Does seed heteromorphism have different roles in the fitness of species with contrasting life history strategies?

Abstract

Punagrass ( Achnatherum brachychaetum (Godr.) Barkworth) and flechilla grande (Nassella clarazii (Ball) Barkworth) are perennial grasses native to Argentine grasslands, with contrasting life history strategies. Both species have seed heteromorphism, but punagrass is an invasive whereas flechilla is a climax species. Experiments were conducted to determine the relative contribution of chasmogamous (CH) seeds and cleistogamous (CL) seeds to growth and reproduction of the two species. CH progeny of punagrass grew and developed fast, but progeny of the largest CL seeds of the first node allocated more resources to the production of CH seeds. Growth and seed production of CH progeny of punagrass were enhanced by increasing nutrients, displaying adaptations to nutrient-rich environments. Thus, the slow-growing CL progeny may have potential to persist as a source of CH propagules until the creation of conditions favouring the less competitive CH progeny. Since CH progeny allocated more to CL seeds, but produced fewer CH seeds under low nutrient conditions, it is suggested that competitive environments would increase the production of CL seeds. Furthermore, when nutrients were abundant, punagrass allocated more to CH seeds. Under limiting nutrients, flechilla grande produced a few large CH seeds. The production of large seeds in flechilla grande may confer superior competitive ability to its progeny, but it may be disadvantageous under heavy grazing, which favours invasive species such as punagrass with both large CL seeds and many small CH seeds of high colonizing potential.