Temperature characteristics in seed germination and growth of Zostera japonica Ascherson and Graebner from Ago Bay, Mie Prefecture, central Japan

The optimal water temperature in seed germination and the upper critical water temperature in seedling growth were determined for Zostera japonica collected from Ago Bay, Japan. The relationship between the seed germination rates and seed storage period (0, 30, and 60 days) at 0degC was also examined. The optimal water temperature in seed germination was in the range 15-20degC regardless of the storage period, in which germination rates were up to 14%. Seedlings, grown from seeds up to 10 cm in total length, were cultured for 1 week at various water temperatures to measure their relative growth rates. The optimal water temperature in early growth was in the range 20-25degC; relative growth rates ranged from 3.8 to 4.2%. Seedlings could survive up to a water temperature of 29degC, but most seedlings withered at 30 or 35degC. The optimal water temperatures for seed germination and seedling growth were related to the seasonal changes of water temperature in the sampling site. Although seedlings were hardly observed in Ago Bay in summer, Z. japonica might extend its distribution as far as where the summer water temperature is lower than 29degC.