The Roles For Brassinosteroids In The Regulation Of Photosynthesis And Antioxidant System In Cucumis Sativus L.

The horticultural facilities in our country are relatively simple and crude, lacking of the special variety for greenhouse. Accordingly, greenhouse crops are frequently subject to chilling injury, which result in the low single productivity, the yield per m2 only 1/4-1/3 of that in developed country. Now, improving cultivation manner and application of growth regulators are the most effective means to enhance yields and qualities of vegetables. Brassinosteroids (BRs) is the 6th class of hormone and it has a long studying history in plant growth regulation. Although BRs was applied widely in increasing yields and stress tolerance, even now, the mechanism is not very clear. Therefore, it is important to study the regulation on photosynthesis and antioxidant system by BRs both in scientific and practical fields. In this study, we investigated the effect of EBR on photosynthesis of cucumber seedlings (Cucumis sativus L.) and discussed by which mechanism EBR regulate the photosynthesis. Also, experiments were carried out to examine the effect of EBR on antioxidant system activities and the protection of photosynthetica apparatus by EBR under chilling and MV stress conditions. At the same time, we verified the regulations on photosynthesis, stress tolerance and nitrogen assimilation by BRs through the Brz, synthesis inhibitor of BRs, involved in the study. The results are as follows:1), The effects of 24-epibrassinolide (EBR) spray application on gas-exchange, chlorophyll fluorescence characteristics, Rubisco activity, and carbohydrate metabolism were investigated in cucumber (Cucumis sativus L. cv. Jinchun No. 3) plants grown in a greenhouse. EBR significantly increased the light-saturated net CO2 assimilation rate (Asat) from 3 h to 7d after spraying, with 0.1 mg L-1 EBR proving most effective. Increased Asat in EBR-treated leaves was accompanied by increases in the maximum carboxylation rate of Rubisco (Vc,max) and in the maximum rate of RuBP regeneration (Jmax). EBR-treated leaves also had a higher quantum yield of PSII electron transport (ΦPSII) than the controls, which was mainly due to a significant increase in the photochemical quenching (qP), with no change in theef(?)ciency of energy capture by open PSII reaction centres (Fv/Fm ). EBR did not influence photorespiration. In addition, significant increases in the initial activity of Rubisco and in the sucrose, soluble sugars, and starch contents were observed followed by substantial increases in sucrose phosphate synthase (SPS), sucrose synthase (SS), and acid invertase (AI) activities after EBR treatment. It was concluded that EBR increases the capacity of CO2 assimilation in the Calvin cycle, which was mainly attributed to an increase in the initial activity of Rubisco.2), Application of Brz resulted in the appearance as phenotype of BRs depletion mutant, like dwarf, dark green etc, as well as, the decreases in sucrose, total sugar and reducing sugar contents were also observed. The decrease of photosynthesis by Brz treatment was not attributed to stomatal closure or photoinhibition, but to the decrease of the initial activity of Rubisco. However, applying BR can reverse these changes, that is, the morphological and physiological data can recover to the control level. At the same time, the expression of rbcS and rbcL were restrained by Brz treatment but promoted by EBR, which suggest that the level of BRs in plants was closely correlated with the expression of rbcS and rbcL, and also make sure that BRs affect photosynthesis of plants by regulating Rubisco activity.3), Exogenous EBR has concentration effect on antioxidant system of cucumber seedlings, and EBR at 0.1mgL-1 is the most effective in increasing antioxidant system activities. Cucumber seedlings having different level in BRs, owing to being sprayed with BR or Brz for a long time, have different activities in antioxidant system. The disturbance in ROS metabolism in plants with inhibited BRs level was probably one of the physiological factors responsible for the more injury in oxidative and chilling stre...