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INTERACTIONS BETWEEN INTERNAL AND EXTERNAL CO 2 POOLS IN THE PHOTOSYNTHESIS OF THE AQUATIC CAM PLANTS LITTORELLA UNIFLORA (L.) ASCHERS AND ISOETES LACUSTRIS L.
Author(s) -
MADSEN TOM VINDBÆK
Publication year - 1987
Publication title -
new phytologist
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 3.742
H-Index - 244
eISSN - 1469-8137
pISSN - 0028-646X
DOI - 10.1111/j.1469-8137.1987.tb04789.x
Subject(s) - malic acid , crassulacean acid metabolism , photosynthesis , decarboxylation , carbon dioxide , botany , total inorganic carbon , chemistry , carbon fibers , oxygen , photorespiration , dissolved organic carbon , environmental chemistry , biology , biochemistry , organic chemistry , materials science , composite material , composite number , citric acid , catalysis
S ummary The significance of CAM as a carbon‐conserving mechanism in two submerged aquatics, Littorella uniflora (L.) Aschers and hoetes lacustris L., was evaluated by determining (1) the loss of previously fixed CO 2 , released through decarboxylation of malic acid and (2) the quantitative importance of CAM relative to external CO 2 uptake in photosynthesis. Using a 14 C‐labelling technique it was found that the loss of CO 2 derived from decarboxylation of malic acid constituted less than 2 % of nocturnal carbon uptake, confirming that the diurnal rhythm of acidity provides a good measure of the incorporation of carbon via CAM. The exchange pattern of inorganic carbon and oxygen was measured for plants incubated in open flow‐through systems. The contribution of internal and external CO 2 to photosynthesis was determined as the difference in CO, uptake and oxygen release, where excess oxygen release reflected the assimilation of CO 2 released from deacidification of malic acid. Despite a rapid deacidification, uptake of external CO 2 was stimulated by 15 to 30% at intermediate external CO 2 concentrations. It is suggested that this effect was due to a reduced photorespiratory activity caused by an enhanced internal CO 2 concentration generated from malic acid. The simultaneous uptake of inorganic carbon from high internal and low external CO 2 concentrations can only be explained by assuming a non‐linear CO, gradient from the lacunal air to the bulk medium, with the CO 2 concentration in the outermost cell layers being lower than both the bulk medium and the lacunal air. The relative contribution of CAM to the total uptake of CO 2 in daytime declined from 95% (both species) at an external CO 2 concentration of 30μ CO 2 to 38% ( Littorella ) and 34% ( Isoetes ) at 200μ CO 2 . This resulted from increased uptake of external CO 2 at high external CO 2 concentrations and a parallel suppression of internal decarboxylation of malic acid. The observed suppression of decarboxylation was confirmed by following the time course in the content of titratable acidity of the leaves. A reversible inhibition of daytime deacidification was seen for external CO 2 concentrations higher than 3.0 to 5.4 mM CO 2 in both Littorella and Isoetes. The functional significance of CAM for aquatics rests in the enhanced capacity for obtaining inorganic carbon resulting from the extension of the diel period in which inorganic carbon can be accumulated and in the high reassimilation efficiency of nocturnal respiratory CO 2 .