Changes in Photosynthetic Characteristics and Antioxidative Protection in Male and Female Ginkgo during Natural Senescence

Ginkgo (Ginkgo biloba), a dioecious tree species, is widely distributed throughout the world, yet little is known about sex-related responses to autumnal senescence in ginkgo. The aim of this study was to investigate changes in photosynthetic activities, concentration of oxidative stress parameters [malondialdehyde (MDA) and H2O2] and antioxidant systems, and ultrastructure of chloroplasts in the naturally senescing leaves of two ginkgo sexes and to examine whether progression of senescence is sex-specific in ginkgo. Photosynthesis in ginkgo leaves of both sexes was not limited by stomatal factors, but rather non-stomatal factors such as decreased photosynthetic pigments and photochemical activities that became more important during autumnal senescence. The responses of antioxidative enzymes were different from those of antioxidants to leaf senescence. Correlation analysis revealed that autumnal leaf senescence was significantly correlated to antioxidative enzymes changes but not to antioxidants such as ascorbate (ASA) and glutathione (GSH). Guaiacol peroxidase (POD) became more important in senescing leaves and played a major protective role, especially at the late stage of senescence. The shape of chloroplasts of both sexes changed from oblong to round, and there was an increase in the number and size of osmiophilic granules during senescence; swollen thylakoid membranes in the stroma and grana with a significant increase in MDA content were also observed. During autumnal senescence, female ginkgo plants showed smaller decreases in net photosynthetic rates, photosynthetic pigments, photochemical activities, superoxide dismutase, ascorbate peroxidase and catalase activities, higher POD activity, ASA and GSH contents, and smaller increases in H2O2 and MDA contents than did males. In addition, female plants had a later senescence of chloroplasts, a smaller accumulation of osmiophilic granules, and a slower rate of membrane damage. These results show that female ginkgo exhibit slower leaf senescence, which may be related to increased reproductive costs.