Methods and compositions are disclosed wherein exogenous chemicals are applied to plants to generate a desired biological response. One embodiment of the present invention is a plant treatment composition that comprises an exogenous chemical and a first excipient substance. The first excipient substance is an amphiphilic quaternary ammonium compound or mixture of such compounds, having the formula R.sup.8 --W.sub.a --X--Y.sub.b --(CH.sub.2).sub.n --N.sup.+ (R.sup.9)(R.sup.10)(R.sup.11) T.sup.- wherein R.sup.8 represents the hydrophobic moiety and is a hydrocarbyl or haloalkyl group having from about 6 to about 22 carbon atoms, W and Y are independently O or NH, a and b are independently 0 or 1 but at least one of a and b is 1, X is CO, SO or SO.sub.2, n is 2 to 4, R.sup.9, R.sup.10 and R.sup.11 are independently C.sub.1-4 alkyl, and T is a suitable anion.