Growth, photosynthesis and acclimation by two submerged macrophytes in relation to temperature

In this study we examine the influence of temperature on growth, photosynthetic performance and acclimation of two submerged macrophyte species, Elodea canadensis L.C. Rich and Ranunculus aquatilis (L.) Wimmer. The plants were grown at 5, 10 and 15°C and a photon flux density of 300 μmol m^-2 s^-1 (PAR) in a medium with an alkalinity of 0.85 meq l^-1 bubbled with atmospheric air containing 400 ppm CO₂. In general, growth rates of both species increased with temperature with a Q ¹⁰ varying from 2.3 to 3.5. An exception was Elodea at 5°C, where growth was nearly arrested. Temperature effects on ambient rates of net photosynthesis and photosynthetic capacity followed the pattern observed for growth. Dark respiration was not suppressed for Elodea at 5°C and both species had a Q ¹⁰ of 2.3. The light-use efficiency (α^I) for photosynthesis declined with increasing growth temperature for Ranunculus. For Elodea no difference in α^I was observed between 10 and 15°C; at 5°C, however, α^I was reduced by about 30%. Both species acclimated to temperature as shown in a series of experiments in which the plants were exposed to a change in temperature. Acclimation was faster for shoots transferred from low to high temperature, where growth rates stabilised after a few days; for shoots transferred to low temperature growth rates still changed after 22 days. Although acclimation was evident, the changes in the metabolic apparatus were insufficient to balance effects of temperature. It is suggested that temperature may affect local distribution of the two species and their ability to grow in turbid or deep water.