Photosynthetic performance of submerged macrophytes from lowland stream and lake habitats with contrasting C02 availability

摘要

We examine the photosynthetic response of submerged plants from streams and lakes with contrasting free-C02 and nitrogen (N) availability. We hypothesized that: the photosynthetic capacity of stream plants is higher because of higher N availability; the photosynthetic N-use efficiency (PNUE) is also higher because stream plants are acclimated to higher free-C02; and PNUE is lower in aquatic compared to terrestrial plants. We tested these hypotheses by measuring tissue-N, photosynthetic capacity and inorganic C extraction capacity in plants collected from streams and lakes and by comparing the PNUE of aquatic plants with previously published PNUE of terrestrial plants. We found that the organic N content was consistently higher in stream (3.8-6.3% w/w) than in lake plants (1.2^.3% w/w). The photosynthetic capacity correlated positively with tissue-N. The relationships were similar for stream and lake plants, indicating that N allocation patterns were similar despite variability in free-C02 between the two habitats. The slope of the relationship between photosynthetic capacity and tissue-N was lower than found for terrestrial plants, whereas the compensatory N content for photosynthesis was similar. This suggests that PNUE is lower in aquatic plants, perhaps reflecting that the selection pressure for a high C fixation rate per unit N is reduced as a result of low inorganic C availability in the aquatic environment.