Temperature effects on seed maturity and dormancy cycles in an aquatic annual, Najas marina , at the edge of its range

Summary1 Najas marina is a summer annual of shallow lakes that reaches its distributional limit in the Norfolk Broads of eastern England. Seeds are shed in autumn and overwinter in lake sediments. 2 We investigated the influence of growing‐season length on seed maturity and germinability and the cumulative effects of winter temperatures on patterns of germinability. Seeds were stratified for up to 1 year and tested for germinability at 4‐week intervals. Data‐loggers recorded temperatures simultaneously at the field site and in the cold room. Cumulative temperature below a threshold (thermal time) was used to relate behaviour in cold storage to phenology. 3 Most seeds were dormant at collection. Cold storage of imbibed seed resulted in loss of dormancy. The proportion of non‐dormant seeds peaked after a cold treatment equivalent to one winter, i.e. coinciding with favourable temperatures for germination in the field. Continued cold stratification resulted in a cycle of secondary dormancy and its subsequent relief; the thermal time characterizing the second, lower, peak in germinability coincided approximately with that attained in the field in the second spring after collection. 4 Shortening the growing season of Najas marina , by collecting material before natural senescence was complete, reduced the proportion of mature seeds, lowered average germinability and increased germination times. 5 The thermal capacity of water bodies causes a lag in spring warming, whereas declining light becomes limiting in autumn. Najas marina is crucially dependent on the length of growing season to produce sufficient viable seed. A low‐temperature requirement to break seed dormancy prevents inappropriate autumn germination. In a prolonged winter (necessarily followed by a shorter growing season), a significant proportion of seed would go into secondary dormancy and remain in the seed bank until the second spring. 6 In annual plants, such requirements for warm summers and cold winters could contribute to a continental‐climate distribution in northern Europe. In the case of Najas marina this might explain its limit at the eastern extremity of England and thus its rarity.