Diurnal variation in light and carbon limitation of photosynthesis by two species of submerged freshwater macrophyte with a differential ability to use bicarbonate

摘要

SUMMARY.1Rates of photosynthetic oxygen evolution byCallitriche cophocarpaandRanunculus peltatusin 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.2Despite an ambient CO2concentration 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 CO2were about 130 and 425 μmol O2g−1DW h−1forC. cophocarpaandR. peltatus, respectively. These rates as a percentage of carbon saturated rates were only about 35 forC. cophocarpaand about 60 forR. peltatus.Ambient rates as a percentage of light saturated rates were about 80 forC. cophocarpaand about 95 forR. peltatus.Only in early morning and late evening where the photon irradiance was below 160 μmol m−2s−1was there evidence for slight light limitation.3Based on results from pH‐drift experiments and from rates of photosynthesis as a function of CO2concentration in the presence and absence of HCO3−,C. cophocarpawas unable, butR. peltatusable to use HCO3−at an ambient HCO3−concentration of about 0·84 mm. The greater rates of photosynthesis at ambient CO2concentration and the lesser limitation by inorganic carbon shown byR. peltatuscompared toC. cophocarpawas the result of HCO3−‐use as laboratory experiments showed thatR. peltatusperformed similarly toC. cophocarpaif the HCO3−conce