The effect of light and number of diurnal temperature fluctuations on germination of Phragmites australis

Seed germination of the wetland emergent perennial Phragmites australis is stimulated by diurnally fluctuating temperatures. A germination experiment in darkness and light at different temperature regimes showed that P. austr alis germinated as well in darkness as in light over most of the temperatur e regimes tested. The germination requirements could partly explain why thi s species, despite a large annual production of small seeds, does not accum ulate a persistent soil seed bank. A second experiment examined the effect of one to four diurnal temperature cycles with amplitude of fluctuations ra nging from 0 to 30 degrees C; diurnal mean temperature of 15 degrees C. Ger mination in the absence of fluctuations was low, and logistic model estimat es showed a positive effect of number of temperature cycles which was stron gly influenced by amplitude size. For seeds that have fallen on moist groun d during the winter, these laboratory results predict that a few large diur nal temperature fluctuations could be sufficient for onset of germination i n the subsequently warmer spring period. Seeds that are located under water , however, are predicted to remain ungerminated until the water level falls . Hence, we suggest that the requirement for fluctuating temperature, in ad dition to being a sensor for 'exposed' seed sites, is also effectively acti ng as a germination timing mechanism.