Photosynthesis in the Aquatic MacrophyteEgevia densa. I. ¹⁴CO₂ Fixation at Natural CO₂ Concentrations

Abstract

In *14C time-course and pulse-chase experiments, the photosynthetic fixation rate for Egeria densa Planch. was about 19 µmol C (g dry wt)^-1 min^-1 (13.7 mg C (g dry wt)^-1 h^-1) at a total inorganic carbon concentration of 1.0 x 10^-3 M, pH 6.8 and 75-80% light saturation. At this high rate, under conditions closely approximating those of plants in the field, ¹⁴C distribution into 3-phosphoglyceric acid, sugar phosphates and sucrose indicates that the Calvin cycle is the primary carboxylation mechanism and responsible for over 90% of the ¹⁴C initially incorporated. Malate accounts for only 5 % of the initial ¹⁴CO₂ fixation and is not actively further metabolized. Additional label from Calvin cycle intermediates enters malate during the cold chase treatment.

It is suggested that the predominant ¹⁴C fixation into malic acid, noted in previous reports on Hydrocharitacean species, may be due to very low experimental CO₂ concentrations and a consequent suppression of the Calvin cycle activity.

Compounds of the glycollate pathway were detected in sufficient quantities to give some support to the previous suggestion of photorespiratory activity in Egeria.