Temperature response of photosynthesis, excitation energy dissipation and alternative electron sinks to carbon assimilation in Beta vulgaris L.

Photosynthesis, excitation energy dissipation and alternative electron sinks to carbon assimilation at different temperatures (5–35 °C range) were studied in plants of Beta vulgaris L. grown outdoors at springtime. Gas exchange and chlorophyll fluorescence measurements were performed in plants exposed to different temperatures for two hours at saturating irradiance (1000 μmol photons m−2 s−1). The photosynthetic rate decreased significantly either by lowering the temperature below 25 °C or by increasing it above 25 °C. At low temperatures (5, 10 and 15 °C) a significant decrease of electron transport rate (ETR) was found and processes alternative to CO2 fixation, as sinks of electrons, occurred but they were not affected by the temperature. However, at 5 °C a low and statistically significant photorespiratory rate was observed and the non-photochemical quenching (NPQ) was the highest. At higher temperature (30–35 °C) ETR saturated and hence non-assimilative processes, alternative to CO2 fixation, sustained the photochemical activity. Among processes consuming O2 photorespiration increased at 30–35 °C, while processes non-consuming O2 were non-affected. By increasing the temperature up to 30–35 °C NPQ values did not show appreciable differences compared to 25 °C. The obtained results indicate that in B. vulgaris plants the thermal dissipation represents the main dissipative process of the excess excitation energy at low temperatures. On the contrary, at high temperatures (30–35 °C) the main dissipation of the excess excitation energy is due to alternative pathways to CO2 assimilation.