Role of thermal dissipation in the photoprotection in cucumber plants after exposure to a chill stress

Experiments were carried out to investigate the changes in CO₂ assimilation, photon allocation, and photosynthetic electron flux in leaves of cucumber (Cucumis sativus L.) plants after chilling stress. Chilling significantly decreased CO₂ assimilation, the energy flux via linear electron transport (J _PS2) and non-constitutive thermal dissipation (J _NPQ) but increased fluorescence and constitutive thermal dissipation (J _f,D) in chilling-sensitive genotype Jinyan No. 4. In contrast, chilling had little effects on J _NPQ and J _f,D although CO₂ assimilation and J _PS2 were inhibited in chilling-tolerant genotype Jinchun No. 3. In parallel with the reduction in J _PS2, electron flux to oxygenation and carboxylation by ribulose-1,5-bisphosphate carboxylase/oxygenase all significantly decreased while electron flux to O₂ significantly increased, especially in chilling-sensitive genotype. Thermal and fluorescence dissipation were the main energy dissipation pathways whilst water-water cycle was an important electron sink when photosynthetic carbon reduction was suppressed after chilling. Chilling sensitivity of the photosynthetic apparatus was related to the operation of different photoprotection mechanisms.