Effects of leaf age on chlorophyll fluorescence and antioxidant enzymes activity in winter rapeseed leaves under cold acclimation conditions

Cold acclimation-dependent physiological changes were investigated through photosynthetic activity and some antioxidant enzymes in two winter oilseed rape cultivars ( Brassica napus L. ssp. oleifera cvs. Eurol and Hansen) on the basis of leaf age. Chlorophyll fluorescence measurements demonstrated that cold acclimation caused decreased photosynthetic activity in Eurol and Hansen as the leaf age increased. Photosynthetic pigment content declined in both cultivars under low temperature irrespective of leaf age. In Eurol, however, relatively higher total carotenoid content than Hansen may provide a protection against photooxidative damage. Superoxide dismutase, ascorbate peroxidase and glutathione reductase activity in the cold-acclimated leaves of Hansen significantly increased, indicating an effective ascorbate–glutathione cycle activity. In Eurol, however, ascorbate peroxidase and glutathione reductase activity in the cold-acclimated younger leaves lowered and hydrogen peroxide detoxifying system collapsed. As a result, decreased photosynthetic activity in the young leaves of Hansen under low temperature could be manifested as a metabolic response that downregulate photosynthesis to attain cold-acclimated state. In the case of Eurol, cold acclimation process caused oxidative stress. In summary, the changes in the chlorophyll fluorescence parameters in the young leaves of Hansen are derived from some metabolic regulation as a result of cold acclimation, whereas in Eurol, it is due to oxidative stress triggered by low temperature. Leaf maturation mitigated the adverse effects of low temperature in the leaves of Eurol and Hansen.