Physicochemical aspects of foliar uptake

Most chemicals penetrate most plants poorly when applied alone and so require an adjuvant for uptake to occur. The adjuvant increases the amount of uptake into the lipophilic environment of the cuticle. This uptake can be predicted from the octanol / water partition coefficient (Kow), the dissociation constant pKa and the parameter A log P. This latter term, derived from work on the penetration of the blood-brain barrier, viz.

A logP = logP0w - log Palk subtracts the alkane/partition coefficient (generally determined using hexane or cyclohex-ane) from the octanol/water value and is thought to have considerable relevance to cuticular penetration, since epicuticular wax is essentially hydrocarbon in character. Thus, Briggs and Bromilow (1994) consider that permeability through the cuticular wax varies inversely with A logP, while permeability along the aqueous, polar cuticular route is inversely related to log Kow. Since A logP is positive for most compounds, those entering the wax then move readily into the cuticle, as they are more strongly absorbed by the octanol-like cuticle. On the other hand, uptake via the aqueous route occurs for compounds with high water solubility. This relationship is illustrated in Figure 3.4.

Surfactants are amphipathic molecules (i.e. possess a hydrophilic head and a hydropho-bic tail) and are often added to aqueous formulations for many purposes, including cuticle retention on leaf surfaces, absorption and penetration to the target site. Four classes of these surface-active agents have been identified:

1 Anionic. Here, surface-active properties are provided by a negatively charged ion. For example, a hydrophobic group is balanced by a negatively charged hydrophilic group, such as a carboxyl (-COO-).

2 Cationic. In this case, the surface-active properties are provided by a positively charged ion. Thus, a hydrophobic group is balanced by a positively charged hydrophilic

group, such as a quaternary ammonium (- N ).

6 - No aqueous Good wax (diclofop methyl)

No aqueous Int. wax

No aqueous Impermeable Poor wax

Poor aqueous Good wax Log Pow (sethoxydim)

Poor aqueous Int. wax (atrazine)

Poor aqueous Poor aqueous Poor wax No wax

(tween 20)

0 - Int. aqueous Good wax

Int. aqueous Int. wax (phenylureas)

Int. aqueous Int. aqueous Poor wax No wax

(sulphonylureas)

Good aqueous Good wax (acetone)

Good aqueous Int. wax

Good aqueous Good aqueous Poor wax No wax

(water, glufosinate, glyphosate)

A Log P

Figure 3.4 Effect of log Pow and 4 log P on foliar uptake through the cuticle via epicuticular wax or aqueous routes. Int., intermediate; names in parentheses refer to estimates for some active ingredients and adjuvants (after Briggs and Bromilow, 1994). © Ernst Schering Research Foundation.

3 Non-ionic. No electrical charge is evident. Thus, a hydrophobic group consists of alkylphenols, alcohols or fatty acids and is balanced by non-ionisable hydrophilic groups, such as ethylene oxide (-CH2CH2O-).

4 Amphoteric. These molecules have hydrophilic groups with the potential to become cationic in an acid medium or anionic in alkaline conditions.

The ratio between the hydrophilic and lipophilic groups is termed the hydrophilic--ipophilic balance (HLB). Thus, compounds of low HLB are relatively water- -oluble and so surfactants can be selected for specific purposes. They can also be further categorised as spray modifiers or activators. Spray modifiers reduce the surface tension of the spray droplets and so improve the wetting and spreading properties of the formulation, resulting in a greater degree of retention on the leaf. Activators are added to improve foliar absorption, an example being a range of polyoxyethylenes with HLB values of 10-13.

How surfactants aid cuticular penetration still remains uncertain and large differences are observed between plant species. The most obvious effect of surfactants is to reduce leaf surface tensions and contact angles, thus increasing the spread of the chemical to cover a greater proportion of the leaf surface. A high contact angle (Figure 3.5A) between the droplet and the surface also means that the droplet is easy to dislodge and run off the leaf. The lowest values for contact angle and surface tension are reached at the critical

No surfactant

With surfactant

With surfactant

Figure 3.5 Schematic representation of droplets on a leaf surface in the absence (A) and presence (B) of a surfactant. 9, contact angle.

Figure 3.5 Schematic representation of droplets on a leaf surface in the absence (A) and presence (B) of a surfactant. 9, contact angle.

micelle concentration of the surfactant, usually about 0.1% to 0.5%, on a volume to volume proportion of the formulation. At such conditions contact angles may be lowered, for example, from 110 to 60 degrees (Figure 3.5B) and surface tensions from 70 to 35mNm-2.

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