Growth and reproductive characteristics of an aquatic macrophyte Ottelia alismoides (L.) Pers. (Hydrocharitaceae)

Life history and reproductive traits of a submerged aquatic weed, Ottelia alismoides (L.) Pers., were studied in irrigation ponds in southwestern Japan. In addition, growth and seed production were investigated in relation to nutrient condition and water depth. The size of plants was very variable but reproductive allocation was nearly constant irrespective of the plant size at reproductive stage, amounting to more than 30% at maximum. Bagging experiments confirmed autogamy of the species. Ottelia alismoides grew best in nutrient-rich water with mud substratum. The optimal water depth for the growth of the species was approximately 50 cm compared with 20 and 90 cm. However, the reproductive allocation and seed set rate did not change much in different nutrient and water depth conditions. The seeds were produced by cleistogamous flowers in a depth of 90 cm. Morphological and ecological plasticity revealed in the present study was considered to be adapted to unstable environments such as rice fields and irrigation ponds liable to water-level fluctuation. The conditions required for the restoration of the populations of O. alismoides are discussed based on the present results.