Models of the Use of Root-zone CO2 by Selected North American Isoetids

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Abstract
Sediment CO2, entering via the roots, contributes a significant portion of the total carbon uptake for isoetids (small, evergreen, submersed, vascular plants). Laboratory studies of inorganic carbon uptake via the roots and shoots by five isoetids were used to model the use of root-zone CO2. Simple first-order linear models accounted for at least 75 per cent of the variation in the data for Gratiola aurea, Isoetes macrospora, Littorella uniflora and Lobelia dortmanna. For Eriocaulon septangulare, which relies almost exclusively on root-zone CO2, models could account for only about 62 per cent of the variation in root-zone CO2 use. For each species, we present the best fitting regression of root-zone CO2 use as a function of root- and shoot-zone CO2 concentrations. For the species studied, carbon uptake was not saturated at field concentrations of root and shoot-zone CO2. Maximum rates of carbon uptake were lower for species that naturally occurred at greater depths, compared with species more common in shallow water. At equal external CO2 concentrations carbon entry per unit root surface area was several times more rapid than entry per unit shoot surface area for L. dortmanna. The entry rates per unit root and shoot surface area were about equal for G. aurea and E. septangulare. Shoots were equally or more permeable than the roots of L. uniflora and I. macrospora, a fact that may be related to the functioning of crassulacean acid metabolism in these plants.