Surfactants and the uptake and movement of paraquat in plants

Surfactants are still considered to be agents which either increase spray coverage of leaves with herbicidal solutions and/or increase herbicide penetration into the leaves. Experiments where the method of application largely eliminates leaf wetting as a factor in paraquat uptake show that this is an over-simplification. Its efficiency in the plant is influenced by penetration and most of all by the degree of movement down the plant into untreated leaves. Results on the uptake, movement and biological activity of paraquat are reported using cocksfoot and wheat. A relation is found between paraquat movement and the hydrophilic nature of the surfactant. It moves most when the number of ethylene oxide residues is less than six and it is minimal when the number is 10 to 15. Leaf penetration, however, is at a maximum when movement under the influence of surfactant is least. Partition studies in which surfactants are distributed between leaf wax and water are described. There is a direct correlation between the degree of partition of the surfactant into the wax and the degree of movement of paraquat in cocksfoot and wheat. Surfactants are essential components of a paraquat formulation to wet the leaf surface and increase penetration but, when the surfactant also penetrates into the leaf, it reduces the mobility of paraquat and hence its efficiency.     

Uptake and movement of paraquat in cocksfoot and wheat as influenced by surfactants

Surfactants are used to increase the efficiency of herbicide formulations mainly because they wet out leaf surfaces, thereby stabilising and increasing the contact area of droplets on the surface. Herbicide penetration through the cuticle may also be facilitated. The work described eliminates effects on wetting and contact area in order to study the effect of surfactants on the penetration and movement of paraquat in cocksfoot. Surfactants were various types of alcohols and amine oxides condensed with 2 to 30 moles of ethylene oxide used at 0.1 to 0.5%. An adult leaf of cocksfoot (Dactylis glomerata) was immersed briefly to constant area in paraquat solutions containing surfactant and uptake and movement of paraquat is recorded. Uptake was little affected by differences in surfactant structure except where surface activity was low and solutions failed to wet out the leaf surface. Percentage movement with 0.5% surfactant was often less than that with 0.1% and a high ethylene oxide content also reduced percentage movement. Paraquat activity was influenced by both the degree of uptake and movement, but movement was the greater influence. Amine oxide surfactants reduced movement less than those based on alcohols. The action of surfactants is discussed in terms of a hydrophobic/hydrophilic balance in the surfactant molecule.     

Studies on octylphenoxy surfactants. Part 2: Effects on foliar uptake and translocation

The effects of octylphenol (OP) and four of its ethoxylated derivatives on uptake into, and distribution within, maize leaf of 2-deoxy-glucose (2D-glucose), atrazine and o, p′-DDT are reported. The surfactants and OP (2 g litre^?1 in aqueous acetone) increased the uptake, at both 1.5 and 24 h, of the three model compounds (applied at 1 g litre^?1) having water solubilities in the g, mg and μg litre^?1 ranges. The uptake of 2D-glucose was positively correlated with the hygroscopicity of the surfactants. The uptake of DDT and atrazine increased with the uptake of the surfactants, being inversely related to their hydrophile:lipophile balance (HLB). Uptake of 2D-glucose and atrazine was enhanced at high humidity, the relative enhancement for atrazine increasing with increasing ethylene oxide (EO) content of the surfactants. A significant proportion of the atrazine and DDT entering the leaf was recovered from the epicuticular wax, the amount of atrazine recovered from the wax increasing with the EO content of the surfactants. The proportion of the surfactants taken up which was recovered from the epicuticular wax was minimal at an EO content of 12.5–16 mole equivalents. The appearance of the deposits on the leaf surface differed markedly among the surfactants, with similar trends for all three chemicals and without visible evidence for infiltration of the stomatal pores. The total quantities of glucose and atrazine translocated were increased by all surfactants but that of DDT was not, despite increases in uptake of up to 7.5-fold. Relative translocation (export from treated region of leaf as a percentage of chemical penetrating beyond the epicuticular wax) was reduced in all cases in the presence of surfactant. Up to 30% of the applied [¹⁴C]chemicals was not recovered from the treated leaf after 24 h. The reduced recovery of 2D-glucose, but not that of atrazine and DDT, was largely attributable to movement out of the treated leaf, with approximately 70% of the chemical taken up being translocated basipetally. Loss of atrazine and DDT was a result of volatilisation. There was no evidence that either [14C]2 D-glucose or [¹⁴C]atrazine was metabolised to [¹⁴C]carbon dioxide.     

Analysis of foliar uptake of pesticides in barley leaves: Role of epicuticular waxes and compartmentation

Uptake of pesticides into barley leaves was measured under controlled conditions. Leaves detached from plants were submerged in aqueous solutions of ¹⁴C-labelled (2,4-dichlorophenoxy)acetic acid, triadimenol, bitertanol and pentachlorophenol. Uptake was biphasic. A short (30-min) period with high rates of uptake was followed by uptake that proceeded more slowly and was steady over hours. Compartmentation of pesticides was studied by desorbing pentachlorophenol from leaves previously loaded with [¹⁴C]pentachlorophenol. From the uptake and desorption kinetics it was concluded that penetration of pesticides proceeds as follows: the compounds are first sorbed at the surface of epicuticular wax aggregates where they are in contact with the donor solutions. Solutes then diffuse through the surface wax aggregates into the cuticle. Equilibrium between donor solutions, surface wax and cuticle is established in about 30 min. After this time the amounts of solutes in these compartments no longer increase. Uptake after this time represents penetration into the leaf cells. This fraction of the pentachlorophenol is retained irreversibly, while that sorbed in wax and cutin can be desorbed again. All compounds were sorbed in cuticular waxes and partition coefficients wax/water were determined. On a mass basis only 5 to 10% of the amounts sorbed in cutin are sorbed in wax. This comparatively low solubility in wax contributes to the barrier properties of cuticular waxes. The other determinant of permeability is the very low mobility of solutes in cuticular waxes.     

Metabolism, uptake and translocation of 14C-paraquat in resistant and susceptible biotypes of Crassocephalum crepidioides (Benth.) S. Moore

The metabolism, uptake and translocation of paraquat in resistant (R) and susceptible (S) biotypes of Crassocephalum crepidioides (Benth.) S. Moore (redflower ragleaf) at the 10-leaf stage was investigated. A study on the properties of leaf surface was carried out to examine the relationship between leaf surface characters and paraquat absorption. The extractable paraquat was not metabolized by the leaf tissue of either the resistant or susceptible biotypes. Differential metabolism, therefore, does not appear to play a role in the mechanism of resistance. Both biotypes did not show any significant difference in the amount of cuticle and trichome densities. Furthermore, both biotypes are identical in the structure of stomata, trichomes and epicuticular wax. The results of the leaf surface studies are in agreement with the findings of the uptake study. Both biotypes demonstrated no significant difference in absorption between the resistant and susceptible biotypes. However, 10% of the absorbed ¹⁴C-paraquat into the S biotype was translocated basipetally, but not in the R biotype. The results of this study suggest that in C. crepidioides , differential translocation may contribute to the mechanism of resistance at the 10-leaf stage.     

Penetration of bean leaves by asulam as influenced by adjuvants and humidity

Penetration of bean (Vicia faba var. Maris Bead) foliage by asulam was found to be affected differently by different surfactants. Some anionic and non-ionic surfactants brought about a significant increase compared with the aqueous control, others were similar to the aqueous control and one caused a significant reduction. High humidity increased asulam uptake and the effect was still further enhanced in the presence of Tween 20. High humidity was also found to influence asulam penetration in the presence of glycerol. With urea, uptake was greater under high humidity conditions and in the presence of Tween 20. A similar enhancement of penetration was found with potassium ethyl xanthate at lower concentrations (0–0.20%) in the presence of Tween 20. On the other hand, reduced uptake resulted at higher concentrations (0.40–1.00%). Tributyl phosphate brought about an increase in penetration at concentrations greater than 0.75%. However, both potassium ethyl xanthate and tributyl phosphate were shown to damage the leaf surface.     

Structural and physiological studies of paraquat-resistant Conyza

Structural and physiological studies were conducted with a population of Conyza bonariensis (L.) Cronq. that segregates into paraquat-resistant and -susceptible biotypes. Leaf disks from resistant seedlings, when incubated on 10 μM paraquat for 24 hr, exhibited little difference from the control disks incubated on H₂O as measured by conductivity change, malondialdehyde formation, or plastid ultrastructure. Leaf disks from the susceptible seedlings incubated on 10^?5M paraquat for 24 hr were uniformly bleached, had elevated malondialdehyde content, and leaked more electrolytes than control disks. Plastids of the susceptible biotype incubated on 10^?5M paraquat for 24 hr were swollen organelles with gross rearrangements of the lamella system. Most of the chloroplasts from the central area of the leaf disk of the resistant biotype incubated on a paraquat solution were structurally normal. Swollen plastids and plastids with twisted lamellae were also noted, although much less frequently. Plastids from the edges of the leaf disks of paraquat-resistant clones were structurally similar to those found throughout the leaf disks in susceptible seedlings. When the size of the leaf disk was increased, paraquat-resistant clones exhibited more “resistance” toward paraquat compared to similar-sized leaf disks of the susceptible seedlings. These data are consistent with the hypothesis that the paraquat-resistant seedlings have an altered uptake and/or compartmentalization of paraquat. Superoxide dismutase isozymes, which were previously considered to be related to paraquat resistance in Conyza, did not correlate with the segregation of paraquat resistance in this population.     

Effect of light environment on the activity and behaviour of diquat and paraquat in plants

The importance of controlling the light environment of experiments involving diquat and paraquat is shown in experiments where activity is markedly dependent on the light quality and intensity before treatment and on the time of day of treatment. Activity and uptake are not directly related since treatments after low light intensities give increased activity associated with reduced uptake; following afternoon treatments, reduced activity is associated with increased uptake. Uptake increased when plants were darkened after treatment but the increase was not directly related to its duration, because after a time, uptake decreased. Three possible explanations for this decrease are considered: diquat exudation from the leaves, downward movement into the roots, and the adsorption of diquat in plant tissue. Evidence did not support exudation from leaves or downward movement into the roots.     

Effects of nonionic surfactants on cuticular sorption and penetration of 2, 4-dichlorophenoxy acetic acid

Surfactants increase the uptake of some foliar-applied chemicals to a greater extent than would be expected from their effects on surface tension and spray coverage. This study of the uptake of 2, 4-D [(2, 4-dichlorophenoxy)acetic acid] evaluated the effect of surfactants on penetration through and sorption by isolated cuticles of apple leaves. [¹⁴C]2, 4-D was placed in glass chambers affixed to enzymatically isolated adaxial apple leaf cuticles after the cuticle segments had been treated with various surfactants. The same surfactant pretreatments were included in sorption studies in which cuticle segments were immersed in [¹⁴C]2, 4-D for 96 h. Quantities of 2, 4-D passing through or sorbed by the cuticle were determined. Similar experiments were conducted with unaltered cuticles and cuticles dewaxed with chloroform. The hydrophile-lipophile balance (HLB) of polyethylene-glycol-based surfactants was inversely related to the sorption of those surfactants by the cuticles and penetration of 2, 4-D. Sorption of 2, 4-D by apple leaf cuticles was unaffected by surfactant pretreatment. Dewaxed cuticle membranes showed a similar response to 2, 4-D penetration and sorption following the surfactant pretreatment.     

Effects of some nonionic polyoxyethylene surfactants on uptake of ethirimol and diclobutrazol from suspension formulations applied to wheat leaves

The influence of a number of commercial nonionic polyoxyethylene surfactants on the foliar penetration and movement of two systemic fungicides, ethirimol and diclobutrazol, was studied in outdoor-grown wheat plants at different growth stages and post-treatment temperatures in two consecutive growing seasons. Both fungicides were applied as ca 0·2 μl droplets of aqueous suspension formulations containing 0·5 g litre^?1 of ¹⁴C-labelled active ingredient; surfactants were added to these suspensions at concentrations ranging from 0·2-10 g litre^?1. To achieve optimum uptake of each fungicide the use of surfactants with different physicochemical properties was required. For diclobutrazol, a lipophilic compound, uptake of radiolabel was best with surfactants of low mean molar ethylene oxide (E) content (5-6) but it was necessary to use concentrations of ca 5 g litre^?1 to attain this. The surfactant threshold concentration for uptake enhancement of radiolabel from ethirimol formulations (< 2 g litre^?1) was much lower than that for diclobutrazol but surfactants with E contents > 10 induced the greatest amount of uptake. For both fungicides, surfactants with an aliphatic alcohol hydrophobe were generally more efficient in promoting their uptake than those with a nonylphenol moiety. The sorbitan-based surfactant ‘Tween 20’ proved to be an effective adjuvant only for the ethirimol formulation; the uptake enhancing properties of the block copolymer ‘Synperonic PE/F68’ were weak. Uptake performance could not be related to the spreading properties of the respective formulations on the wheat leaf surface or to differences in solubilisation of the two fungicides by the surfactants. Although surfactants could substantially increase the amount of acropetal transport of radiolabel from both fungicides, none of those tested specifically promoted it; a constant proportion of the radioactive dose absorbed by a treated leaf was usually exported away from the site of application. The results are discussed in the light of current theories about the mode of action of surfactants as spray adjuvants.     

Contributions of stomatal infiltration and cuticular penetration to enhancements of foliar uptake by surfactants

Radiolabelled deoxyglucose (DOG) and glyphosate were used to investigate the effects of certain non-ionic surfactants on the kinetics of foliar uptake in three species. ‘Silwet L-77’ (5 g litre^?1), an organosilicone surfactant, enabled spray solutions to infiltrate stomata, providing uptake of DOG into Vicia bean (50%), oat (35%) and wheat (20%) within 10 min of application. ‘Silwet Y-12301’, another organosilicone, also induced stomatal infiltration but to a lesser extent; unlike L-77, this was attenuated by partial stomatal closure. A third organosilicone, ‘Silwet L-7607’, and two conventional surfactants, ‘Triton X-45’ (OP5) and ‘Agral 90’ (NP9), did not induce stomatal infiltration. The effective minimum concentration of L-77 required to enable infiltration of stomata was 2 g litre^?1. The uptake of glyphosate into bean did not differ from that of DOG but the ‘Roundup’ formulation of glyphosate partially antagonised the infiltration provided by L-77. Addition of surfactants did not increase the rate of cuticular penetration of DOG into bean but total uptake was increased, except by NP9, either via infiltration (L-77 and Y-12301) or by extending the period during which penetration occurred (L-7607 and OP5). The surfactants had a variable effect on rates of penetration of DOG into wheat and oat. In general, foliar uptake followed an exponential timecourse which was largely complete within 6 h and only rarely approached 100% of the applied chemical. The stomatal infiltration provided by L-77 caused an increase in translocation of DOG in bean.     

Influence of cuticular waxes on penetration of pear leaf cuticle by 1-naphthaleneacetic acid

Removal of waxes from the upper pear leaf cuticle by chloroform extraction greatly increased sorption of naphthaleneacetic acid (NAA) into the cuticle. Penetration of NAA through isolated upper pear leaf cuticle was likewise increased following wax removal by chloroform extraction. Plating pear leaf wax back onto dewaxed cuticle surfaces reduced sorption and penetration.     

Tolerance to paraquat-mediated oxidative and environmental stresses in squash (Cucurbita spp.) leaves of various ages

Paraquat is labeled for row-middle application on cucurbits, but drift to crop foliage is inevitable. Experiments were conducted to determine whether differential tolerance to paraquat existed among leaves of various ages in Cucurbita spp. (squash) and other plants, and to examine whether leaves tolerant to paraquat are also tolerant to other herbicides and abiotic stresses. Physiological responses to paraquat, including antioxidant activity, were investigated in squash leaves to identify mechanisms of paraquat tolerance. Although the level of paraquat tolerance differed by leaf age, cultivar, and species, the level of paraquat injury was lower in younger leaves than in older leaves in 14 of 18 squash cultivars and 5 of 12 other species tested. Cellular leakage and lipid peroxidation were consistently lower in the youngest leaf (leaf 4) than in the older leaves. Quantum yield and relative chlorophyll content were the same in all leaves of nontreated plants. Epicuticular wax content was higher in the youngest leaf than in leaves 1, 2, and 3 of cv. ‘Joongangaehobak’ and ‘Wonbiaehobak’. However, leaf cuticle content was not consistent with leaf ages. Differential leaf response to paraquat was partially correlated with the change in catalase, peroxidase, ascorbate peroxidase (APX), and glutathione reductase activities in nontreated and treated leaves. The APX activity in the youngest leaf was generally 2 times higher than in leaves 1-3 in both nontreated and treated plants. Ascorbate antioxidant levels were also higher in the youngest leaf than those in leaves 1-3. Leaves tolerant to paraquat were also tolerant to diquat and to abiotic stresses, low temperature and drought. However, tolerance to oxyfluorfen, which has a different mode of action than paraquat and diquat, was higher in older than in younger leaves. Higher tolerance to paraquat-mediated oxidative and abiotic stresses in young leaves of most squash cultivars might contribute to the differential prevention of oxidative damage in leaves of various ages.     

Status of biological control of the weed,Lantana camara in India

Abstract

A glasshouse study was conducted to determine the effect of shade on the uptake, translocation and activity of paraquat on Mikania micrantha H.B.K. The uptake, translocation and activity of paraquat were significantly enhanced at low light intensity. Placement of plants grown initially under 75% shade and transferred to 0% shade resulted in significantly higher uptake of paraquat than those initially grown under 0% and 50% shade. The uptake and activity of paraquat were also significantly increased when treated plants were placed in darkness. The rate of efflux of paraquat from leaf segments of plants grown in high light intensity was considerably slower than from plants grown in low light intensity.     

Polydisperse ethoxylated fatty alcohol surfactants as accelerators of cuticular penetration. 2: Separation of effects on driving force and mobility and reversibility of surfactant action

Polydisperse ethoxylated fatty alcohol (EFA) surfactants can improve the performance of crop protection agents. At the cuticular level they act as accelerators of penetration by increasing the mobility of active ingredients in the cuticle, the barrier properties of which are mainly caused by cuticular waxes. Polydisperse Genapol C-050 (GP C-050, average formula C12.5E5.8) was also found to increase mobility in wax-extracted polymer matrix membranes (MX) of bitter orange and pear, indicating that sorption of surfactants increased segmental mobility of polymethylene chains in cutin and wax. Sorption into MX of the active fraction of GP C-050 from 5g litre⁻¹ micellar solutions was in equilibrium in less than 1 h after establishing contact. This is almost 100-fold faster than with cuticular membranes (CM). Temperature dependence of solute mobilities in CM was studied in order to measure activation energies (E_D) of diffusion in the presence and absence of aqueous surfactant solutions. Monodisperse fatty alcohol ethoxylates C8E3, C8E4 and C12E6, and (non-surface-active) tributylphosphate decreased E_D of the model compounds WL 110547 and bifenox in Citrus, Pyrus and Stephanotis CM by more than 100 kJ mol⁻¹. This corresponds to 50 to 275-fold increases of mobilities at 15 °C. Our data suggest that the decrease in activation energies with the concomitant accelerating effect on mobility contributes considerably to the effects of so-called activator surfactants. High temperature and accelerators act similarly on barrier properties of CM. It is shown that effects of both monodisperse and polydisperse EFA surfactants on solute mobility are reversible and that radiolabelled C12E8 penetrated pear CM rapidly. However, rates of penetration were lowered by excess amounts of WL 110547 and especially phenylurea. Partition coefficients of seven organic solutes between Capsicum fruit cuticles and GP C-050 were very low and, with the exception of methylglucose, smaller than 1. They decreased with lipophilicity and differed about 100-fold. Especially for the lipophilic compounds they were orders of magnitude lower than octanol/water or cuticle/water partition coefficients, which indicates the limited usefulness of these values for an understanding of penetration of active ingredients from formulation residues. © 1999 Society of Chemical Industry     

THE MOVEMENT OF PARAQUAT IN PLANTS

Summary. A qualitative study of the movement of the herbicide paraquat from droplets applied to leaves of tomato plants, using ¹⁴C-methyl-labelled and ¹⁴C-ring-labelled paraquat dichloride and di(methylsulphate), has shown that it moves in the xylem with the transpiration stream. The chemical is as well transported from young leaves as from mature ones, and will move through a steam-ringed petiole. The enhancement of the amount of paraquat transported from the treated leaves which occurs when treated plants are kept in darkness for a period following treatment and then exposed to light, is probably due to the greater movement into the xylem through undamaged tissue which can occur in the dark. Once the chemical has been absorbed into treated leaves, light-induced damage is required for significant movement through the rest of the plant to take place, but the damage then inhibits further entry of paraquat into the xylem. The movement of paraquat in broad bean and maize is essentially similar, though the enhancement of movement by a period of darkness after application is much less marked.
La migration du paraquat dans les plantes     

核盘菌侵入油菜超微结构及侵染机制的研究

 通过电子显微镜观察核盘菌在油菜叶片上侵染过程,发现该菌首先在叶片上形成复合附着器。每个分枝末端一般生出一个侵染钉。侵染钉侵入叶表面腊质、角质层和表皮细胞壁时.不仅靠附着器产生的压力,而且供助于酶对寄主表面的软化、消解作用。该菌通过角质层和表皮细胞壁侵入油菜叶片,尚未发现通过气孔侵入的现象。侵入叶片后,该菌的继续生长,导致了油菜组织的溃烂。然后菌丝在腐烂的叶片上集结形成菌核。     

Polyamines can inhibit paraquat toxicity and translocation in the broadleaf weed Arctotheca calendula

Reduced paraquat transport from the site of application to the site of action in the chloroplast seems a likely mechanism for paraquat resistance in several weed species including Arctotheca calendula. Recently, it has been shown that paraquat translocation in A. calendula is correlated with paraquat-induced injury and is reduced in paraquat-resistant A. calendula. Studies with leaf slices have shown that some polyamines when applied concomitantly with paraquat can reduce the toxic effects of paraquat. This study examined the effects of three polyamines, putrescine, cadaverine, and spermidine, on paraquat translocation to examine the possibility that paraquat translocation in susceptible plants would be reduced in the presence of polyamines due to competition of the polyamines with cellular paraquat uptake. Two polyamines, spermidine and cadaverine were effective in reducing paraquat translocation in susceptible A. calendula inducing these plants to perform more like resistant A. calendula in terms of translocation. Quantification of the polyamine contents of resistant and susceptible A. calendula showed that resistant plants have higher constitutive spermidine levels than susceptible plants, which infers a possible role of either polyamines or a polyamine transporter in paraquat resistance.     

THE BIPYRIDYLIUM QUATERNARY SALTS

Summary. In a number of plant species, the biological activity of diquat and paraquat was increased by an increase in environmental humidity. This improved activity resulted from an increase in both uptake and movement.
High humidity was more effective after treatment than before it, and durations of 8 hr or more were required to produce the maximum effect. When periods of low humidity of up to 16 hr were interposed between treatment and high humidity, there was no significant effect on the activity of diquat in darkened tomato or sugar beet.
The increase in activity occurred both in the dark and in the light, and it is therefore concluded that humidity does not exert its effect by modifying the degree of stomatal opening.
Experiments were carried out with wheat using two air humidities combined with different soil moisture contents, ranging from saturated down to only 30% of water-holding capacity. Greatest movement occurred where high air humidity was combined with low soil moisture, and least where low air humidity was combined with high soil moisture. It is concluded that diquat and paraquat would be most effective in the field when sprayed under dry soil conditions in late afternoon or evening when increased humidity (and darkness) can follow soon after treatment.
Les sels quatemaires de bipyridylium
Effet de l'humidité atmosphérique et de l'humidité du sol sur l'absorption et la migration du diquat et du paraquat dans les plantes     

A new method for assessing foliar uptake of fungicides using Congo Red as a tracer

In order to develop a new method for measuring foliar uptake of fungicides, Congo Red was selected as a tracer, and optimum procedures were established for washing, extracting and analyzing it and fungicides from leaf surfaces. Congo Red, a water‐soluble dye, was not absorbed into cucumber or rice leaf, even in the presence of various surfactants, and was completely washable from leaf surfaces by aqueous acetonitrile solutions. Congo Red and fungicides in washings were quantified to calculate the amount of foliar uptake of the latter, by comparing the ratio to Congo Red. The optimum concentration of Congo Red in a formulation should be established in order to minimize its influence on fungicide uptake. Although Congo Red has proved to be useful with a conventional droplet application method, it will give more realistic and practical results with the spraying method used in the present study. © 2001 Society of Chemical Industry