A discussion of the adaptations for carbon acquisition in relation to the growth strategy of aquatic isoetids

Abstract

Annual productivity of populations of the CAM-isoetids Littorella uniflora (L.) Aschers. and Isoetes macrospora Durieu. in an oligotrophic and mesotrophic lake were determined as part of a study of adaptations for carbon acquisition. Leaf turnover rates were 1.58 year −1 and 1.72 year −1 for Littorella and 0.85 year −1 and 1.00 year −1 for Isoetes. Areal productivity ranged from 6.3 to 33 g C m −2 year −1. The productivities were related to site-specific factors and could not be predicted based on lake trophy Site characteristics and the low annual productivities were typical for isoetids. Tissue nutrient concentrations were fairly constant throughout the growing season and plants in the oligotrophic lake had similar or greater tissue nutrient concentrations relative to plants in the mesotrophic lake. The results of this study and other reports for isoetids suggest that isoetids inhabit nutrient-poor, carbon-poor sites. The growth characteristics of several common isoetids show a similar response to environmental constraints on productivity, as do terrestrial plants from chronically unproductive habitats. Furthermore, isoetids possess several uncommon photosynthetic adaptations: (1) the utilization of sediment CO 2; (2) CAM; and (3) efficient recycling of endogenous CO 2. The utilization and apparent importance of these adaptations may be restricted largely to the isoetids because; (1) the growth form facilitates these adaptations and (2) these adaptations provide a small but constant carbon supply for slow growing plants in carbon-poor habitats.