Inorganic carbon assimilation in the Isoetids, Isoetes lacustris L. and Lobelia dortmanna L

The inorganic carbon fixation patterns of Isoetes lacustris and Lobelia dortmanna from an oligotrophic Scottish loch have been examined by following titratable acidity changes in plant sap and light/dark ¹⁴CO₂ incorporation by roots and shoots. The diurnal pattern of titratable acidity changes in I. lacustris suggests crassulacean acid metabolism (CAM) while the lack of any change in titratable acidity in L. dortmanna suggests C₃ metabolism. Of the carbon fixed by L. dortmanna, 99.9% was taken up through the roots and fixation occurred primarily during the day. In Isoetes, CO₂ was taken up by both roots and shoots and during both day and night. Regardless of the site of CO₂ uptake, fixation occurred only in the shoots of both plants. Analysis of carbon isotope ratios of plant organic material was used to further investigate the photosynthetic mechanisms of these Isoetids. Considering the absence of a nighttime peak in titratable acidity in L. dortmanna, the Δ¹³C (Δ=δ¹³C plant-δ¹³C source) value of the shoots of L. dortmanna (-14.2‰) is indicative of C₃ photosynthesis limited by the rate of CO₂ diffusion. The less negative Δ of I. lacustris (-6.0‰) is consistent with both dark acidification of CAM and CO₂ limited C₃ photosynthesis. This is in contrast to the terrestrial Isoetes durieui which is shown to have a Δ value which is similar to a terrestrial C₃ plant. The carbon fixation patterns of these Isoetids suggest that the CO₂ concentration in the loch may be growth limiting, and that root uptake and/or dark acidification are means of optimising CO₂ supply. However, in view of the relatively high levels of CO₂ in sediment and bulk water, it is suggested that low levels of nutrients may also limit growth in these plants.