Ecological life history and reproductive traits of two threatened aquatic macrophytes, Blyxa aubertii L.C. Richard and B. echinosperma (Clarke) Hooker, were studied in irrigation ponds in south‐western Japan. The size of plants varied greatly in both species but reproductive allocation was nearly constant irrespective of plant size at reproductive stage, amounting to 31.8% and 45.9% at maximum in B. aubertii and B. echinosperma, respectively. The number of flowers and total number of seeds produced per individual were associated significantly with plant size. Seeds were produced by self‐pollination both in emerged and submerged cleistogamous flowers and seed set rate of the two species was very high. These traits are considered to be adaptive to unstable environments that are liable to water level fluctuations, such as rice‐fields and irrigation ponds.
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.
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