pH range from 6.2 to 6.9. Highest specific growth rate using 2,4-DCP was 0.12/h at 25 C in a pH range from 7.1 to 7.8. Growth was strongly inhibited by 2,4-DCP above a concentration of 25 mg/liter whereas no appreciable inhibition was observed with 2,4-D at concentrations up to 2,000 mg per liter. Growth on 2,4-DCP was described by Monod kinetics at subinhibitory concentrations but the inhibition by 2,4-DCP exhibited an unusual linear response to substrate concentration, and did not fit a model based on noncompetitive inhibition. The lag phase of batch cultures was found to depend on both 2,4-DCP concentration and prior adaptation of the inoculum. A study such as this on the kinetics of growth on related substrates may be useful as a method of finding the rate-limiting step in a metabolic sequence.     

The Effects of 2,4-Dichlorophenoxyacetic Acid on Ion Uptake by Maize Roots

The effects of the synthetic auxin and herbicide 2,4-dichlorophenoxyaceticacid (2,4-D) on K^$ and Cl^– uptake and H^$ release by youngexcised maize roots has been studied. Brief exposure to 2,4-D(0.01 mmol dm^–3) at pH 3.5 causes a large depolarizationof the electrical potential across the root plasma membranesand converts K^$ uptake to K^$ leakage into the bathing solution.These results can be explained by the increased H^$ permeabilityof the membranes induced by the weak acid 2,4-D. The depolarizationresults in a less favourable electrochemical potential gradientfor K^$ uptake across these membranes. These effects are notrelated to the auxin properties of 2,4-D as the nonauxin 3,5-dichlorophenoxyaceticacid (3,5-D) gives rise to similar effects. The relative depolarizationsinduced by a range of weak acids appear to be unrelated to theiroil/water partition coefficients. In contrast, on bathing the roots for longer periods in solutions(pH > 5) containing 2,4-D (0.01 mmol dm^–3) K^$ and Cl^–uptake and H^$ release are inhibited. These effects are not shownwith 3,5-D suggesting an auxin-linked action for 2,4-D. Alsothe electrical potential across the plasma membranes is onlyslightly depolarized so that a change in the electrochemicalpotential gradient cannot be invoked to explain the loweredion fluxes. The evidence is consistent with the removal of anenergy supply to a metabolically linked K^–/H^– exchangemechanism in the plasma membranes. It is likely that both modes of action would operate to lowerion uptake under soil-grown conditions, the former becomingmore manifest in acidic soils.     

2,4-二氯苯氧乙酸的研究进展

2,4-二氯苯氧乙酸经常作为除草剂和植物生长调节剂使用,在农林业中发挥了重大作用,尤其在水果(如柑橘)保鲜中应用广泛,但其毒性问题也受到广泛关注,因此了解2,4-D的生理作用、在生物及环境中的代谢降解、残留毒性和提取鉴定等的研究进展很有必要.     

A Bioluminescent Whole-Cell Reporter for Detection of 2,4-Dichlorophenoxyacetic Acid and 2,4-Dichlorophenol in Soil

A bioreporter was made containing a tfdRP_DII-luxCDABE fusion in a modified mini-Tn5 construct. When it was introduced into the chromosome of Ralstonia eutropha JMP134, the resulting strain, JMP134-32, produced a sensitive bioluminescent response to 2,4-dichlorophenoxyacetic acid (2,4-D) at concentrations of 2.0 μM to 5.0 mM. This response was linear (R² = 0.9825) in the range of 2.0 μM to 1.1 × 10² μM. Saturation occurred at higher concentrations, with maximal bioluminescence occurring in the presence of approximately 1.2 mM 2,4-D. A sensitive response was also recorded in the presence of 2,4-dichlorophenol at concentrations below 1.1 × 10² μM; however, only a limited bioluminescent response was recorded in the presence of 3-chlorobenzoic acid at concentrations below 1.0 mM. A significant bioluminescent response was also recorded when strain JMP134-32 was incubated with soils containing aged 2,4-D residues.     

Interaction between 2,4-Dichlorophenoxyacetic Acid and Vitamin K in Datura stramonium

Plants of the cross Datura stramonium L. var. stramonium×Datura stramonium L. var. inermis (Juss. ex Jacq.) Timm. were sprayed either with a water solution of the sodium salt of 2,4-dichlorophenoxyacetic acid (2,4-D) or with a combination of 2,4-D (sodium salt) and the vitamin K compound menadione sodium bisulfite in water solution. The plants were found to differ in one of their responses to the treatments, namely the development of necrotic spots on the leaf blades. Many necroses appeared on the plants treated with 2,4-D plus menadione sodium bisulfite while only a few necroses or none at all developed on the plants treated with 2,4-D alone. Furthermore, 2,4-D in combination with menadione sodium bisulfite induced much larger necrotic spots than did 2,4-D by itself. Spraying with menadione sodium bisulfite in water solution did not produce any observable effects upon Datura plants. The great increase in necrotic lesion formation was probably due to an interaction between 2,4-D and menadione sodium bisulfite (or a metabolite of this compound) within leaf cells.     

Predicting 2,4-Dichlorophenoxyacetic Acid Ester Transformation Rates in Periphyton-Dominated Ecosystems

Using batch cultures, we determined transformation rate coefficients for microbial transformation of 2,4-dichlorophenoxyacetic acid butoxyethyl ester (2,4-DBE) in periphyton-dominated ecosystems. Rates of 2,4-DBE loss were measured over short periods of time (usually less than 10 h), and first-order transformation rate coefficients (k₁) were determined under the specific conditions of low 2,4-DBE concentrations and no growth. Values for k₁ were divided by total plate counts and by biomass measured as ash-free dry weight to give second-order rate coefficients (k_b and k_AFDW, respectively) for use in predictive models. Using periphyton attached to Teflon strips, we also determined second-order rate coefficients based on the ratio of colonized surface area to container volume (k_A). Mean second-order rate coefficients were used to predict 2,4-DBE transformation rates in microcosms having diverse chemical and biological environments. The observed transformation rates among the microcosms were most accurately predicted by using k_A.     

Some Physiological Responses of Chlorella pyrenoidosa to 2,4-Dichlorophenoxyacetic Acid

An 18-h treatment of synchronously-grown Chlorella pyrenoidosawith 2,4-D did not significantly alter the size, dry weight,degree of synchrony, or pigment content of the cells, nor weredetectable quantities of ethylene produced. When Chlorella pyrenoidosawas treated with 5?10^–4 M 2,4-D, there was a statisticallysignificant stimulation of both net oxygen uptake and productionwhile 5?10 M 2,4-D inhibited both processes. When Chlorellapyrenoidosa was treated with 5?10^–4 M and 5?10^–3M 2,4-D, significantly greater amounts of glycollate were presentin the culture medium, even though an assay for glycollate dehydrogenaseshowed that the activity of this enzyme from 2,4-D-treated Chlorellapyrenoidosa was three times greater than in control cells. Looselybound 2,4-D was partitioned from a nonaqueously isolated chloroplastfraction, while other cell fractions failed to show detectablequantities of 2,4-D. It is postulated that in Chlorella pyrenoidosathe chloroplast is a target for 2,4-D action and that interferencein photorespiratory processes may underlie the observed responses.