| In vitro catalytic characteristics of the violaxanthin de-epoxidase (VDE) in wheat leaves were studied. VDE activities from leaves of wheat cultivars with different photooxidation tolerance were also investigated under weak and strong light. Changes of the VDE activities, the de-epoxidation of the xanthophyll cycle, (A+Z)/(V+A+Z), the non-photochemical quenching(NPQ) and its components, e.g. qE and qI and the effect of the xanthophyll cycle in photoprotection under different stress conditions were examined. Special attention has been paid to whether the VDE activities measured in vitro could reflect the capability of thermal dissipation through xanthophyll cycle in vivo. The main results are as follows:1. The method for measuring the VDE activities in vitro was established.2. The factors affecting catalytic reaction of VDE was studied.Effects of the substrate (V) concentration on the reaction rate answer for the "three-phase-reaction rule". Increase in the substrate concentration could hardly affect the reaction rate when the absorbing value of V at 441nm was higher than 2. Effects of temperature on the reaction rate exhibited a "bell" curve, and 30℃ is the optimum temperature for VDE activities. The reaction rate declined gradually with the increase in DTT concentration added to the reaction system, and the reaction almost stopped when 2mM DTT was added to the reaction system. Effects of pH on the VDE activities also exhibited a "bell" curve, and the optimum pH rose with the increase in ASA concentration. The reaction rate decreased as the ASA concentration decreased from 3mM to 0.5mM.3. The VDE activities of different photooxidation tolerant wheat cultivars:Wheat with different photooxidation tolerant exhibited different VDE activities under weak and strong light. The activities were higher under strong light than under weak light. The VDE activities in the more photooxidation tolerate cultivars were higher than that in less photooxidation tolerant cultivars, which was in accordance with the photooxidation tolerance. 4. Changes of the VDE activities during the course of combined stress of salt and lightФPSII and Fv/Fm declined while NPQ, qE and (A+Z)/(V+A+Z) increased with the increases in NaCl concentration and light intensity. The VDE activities increased with the enhancement of salt and light intensity, but declined when severe salt stress and strong light were imposed to the leaves.The measurement temperature in vitro could affect the VDE5. activities.The optimum measurement temperature could increase VDE activities in wheat leaves incubated at different temperature and light intensity. Leaves incubated at 38℃ showed the highest VDE activities either measured at optimum measurement temperature or measured at incubating temperature. whereas, the leaves incubated at 45℃ showed the lowest activities, which suggested that 38℃ was the nearest temperature to the optimum temperature for VDE activities in vivo among the four temperature examined, and 45℃ exceeded the thermal stability temperature of VDE and caused the VDE protein denatured and inactivated.6. A period of light is necessary for the induction of the VDE activities.The VDE activities in leaves incubated at different temperature increased, with prolonging of incubation time under light. The maximal VDE activities were observed in leaves incubated at different temperature and light for 6h. 7. Changes of the VDE activities during the course of combined stress of drought and lightФPSII and Fv/Fm declined while NPQ, qE and (A+Z)/(V+A+Z) increased with the increases in PEG concentration and light intensity. The VDE activities increased with enhancement of drought and light, but declined when severe drought stress and strong light were imposed to the leaves.8. Changes of the VDE activities during the course of combined action of light and senescence.ФPSII and Fv/Fm declined while NPQ, qE and (A+Z)/(V+A+Z) aswell as VDE activities increased with senescence .In general,... |
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