Reproductive Allocation in Annual and Perennial Species of the Submerged Aquatic Halophyte Ruppia

Abstract

(1) The allocation of resources to reproductive and non-reproductive parts was investigated in annual and perennial species of the submerged aquatic halophyte Ruppia. Dry weights were recorded for plants from permanent and ephemeral ecosystems in the south-east of South Australia over 13 months in 1977 and 1978. One perennial species, R. megacarpa, and two annual species R. tuberosa and R. polycarpa, were studied. (2) The proportion of plant dry weight allocated to reproduction, both sexual (seeds, flowers, fruit parts) and asexual (turions) was much greater in all seasons for the two annual species than for the perennial. Seeds and turions of the annuals comprised 100% of live parts (3 g m-2 for R. tuberosa, 2 g m-2 for R. polycarpa) during the 7-8 months when the lakes were dry, in contrast to the maximum allocation to reproduction of 4.5% (seeds, 10.8 g m-2) for the perennial R. megacarpa. When vegetative growth was maximal for the annuals the minimum allocation to reproduction occurred (13%J/o (9.1 g m-2) for R. tuberosa and 5% (4-8 g m-2) for R. polycarpa). (3) Rhizomes alone survived desiccation and formed separate reproductive units in the modified form of dormant turions in the two annual species. (4) Although the two annual species were similar in their total reproductive effort they showed differences in the distribution of this effort between asexual and sexual reproductive units: R. tuberosa had 89% turions and 11% seeds whereas R. polycarpa had 14% turions and 86% seeds in the dry season. (5) These results are consistent with those reported for terrestrial annual and perennial species in other studies. The monocarpic reproductive pattern, rapid development of the plants, early maturity and large amount of energy allocated to the production of a large number of small propagules are characteristic of both annual species. This contrasts markedly with the polycarpic reproductive pattern, slow plant development, late maturing and the smaller amount of energy allocated to producing a small number of large propagules in the perennial.