Changes in electron transport, superoxide dismutase and ascorbate peroxidase isoenzymes in chloroplasts and mitochondria of cucumber leaves as influenced by chilling

In order to clarify the relationship between chill-induced disturbance in photosynthetic, respiratory electron transport and the metabolism of reactive oxygen species ( ROS), leaf gas exchange, chlorophyll fluorescence quenching, respiration, and activities of superoxide dismutase ( SOD) and ascorbate peroxidase ( APX) were investigated in chloroplasts and mitochondria of cucumber ( Cucumis sativus) leaves subjected to a chill ( 8 degrees C) for 4 d. Chilling decreased net photosynthetic rate ( P-N) and quantum efficiency of photosystem 2 (Phi(PS2)), but increased the ratio of Phi(PS2) to the quantum efficiency of CO2 fixation (Phi(CO2)) and non-photochemical quenching (NPQ) in cucumber leaves. While chilling inhibited the activity of cytochrome respiration pathway, it induced an increase of alternative respiration pathway activity and the reduction level of Q-pool. Chilling also significantly increased O-2(center dot) production rate, H2O2 content, and SOD and APX activities in chloroplasts and mitochondria. There was a more significant increase in SOD and APX activities in chloroplasts than in mitochondria with the increase of membrane-bound Fe-SOD and tAPX in chloroplasts being more significant than other isoenzymes. Taken together, chilling inhibited P-N and cytochrome respiratory pathway but enhanced the photosynthetic electron flux to O-2 and over-reduction of respiratory electron transport chain, resulting in ROS accumulation in cucumber leaves. Meanwhile, chilling resulted in an enhancement of the protective mechanisms such as thermal dissipation, alternative respiratory pathway, and ROS-scavenging mechanisms ( SODs and APXs) in chloroplasts and mitochondria.