Induction of resistance and stress reduction by Glycyrrhiza glabra L. (licorice) leaf extract in cucumber and tomato
Abstract: In tomato leaves, production of H2O2 took place after treatment with licorice leaf extract (P1) and two fractions of P1, F6 (mainly flavonoids) and F4 (terpenoids and sterols), similarly to former findings in cucumber. Up to 15-fold increase of ethylene concentration compared to the control was measured in cucumber leaf samples treated with P1 and F6. Moreover, chlorophyll fluorescence (Fv/Fm) measurements in non-infected cucumber leaf discs treated with P1 and F6 showed higher Fv/Fm values than the control over 14 to 21 days (21 and 35°C, respectively). Infected cumber plants prophylactically treated with the fractions F6 and F4 as well as the raw extract also exhibited a better function of the photosynthetic apparatus than non-treated plants. The results give evidence for induced resistance by ethylene signal transduction and for general stress reduction as modes of action of G. glabra extract.