CO2 concentration mechanisms in Egeria densa, a submersed aquatic species

LARA, MARIA V.; CASATI, PAULA; ANDREO, CARLOS S.

PHYSIOLOGIA PLANTARUM

WILEY-BLACKWELL PUBLISHING, INC

Año: 2002 vol. 115 p. 487 - 495

0031-9317

Egeria densa is an aquatic higher plant which has developeddifferent mechanisms to deal with photosynthesis under conditionsof low CO2 availability. On the one hand it shows leafpH-polarity, which has been proposed to be used for bicarbonateutilization. In this way, at high light intensities and lowdissolved carbon concentration, this species generates a lowpH at the adaxial leaf surface. This acidification shifts theequilibrium HCO3?/CO2 towards CO2, which enters the cellby passive diffusion. By this means, E. densa increases theconcentration of CO2 available for photosynthesis inside thecells, when this gas is limiting. On the other hand, under stressconditions resulting from high temperature and high light intensities,it shows a biochemical adaptation with the induction of a C4-like mechanism but without Kranz anatomy. Transferfrom low to high temperature and light conditions inducesincreased levels of phosphoenolpyruvate carboxylase (PEPC,EC 4.1.1.31) and NADP-malic enzyme (NADP-ME, EC1.1.1.40), both key enzymes participating in the Hatch-Slackcycle in plants with C4 metabolism. Moreover, one PEPCisoform, whose synthesis is induced by high temperature andlight, is phosphorylated in the light, and changes in kineticand regulatory properties are correlated with changes in thephosphorylation state of this enzyme. In the present review,we describe these two processes in this submersed angiospermthat appear to help it perform photosynthesis under conditionsof extreme temperatures and high light intensities.