Exudation, analysis Subject

Petunia leaves are typically coated with a sticky exudate, and it was of interest to determine if this exudate contained any of the insect-inhibitory steroids. We subjected fresh leaves to an initial five minute soak in water followed by a one minute dip in chloroform with gentle agitation. This chloroform extract contained nonpolar surface lipids and substances tentatively identified as carbohydrate esters (ca 1 % of fresh wt.), but no biologically active materials were shown to be present by HPLC analysis. Further workup of the solvent-washed leaves by grinding with additional chloroform then yielded the usual quantities of active materials, showing that the insect-inhibitory components were definitely neither part of the exudate nor of the leaf surface coating. 

Another subjective test is run per ASTM D3291, generally on wire coverings. Samples are clamped in 180° loops of 9.5-mm inside diameter so as to induce exudation under strain. In this case, after various time periods, the surface is wiped with absorbent paper and the extent visually estimated and compared with the behavior of control samples. In both tests, it is more useful to the formulator instead to remove the exudate with a small amount of hexane and determine by analysis how much of what has exuded. 

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