Most formulants and adjuvants are low molecular weight solutes and can principally be sorbed in the surface lipids and deeper layers of the cuticle. This can have huge impact on the sorption potential of actives and their mobility in the rate limiting barrier of the cuticle. Factors like volatility, photostability, plant compatibility, selectivity, salt compatibility, rainfastness, speed of action, weed control, residual efficacy, the possibility of product combinations etc. depend often primarily on the relative absorption and penetration of active and adjuvant and their interaction. However, many factors affect the manifestation of these effects in practice like separation of active and adjuvant from the respective dispersion during evaporation, unmatched speed of penetration, or precipitation of active or adjuvant, respectively. Typically, adjuvants have several functions and -ignoring wetting effects in this contribution -they can act in the dry spray deposit as much as in the cuticle, for example they can solubilize actives in the former and mobilize them in the latter. For best results a timely and rate fit is needed, robust enough to withstand practical variability. Important commercial formulations show a perfect match of active(s) and adjuvant penetration or non-penetration, respectively. Examples on interactions among formulation or spray ingredients will be shown for a model compound and leaf cuticle system. On the

Aerial and non-aerial CM differ largely in their absolute barrier properties. This difference is related to the absence of embedded cuticular waxes in the non-aerial hypocotyl membrane, which make the CM physically dense and cause low solubility of hydrophilic solutes. The free volume for diffusion at the interface of the non-aerial hypocotyl cuticle to the environment is much larger resulting in higher penetration rates. It is suggested that diffusion through the non-aerial hypocotyl does not proceed in a real channel system with continuous aqueous phase but is more like transport through a filter with restricted diffusion in the pore openings.

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