Growth and physiological acclimation to temperature and inorganic carbon availability by two submerged aquatic macrophyte species, Callitriche cophocarpa and Elodea canadensis

1. Interactive effects of temperature and inorganic carbon availability on photosynthetic acclimation and growth of two submerged macrophyte species, Elodea canadensis and Callitriche cophocarpa, were examined to test the hyp otheses that: (1) effects of temperature on growth rate and photosynthetic acclimation are suppressed under low inorganic carbon availability; (2) the plants compensate for the reduction in activity of individual enzymes at l ower temperatures by increasing the activity per unit plant mass, here exem plified by Rubisco. The experiments were performed in the laboratory where plants were grown in a factorial combination of three temperatures (7-25 de grees C) and three inorganic carbon regimes. 2. The relative growth rate of both species was strongly affected by growth conditions and increased by up to 4.5 times with increased temperature and inorganic carbon availability. The sensitivity to inorganic carbon was gre atest at high temperature and the sensitivity to temperature greatest at hi gh carbon concentrations. 3. Photosynthetic acclimation occurred in response to growth conditions for both species. The affinity for inorganic carbon and the photosynthetic cap acity, both measured at 15 degrees C, increased with reduced inorganic carb on availability during growth and were greater at warmer than at cooler gro wth temperature. The acclimative change in photosynthesis was related to th e extent of temperature and inorganic carbon stress. Using data for Elodea, a negative relationship between degree of temperature stress and photosynt hetic performance was found. In relation to inorganic carbon, a linear incr ease in CO2 affinity and photosynthetic capacity was found with increased i norganic carbon stress during growth. 4. The total Rubisco activity declined with increased inorganic carbon avai lability during growth and with enhanced growth temperature. In addition, t he activation state of Rubisco was higher at cooler than at warmer temperat ures for Callitriche. This suggests that low-temperature grown plants compe nsate for the temperature-dependent reduction in activity of the individual Rubisco molecules by enhancing resource allocations towards Rubisco.