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.