Germination characteristics of Zannichellia palustris from a northern California spring-fed river

• Light and temperature affected Zannichellia palustris germination.
• Photoperiod and light intensity have no effect above stimulatory light levels.
• Germination is temperature-dependent above stimulatory light levels.
• Maximum germination rate and fraction was between 25 and 30 °C.
• Germination time was log-normally distributed across the seed population.

The germination characteristics of Zannichellia palustris seeds collected from the spring-fed Fall River of northern California were investigated as affected by temperature, darkness, photoperiod, and light intensity. Darkness significantly inhibited germination; however, once stimulatory light levels were reached, germination time and the fraction of seeds that germinated were neither affected by photoperiod nor light intensity. The effects of temperature were assessed across a range of constant values from 4.2 to 40.8 °C. Seeds germinated at all temperatures except 40.8 °C, while the maximum fraction of seeds that germinated was observed at 24.4 and 29.5 °C, with 69 ± 5 and 73 ± 3% total germination, respectively. Germination times were also fastest at 24.4 and 29.5 °C, where median germination was 9 d. The cumulative spread in germination over time was determined to be log-normally distributed, and was best described by a probit function for both individual temperatures and thermal units derived from linear regressions of subpopulations. These results, which deviate from those reported for Z. palustris in other parts of the world, suggest that Z. palustris may be able to adapt to local conditions, such as cold spring-fed waters as found in the Fall River of northern California.