Photosynthesis in the Aquatic Macrophyte Egeria densa. III. Gas Exchange Studies

Abstract

When free CO₂ alone was present in the ambient medium, photosynthesis by Egeria densa Planch displayed an apparent K_m of 77 μM. A light- and CO₂-saturated rate of 100 μmol C (mg Chl)^-1 h^-1 was achieved only in 400 μM CO_2(aq) [c. 1% CO_2(g)]. The CO₂ response data and other considerations suggest that, although the carboxylation and mesophyll resistances (3800 s m^-1 and <9000 s m^-1 respectively) are considerably higher than in aerial plant leaves, the boundary layer is the highest component (> 27 000 s m^-1) of the total resistance. An increase in the total resistance of 7200 s m^-1 between 0.02 and 0.21 atm O₂ (2 and 21 kPa O₂) is attributed to photorespiration.

Closed and open circuit gas exchange experiments demonstrated that bicarbonate is taken up by the plant cells and does not act merely as a reservoir of inorganic carbon for production of CO₂ at the plasmalemma. Bicarbonate stimulated photosynthesis, even when the free CO₂ concentration was below the CO₂ compensation point. The total resistance to bicarbonate uptake appears to be 8-12 times that for CO₂ uptake presumably due to the processes of active uptake, transport and/or conversion to CO₂ involved in bicarbonate but not CO₂ assimilation.