Diurnal variation in light and carbon limitation of photosynthesis by two species of submerged freshwater macrophyte with a differential ability to use bicarbonate
1991
MADSEN, TOM V. | MABERLY, STEPHEN C.
1 Rates of photosynthetic oxygen evolution by Callitriche cophocarpa and Ranunculus peltatus in stream were measured on live occasions during the light period on 2 days at ambient light and ambient inorganic carbon, ambient light and saturating inorganic carbon, saturating light and ambient inorganic carbon, saturating light and saturating inorganic carbon and air‐equilibrium inorganic carbon and ambient light. 2 Despite an ambient CO2 concentration of about 220 μm, which is about ten times air‐equilibrium, the concentration of inorganic carbon was more limiting than light on all the occasions that rates were measured. On average, rates of photosynthesis at ambient concentrations of CO2 were about 130 and 425 μmol O2 g−1 DW h−1 for C. cophocarpa and R. peltatus, respectively. These rates as a percentage of carbon saturated rates were only about 35% for C. cophocarpa and about 60% for R. peltatus. Ambient rates as a percentage of light saturated rates were about 80% for C. cophocarpa and about 95% for R. peltatus. Only in early morning and late evening where the photon irradiance was below 160 μmol m−2 s−1 was there evidence for slight light limitation. 3 Based on results from pH‐drift experiments and from rates of photosynthesis as a function of CO2 concentration in the presence and absence of HCO3 −, C. cophocarpa was unable, but R. peltatus able to use HCO3 − at an ambient HCO3 − concentration of about 0·84 mm. The greater rates of photosynthesis at ambient CO2 concentration and the lesser limitation by inorganic carbon shown by R. peltatus compared to C. cophocarpa was the result of HCO3 −‐use as laboratory experiments showed that R. peltatus performed similarly to C. cophocarpa if the HCO3 − concentration was reduced to 60 μm.
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