The acclimation of photosynthesis and respiration to temperature in the C3-C4 intermediate Salsola divaricata: induction of high respiratory CO2 release under low temperature

Photosynthesis in C-3-C-4 intermediates reduces carbon loss by photorespiration through refixing photorespired CO2 within bundle sheath cells. This is beneficial under warm temperatures where rates of photorespiration are high; however, it is unknown how photosynthesis in C-3-C-4 plants acclimates to growth under cold conditions. Therefore, the cold tolerance of the C-3-C-4 Salsola divaricata was tested to determine whether it reverts to C-3 photosynthesis when grown under low temperatures. Plants were grown under cold (15/10 degrees C), moderate (25/18 degrees C) or hot (35/25 degrees C) day/night temperatures and analysed to determine how photosynthesis, respiration and C-3-C-4 features acclimate to these growth conditions. The CO2 compensation point and net rates of CO2 assimilation in cold-grown plants changed dramatically when measured in response to temperature. However, this was not due to the loss of C-3-C-4 intermediacy, but rather to a large increase in mitochondrial respiration supported primarily by the non-phosphorylating alternative oxidative pathway (AOP) and, to a lesser degree, the cytochrome oxidative pathway (COP). The increase in respiration and AOP capacity in cold-grown plants likely protects against reactive oxygen species (ROS) in mitochondria and photodamage in chloroplasts by consuming excess reductant via the alternative mitochondrial respiratory electron transport chain.