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Crop Protection 26 (2007) 312–319
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Spray formulation efficacy—holistic and futuristic perspectives
Jerzy A. Zabkiewicz�
Plant Protection Chemistry NZ, 49 Sala Street, Rotorua, New Zealand
Received 10 January 2005; accepted 17 August 2005
Abstract
This overview considers the use of liquid agrichemical sprays for different applications, their complexities and current limitations, as
well as future requirements needed to increase overall efficacy. This has been done by considering the characteristics and limitations in
each of the steps of deposition, retention, uptake and translocation. It has identified the lack of understanding of the plant component in
relation to the processes involved in retention, uptake and translocation. Satisfactory progress will not be made until appropriate models
are developed for each process and integrated into a comprehensive agrichemical efficacy system.
r 2006 Elsevier Ltd. All rights reserved.
Keywords: Spray formulation efficacy; Deposition; Retention; Uptake; Translocation; Models
1. Introduction
The challenges facing users of agrichemicals have beenincreasing in complexity over recent years. On the onehand consumers require the highest quality of produce, onthe other regulators insist on safety (to the consumer fromresidues) and risk reduction (to the operator or environ-ment). Assuming dependence on agrichemicals will con-tinue, then clear and reasonable operational proceduresmust be developed by agrichemical producers, equipmentmanufacturers, applicators, and by the regulators, torespond to the new criteria. As quoted recently ‘‘we havegone from the goal of pest death, to the more elaborateterms of yield increases, quality benefits, resistancemanagement, IPM goal, and combinational tactics, andnow towards the more sustainable and bio-intensive levelsto include whole farm and agro-ecosystems approaches’’(Hall, 2004).
The requirement to increase biological efficacy andreduce detrimental environmental effects can only be metby improved spray application and formulation technolo-gies. They must be considered together as they are linkedinextricably: the proportion of product intercepted by thetarget will vary with plant growth stage and crop type.
e front matter r 2006 Elsevier Ltd. All rights reserved.
opro.2005.08.019
343 5491; fax: +64 7 343 5811.
ess: [email protected].
There are many reviews on the choice and selection ofnozzles and spraying systems for specific applications.More importantly, there is now an awareness that thesehave to be considered in relation to the spray formulation,as droplet size can vary significantly with the same sprayformulation, if different nozzles or operating conditions areused (Combellack, 2004). However, although some gen-eralisations can be made, we are still a long way from beingable to predict appropriate combinations accurately. Inparticular, the characteristics, susceptibility, receptivity, orwhatever term may be applied, of the target, whether plantor other organism, is still poorly understood and may stillbe the dominant variable in the overall efficacy processes.An illustration of possible overall spray efficiency for an
herbicide application is given in Table 1. This is a situationwhere the compounding effects of poor to good efficiencyin individual steps of the deposition:plant retention:upta-ke:translocation interactions will give a very variablesystem efficiency. When individual steps are optimised,the amount received by the target site can be over ahundred and 50-fold more than the worst case.The study of spray formulation efficacy has evolved
from empirical tests and field trials, through laboratoryscreening methods, to more detailed fundamental studiesfor each of the specific steps. Pesticide efficacy can bedefined, and the dose received by the target site ororganism modelled as a function of deposition, retention,
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Table 1
Representative spray application efficiency, system efficiency and off-target loads for an herbicide application
Spray efficacy processes Process efficiency (%) System efficiency (%) Off target component (%)
Depositiona 80–95 80–95 20–5
Retentionb 10–100 8–95 92–5
Uptakec 30–80 2.4–76 97.6–24
Translocationd 10–50 0.24–38 99.6–62
aAmount deposited within target area.bAmount captured by plant.cAmount of retained material taken up into plant foliage.dAmount of absorbed material translocated from absorption site.
J.A. Zabkiewicz / Crop Protection 26 (2007) 312–319 313
uptake, and translocation (Zabkiewicz, 2003). Most workhas been done with herbicides as they involve all the factorsin the spray efficacy processes and account for over half ofall agrichemicals used. Where fungicides or insecticides areconcerned, good plant surface distribution and persistence(due to a lack of systemic compounds), are the predomi-nant requirements. The behaviour and performance ofspray formulations is greatly influenced by adjuvants,either by affecting flow rate, droplet size, dynamic surfacetension, spreading on and wetting of foliage, or byinfluencing uptake and translocation (in plants). A lackof coherent information on the interactions of sprayformulations with fungal and insect organisms precludesany substantive discussion of the factors involved in thesesystems.
This overview will attempt to consider the use of liquidsprays for different applications, their current limitations,and future requirements needed to increase overall efficacyand minimise eco-system loading. It can be done best byconsidering the characteristics and limitations in each ofthe steps of deposition, retention, uptake and transloca-tion.
2. Deposition
Considerable attention has been given to the physicalprocess of spray deposition, and its off-site component,‘‘drift’’. Are the principles applied to aerial application ofsprays relevant to ground-based sprays or to air-blastapplications in orchards? The common denominators aredroplet size, release height and meteorological effects. It isinevitable that aerial sprays will be released at greaterheights above a crop canopy than for ground-basedsystems. Since fall distance is also greater, dropletevaporation can be an issue, leading to potentially moredrift. In this case the addition of anti-evaporant or anti-drift agents (either to retain or increase droplet size) hasbeen exploited. A clear case of formulation technologyused to decrease drift and increase on-site deposition.Access to real-time meteorological information could easilyprevent application when the wind direction is inappropri-ate. Easy to say but difficult to implement or justifyeconomically. The ultimate solution is real-time interactivespray formulation production and application, based on
operating conditions and location of the applicator withinthe spray zone. Smart systems such as these have beensuggested and are under development for ground-basedapplication systems (Ganzelmeier, 2005). To date, im-provements in aerial application have tended to be focusedon the development of spray deposition models, whichhave been very useful in pre-application depositionoptimisation. All operations are at the mercy of meteor-ological conditions, so with such a system, actual deposi-tion profiles can be calculated post-application, using realconditions.In contrast, ground-based application systems have used
a more prescriptive approach, than through optimisationof deposition. However, this may be changing, as concernsdevelop over eco-system loading, and off-site movementinto irrigation ditches or waterways. It has been pointedout that nozzle selection and operating parameters, as wellas formulation factors, need to be tailored to specificapplications (Miller et al., 2001). In the case of groundapplications, drift effects are of the order of a few metres,compared with the hundreds of metres more likely withaerial applications. However, short-range drift may in turngenerate a higher pesticide deposit per unit area, and ifclose to water bodies, exceed permitted levels. One optionis that spraying at field edges should be through nozzlesthat produce courser droplets. This could again be pre-programmed or under the control of the operator. Thoughthis would improve deposition on-site, it may not do so on-target, as it is well known that there is less retention oflarger drops by (hard to wet) plant foliage. It is possiblethat a deliberate change in formulation during application,by the addition of adjuvants, could be a solution. In thecase of systems used on certain fruit crops, shrouded unitswith capture and recycling of the spray solution can reduceboth drift and ground deposition (Ganzelmeier, 1999).
3. Adhesion, retention, re-distribution
There has been considerable focus and study of actualspray retention by crops throughout their growing season,and this approach can quantify soil deposition as well asplant retention (Gyldenkaerne et al., 1999; van de Zandeet al., 2003). Though such empirical field determinationsmay be possible for a range of arable crops, it is unlikely to
ARTICLE IN PRESSJ.A. Zabkiewicz / Crop Protection 26 (2007) 312–319314
be feasible for all crops or weeds in all situations.Furthermore, if pests or diseases are involved, targeteddistribution and placement of the pesticide on or within theplant canopy is even more important. To enhancebiological efficacy in such cases, correct formulationproperties are even more important.
It is well known that droplets of between 50 and 250 mmdiameter will adhere to plant surfaces, but larger dropletsmay rebound or shatter (Hartley and Brunskill, 1958).However, smaller droplets are more prone to drift, so wehave the classic conundrum of how to achieve maximumon-target adhesion/retention, with minimum off-site drift.The situation is complicated by different crops havingdifferent characteristics or pests. The surfaces of targetscan vary widely, from ‘‘easy to wet’’ to almost totallyreflective (with conventional formulations). For example,‘‘wettability’’ (as measured by the contact angle of 20%acetone:water droplets: Forster and Zabkiewicz, 2001) ofupper and lower leaf surfaces and fruit varies from easy tohard to wet (Table 2). This implies not only that adhesion/retention will vary, but also droplet spread, which mayhave considerable significance for pest or disease control.
Since adhesion and retention are dependent on physicalprocesses, they also lend themselves to modelling ap-proaches. A model for droplet primary adhesion (i.e. onfirst impact) has been developed (Forster et al., 2005) andidentifies the main influences as leaf surface character,droplet velocity and spray solution dynamic surfacetension. Similar plant and spray solution parameters areinvolved in retention models (Forster et al., 2004a;
Table 2
Variation in wettablity of plant leaves and fruit surfaces using 20% acetone:w
Species Contact angle (1)
Variety Uppe
Grape Cabarnet sauvignon 93
Apple Pacific Rose 64
Avocado Hass 64
Kiwifruit Hayward 79
aAbstracted from Gaskin et al. (2005).
y = 0.6265x + 0.1837
R2 = 0.9891
0
0.5
1
1.5
2
2.5
3
3.5
0 1 2 3 4 5
Pla
nt r
eten
tion
Surface Deposition µl cm-2
µl cm-2
Fig. 1. Retention of the same series of spray formulations by potato and oni
(Forster et al., 2004a). Dashed line represents 100% retention.
Grayson et al., 1993) but such models are for specific croptypes and plant phenology (which still requires bettercharacterisation), so a universal model does not yet exist.The influence of leaf surface wettability and plantphenology is illustrated in Fig. 1. The same series of sprayformulations were applied to potatoes and onions, twovery dissimilar crops (easy and hard to wet, as well ashaving horizontal and vertical foliage). Retention bypotato foliage followed a simple linear relationship,regardless of spray volume or added adjuvant. On onions,retention did not follow any simple relationship, with poorretention at higher volumes or with less adjuvant. In thissituation, the spraying parameters must be tailored to theplant’s phenology, which is obviously predominant, andwhich is inadequately described in current models.Such retention models are also limited because they do
not include droplet velocities. In the case of field cropsprayers or air-assisted systems, droplet velocities onimpact may be higher than terminal, so primary adhesionand retention efficiencies will be reduced. A practicalsolution which has met with some success, is the additionof ‘‘stickers’’, which are generally high molecular weightadjuvants, to prevent droplet rebound. Although providingbetter adhesion, such additives may not be very usefulwhere good foliar coverage is essential, as they do notspread after adhesion, so need to be applied in smalldroplets. An alternative approach may be the oppositestrategy, which is the use of additives to make largerdroplets shatter on impact with the leaf, and create smallerdroplets in situ within the canopy. This would be an elegant
ater contact angle measurementsa
r leaf Lower leaf Fruit
132 133
64 100
140 86
120 Not available
y = 0.0627x + 0.7612
R2 = 0.0238
0 1 2 3 4 5
Surface Deposition µl cm-2
0
0.5
1
1.5
2
2.5
3
3.5
Pla
nt r
eten
tion
µl cm-2
on plants showing influence of plant leaf wettability and plant phenology
ARTICLE IN PRESSJ.A. Zabkiewicz / Crop Protection 26 (2007) 312–319 315
solution to spray drift but may only be applicable tocertain canopy types. These adhesion enhancing adjuvants(such as organosilicones which provide low surface tensionsolutions) may give not only good adhesion/retention, butby their nature also cause droplet spread over surfaces,including those of insects and fungal bodies. Where topicalplacement and good coverage is paramount, these adju-vants are being used with insecticide and fungicideformulations, particularly on difficult-to-wet plant surfaces(Gaskin et al., 2001). Though adhesion and retentionproperties can be modified substantially by adjuvants, theirinfluence on subsequent performance must also be takeninto account, in particular their effect on uptake andtranslocation.
4. Uptake
In the case of systemic pesticides, if there is little or nouptake, then regardless of the efficiency of deposition andretention, this will have been to no avail. Obviouslyenvironmental factors, such as rain (causing wash-off),wind and relative humidity (affecting droplet drying), willmaterially affect pesticide uptake. However, a major factorin all cases is the plant leaf itself, including its surface andcuticle. One of the main functions of spray adjuvants is toovercome or minimise the effect of leaf waxes and thecuticular barrier. Appropriate adjuvants will substantiallyaid droplet spread, redistribution and leaf coverage, butcan also enhance uptake of the active ingredient. This mayappear obvious, but it is a difficult task for a number ofreasons.
Mass flow of spray solution into leaves through theirstomata was postulated several decades ago (Schonherrand Bukovac, 1972) and demonstrated with foliar fertilizertreatments involving silicone surfactants around the sametime (Neumann and Prinz, 1974) but not put into wider use
Fig. 2. Illustration of droplet spread effects on Chenopodium album with
monododecylether (1.2mm2) or trisiloxane ethoxylate (31.2mm2).
until the 1980s (Stevens and Zabkiewicz, 1988) through theintroduction of the Silwet L-77 organosilicone surfactant.This mechanism, and the properties of sprays required toachieve this effect, have been studied extensively since then(Stevens et al., 1992; Stevens, 1993), and used veryeffectively for certain weed types. However, this mechan-ism is not effective universally for the reason that not allplants have stomata on upper leaf surfaces nor are they ofthe necessary dimensions (Schonherr and Bukovac, 1972).Furthermore, under stress and at night, stomata close,making this mechanism inapplicable.Cuticular uptake (diffusion of the chemical directly
through the cuticle) of pesticides has been studied for alonger time, but progress in understanding the factorscontrolling this mechanism has been much slower. Atten-tion was focussed on relating the physical properties of theformulation to percentage uptake (Stevens and Bukovac,1987; Stock et al., 1993), with less focus on the interactionsof adjuvant(s) in a spray formulation, or the importance ofplant cuticle and epidermal layers. Just as it is essential toconsider application and formulation together in the caseof adhesion and retention by plants, so the interactions ofplant, active and adjuvant must all be considered in theuptake process.The fundamental mechanism of uptake has been
considered (Riederer and Markstadter, 1996; Schonherret al., 1999; Kirkwood, 1999), with most attention givento the epicuticular lipids and their role in modifying activeingredient diffusion through cuticles. However, thereis a much simpler effect on the leaf surface that needs tobe considered first. If a spray formulation containsadjuvants that cause droplet spread on a leaf surface(Fig. 2), this will in effect lower the mass of active per unitarea, without any change in concentration until the spraysolution begins to evaporate. In any case, there will be a‘‘solution residue’’, where the concentration of the active is
different spray formulations. 2-Deoxyglucose with hexaethylene glycol
ARTICLE IN PRESSJ.A. Zabkiewicz / Crop Protection 26 (2007) 312–319316
many times more than in the starting spray solution(Zabkiewicz, 2003).
It has been found (Forster et al., 2004b) that thissolution residue or ‘‘initial dose’’ (ID) can be related to themass uptake of xenobiotics. This relationship has beenvalidated with a wide range of formulations and plants,representing typical field rates and formulations (Forsteret al., 2006). An illustration of such a relationship is givenin Fig. 3 for 2,4-D acid in the presence of various adjuvantsinto Chenopodium album, Hedera helix and Stephanotis
floribunda, where very similar trends are seen for eachspecies. Uptake per unit area at 24 h can be represented bythe relationship: Uptake ¼ a[ID]b, where a and b areconstants specific to the active on these species.
Although the cuticle is not a homogeneous membrane,the diffusion of substances through it may be described byFick’s first law (Price, 1982), where the flux is the amountof solute that diffuses through a unit area per unit of time(i.e. Mass/Area�Time). The flux is proportional to theconcentration gradient and the diffusion coefficient of thexenobiotic. In reality the equation has to include furtherinteractions (Price, 1982), as the solute has to initiallypartition between the external solution and the membrane(partition coefficient between aqueous and lipophilicphases). Other studies (Schonherr and Baur, 1996), usingisolated leaf cuticles, considered transport through thecuticle to be a three stage mechanism. Namely absorptioninto the cuticle, diffusion through the cuticle and finallydesorption from the cuticle into the internal leaf cells.
Schonherr and co-workers (Schonherr et al., 1999) haveidentified that the principal factors affecting uptake rates are:
Solute mobility, which is affected by temperature, solutemolar volumes, and cuticular wax composition.Limiting skin, or the limiting skin tortuosity which is thelength of the diffusion path through the ‘‘limiting skin’’.
0.0001
0.001
0.01
0.1
1
10
100
0.0001 0.01 1 100
Upt
ake
Initial Dose Applied
nmole mm-2
(nmole mm-2)
Fig. 3. 2,4-D mass uptake in the presence of polyethylene glycol
monododecyl ether C12EO3 (open symbols) or a trisiloxane ethoxylate
with mean EO of 7.5 (filled symbols) into Chenopodium album (n,m),
Hedera helix (J,K) and Stephanotis floribunda (&,’) respectively; (- - -)
maximum uptake line, representing 100% uptake over the initial dose
range.
This is only a proportion of, and not the entire cuticle,and is influenced by the size and orientation of thecuticular wax crystals.Driving force, which is affected by the starting andcontinuing concentration of a.i. in the ‘‘solution’’ on thecuticle surface, in the cuticular layers, and in theepidermal cell wall.
Overall, in simple terms,
uptake ¼ solute mobility� cuticle tortuosity
� driving force:
Adjuvants have been classified into ‘‘accelerator’’ and‘‘passive’’ categories, and it is probable that they will affecteach of these terms. In the case of solute mobility, bymodifying xenobiotic solubility in the cuticular waxes; forcuticle tortuosity, by modifying (e.g. swelling) the cuticleand changing its properties; and by modifying the IDthrough droplet spread and hence the concentrationgradient controlling the driving force. The results ofForster et al. (2004b) would indicate that the driving force(related to the ID) is dominant in typical plant systemsusing accelerator adjuvants and hydrophilic to moderatelylipophilic xenobiotics. However, it is unlikely to bepredominant in all situations and all plant species. There-fore, a much better understanding of plant leaf cuticularstructures, as well as structure activity relationships foradjuvants, is still required to progress to a successfulquantitative model of uptake.
5. Translocation
Translocation has received the least attention, thoughlong distance transport has been well studied and reviewed(Price, 1976; Coupland, 1988). In the case of foliar appliedpesticides, it is known that lipophilicity is important, asthese compounds have to cross hydrophobic membranes orstructures other than the cuticle proper. Hydrophilicmolecules are readily transported in either the phloem orthe xylem, though their initial movements through thecuticle, epidermal cells and into the mesophyll are not wellunderstood (Devine and Hall, 1990). The presence ofseparate ‘‘hydrophilic’’ and ‘‘lipophilic’’ pathways as partof the uptake process, may in turn determine the efficiencyof the subsequent translocation pathway, but it isalso difficult to define when uptake becomes translocation(Fig. 4).Previous attempts have been made to relate herbicide
translocation to the concentration of a.i. per droplet, butgave conflicting results, depending on herbicide and con-centration, plant species and formulation differences (Wolfet al., 1992; Liu et al., 1996). Studies have also identifiedthat localised contact phytotoxicity due to formulation orconcentration of specific a.i.s can reduce translocation(Wolf et al., 1992; Forster et al., 1997). It is clear that‘‘long-term’’ translocation will be affected by plant growth
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Fig. 4. Representation of differing trans-cuticular pathways and sub-
sequent apoplastic (polar) and symplastic (non-polar) pathways (adapted
from Steurbaut et al., 1989).
0.00001
0.0001
0.001
0.01
0.1
1
10
100
0.00001 0.001 0.1 10 1000
Mass absorbed nmol mm-2
Mas
s tr
ansl
ocat
ed
nmol mm-2
100% line
Fig. 5. 2-Deoxyglucose mass translocation vs. mass absorbed, in
Chenopodium album, in the presence of different surfactants. Surfactants
are polyethylene glycol monododecyl ethers C12EO3 (’), C12EO6 (K),
C12EO10, (m) and a trisiloxane ethoxylate with mean EO of 7.5 (J); (- - -)
maximum uptake line, representing 100% uptake over the initial dose
range (Zabkiewicz et al., 2004).
0.1
1
10
100
0.1 1 10 100
Mas
s tr
ansl
ocat
ed
nmol mm-2
Mass absorbed nmol mm-2
100% line
Fig. 6. Glyphosate mass uptake into Chenopodium album, vs. mass
absorbed, from Roundup Ultra (m) and Touchdown (’) commercial
formulations; (- - -) maximum uptake line, representing 100% uptake over
the initial dose range (Zabkiewicz et al., 2004).
J.A. Zabkiewicz / Crop Protection 26 (2007) 312–319 317
stage and environmental conditions, but ‘‘short-term’’translocation (over hours rather than days) may haveother rate limiting or modifying features.
Adjuvants are known to facilitate cuticular ‘‘transport’’(foliar uptake) but are not thought to play any significantpart in further short or long-distance translocationprocesses. However, in theory, if adjuvants could reachthe cellular plasmalemma, then they could affect the initialstage of the sub-cuticular transport process (Fig. 4). Therecent use of mass or molar relationships, instead ofpercentages, for xenobiotic uptake into plants fromdiffering formulations (Forster et al., 2004b), may be ameans of elucidating some of the interactions amongactives, adjuvants and plants.
It has been demonstrated (Zabkiewicz et al., 2004) that arelationship similar to that developed for mass uptake, fortranslocation vs. mass absorbed (which is related to the IDapplied) can also apply to the translocation process inmodel systems (Fig. 5) but less well with complexformulations (Fig. 6). The mass uptake relationship is anexpression of the driving force component in Fick’s law ofdiffusion. The fact that a similar relationship has beendemonstrated for translocation with these polar molecules,implies that short distance, or short-term translocation isalso controlled by a concentration gradient effect. Asshown in Fig. 5, there is no difference, either in slope ormolar amount, among the four formulations, indicatingthat once taken up, 2-deoxyglucose translocation is notaffected by these adjuvants. Either they do not penetratefar into the leaf tissues, or they do not have any effect oncell permeability, at these concentrations.
In contrast (Fig. 6) the commercial glyphosate formula-tions show considerable interaction, if the reduction inboth mass uptake and translocation at higher initialdosages, is a correct indication. Previous mass uptakeresults with model glyphosate formulations into otherplant species (Zabkiewicz and Forster, 2001) have alsoshown a divergence from linearity at lower initial dosages
with different adjuvants. This manner of presenting massuptake and translocation has the potential to identifyanomalous behaviour or complex interactions.The advantage of the mass relationship is that it can
provide information similar to that used for drug deliverydose prescriptions. Knowing the mass uptake, an estima-tion can be made of the mass translocated in specificsystems; with subsequent studies on the influence ofphysiological and environmental influences, appropriate‘‘dosages’’ could be applied at specific growth stages orconditions. It can also be used to estimate the change inoverall efficacy of spray formulations, as to date no genericquantitative relationship has been identified.
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6. Conclusions
The future focus in the use of pesticides should be ondelivery of the active to the target site, which is not thetotal planted area, but the plant(s) within it, usingappropriate formulations to control spray retention,uptake and translocation. Due to the complexity of thesystem, it is inevitable that this can only be completedthrough the development of computer-based decisionsupport systems (DSS). This in turn requires the develop-ment of models for each of the processes. Depositioninto target areas can be modelled, though this approachis not used much by most of the primary sectors. Therequirement to place deposits onto specific parts of aplant, or all over a plant, needs to be addressed througha much better approach to modelling retention (insteadof empirical data sets) which in turn requires a betterdescription of plant development and (foliar) sur-face characteristics. The principal factors controllingfoliar uptake appear to be solute mobility in the cuticle,cuticle tortuosity and solute-driving force. Apart fromthe last factor, the other two are poorly understoodand not easy to measure. Whole plant translocationmodels exist, but it is the short-term or short-distancecontrols that need to be elucidated and related to the initialmass uptake.
More complex models of uptake or total pesticideefficacy are being developed and show promise (Satchiviet al., 2000a, b; Lamb et al., 2001), but their universalapplicability and acceptance will not occur until the inputparameters have been substantially simplified or can bemeasured easily. At present, as with all models, theirgreatest value may lie in identifying the gaps in ourknowledge, so that we focus on the most importantrequirements
The integration of spray deposit profiles within geo-graphic information systems can provide information thatcan be subsequently related to pest or weed competitionlevels, crop productivity and cumulative residue profiles.The development of a comprehensive DSS has the furtherbenefits of providing tailored sub-sets of information thatcan be used by individual operators, for operationaloptimisation, such as selection of appropriate pesticidesor application technology and maximising on-targetdeposits. This DSS can in turn become part of anagrichemical management system, which can be appliedon a local, regional or national scale and related to long-term pesticide eco-system interactions.
Acknowledgements
This work was supported by the New Zealand Founda-tion for Science and Technology. Thanks are due to W.A.Forster and R.E. Gaskin for constructive advice and use ofdata, and M. Haslett for assistance with the preparation ofthe figures for this manuscript.
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