A Comparative Study of Germination Responses to Diurnally-Fluctuating Temperatures

(1) Seeds collected from populations of a range of native herbaceous plants in the Sheffield region were subjected to experimental treatments providing a range of amplitudes of diurnal fluctuations in temperature. All experiments involved a photoperiod of 18 h combined in some species with a comparable series of temperature treatments applied in continuous darkness. (2) In the light a stimulation of germination by fluctuating temperature was observed in forty-six of the 112 species tested. The responses varied from those indicating an absolute requirement in all seeds for large (>5 OC) fluctuations (e.g. Rorippa islandica) to polymorphisms (e.g. Carex otrubae) in which some seeds germinated at constant temperatures whilst the remainder responded to small (<5 OC) fluctuations. (3) The capacity to respond to fluctuating temperatures in the light was prominent in species of wetland where it appears to provide a mechanism whereby spring germination may be initiated by increasing irradiance and falling water table. (4) In certain species (e.g. Ranunculus repens) the effect of continuous darkness was to increase the amplitude of temperature fluctuation necessary for germination. Small-seeded species known to form buried seed reserves (e.g. Juncus effusus) were inhibited by darkness and in the absence of light did not respond to fluctuating temperatures. (5) Sensitivity to temperature fluctuations in darkness was observed in species of grassland, wetland and disturbed habitats, and was conspicuous among species forming persistent seed banks. It seems likely that responses to temperature fluctuations in darkness provide mechanisms of depth-sensing by buried seeds and may initiate establishment from seed within canopy gaps. (6) Many of the species found to respond to temperature fluctuations in darkness are successful weeds of arable land or pasture. A polymorphic response to temperature fluctuations may account partially for the unpredictability of appearance of infestations of these weeds. The ability to invade canopy gaps throughout the year is important to the success of many pasture weeds (e.g. Rumex obtusifolius, Ranunculus repens) and is notably absent from the major sown species, Lolium perenne.