Uptake and development of phytotoxicity following exposure to vapour of the herbicide C 2,4-D butyl by tomato and lettuce plants

Tomato and lettuce plants were exposed to vapour of the herbicide ¹⁴C 2,4-D butyl at concentrations in the range of 1–660 pg/l for 6, 24 or 72 hr. The relationship between herbicide uptake, measured as amount of radiolabel in the plant, and vapour concentration was linear and independent of the duration of exposure for both species. The rate of uptake of herbicide vapour at any one concentration by tomato was about twice that of lettuce. Immediately after exposure, the leaves of lettuce plants contained similar amounts of herbicide residue, whereas leaves at the base of tomato plants usually contained the least herbicide, and those at the apex the most. In terms of leaf area, both species had highest residue contents in the apical leaves. Different exposure periods did not affect the amounts of herbicide residue in the apical leaves or the leaf below, indicating that herbicide present in the apex is largely due to uptake and not to translocation. Both species developed visible symptoms of phytotoxicity following exposure, the severity being directly related to the amount of herbicide vapour received. Leaf contents of herbicide residue in tomato plants 40 days after exposure were related to the total plant content measured immediately after exposure; the leaf below the apex at the time of exposure retained about 17% of the herbicide originally present. Shoot dry weight, 40 days after exposure, was reduced by 10% following uptake of about 250 ng/plant in tomato, whereas for lettuce, about twice this amount of 2,4-D butyl was required to cause noticeable weight reduction. Doses of this magnitude are of the order of 10⁻⁵ the amount used in field application, and so it is clear that 2,4-D vapour is a potential hazard to tomato and lettuce crops.     

Auxin-resistant mutants of Arabidopsis thaliana with an altered morphology

Summary Mutant lines of Arabidopsis thaliana resistant to the artificial auxin 2,4-dichloro phenoxyacetic acid (2,4-D) were isolated by screening for growth of seedlings in the presence of toxic levels of 2,4-D. Genetic analysis of these resistant lines indicated that 2,4-D resistance is due to a recessive mutation at a locus we have designated Axr-1. Mutant seedlings were resistant to approximately 50-fold higher concentrations of 2,4-D than wild-type and were also resistant to 8-fold higher concentrations of indole-3-acetic acid (IAA) than wild-type. Labelling studies with (¹⁴C)2,4-D suggest that resistance was not due to changes in uptake or metabolism of 2,4-D. In addition to auxin resistance the mutants have a distinct morphological phenotype including alterations of the roots, leaves, and flowers. Genetic evidence indicates that both auxin resistance and the morphological changes are due to the same mutation. Because of the pleiotropic morphological effects of these mutations the Axr-1 gene may code for a function involved in auxin action in all tissues of the plant.     

Effects of pH Changes on Ion and 2,4-D Uptake of Wheat Roots

The quantitative relationships between pH-dependent ion and 2,4-D uptake in winter wheat seedlings (Triticum aestivum L. cv. Yubileynaya 50) have been investigated. The movement of various ions (potassium, phosphate, nitrate and ammonium) and 2,4-D across the root membranes was monitored with radioactive and stable isotope tracer methods. It was found that the H₊ ion concentration of the absorption solution strongly influences the 2,4-D uptake of the roots. Simultaneously, the 2,4-D uptake stimulates secretion of H⁺ into the absorption solution, that is, a H⁺ efflux can accompany the uptake of 2,4-D. This finding is consistent with the acid secretion theory of auxin and fusicoccin action. At pH 4 the 2,4-D uptake was much higher than at pH 6, thereby inhibiting the ion uptake and increasing the phytotoxicity in the plant. The results indicate that 2,4-D enters the root cells rapidly at the lower pH, mostly as undissociated molecules. With reference to the 2,4-D concentration in the roots at pH 4, a possible transport mechanism of the auxin herbicide is briefly discussed.     

Action de l'acide 2,4 dichlorophenoxyacetique (2,4-D) sur la migration du42K fourni par voie foliaire chez la laitue et le maïs

The 2,4 dichlorophenoxyacetic acid (2,4-D) applied in solution (10μg per plant) on a leaf of lettuce plants (Lactuca sativa, cv. Grosse Blonde Paresseuse) decrease the translocation of⁴²K from the treated leaf, both in pretreatment and in simultaneous action if the interval of time is sufficiently long between the foliar applications. The metabolic action of 2,4-D seems to be located in the treated leaf. The results may be explained by a stimulation of the metabolic activity and consequently of the retention, already observed by several authors, with 2,4-D and other phytohormones. The 2,4-D had not a comparable effect on translocation of⁴²K from the leaves of maize plants (Zea mays Dekalb 202) even if the amount applied is higher and the duration of the pretreatment more important than previously. However, it must be noted for this species a light stimulation with 2,4-D on the retention of⁴²K in the sheath of the treated leaf. The results obtained with lettuce and maize might be fitted in a pattern of selectivity which appears for other aspects of the metabolism.     

2,4-D-Induced Changes in the K+ Uptake of Wheat Roots at Different pH Values

The effects of an auxin herbicide, 2,4-D, at a concentration of 0.01 mM, on the K⁺ uptake and efflux of excised roots of wheat (Triticum aestivum L. cv. Rannaya) were investigated at different pH values. The K⁺ movement was monitored with a K⁺ (⁸⁶Rb) tracer. In parallel experiments the ATPase activities of microsomal fractions were determined by the inorganic phosphate liberation method. 2,4-D inhibited the K⁺ uptake especially at low pH, irrespective of whether Ca²⁺ was present or not. No marked changes were observed in the K⁺ efflux properties at pH values above 4. The inhibitory effect on K⁺ uptake exhibited a correlation with the hydrocarbon solubility of the herbicide, but not with the 2,4-D-induced decrease of the ATPase activity. It is suggested that 2,4-D exerts a non-specific effect on the lipid-protein interactions, giving rise to a generalized alteration of the transport barrier properties of the plasma membrane even at as low a concentration as 0.01 mM.     

2,4-Dichlorophenoxyacetic Acid (2,4-D) Utilization by Delftia acidovorans MC1 at Alkaline pH and in the Presence of Dichlorprop is Improved by Introduction of the tfdK Gene

Growth of Delftia acidovorans MC1 on 2,4-dichlorophenoxyacetic acid (2,4-D) and on racemic 2-(2,4-dichlorophenoxy)propanoic acid ((RS)-2,4-DP) was studied in the perspective of an extension of the strain’s degradation capacity at alkaline pH. At pH 6.8 the strain grew on 2,4-D at a maximum rate (μ_max) of 0.158 h⁻¹. The half-maximum rate-associated substrate concentration (K_s) was 45 μM. At pH 8.5 μ_max was only 0.05 h⁻¹ and the substrate affinity was mucher lower than at pH 6.8. The initial attack of 2,4-D was not the limiting step at pH 8.5 as was seen from high dioxygenase activity in cells grown at this pH. High stationary 2,4-D concentrations and the fact that μ_max with dichlorprop was around 0.2 h⁻¹ at both pHs rather pointed at limited 2,4-D uptake at pH 8.5. Introduction of tfdK from D. acidovorans P4a by conjugation, coding for a 2,4-D-specific transporter resulted in improved growth on 2,4-D at pH 8.5 with μ_max of 0.147 h⁻¹ and K_s of 267 μM. Experiments with labeled substrates showed significantly enhanced 2,4-D uptake by the transconjugant TK62. This is taken as an indication of expression of the tfdK gene and proper function of the transporter. The uncoupler carbonylcyanide m-chlorophenylhydrazone (CCCP) reduced the influx of 2,4-D. At a concentration of 195 μM 2,4-D, the effect amounted to 90% and 50%, respectively, with TK62 and MC1. Cloning of tfdK also improved the utilization of 2,4-D in the presence of (RS)−2,4-DP. Simultaneous and almost complete degradation of both compounds occurred in TK62 up to D = 0.23 h⁻¹ at pH 6.8 and up to D = 0.2 h⁻¹ at pH 8.5. In contrast, MC1 left 2,4-D largely unutilized even at low dilution rates when growing on herbicide mixtures at pH 8.5.     

Effects of 2,4-D butyl vapour on the growth of six crop species

Six species of crop plants (tomato, lettuce, clover, cabbage, sunflower and field bean) were exposed to concentrations of 2,4-D butyl in the range 3–50 ng/litre, for 3.5 h, using an air-flow system. Symptoms of phytotoxicity were recorded during the following 4 wk, after which the plants were harvested. Sunflower and field bean were found to be most sensitive, with tomato, lettuce and cabbage less affected and clover least sensitive, on the basis of weight. Small tomato plants were more sensitive than ones approximately twice the weight. A 50% reduction in dry weight was caused by a 3.5h exposure to less than 3 ng/litre 2,4-D butyl for sunflower, compared with over 50 ng/litre for clover. The ranking of sensitivities did not agree with reports based on liquid doses of 2,4-D, especially for field bean, which was found to be more sensitive than previously thought.     

三种除草剂对五爪金龙的防除作用及2,4-D丁酯对环境的影响

用3种除草剂(2,4-D丁酯、麦草畏和塔隆)对五爪金龙(Ipomoea cairica L.Sweet)进行化学防除试验。结果表明:1.00 mL L-1的2,4-D丁酯可以彻底杀灭五爪金龙。喷施1.00 mL L-12,4-D丁酯20 d后,五爪金龙茎叶枯死率接近100%;60 d后五爪金龙的总生物量显著低于其它处理及对照;90 d后未出现生长恢复,最终盖度防效为99.8%。而喷施1.00 mL L-1的麦草畏40 d后,五爪金龙的茎叶枯死率为99.0%,但仍有少量存活的根,90 d后再次萌生率为10.0%;喷施1.00 mL L-1塔隆40 d后,五爪金龙的茎叶枯死率为100%,90 d后再次萌生率为100%。土壤残留分析表明:在有机质含量较高[(10.14±1.01)g kg-1]的土壤中2,4-D丁酯降解速率较快,半衰期为14 d,施药后80 d的土壤中已检测不到2,4-D丁酯。此外,在野外喷洒1.0 mL L-1的2,4-D丁酯对其它植物是安全的,施药1年后,样地内的植物均能恢复生长。因此,实践中可用1.00 mL L-1的2,4-D丁酯来防除五爪金龙。     

Does transpiration control stomatal responses to water vapour pressure deficit?

Three types of observations were used to test the hypothesis that the response of stomatal conductance to a change in vapour pressure deficit is controlled by whole-leaf transpiration rate or by feedback from leaf water potential. Varying the leaf water potential of a measured leaf by controlling the transpiration rate of other leaves on the plant did not affect the response of stomatal conductance to vapour pressure deficit in Glycine max. In three species, stomatal sensitivity to vapour pressure deficit was eliminated when measurements were made at near-zero carbon dioxide concentrations, despite the much higher transpiration rates of leaves at low carbon dioxide. In Abutilon theophrasti, increasing vapour pressure deficit sometimes resulted in both decreased stomatal conductance and a lower transpiration rate even though the response of assimilation rate to the calculated substomatal carbon dioxide concentration indicated that there was no ‘patchy’ stomatal closure at high vapour pressure deficit in this case. These results are not consistent with stomatal closure at high vapour pressure deficit caused by increased whole-leaf transpiration rate or by lower leaf water potential. The lack of response of conductance to vapour pressure deficit in carbon dioxide-free air suggests that abscisic acid may mediate the response.     

Influence of Two Phenoxy Growth Regulators on the Uptake and Accumulation of Naptalam by Bean Plants

Bean plants (Phaseolus vulgaris L. cv. Black Valentine) treated with 2,4-dichlorophenoxyacetic acid (2,4-D) and 4-chlorophenoxyacetic acid (PCPA) absorb and accumulate considerably more N-1-naphthylphthalamic acid (naptalam) than untreated plants. All concentrations of 2,4-D from 5 x 10^-8 to 5 x 10^-4M were effective, peak stimulation occurring at 3 x 10^-5M. Plants treated with this concentration took up 186% more naptalam than control plants. It was shown that the leaf area (on a per g dry weight basis) was influenced most by growth regulator treatment. The leaf area of plants treated with 5 x 10^-5M 2,4-D contained 575% more naptalam than the leaf area of untreated plants. The influence of PCPA on naptalam uptake by bean plants was similar to that of 2,4-D but less effective.     

镍离子对尖叶莴苣氮素吸收和生长生理的影响

以‘全年油麦菜’尖叶莴苣为试验材料,采用水培方式,研究3个浓度(0 mg·L^-1、0.1 mg·L^-1、1 mg·L^-1)Ni²⁺在22.4 mg·L^-1 N处理下对尖叶莴苣氮素吸收的生长及生理影响。结果显示：(1)尖叶莴苣根系和地上部生物量随处理时间的增加呈上升趋势。与对照T₁(0 mg·L^-1 Ni²⁺、112 mg·L^-1 N)相比,T₂处理(0 mg·L^-1 Ni²⁺、22.4 mg·L^-1 N)对尖叶莴苣根系及叶片生长具有一定抑制作用,植株鲜重、干重、根冠比、根系长度、平均直径、表面积、体积、根尖数、分根数、叶片表面积和体积在T3处理(0.1 mg·L^-1 Ni²⁺、22.4 mg·L^-1 N)下显著高于对照,T₄处理(1 mg·L^-1 Ni²⁺、22.4 mg·L^-1 N)对尖叶莴苣根系及其叶片生长具有一定促进作用,但对其根尖数和分根数表现出一定抑制性。(2)随着Ni²⁺浓度的增加,尖叶莴苣叶片叶绿素a、叶绿素b和总叶绿素含量呈先升后降的变化规律,且均在T₃处理下显著提高。(3)随着处理时间的增加,尖叶莴苣叶片的净光合速率(P_n)、气孔导度(G_s)和蒸腾速率(T_r)逐渐上升,胞间CO₂浓度(C_i)逐渐下降,且T₃处理叶片的G_s显著高于对照,其C_i最低,P_n最大。(4)施加Ni²⁺对尖叶莴苣有机酸、可溶性蛋白和可溶性糖含量以及SOD和POD活性有显著影响,在T₃处理下有机酸含量降低,可溶性糖和可溶性蛋白含量显著增加,SOD和POD活性显著提高。(5)T₃处理尖叶莴苣根系中N及叶片中B和Ca含量较高;根系中Ni含量高于叶片,T₃处理叶片中的Ni含量较低,Mg含量较高;植株体内Cu含量随Ni²⁺浓度增加而下降。研究表明,外源Ni²⁺处理能影响低氮条件下(22.4 mg·L^-1 N)尖叶莴苣幼苗生长及生理状况,适宜浓度(0.1 mg·L^-1)Ni²⁺可有效提高尖叶莴苣根系对氮素的吸收利用效率,减少氮素施用量,促进尖叶莴苣根系和地上部叶片生长,增加光合色素含量,并提高净光合速率,进而改善植株的产量和营养品质。     

Translocation and Complex Formation of Picloram and 2,4-D in Rape and Sunflower

Uptake, translocation and complex formation of ¹⁴C-labelled 4-amino-3,5,6-trichloropicolinic acid (picloram) and 2,4-dichlorophenoxyacetic acid (2,4-D) in seedlings of rape (Brassica napus L. cv. Nilla) and sunflower (Helianthus annuus L. var. uniflorus) were studied. Sunflower is susceptible both to 2,4-D and picloram, while rape is susceptible to 2,4-D but more tolerant to picloram. The uptake of the herbicides through the leaves was almost complete in both species. Translocation of 2,4-D into the roots took place more readily than that of picloram. In sunflower about 50 per cent of the applied 2,4-D was extruded through the roots into the nutrient solution after 9 days. In the picloram-treated sunflower most of the activity was found in the aerial parts, while in picloram-treated rape most of the activity still occurred in the treated leaf after 9 days. No activity at all was found in the roots or in the nutrient solution of the picloram-treated rape seedlings. While the major part of 2,4-D always was found in the state of free herbicide, a large fraction of picloram was rapidly bound into water-soluble complexes. This binding was especially pronounced in rape. Separation by paper chromatography showed that different radioactive compounds were formed. Most of these could be hydrolyzed, thereby releasing free herbicide. The results support the hypotheses that complex formation could counteract herbicide translocation and toxicity of auxin herbicides.     

Translocation and Complex Formation of Picloram and 2,4-D in Wheat Seedlings

Uptake, translocation and metabolism of ¹⁴C-labelled 4-amino-3,5,6-trichloropicolinic acid (picloram) and 2,4-dichlorophenoxyacetic acid (2,4-D) in seedlings of wheat (Triticum aestivum L.) were studied. The uptake of the herbicides through the upper surface of the first leaf was slow but was almost complete after nine days. Picloram was absorbed faster than 2,4-D. Picloram was also translocated into the stem and the untreated leaves to a greater extent than 2,4-D. Only small fractions of the activity were recovered from the roots and from the nutrient solution. Picloram and 2,4-D formed water-soluble conjugates in the tissues. These conjugates were very labile and hydrolyzed under release of the unchanged herbicides. The isotope from 2,4-D was also incorporated in an insoluble fraction, containing cell walls and proteins. Also from this fraction biologically active 2,4-D could be released by hydrolysis. The formation of the complexes was partly prevented by cycloheximide. It is suggested that herbicide detoxification through complex formation is of importance for the relatively low sensitivity of wheat to auxin herbicides.     

Investigations on the causes of low susceptibility of scentless mayweed to 2,4-D

White mustard (Sinapis alba L.) plants, susceptible to 2,4-D, and scentless mayweed (Tripleurospermum maritimum L.) plants, resistant to 2,4-D, markedly differ in the distribution and metabolism of 2,4-D. When 2,4-D-¹⁴C was applied onto the leaf, radioactivity was found in mustard after 8 days in the entire plant, whereas in scentless mayweed radioactivity occurred mainly in the leaf onto which it was applied and in another one or two leaves, with the roots showing only traces of radioactivity. The two plant species differed both in the character and in the number and amount of metabolites detected in aqueous and ether fractions. The metabolite with R_f 0.63 -0.65, soluble in ethylether (39% of the total radioactivity applied), and that with R_f 0.35 -0.36 (23% of the total radioactivity) were present exclusively in the roots of scentless mayweed plants. The R_f values established indicate that the former metabolite may be a conjugate with the amino acids alanine and valine and the latter a hydroxylated derivate of 2,4-D. In the shoots of both plants, 2,4-D-¹⁴C was metabolized to identical metabolites with R_f 0.59 -0.61, which occurred in mustard plants only in trace amounts. Free 2,4-D-¹⁴C occurred in the shoots of both plants and in considerable amounts also in mustard roots; it could not be demonstrated in the roots of scentless mayweed. The two species did not differ in the uptake rate or in the amount of absorbed 2,4-D-¹⁴C.     

Effect of Gibberellin and Auxin on Parthenocarpic Fruit Growth Induction in the cv Micro-Tom of Tomato

The effect of applied gibberellin (GA) and auxin on fruit-set and growth has been investigated in tomato (Solanum lycopersicum L.) cv Micro-Tom. It was found that to prevent competition between developing fruits only one fruit per truss should be left on the plant. Unpollinated ovaries responded to GA₃ and to different auxins [indol-3-acetic acid, naphthaleneacetic acid, and 2,4-dichlorophenoxyacetic acid (2,4-D)], 2,4-D being the most efficient. GA₃- and 2,4-D-induced fruits had different internal morphology, with poor locular tissue development in the case of GA, and pseudoembryos development in the case of 2,4-D. Also, GA₃ produced larger cells in the internal region of the mesocarp (IM) associated with higher mean C values, whereas 2,4-D produced more cell layers in the pericarp than pollinated fruits. The smaller size of GA₃- compared with 2,4-D-induced fruits was due to them having fewer cells, only partially compensated by the larger size of IM cells. Simultaneous application of GA₃ and 2,4-D produced parthenocarpic fruits similar to pollinated fruits, but for the absence of seeds, suggesting that both kinds of hormones are involved in the induction of fruit development upon pollination. It is concluded that Micro-Tom constitutes a convenient model system, compared to tall cultivars, to investigate the hormonal regulation of fruit development in tomato.     

Effect of 2,4-Dichlorophenoxyacetic Acid and NaCl on the Establishment of Callus and Plant Regeneration in Durum and Bread Wheat

Optimal callus induction and plant regeneration were obtained in bread and durum wheat by manipulating the NaCl concentration in the induction medium. Immature embryos from a high regeneration line of spring wheat (Triticum aestivum L.), 'MPB-Bobwhite 26', and an elite durum wheat (Triticum turgidum var. durum L.), 'Mexicali', were cultured in E3 induction medium consisting of Murashige and Skoog (MS) medium, 2.5 mg l^–1 2,4-dichlorophenoxyacetic acid (2,4-D), 2% sucrose and 0.9% Bacto agar. The treated embryos were transferred to E3 liquid medium supplemented with various levels of 2,4-D and NaCl. Incubation on medium containing 2.5 mg l^–1 2,4-D for 45 days produced callus and plant regeneration in 'MPB-Bobwhite 26', but lower callus yield and plant regeneration in 'Mexicali', indicating that 2,4-D alone was not sufficient for callus induction and plant regeneration in this durum variety. Callus yield and regeneration frequencies were higher in 'Mexicali' embryos that were incubated in media containing 2 mg l^–1 2,4-D and 2 mg l^–1 NaCl. The presence of NaCl in the medium beyond the initiation phase was detrimental to plant regeneration. The use of NaCl in the callus formation could form the basis for improved transformation of durum wheat varieties.     

Auxin-stimulated somatic embryogenesis from immature cotyledons of white clover

Cotyledons from immature embryos of white clover (Trifolium repens L.) cv. Osceola were exposed to 2,4-D or NAA to induce somatic embryogenesis. NAA at 10 or 20 mg 1^–1 was very inefficient at stimulating embryogenesis, while concentrations of 30 or 40 mg 1^–1 resulted in death of the explant tissue. Continuous exposure of cotyledons to 40 mg 1^–1 2,4-D resulted in somatic embryos which were arrested at the globular stage, or which underwent cycles of secondary embryogenesis, never proceeding beyond the globular stage. A 10 day exposure time to 2,4-D at the same concentration led to formation of somatic embryos, most of which had poorly developed cotyledons. Almost 10% of the somatic embryos converted into plants following transfer to medium devoid of growth regulators. Attempts to improve morphology of somatic embryos by using shorter exposure times to 2,4-D at 40 mg 1^–1, or by maintaining the 10 day exposure time while varying the concentration of 2,4-D, were not successful. Plants were obtained from all parents evaluated, although at different frequencies.     

Morpho-histological study of direct somatic embryogenesis in endangered species Frittilaria meleagris

Direct somatic embryogenesis of Frittilaria meleagris L. was induced using leaf base explants excised from in vitro grown shoots. Somatic embryos occurred at the basal part of leaf explants 4 weeks after culture on a Murashige and Skoog (MS) medium supplemented with various concentrations of 2,4-dichlorophenoxyacetic acid (2,4-D) or kinetin (KIN). The highest number of somatic embryos (SEs) were formed (9.74) from leaf explant on MS medium supplemented with 0.1 mg dm⁻³ 2,4-D after 4 weeks of culture initiation. An initial exposure to a low concentration of KIN in the medium also enhanced SEs induction. Our observations by light and scanning electron microscopy revealed that SEs originate directly from the epidermal and subepidermal layers of leaf explant. The developmental stages of somatic embryogenesis from the first unequal cell division through the meristematic clusters, multi-cellular globular somatic embryos to the fully formed cotyledonary embryos were determined. After 4 weeks on MS medium without plant growth regulators, SEs developed into bulblets.     

Transport of the Auxin 2,4-Dichlorophenoxyacetic Acid Through Absiccion Zones, Pulvini, and Petioles of Phaseolus vulgaris

Measurements were made of the transport of 2,4-dichlorophenoxyacetic acid-¹⁴C (2,4-D) through segments cut from the region of the distal abscission zone in young and old primary leaves of Phaseolus vulgaris L. When old leaves were used basipetal transport of 2,4-D in segments including pulvinar tissue, abscission zone, and petiolar tissue was much less than in wholly petiolar segments. In both young and old plants, segments consisting entirely of pulvinar tissue transported 2,4-D basipetally at a velocity about half that in petiolar tissue. At both ages the flux of 2,4-D through pulvinar tissue was less than that through petiolar tissue. In segments from old leaves the flux through pulvinar tissue was much less than in young plants; the flux through petiolar tissue changed little with age. There was no change with age in the velocity of basipetal transport. The distribution of ¹⁴C along segments including the abscission zone showed no marked discontinuity. It was concluded that the pulvinus limited the basipetal movement of 2,4-D through segments from old leaves which included both pulvinar and petiolar tissue, but there was no evidence that the abscission zone itself was a barrier to auxin transport.