PHOTOCONTROL OF TURION FORMATION BY POTAMOGETON CRISPUS L. IN THE LABORATORY AND NATURAL WATER

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Abstract
Summary: The freshwater angiosperm Potamogeton crispus produces specialized vegetative buds, known as turions, in early summer, in contrast to their autumn production by other temperate climate species. Laboratory experiments showed that turion production by P. crispus is triggered by long days (16 h or longer) and high temperatures (16 °C and above) together. Long days represent a photoperiodic signal apparently perceived by phytochrome. A midnight exposure to dim R light is necessary to induce turion formation in plants kept under short days, irrespective of total daily light dose. A single long day suffices to initiate turion formation.Under inductive long warm days few turions form at low photon fluence rates (whatever the total daily light dose) or at low R: FR ratios near 1 during the photoperiod. Rate and magnitude of turion production are greatly increased by increased photon fluence rates and R: F ratios above 1. Sucrose feeding hastens turion development, but turion initiation is not significantly altered.As predicted from experimental findings and natural lakewater photoperiods and temperatures, turions were observed on plants in lakes studied from June until August. Also, turions were confined as predicted to approximately the upper 15 m depth range in lakes where R: FR ratios and photon fluence rates were inductively high. Below these depths the R: FR ratio remains high but photon fluence rate is inadequate. Thus laboratory and field findings concur. In north temperate waters, turions of P. crispus appear to act as summer propagules which settle and germinate by autumn, rather than as overwintering hibernaculae.