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The influence of PPMTM on somatic embryogenesis in melon, adventitious shoot organogenesis in petunia, and androgenesis in tobacco was studied by culturing explants in regeneration media supplemented with 0, 2, 5 or 10 ml l−1 PPM for 8-12 wk. The percentage of melon cotyledon explants that produced callus and somatic embryos and the number of embryos per explant were reduced when incubated in embryo initiation and embryo development media containing more than 5 ml l−1 PPM. Less PPM was required to inhibit petunia shoot organogenesis. The number of shoots and number of buds per Petri dish were reduced 3-6.9-fold when leaf explants were incubated in shoot regeneration medium containing more than 2 ml l−1 PPM. In contrast, the addition of up to 10 ml l−1 PPM to tobacco anther culture medium had no effect on androgenesis. Our results suggest that the influence of PPM on plant regeneration depends on the plant species. We recommend that experimenters examine a range of PPM concentrations when using it for the first time on an untested plant species.