Characteristics of Herbicide Distribution As Patterns for Nematicide Behavior 1 D

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Characteristics of Herbicide Distribution as Patterns for Nematicide Behavior 1 D. E. BAYER. 2 ing developed regarding the use of foliar Key words: pest management, chemical control, applied herbicides with data to illustrate foliar applications, absorption, mode of action, translocation. the significance of the major concepts that may be applicable to nematicides. In current agricultural practices the foliar application of pesticides for the con- LEAF SURFACES trol of unwanted pests plays a major role The topography of the leaf surface has in economic crop production. The pene- been shown to be closely correlated with tration of foliar-applied pesticides is the type and amount of waxes present (2,6). strongly influenced by the plant surface, This waxy layer on the outer surface of the spray solution, and the environmental the leaf is generally the first barrier through conditions before, during, and following which a foliar-applied pesticide must pass. application. The biological activity of a The morphology of the surface waxes is herbicide has been shown to be influenced extremely variable, and the extent to which by the physico-chemical properties as well the surface waxes impede penetration is as the rate and manner in which it was unclear. However, they do influence the applied to the leaf (11). The interactions overall process by interfering with intercep- between the plant and the pesticide will tion and retention of the spray droplet by govern the quantity that will enter the the leaf. Of primary importance is the need plant. The question of why some pesticides to obtain intimate contact between the are translocated within the plant, and spray droplet and the surface of the leaf. others are not, is of considerable impor- The liquid-solid interactions that occur tance and could be used to predict the sys- on the leaf surface depend on the pesticide temic properties of pesticides. Many pesti- (formulation) and the leaf surface. Sargent cides are incapable of being transported in (8) has suggested that there are three prop- the symplast from external applications be- erties at the leaf surface that will affect this cause the physico-chemical properties, such interaction: the degree of surface topogra- as polarity, inability to move across mem- phy, the presence or absence of a film of branes, adsorption to various cellular com- air between the pesticide droplet and the ponents, etc., prevent their entry into the leaf surface, and the nature of the chemical symplasm. If they do enter, their movement groups associated with the wax or cutin is prevented. on the surface of the leaf. Translocation of assimilates (photosyn- The size and shape of the epidermal thates) in plants involves both a metabolic- ceils, including anticlinal cell wall depres- ally active source (area of synthesis or high sions, position of stomates, and guard cells, concentration) and a metabolically active as well as the presence or absence of tri- sink (area of utilization or storage). Thus, chomes, have a strong influence on the de- assimilates will move through the sieve ele- gree of droplet spreading and ultimate ments in the direction of the sink and at a penetration of the applied herbicide. rate determined by the sink strength. A The role of the stomate in uptake of unique feature of the phloem is the ability foliar-applied materials is still under dis- of the materials in the phloem to move out cussion. However, it has been shown that of the sieve elements or into the sieve ele- significant uptake of herbicides does occur ments at any point along the system. through astomatous tissue (1,6). Stomatal This report will present some of the uptake is enhanced when leaf morphology current ideas or hypotheses currently be- and surface energies allow the sway solution to drain into depressions and stomatal Received for publication 26 January 1983. 1Presented in a symposium to the Society of Nematolo- chambers. In the leaf epidermal layer, the gists, July 1989, Knoxville, Tennessee. guard cells and the regions over the anti- 2Professor, Department of Botany, University of f'ali- fornia, Davis, CA 95616. clinal walls appear to be the preferred 602 Patterns for Nematicide Behavior: Bayer 603 sites of absorption. The fact that the herbi- the other layers. Even cellulose is occasion- cide tends to accumulate in the depressions ally found impregnating the cuticle. Both overlying the anticlinal walls might in the wax and cutin layers are considered to part explain this phenomenon. When a be lipophylic in nature, while the pectin herbicide is applied at 748 or 374 liters/ha, and cellulose are more hydrophylic. The it tends to accumulate over the anticlinal degree of ramification of the cutin layer walls leaving a honeycomb appearance on with pectin varies significantly between spe- the leaf surface. At 117 liters/ha or less, the cies. In some species, such as Indian rubber herbicide is randomly deposited in small plant (Ficus elastica Roxb.), the pectin deposits on the leaf surface indicating that layer is quite discrete, while in others it the herbicide is covering both the anti- is very diffuse, ramifying tile entire cutin clinal and periclinal walls. The smaller de- layer. Even though the cutin is a highly posits may be a result of decreased droplet polymerized layer and considered to be size of the spray solution as a resuh of lipophilic in nature (2,6), the embedded decreased nozzle orifice used in application. waxes or waxes dispersed in the cutin are Even on very waxy leaf surfaces these small considered to be tile nlajor limiting com- droplets have been shown to adhere to the ponent of the cuticular layer to the uptake surface of the leaf, thus allowing a greater of foliar applied materials. amount of lterbicide to be retained on the Differences in foliar absorption of a leaf (4). single herbicide has been shown to be di- The absorption of a foliar-applied ma- rectly correlated with tile formulation (Ta- terial can be considered in terms of three ble 1). The more lipophilic formulations, criteria: the quantity of active ingredient such as tlte ester formulations, will wet the deposited on the target surface, the extent leaf surfaces better and penetrate the plant to which the target surface is covered, and cuticle more readily than will the nlore the presence of the active ingredient in a hydrophilic salt formulations (5). It should form readily absorbed by the target surface. be recognized that pesticides are formulated Retention of the spray droplets on the tar- to benefit haudling and to enhance biolog- get surface depends on the number of drop- ical activity. Often the type of formulation lets lost by reflection and/or runoff after is dictated by the chemical properties of aggregation. The runoff is especially im- the pesticide. Perhaps there is no readily portant when high volumes are applied. available solvent system for the pesticide, The frequency and morphology of so the manufacturer may resort to a wet- trichomes on the leaf surface presents a table powder. Or perhaps the dry formula- problem to foliar-applied materials because tion presents a handling problem, so a flow- the trichomes do not function efficiently as able formulation is prepared. It should al- routes of uptake. Spray volume, droplet ways be remembered that for absorption size, and surface tension of the spray solu- to take place, the active ingredient must tion will all influence herbicide perfor- be in a form that can be absorbed by the mance. High volumes of spray solution are plant. This effect on efficacy may be seen often used in an attempt to thoroughly wet in Table 2, where propanil was formulated the surface (4). Large spray droplets tend to as five different emulsifiable concentrates shatter when they impinge on a stellate and sprayed on barnyardgrass [Echinochloa hair, allowing the small droplets to pene- crus-galli (L.) Beauv.]. Upon drying, three trate deeper to the cuticle surface. Small Table I. Influence of different types of formula- spray droplets tend to be reflected off the tions on efficacy of 2,4-D applied at 0.6 kg/ha for hair back into the atmosphere or may just the control of mustard. remain lodged on the hair; thus efficiency of absorption is much reduced. Formulation Percent controlled The cuticle refers to the outermost layers of the epidermal cells. It is most High volatile ester 100 generally thought to be composed of wax, Low volatile ester 96 cutin, and pectin (2). Each layer should not Oil-soluble amine 89 Water-soluble amine 63 be considered as discrete and separate but Metallic salt 54 rather as a gradual transition into each of 604 Journal of Nematology, Volume 15, No 4, October 1983

Table 2. Influence of five emulsifiable concen- the plant to an area where they exert their trate formulations* on efficacy of propanil applied phytotoxic effect are considered to be sys- at 4.5 kg/ha for control of barnyardgrass in rice. temic, and those tltat are retained and exert their effect in the underlying tissue are Formulation Percent controllext termed contact chemicals. All herbicides must move into the underlying plant tissue # 1 56 before translocation from the point of ap- #2 61 plication can occur or, in the case of con- #3 47 #4 81 tact herbicides, before a phytotoxic re- # 5 93 sponse can be accomplished. This movement constitutes the absorption process. *The formulations are identical to those used This process involves the movement of the in Tab!c 3. penetrating molecules through the wax, cutin, pectin, and cell wall layers (cuticle of the formulations formed crystals on tile plus the cell wall) to the plasma membrane. surface of the leaf and were no longer in For herbicides whose primary mode of ac- a form that could be readily absorbed by tion is the disruption of membranes, the the barnyardgrass. Tile other two remained absorption process may be considered to in a form that was readily absorbed, and be complete. However, for most herbicides the barnyardgrass plants were killed. the absorption process also entails the Another aspect emphasizing the need movement across the plasmalemma. for proper formulation is the need to keep Little is known about the movement of the active ingredient on the target plant. pesticides across the plasmalemma. Many In a laboratory study, the same five formu- studies have suggested that passive move- lations of propanil were applied to rice ment, as well as a metabolic component of plants which were then placed in a cham- uptake, is involved in the movement of ber. Air at 10 mph was passed over tile organic compounds across the plasma- foliage and filtered, and the amount of lemma. propanil trapped was determined (Table To study the absorption process inde- 3). Those formulations that formed loose pendently of transport, a somewhat ideal- crystals on the surface of the plants not ized method was developed (9), Thin sec- only provided poorer control of the barn- tions of leaf tissue (4-8 cells wide) were yardgrass plants, but the crystals were free floated in a buffered treatment solution to be dislodged by leaf movement or the containing radioactive herbicide. Depend- wind and fall to the soil or be blown to ing on treatment time, the sections were nontarget areas. removed from the solution, rinsed briefly, and radioactivity determined, or the leaf ABSORPTION OF FOLIAR sections were desorbed in an equivalent APPLIED HERBICIDES solution containing nonradioactive solu- Those chemicals translocated within tion. In this manner, the various fractions Table 3. Influence of formu'ation* on the (total, exchangeable, and nonexchangeable) amount of propanil dislodged from rice leaves by were determined. The "total" fraction rep- air moving at 10 mph following a 4.5-kg/ha appli- resented the total amount of herbicide cation. taken up by the leaf sections. The "exchangeable" fraction represented the Propanil dislodged amount of herbicide that could be removed Formulation (~g) by washing for 1 hour. The "nonexchangeable" fraction represented the amount of #1 2.4 herbicide that remained in the leaf sections #2 1.9 following the 1-hour wash period . #3 3.3 Certain herbicides, such as 2,4-D, tend #4 1.0 #5 .3 to be accumulated to very high concentrations in the metabolically active tissue, *The formulations are identical to those used while other herbicides, such as diuron, do in Table 2. not accumulate to high concentrations in Patterns for Nematicide Behavior: Bayer 605 tile symplast. Herbicides like 2,4-D may ac- location on absorption, it appeared that cumulate in the symplast to a concentra- internal plant moisture stress directly af- tion when egress is more rapid than influx. fected the absorption process of 2,4-D. This During this period net concentration will does not mean, however, that internal plant decrease in the cell. At some lesser con- moisture stress would not influence the centration, influx will again exceed egress translocation process. and tile concentration within tile cell will increase. Other herbicides, such as picloram, do not appear to show a major phase TRANSPORT OF FOLIAR of net loss from the tissue. APPLIED HERBICIDES Temperature has a strong influence on Once a herbicide has penetrated into both the absorption and trans'.ocation pro- the plant, two predominant types of trans- cesses. For those herbicides that tend to ac- port have been reported (3). Herbicides cumulate in the symplast, absorption in- may be grouped into four general categories creases as temperature increases within the depending on their translocation charac- limits of biological activity. However, when teristics: herbicides that are translocated translocation is considered, the curve is primarily in the apoplast, herbicides that modified by the physiology of the plant. are translocated primarily in the symplast, When temperature approaches approxi- herbicides that are translocated in both mately 35 C, the absorption process is rapid. the symplast and the apoplast, and herbi- However, transport in some plants may be cides that have restricted or no movement slowing due to physiological reasons, so in either the symplast or apoplast. Those the concentration of the herbicide in those herbicides that enter via the apoplast, for cells directly ftmctioning in the uptake the most part, are transported and/or re- process accumulate phytotoxic levels of main ill the apoplast. Those that enter the herbicide and are killed. Movement out symplast remain in the symplast. In addi- of those areas is stopped or much reduced. tion to these two major classifications are This may be observed as a necrotic spot herbicides that exhibit all combinations or or contact type burn on the plant. degrees between these two extremes. Tran- The absorption process of these same spiration is the major driving force for herbicides is also affected by internal mois- those herbicides that move in the apoplast. ture status of the plant during the time The major routes of movement are in the when influx is occurring. When the inter- cell walls and xylem stream. These herbi- nal water stress of field bindweed (Convol- cides tend to accumulate in areas where vulus arvensis L.) is increased from -9 bars high transpiration or water loss occurs, to-12 bars, the influx to 2,4-D is reduced such as the older, fully developed transpir- over 50 percent (Table 4). There are con- ing leaves and, to a much lesser extent, the tlicting reports in the literature whether stems and younger developing leaves. the absorption process is reduced because Herbicides that move primarily in the of plant water stress, thus limiting translo- apoplast are characteristically applied to cation, or whether translocation is affected the soil where they are taken up by the directly. When controlled experiments were roots of the plant and moved upward in conducted to minimize the impact of trans- the xylem stream to be accumulated in the Table 4. Influx of a~C-2,4-D into foliage of field foliage. If applied to the foliage, coverage bindweed under various internal moisture stress. must be complete to achieve uniform distribution within the plant, because the major Moisture stress Uptake direction of movement is with the water (bars) (percent reduction) to the older, mature foliage where the herbicide will remain until metabolized or 0 (Nonstressed) 0 lost in some other manner. -6 5 Those herbicides that enter the symplast -9 20 and are translocated in the phloem have -12 43 been shown to move with the assimilate -15 51 -18 57 (photosynthate) stream from areas of synthesis (high concentration) to areas of utili- 606 Journal of Nematology, Volume 15, No 4, October 1983 zation (low concentration) or storage. Thus, Table 5. Monthly variation of total carbohy- assimilates move "en masse" through the drates in rhizome tissue of umlisturbed Johnson- grass, Davis, California. sieve elements in the direction and at a rate as determined by the gradient created by the sum total of the osmotically active Carbohydrates solutes. It is important to remember in Month (mg/g dry weight) discussing the fate of herbicide movement in the phloem that the phloem is a distri- January 615 bution system for assimilates serving the February 603 March 610 entire plant. A unique feature of this sys- April* 540 tern is the ability of materials to move into May 500 or out of the sieve elements from adjoining June 475 405 cells. July Augustt 484 Translocation of these herbicides is sire- September 591 liar to the translocation of naturally oc- October 602 curring assimilates except when the phyto- November 598 toxic effects induced by the herbicide pre- December 601 vents such movement (3). In a very young seedling, the percentage of actively metabo- *New growth was being initiated. lizing cells would be high, thus a symplastic +Full seedhead formation, seed maturing. mobile herbicide would be translocated Those herbicides that translocate read- throughout the entire plant and would ac- ily in both the symplast and the apoplast cumulate in the areas of high metabolic need to be considered by relating the char- activity which would include the root tips acteristics of both types. All herbicides, if and apices. As the plant increased in size, taken up by the plant, have the potential the potential for dilution in the plant to be translocated in either the apoplast or would become greater and accumulation symplast, or both. However, many are cap- would become more localized. As the plant able of only limited movement because of flowered and set seed, movement would be physico-chemical factors, including struc- to the flowers and seeds; as the seeds ma- tural requirements. It should be remem- tured, transport would diminish and the bered that these classifications (Table 6) plant would die. are only relative. When a herbicide is listed Carbohydrate reserves in perennial plants are depleted during the early phase Table 6. Translocation patterns of some corn- moll herbicides. of growth and development and are re- plenished later as the foliage of the plant develops (Table 5). The overall major di- Translocation* rection of assimilate movement during the Symplast Apoplast early growth and developmental phase is Herbicide (phloem) (xylem) from the stored reserves to the young de- Amitr&.e ++ +++ veloping foliage. If an application o[ a Atrazine 0 +++ foliar-applied, symplastically mobile herbi- Dalapon ++ + cide was made at this early stage of growth, Dicamba +++ + the herbicide would remain in the foliage Diuron 0 +++ because the stored food reserves are being 2,4-D ++ 0 2,4-DB 0 0 utilized for growth and movement is away EPTC 0 >+ from these areas and towards the foliage. Fenac + ++ If an application of the same herbicide Glyphosate ++ ++ was made later in the growth cycle of the Picloram +++ ++ plant when assimilates are being translo- Simazine 0 +++ 2,4,5-T ++ 0 cated from the foliage to the areas of stor- 2,3,6-TBA ++ +++ age, such as full bloom or mature seed stage, the herbicide would be moved along *0 = restricted to no evidence of movement: with the assimilates and would accumulate + = limited translocation, ++ = intermediate in the developing seed or storage areas. translocation: + + + = translocated readily. Patterns for Nematicide Behavior: Bayer 607 as being mobile only in the apoplast, it ac- Table 7. Amount of herbicide exuded from roots tually moves into the symplast and is swept of bean plants from a foliar application of 100 #g along in this system; however, it has such herbicide. a high affinity for the apoplast it is rapidly partitioned out of the symplast and back Herbicide Exuded (#g) into the apoplast. The reverse is true for those indicated as moving only in the sym- 2A-I) 2 plast. For those herbicides that move in Dicamba 11 2,3,6-TBA 7 both systems, the degree of affinity for the Fenac 8 apoplast or symplast, or the ability to move Picloram 5 into or out of one system and into or out (;lyphosate 1 of the other, will determine which system it moves in and to what extent (10). components of the molecule. The area or Those herbicides that exhibit limited zone of the root most actively involved in movement in plant tissue are usually re- the exudation process is the zone of root stricted because of structural requirements, elongation. solubility, adsorption, etc. These herbicides must be applied in such a manner that LITERATURE CITED limited movement will allow them to reach their site of herbicidal action. 1. Bayer, D. E., and J. M. Lumb. 1973. Penetra- tion and translocation of herbicides. Pp. 387-439 in There is a group of herbicides that will W. Van Vaikenburg, ed. Pesticide formulations. translocate in the symplast, accumulate in New York: Marcel Dekker. the roots (especially root tips), and leak 2. Bukovac, M. J. 1976. Herbicide entry into out of the roots to the surrounding environ- plants. Pp. 335-364 in L. J. Audus, ed. Herbicides: ment (7). They then may be tied up in the physiology, tfiochemistry, ecology. London: Aca- demic Press. surrounding environment or they may be 3. Crafts, A. S., and S. Yamaguchi. 1964. Auto- resorbed by the roots. This phenomenon radiography of plant materials. California Agric. has been known for some time but has be- Exp. Stn. and Ext. Serv., Manual 35. come of interest recently with exudation 4. Hess, F. D., D. E. Bayer, and R. H. Falk. 1974. Herbicide dispersal patterns: I. As a function of of such organic substances as herbicides leaf surface. Weed Sci. 22:394-401. that were applied to the foliage. The most 5. Hess, F. D., D. E. Bayer, and R. H. Falk. 1981. prominent classes or families of herbicides Herbicide dispersal patterns: III. As a function of that leak out of roots following a foliar formulation. Weed Sci. 29:224-229. application are benzoic acids, picolinic 6. Linskens, H. F., W. Heinen, and A. L. Stoffers. 1965. Cuticula of leaves and the residue problem. acid, phenoxyacetic acids, and phenylacetic Residue Rev. 8:136-178. acids. The quantities that are exuded from 7. Mitchell, J. w., B. C. Smale, and W. H. Pres- roots range from less than 1% to over 10% ton, Jr. 1959. New plant regulators that exude from of the amount applied to the foliage (Ta- roots. J. Agric. Fd. Chem. 7:841-843. 8. Sargent, J. A. 1965. The penetration of ble 7). Exudation appears to be dependent growth regulators into leaves. Annu. Rev. Plant on the rate applied, the presence of bind- Physiol. 16:1-12. ing to plant and constituents, and the rate 9. Smith, R. C., and E. Epstein. 1964. Ion ab- of metabolism of the compound by the sorption by shoot tissue: technique and first find- plant. In general, those herbicides that are ings with excised leaf tissue of corn. Plant Physiol. 39:338-341. not translocated readily or are easily me- 10. Tyree, M. T., C. A. Peterson, and L. V. tabolized are not exuded from the roots. Edgington. 1979. A simple theory regarding ambi- The chemical structure of the molecule mobility of xenobiotics with special reference to is a critical factor in determining whether the nematicide, oxamyl.~Plant Physiol. 63:367-374. 11. Wilkinson, C.~ F. 1973, Correlation of bio- a herbicide will be exuded from plant logical activity with chemical structure and physical roots. Such factors as number of substitu- properties. Pp. 1-64 in W. Vail Valkenburg, ed. tions, position, nature, etc., are all critical Pesticide formulations. New York: Marcel Dekker.