Cold acclimation and BnCBF17-over-expression enhance photosynthetic performance and energy conversion efficiency during long-term growth of Brassica napus under elevated CO2 conditions

The effects of cold acclimation and long-term elevated CO2 on photosynthetic performance of wild-type (WT) and BnCBF17-over-expressing line of Brassica napus cv. Westar (BnCBF17-OE) grown at either 20/16 A degrees C (non-acclimated) or 5/5 A degrees C (cold acclimated) and at either ambient (380 mu mol C mol(-1)) or elevated (700 mu mol C mol(-1)) CO2 were studied. Compared with non-acclimated WT, the BnCBF17-OE grown at 20 A degrees C mimicked the effects of cold acclimation on WT B. napus with respect to compact dwarf phenotype and increased rates of light-saturated CO2 assimilation and photosynthetic electron transport. This was associated with enhanced energy conversion efficiency into biomass as assessed by decreased excitation pressure coupled to decreased dependence on non-photochemical energy dissipation for a given irradiance. Growth at elevated CO2 decreased the light and CO2-saturated rates of photosynthesis by 30 % for non-acclimated WT relative to growth at ambient CO2. This was associated with inhibition in electron transport rates (20 %), decrease in amount of rbcL (35 %) and cytosolic FBPase (70 %) and increased excitation pressure and non-photochemical quenching in elevated versus ambient CO2-grown non-acclimated WT. In contrast, light and CO2-saturated rates of photosynthesis, electron transport, excitation pressure, non-photochemical quenching and levels of rbcL, cytosolic FBPase and Lhcb1 were insensitive to growth under elevated CO2 in BnCBF17-OE and cold-acclimated WT. Thus, BnCBF17-over-expression and cold acclimation maintain enhanced energy conversion efficiency and reduced sensitivity to feedback-limited photosynthesis during long-term growth of B. napus under elevated CO2. Our results indicated that CBFs transcription factors regulate not only freezing tolerance but also has major whole plant effects.