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

Sediment CO₂, entering via the roots, contributes a significantportion of the total carbon uptake for isoetids (small, evergreen,submersed, vascular plants). Laboratory studies of inorganiccarbon uptake via the roots and shoots by five isoetids wereused to model the use of root-zone CO₂. Simple first-order linearmodels accounted for at least 75 per cent of the variation inthe data for Gratiola aurea, Isoetes macrospora, Littorellauniflora and Lobelia dortmanna. For Eriocaulon septangulare,which relies almost exclusively on root-zone CO₂, models couldaccount for only about 62 per cent of the variation in root-zoneCO₂ use. For each species, we present the best fitting regressionof root-zone CO₂ use as a function of root- and shoot-zone CO₂concentrations. For the species studied, carbon uptake was not saturated atfield concentrations of root and shoot-zone CO₂. Maximum ratesof carbon uptake were lower for species that naturally occurredat greater depths, compared with species more common in shallowwater. At equal external CO₂ concentrations carbon entry perunit root surface area was several times more rapid than entryper unit shoot surface area for L. dortmanna. The entry ratesper unit root and shoot surface area were about equal for G.aurea and E. septangulare. Shoots were equally or more permeablethan the roots of L. uniflora and I. macrospora, a fact thatmay be related to the functioning of crassulacean acid metabolismin these plants. Carbon, CO₂, photosynthesis, isoetid, Eriocaulon septangulare, Gratiola aurea, Isoetes macrospora, Littorella uniflora, Lobelia dortmanna     

Utilization of Sediment CO2 by Selected North American Isoetids

The photosynthetic uptake of root-zone CO₂ was determined forEriocaulon septangulare, Gratiola aurea, Isoetes macrospora,Littorella uniflora var. americana and Lobelia dortmanna aspart of a study of the photosynthetic carbon economy of submergedaquatic isoetids. The pH and dissolved inorganic carbon (DIC)of the sediment interstitial water in four Wisconsin lakes reflectedthe water column character, where the DIC increased with depthin the sediment to concentrations five to ten times those ofthe water column. Sediment free CO₂ concentrations were 5–50times those in the water column and were similar at all sites(about 05–1.0mM CO₂ in the root-zone). In ‘pH-drift’studies these plants were unable to take up HCO₂^–. Laboratory determinations of the carbon uptake from the rootand shoot-zones were made for all five species. These experimentsshowed that CO₂ in the root-zone accounted for 65–95 percent of external carbon uptake for the five species. For G.aurea and E. septangulare, root-zone CO₂ was > 85 per centof carbon uptake. Carbon, CO₂, photosynthesis, sediment, isoetid, Eriocaulon septangulare, Gratiola aurea, Isoetes macrospora, Littorella uniflora, Lobelia dortmanna