Variable photosynthesis/photorespiration ratios in Hydrilla and other submersed aquatic macrophyte species

Hydrilla verticillata plants collected from Orange Lake in Florida during the summer exhibited higher net photosynthetic rates and lower photorespiratory and dark respiratory rates than plants collected during the winter when measured at a standard temperature of 25 °C. Similar seasonal shifts in the CO 2 compensation point, ranging from 17 μL CO 2 /L in summer to 90 in winter, occurred in Hydrilla from three geographically distinct lakes in Florida. Incubation of Hydrilla and five other submersed angiosperms under a high temperature (27 °C day) and long photoperiod (14 h) reduced the CO 2 compensation point to less than 20. A continuously high CO 2 concentration of 5000 μL/L (gas phase) prevented the decline in the CO 2 compensation point, whereas 300 μL/L induced it. Lake-grown and chamber-grown Hydrilla with low photorespiratory capacity, as determined by low CO 2 compensation points, showed elevated phosphoenolpyruvate carboxylase activity (239 versus 33 μmol/mg Chl per hour). Diurnal gas-exchange measurements at 300 and 3000 μL CO 2 /L (gas phase) demonstrated that low photorespiration Hydrilla exhibited net CO 2 fixation in the dark at the higher CO 2 level, probably via the elevated phosphoenolpyruvate carboxylase activity. Only Hydrilla with low CO 2 compensation points exhibited net dark fixation and then only during the night period. The excretion of 14 C-organic compounds was only 3% of the 14 CO 2 light fixation rate and thus too low to account for the reduced photorespiration. The potential to alter the photosynthesis/photorespiration ratio appears ubiquitous among submersed freshwater macrophytes. This ratio change to a low photorespiratory capacity, and the induction of a dark fixation capacity, would aid the carbon economy under summer daytime conditions of low inorganic carbon and high O 2 , where photorespiratory CO 2 loss would otherwise be substantial.