Characteristics And Antioxidant Enzyme Activity Of Ginkgo Leaf Senescence Shoot Energy Transformation

In this paper, shoots leaves and perennial branches leaves of Ginkgo biloba L were studied. CIRAS-2portable photosynthesis system were used to measure photosynthetic parameters during leaf senescence. Handy PEA plant efficiency analyzer were to measure chlorophyll fluorescence parameters during leaf senescence. Chlorolab-2oxygen electrode instrument were used to measure the electron transport activity of thylakoid membranes during leaf senescence. At the same time, we used traditional plant physiology experiment methods. Antioxidant enzyme activity, active oxygen scavenging material contents and MDA content during leaf senescence of Ginkgo biloba L.were measured by the spectrophotometer. The results show that:1. Ginkgo seedlings grown under natural conditions, the photosynthetic characteristics of leaf:the average net photosynthetic rate of shoots branches leaves is greater than the perennial branches leaves at same period before into the senescence period. On the contrary, when the senescence process was start, the aging speed of shoots branches leaves is faster than perennial branches leaves. Shoots leaves of ginkgo with vigorous growth rate had some internal factors adapt to its high net photosynthetic rate, such as stomatal conductance, intercellular CO2concentration. They also had favorable environmental conditions, such as light temperature, air flow. Then senescence process of shoots leaf is fast performance in the rapid degradation of chlorophyll and rapid decline of net photosynthetic rate.2. In order to research the structure and function of PS Ⅱ from growing season to the aging period and the changes during senescence process of ginkgo. Chlorophyll fluorescence parameters of ginkgo seedlings grown under natural conditions were measured. The results showed:In the vigorous growing season, ginkgo shoots leaves with high quantum yield of primary photochemistry (Fv/Fm) and conformation term for primary photochemistry (Fv/Fo). Density of reaction centers per cross-section (RC/CSo) of shoots leaves is much more than perennial branches leaves. They also have different in absorption flux per reaction center (ABS/RC), electron transport flux per reaction center (ETo/RC) and dissipated energy flux per reaction center (DIo/RC). ABS/RC and DIO/RC of shoots leaves are less than perennial branches leaves, but shoots leaves distribute more energy to electron transport, all of the description above is also consistent with higher net photosynthetic rate of ginkgo shoots leaves. Then ginkgo leaf senescence begin, along with the rapid aging of the shoot leaves, chlorophyll fluorescence parameters decreased rapidly. At later period, PS Ⅱ active centers were destructed rapidly, the number decline, activity reduced, resulting in PS Ⅱ structure and function of irreversible severe damage, leaf photosynthetic capacity recession.3. In the present study, we also measured the internal antioxidant enzyme activity and active oxygen scavenging material contents in ginkgo leaves during senescence process. The study showed:From the beginning of sencescence to medium-term of senescence, antioxidant enzyme activities expressed as upward trend, with the further degree of leaf senescence. In late aging, antioxidant enzyme activities decline. The ROS scavenger ASA and GSH expressed same trend, ASA content increased gradually with the leaf senescence, GSH content gradually increase too. Throughout the senescence process, the mesophyll cell membrane lipid peroxide gradually increased, at the same time MDA content rising. Thus, in the early and medium-term of senescence, the response mechanism enhanced of antioxidant enzymes activities is the main way to eliminate active oxygen; The latter period of senescence scavenger of reactive oxygen species play an important role in the eliminate of reactive oxygen species. However, with the further escalation of senescence, their role is no longer so obvious, a large number of reactive oxygen species accumulate eventually, membrane lipid peroxidation level deepen, leaves physiological function decline.