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Keywords:

  • crassulacean acid metabolism (CAM);
  • isoetids;
  • leaf malate;
  • photorespiration;
  • submergence tolerance;
  • underwater photosynthesis;
  • vernal pool;
  • wetland plant

Summary

  • Underwater photosynthesis by aquatic plants is often limited by low availability of CO2, and photorespiration can be high. Some aquatic plants utilize crassulacean acid metabolism (CAM) photosynthesis. The benefits of CAM for increased underwater photosynthesis and suppression of photorespiration were evaluated for Isoetes australis, a submerged plant that inhabits shallow temporary rock pools.
  • Leaves high or low in malate were evaluated for underwater net photosynthesis and apparent photorespiration at a range of CO2 and O2 concentrations.
  • CAM activity was indicated by 9.7-fold higher leaf malate at dawn, compared with at dusk, and also by changes in the titratable acidity (μmol H+ equivalents) of leaves. Leaves high in malate showed not only higher underwater net photosynthesis at low external CO2 concentrations but also lower apparent photorespiration. Suppression by CAM of apparent photorespiration was evident at a range of O2 concentrations, including values below air equilibrium. At a high O2 concentration of 2.2-fold the atmospheric equilibrium concentration, net photosynthesis was reduced substantially and, although it remained positive in leaves containing high malate concentrations, it became negative in those low in malate.
  • CAM in aquatic plants enables higher rates of underwater net photosynthesis over large O2 and CO2 concentration ranges in floodwaters, via increased CO2 fixation and suppression of photorespiration.