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 Km of 77 μM. A light- and CO2-saturated rate of 100 μmol C (mg Chl)-1 h-1 was achieved only in 400 μM CO2(aq) [c. 1% CO2(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.