Greenhouse and field cucumber genotypes use different mechanisms to protect against dark chilling

Diurnal changes in photosynthetic gas exchange and chlorophyll fluorescence were measured after two consecutive night chills to reveal the photosynthetic characteristics and the mechanism of photoprotection in a greenhouse genotype Jinyou No. 3 (GH), and in a field genotype Jinyan No. 4 (OF) of cucumber (Cucumis sativus L.). Both genotypes showed inhibition of CO₂ assimilation immediately after the dark chill, with OF exhibiting a greater reduction. Dark chilling had little effect on stomatal limitation (l) and RuBP regeneration (J_max) but significantly decreased maximum carboxylation velocity of Rubisco (V_cmax). The reduced capacity for CO₂ fixation in the Calvin cycle induced a downstream regulation of PSII photochemistry, a mechanism that regulates the photosynthetic electron transport to match the lower demand for ATP and NADPH in the stroma of chloroplasts. The reduced quantum efficiency of PSII photochemistry was mainly due to reductions both in the photochemical quenching coefficient (qP) and in the efficiency of excitation energy capture by open PSII reaction centres (F_v' / F_m') for OF, but only to the latter for GH. Night chills resulted in an enhanced photorespiration proportion in GH and an O₂-dependent alternative electron flux in OF, which served as protective mechanisms for the two varieties. These results showed that there are genotypic differences in the limitation factor for CO₂ assimilation and in photo-protection mechanism to night chill in cucumber.