The interactive effects of light and inorganic carbon on aquatic plant growth

Abstract

ABSTRACTSubmerged aquatic macrophytes grow across a wide, often coupled, range of light and inorganic carbon availabilities, and each single factor influences photosynthesis and acclimation. Here we examine the interactive effects of light and inorganic carbon on the growth of Elodea canadensis and Callitriche cophocarpa. The plants were grown in the laboratory at a range of light intensities (0–108 μmol m−2s−1) and four inorganic carbon regimes in a crossed factorial design. Plant growth rates, measured over 3–4 weeks of incubation, increased in response to increasing light intensity and inorganic carbon availability, and significant interactive effects were observed. The light‐use efficiency for growth at low light increased 2‐fold for Callitriche and 6‐fold for Elodea between the lowest and highest inorganic carbon concentrations applied. Also, the growth rate at the highest light intensity increased with inorganic carbon availability, but the relative increase was smaller than at low light. Both species acclimated to the light and carbon regime such that the chlorophyll content declined at low and high light intensities and the initial slopes of the photosynthetic CO2 and HCO3− response curves declined at high levels of CO2. Callitriche responded less markedly than Elodea to changing inorganic carbon availability during growth, and the initial slope of the photosynthetic HCO3− response curve, in particular, was greatly reduced (>90%) in Elodea by high CO2. It is suggested that the coupled responses of aquatic macrophytes to light and inorganic carbon influence their ability to develop dense stands at high light in shallow water and to extend to greater depths in waters rich in inorganic carbon.