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In this study the effect of ontogenetic drift on crassulacean acid metabolism (CAM) was investigated in the aquatic CAM-isoetid Littorella uniflora . The results of this study strengthen the general hypothesis of CAM being a carbon-conserving mechanism in aquatic plants, because high-CAM capacity (45–183 μequiv. g −1 FW) was present in all leaves of L. uniflora irrespective of age. Since possession of CAM in aquatic plants allows CO 2 uptake throughout the light/dark cycle, presence of CAM in all leaves influences the carbon balance of L. uniflora positively. On average for all lakes, different leaf classes accounted for 11–36% of the total dark CO 2 uptake by the individual plant. The capacity for both CAM and photosynthesis declined with increasing leaf age, and was in the oldest leaves only 25–53% of the capacity in the youngest. The photosynthetic capacity was estimated to be sufficiently high to ensure refixation of the CO 2 released from malate during decarboxylation in the daytime. In line with this, a linear coupling between CAM capacity and photosynthetic capacity was found. Parallel to the change in photosynthetic capacity, an age-related change in total ribulose-bisphosphate carboxylase/oxygenase (rubisco) activity from 732 μmol C g −1 DW h −1 in the youngest leaves to 346 μmol C g −1 DW h −1 in the oldest was observed. In contrast, no significant change in phosphoenolpyruvate carboxylase (PEPcase) activity with leaf age was observed (means ranged between 46 and 156 μmol C g −1 DW h −1 ). |
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