Unraveling the effects of cold stratification and temperature on the seed germination of invasive Spartina alterniflora across latitude

Seed germination is critical to the life history of plants, playing an
important role in the successful recruitment, colonization, and even
invasion of new individuals within and outside population distribution
ranges. Cold stratification and temperature are the key factors affecting
seed germination traits. Studying how these two factors drive geographical
variation in seed germination is essential to analyze and predict the
geographical distribution range of alien plants in novel habitats.
Spartina alterniflora, native to the United States, was introduced into
China in 1979 and has spread over 20° of latitude along the eastern coast
of China. Germination plays a crucial role in S. alterniflora’s large-scale
invasion and diffusion across latitude. To evaluate the effects of cold
stratification and temperature on seed germination of S. alterniflora across
latitude, we collected seeds at seven locations across latitude in China.
We exposed these provenances to cold stratification at 4°C (0, 1, 3, and 5
months) and germination temperature (5°C, 15°C, 25°C, and 35°C) treatments
in growth chambers. Seed germination was observed for 98 days, and we
calculated germination rate, germination index, and germination time.
Results indicated that longer cold stratification significantly promoted
germination rate and germination index, but decreased germination time.
Similarly, higher germination temperature significantly promoted
germination rate and germination index, but decreased germination time.
Moreover, there were significant interactive effects on germination traits
between cold stratification and temperature. Seed germination traits showed
linear relationships with latitude, indicating that S. alterniflora seeds
from different provenances germinated at different times and adopted
different germination strategies. The stratification and temperature are
the most important factors regulating the dormancy and germination seeds,
so they can be important drivers of this variation along latitude. Under
scenarios of warmer regional temperature, seeds at higher latitudes could
germinate earlier and higher germination rate, which would favor a
potential northern expansion of this invasive plant.