Characteristics of Herbicide Distribution As Patterns for Nematicide Behavior 1 D

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.

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