Turions of the duckweed Spirodela polyrhiza (L.) Schleiden are dormant immediately after their formation (there is no germination response regardless of the light conditions) but become non-dormant following a cold after-ripening treatment (i.e. responsive to phytochrome-mediated germination response). Application of glucose, sucrose or maltose (but not fructose or mannitol) to dormant turions resulted in a germination response which was enhanced in the presence of light. Therefore, mobilization of endogenous starch was investigated in turions as a possible source of low molecular weight carbohydrates. In dormant turions, starch degradation was not detected under all light conditions applied (continuous red light, red light pulse or complete darkness). In non-dormant turions, a rapid starch degradation was measured in continuous red light but degradation could not be detected if a single red light pulse was given or in complete darkness. The main function of storage starch during turion formation appears to be to secure the survival of turions during periods of unfavourable conditions. Our results indicate that starch does not seem to contribute to early events of germination but to subsequent growth of newly formed sprouts. Further, breaking of dormancy in turions is rather independent of the mobilization of starch.