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 (lmol 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.