Toward predicting reproductive success in submersed freshwater angiosperms

Abstract

Reproduction is central to changes in distribution and abundance patterns of submersed freshwater macrophytes. In this paper we seek to identify potential ‘bottlenecks’ limiting sexual or vegetative reproduction or its predictability for these plants. To this end we consider the regulation of seed production, seed fate, vegetative propagule production, and vegetative propagule fate in turn, both in general terms and for the perennial Vallisneria americana Michx. in particular Plant biomass appears to be an important determinant of the incidence of flowering for greenhouse-grown Vallisneria americana: 88% of 425 plants either failed to flower if below a threshold dry weight of 0.75 g, or flowered if above it. The number of flowers (in females) or inflorescences (in males), however, was only moderately well correlated with Vallisneria biomass, and the flowering ‘threshold rule’ weakened considerably in the field. Closer examination of the dependence of seed output on plant biomass and pollination intensity, and of the importance of seed or fruit abortion is needed for submersed species. Our ignorance of seed fate is exemplified by the paucity of quantitative studies on dispersal distances and seed deposition patterns, on seed banks, and on features of seed microenvironments pertinent to germination, although the sensitivity of seed dormancy and germination to a variety of environmental conditions has been tested in the laboratory for many species. Further, losses of seeds and seedlings as a result of unfavorable environments, sediment disturbance, flotation, pathogens, and herbivores are generally not well characterized. Overall, the principal limiting step for sexual reproduction may in some cases be lack of floral induction, and in others, ineffective pollination, but dispersal away from the local population, failure of seeds to germinate, and the challenges of seedling establishment cannot be ruled out. Plant size is well correlated with vegetative propagule production for both greenhouse and field Vallisneria populations. As with seed germination, the germination of vegetative propagules has been relatively well studied in the laboratory, but important regulators of germination and establishment in the field warrant further study. We suggest that more investigations at two interfaces of physiological ecology and demography may be particularly fruitful. Regarding propagule production, experimental focus on allocation patterns which ‘translate’ accumulated resources into seeds or vegetative propagules should prove rewarding. Regarding propagule fate, post-dispersal mortality of seeds and vegetative propagules may be rooted in phytiological tolerance limits, but could also be attributable to pathogen or herbivore attack.