A comparative study of germination responses to diurnally-fluctuating temperatures.
Seeds collected from native herbaceous plants in the Sheffield region were subjected to a range of amplitudes of diurnal fluctuations in temp. All experiments involved a photoperiod of 18 h combined in some spp. with a comparable series of temp. treatments applied in continuous darkness. In the light, fluctuating temp. stimulated germination in 46 of the 112 spp. tested, the responses varying from an absolute requirement in all seeds for large (>5 deg C) fluctuations to polymorphisms in which some seeds germinated at constant temp. whilst the remainder responded to small (<5 deg C) fluctuations. Wetland spp. showed marked response to fluctuating temp. in the light. In certain spp. (e.g. Ranunculus repens) continuous darkness increased the amplitude of temp. fluctuation necessary for germination. Small-seeded spp. 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 temp. Sensitivity to temp. fluctuations in darkness was observed in spp. of grassland, wetland and disturbed habitats and was conspicuous among spp. forming persistent seed banks. It seems likely that responses to temp. fluctuations in darkness provide mechanisms of depth-sensing by buried seeds and may initiate establishment from seed within canopy gaps. Many spp. that responded to temp. fluctuations in darkness are successful weeds of arable land or pasture. A polymorphic response to temp. fluctuations may account partially for the unpredictability of appearance of infestations of these weeds. The ability to invade canopy gaps throughout the yr is important to the success of many pasture weeds (e.g. Rumex obtusifolius, Ranunculus repens) and is notably absent from the major sown spp., Lolium perenne.