Enzymatic defense responses and protection against Phytophthora nicotianae were studied in tobacco
plants treated with chitosan of different molecular weights (MW) and degrees of acetylation (DA). The
concentration and mode of chitosan application affected enzymatic induction in tobacco leaves. b-1,3-
Glucanase (EC 3.2.1.6) activity required 10 times the polymer concentration relative to the oligochitosan
mixture to induce the highest activity above control when treated by foliar spray, indicating the influence
of molecular weight in this response. PAL (EC 4.3.1.5) and POD (EC 1.11.1.6) activities increased above
control as a result of the influence of polymer degree of acetylation when treatments were applied by
foliar spray. A higher DA favored PAL activity, whereas a lower DA induced higher POD activity. Using
an in vitro bioassay, it was found that the three chitosan compounds caused a reduction of the infection
index of P. nicotianae in tobacco plants that was dependent on elicitor concentrations. There was a significant
relationship between the reduction of the infection index and an increase in PAL activity when
chitosan was applied by foliar spray and by substrate drench to the plant rhizosphere. These results demonstrate
the influence of chitosan physico–chemical properties in plant-induced resistance and the relevance
of particular responses in plant protection against pathogens.