Interdependence of CO2 and inorganic nitrogen on crassulacean acid metabolism and efficiency of nitrogen use by Littorella uniflora (L.) Aschers

The hypothesis is tested that crassulacean acid metabolism (CAM) in isoetid s is a mechanism which not only conserves inorganic carbon but also plays a role in nitrogen economy of the plants, This hypothesis was tested in an o utdoor experiment, where Littorella uniflora (L,) Aschers, were grown at tw o CO2 and five inorganic nitrogen concentrations in a crossed factorial des ign. The growth of Littorella responded positively to enhanced nitrogen ava ilability at high but not at low CO2 indicating that growth was limited by nitrogen at high CO2 only. For the nitrogen-limited plants, the capacity fo r CAM (CAM(cap)) increased with the degree of nitrogen limitation of growth and an inverse coupling between CAM and tissue-N was found. Although this might indicate a role of CAM in economizing on nitrogen in Littorella, the hypothesis was rejected for the following reasons: (1) although CAM(cap) wa s related to tissue-N no relationship between tissue-N and ambient CAM acti vity (CAM(ambient)) was found whereas a close relationship would be expecte d if CAM was regulated by nitrogen availability; (2) the photosynthetic nit rogen use efficiency for high CO2-grown plants declined with increased CAM( ambient) and with CAM(cap); and (3) growth per unit tissue-N per unit time declined with increased CAM(ambient) and CAM(cap).