Germination response of Phragmites australis and Typha latifolia to diurnal fluctuations in temperature

Germination responses to 45 combinations of diurnal mean temperature and am plitude were examined in freshly collected seeds of two wetland perennials: Typha latifolia L. and Phragmites australis (Cav.) Trin. ex Steudel. Nearl y all seeds (> 95%) germinated in favourable temperature regimes. Mean temp erature (range 10-30 degrees C) and amplitude (range 0-20 degrees C) affect ed final germination of both species. P. australis required a high amplitud e (> 10 degrees C) for germination over the entire range of mean temperatur es. Final germination of T. latifolia was more sensitive to mean temperatur e than P. australis. The germinated proportion of T: latifolia had a maximu m around 20 degrees C, above which it decreased, and amplitudes were more s timulating at low than at high levels of mean temperature. The germination rate was rapid and increased with mean temperature for both species. More t han 50% germination was achieved within 1-3 d at favourable temperatures. I t is proposed that the thermal requirements provide the non-dormant seeds w ith a season-sensing mechanism which postpones germination of seeds dispers ed during autumn, winter or early spring, until the soil surface is heated by the sun in the spring and sufficiently large diurnal fluctuations of tem perature occur. Furthermore, the amplitude requirement implies a strong avo idance mechanism for germination of P. australis in sites with small temper ature fluctuations (e.g. below water tables), whereas seeds of T: latifolia appear to be less exacting in the requirements when the soil or water beco mes warmer.