Photosynthetic characteristics (assessed by chlorophyll fluorescence and O2 evolution) were analysed monthly during one year in a tropical population of Nitella cernua from southeastern Brazil (20º50'32" S, 49º26'15" W). Parameters derived from photosynthesis-irradiance (PI) curves by fluorescence (high values of the photosynthetic parameter, Ik, and lack or low values of photoinhibition, beta) suggested adaptation to high irradiance, whereas those by O2 evolution showed a different pattern (low values of Ik and compensation irradiance, Ic, high values of photosynthetic efficiency, alpha, and photoinhibition). Parameters from PI curves by O2 evolution suggested light acclimation: Ik and maximum photosynthetic rate, Pmax (as rETR, relative electron transfer rate) increased, whereas a became significantly lower under higher irradiance (winter). This pattern is an adjustment of both number and size of photosynthetic units. Photosynthetic performance assessed by fluorescence revealed two seasonal periods: higher values of Pmax (rETR), Ik and non-photochemical quenching from October to March (rainy season), and lower values from April to October (dry season). Temperature responses were observed only in summer, but temperature optima were different between methods: peaks of net photosynthesis occurred at 20ºC, whereas rETR increased towards higher temperatures (up to 30ºC). Dark respiration increased with higher temperatures. Current velocity had a stimulatory effect on photosynthetic rates, as suggested by positive correlations with Pmax (rETR) and alpha. pH experiments revealed highest net photosynthetic rates under pH 4.0, suggesting higher affinity for CO2 than HCO3-. This broad range of responses of photosynthetic characteristics of this N. cernua population to irradiance, temperature, and pH/inorganic carbon reflects a wide tolerance to variations in these environmental variables, which probably contribute to the wide distribution of this species.
charophytes; irradiance; Nitella; pH; photosynthesis; seasonal dynamics; temperature
