Effects Of Ozone On Ascorbic Acid-Glutathione Cycle In Soybean (Glycine Max) Leaves

Widespread areas of crop production are presently at risk for damage from ambient atmospheric ozone (O3) concentrations. The concentration of tropospheric O3 has been rising due to aggravating urbanization since the Industrial Revolution, it has had a significant impact on plants, and plants play an important role in adjusting structure and function of the ecosystem, thus, to correctly evaluate how elevated O3 concentration will affect plants is of great significance.In this research, open-top chambers (OTCs) were used to investigate lipid peroxidation, reactive oxygen species (ROS) metabolism, antioxidation enzymes system and AsA-GSH cycle in Glycine max leaves under different O3 stress (80±10 nL·L-1 and 110±10 nL·L-1) and the mechanism of AsA-GSH cycle to eliminate reactive oxygen species and the effects on growth and development of plants. This could establish the foundation for the study of plants adaptability mechanism under elevated O3 on the molecular level. The results show:Compared with control, the elevated O3 concentration of (80±10) nL·L-1 and (110±10) nL·L-1 induced an increase on malondialdehyde (MDA) content, relative electrical conductivity and a decrease on superoxide anion (O2) production rate, hydrogen peroxide (H2O2) content during the whole growth stage. This reveals that O3 stress could increase the content of lipid peroxidation and ROS, and injure the membrane system of leave cell and the higher O3 concentration is; the more the jury effect of O3 stress on soybean is obvious.O3 stress reduced the activity of SOD, CAT and POD of antioxidant enzyme system. SOD activity had a short-term increase at branching stage and at other stages; SOD activity was lower than CK under elevated O3. Elevated O3 decreased CAT activity and POD activity significantly in soybean leaves. Generally, O3 stress performs an injury function antioxidant enzyme system activity in soybean leaves and the higher the O3 concentration is, the more injury function is obvious.AsA-GSH cycle has an important function of eliminating H2O2, and the efficiency of AsA-GSH cycle in plants can be stimulated by moderate stress conditions to scavenge ROS. The results indicated that, compared with control, O3 stress induced the lower content of ascorbic acid (AsA) and glutathione (GSH), and higher content of dehydroascorbic acid (DHA) and oxidized glutathione (GSSG) of AsA-GSH cycle. It showed a trend of increasing in earlier stage and decreasing in later stage of the activities of ascorbate peroxidase (APX), monodehydroascorbate reductase (MDHAR) and glutathione reductase (GR), while the dehydroascorbate reductase (DHAR) activity was increased in earlier period, decreased in middle period and then increased in later period compared to control, respectively. The changing trend of total AsA content was similar with AsA content, and the changing trend of total GSH content was similar with GSH content. At every stage, elevated O3 decreased AsA/DHA ratio and GSH/GSSG ratio; this reveals that O3 stress decreased the efficiency of AsA-GSH cycle in soybean leaves and antioxidant ability.O3 stress increased the yield of soybean significantly. With the increase of O3 concentration, soybean grains number, seed weight,100-seeds weight and number of pods per plant decreased respectively. And the higher the O3 concentration was, the greater the decrease was. Under elevated O3 condition, the yield of soybean plants and the control was remarkable difference.The results above show that elevated O3 concentration accelerates ROS metabolism rates and peroxidation damage, reduces the efficiency of AsA-GSH cycle and antioxidant system can not tolerate O3-induced injure, thus increasing membrane lipid peroxidation, decreasing the soybean grain yield and resulting in a significant negative effect on soybean. Also, in the early of O3 treatment, the intensity of stress was low; the protection of plants played an important role to increase the efficiency of removing ROS and reduce ROS damage by improving the efficiency of AsA-GSH cycle. With stress intensity increasing, the structure of antioxidant system in soybean leaves was damaged, resulting in rapid accumulation of ROS. And O3 stress caused an acceleration of caducity that might be in part responsible for the reduction of antioxidant ability.