The effect of temperature on the phenology of germination of Isoëtes lacustris

Isoëtes lacustris (quillwort) is an aquatic macrophyte commonly dominating oligotrophic softwater lakes in Europe. Reproductive ecology of a relic population of quillwort based on spore germination was studied in an acidified mountain lake in the Czech Republic. In a four-year experiment, we recorded temperature-related temporal changes in micro- and macrospore germination and sporeling establishment in (i) natural in situ conditions in Černé jezero lake and (ii) in the laboratory at various temperatures. Germination of both micro- and macrospores increased gradually over four consecutive growth seasons. Several annual cohorts of germinating macrospores born together in a sporangium indicate that spores remain viable for up to several years and the formation of a spore bank. Minimum temperature necessary for germination was lower for microspores (6 °C) than macrospores (12 °C) and this ensured the availability of spermatozoids for the fertilization of the long-living macrogamethophytes whenever they produced archegonia during growing season(s). Macrogametophyte development started between July and October and sporeling development always followed in the next or subsequent springs. Long germination and embryo development may limit reproduction in I. lacustris by making it sensitive to both episodic and chronic changes in the environment. The relatively high minimum temperature for macrospore germination may set general limits for the reproduction of I. lacustris in lakes, by constraining its distribution along latitudinal and altitudinal gradients, and to particular depths. The mean length of time when the temperature ( 12 °C) was high enough for the germination of I. lacustris in Černé jezero lake was 119 days and occurred during the period June to September in 2004–2011.