JXB Advance Access originally published online on April 23, 2004
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Journal of Experimental Botany, Vol. 55, No. 399, pp. 1135-1143, May 1, 2004
© 2004 Oxford University Press
Plants and the Environment |
A role for brassinosteroids in the regulation of photosynthesis in Cucumis sativus
Received 23 October 2003; Accepted 12 February 2004
1 Horticultural Department, Zhejiang University, Kaixuan Road 268, Hangzhou, PR China 310029
2 Laboratory of Horticultural Plant Growth, Development and Biotechnology, Ministry of Agriculture, PR China 310029
3 Laboratoire d’Ecophysiologie Végétale, Bâtiment 362, Université Paris XI, F-91405 Orsay, France
* To whom correspondence should be addressed at Zhejiang University. Fax: +86 571 86049815. E-mail: yu{at}mail.hz.zj.cn
Abbreviations, AI, acid invertase; Asat, light-saturated net CO2 assimilation; BRs, brassinosteroids; EBR, 24-epibrassinolide; Fv/Fm, the maximal photochemical efficiency of PSII; F'v/F'm, the efficiency of excitation energy capture by open PSII reaction centres; Jmax, maximum potential rate of electron transport contributed to RuBP regeneration; l, stomatal limitation; 
PSII, relative quantum efficiency of PSII photochemistry; qP, photochemical quenching; Rubisco, ribulose-1,5-bisphosphate carboxylase/oxygenase; RuBP, ribulose-1,5-bisphosphate; SPS, sucrose phosphate synthase; SS, sucrose synthase; Vc, max, maximum carboxylation rate of Rubisco.
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 (PSII) than the controls, which was mainly due to a significant increase in the photochemical quenching (qP), with no change in the efficiency of energy capture by open PSII reaction centres (F'v/F'm). 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.
Key words: Carbohydrate metabolism, CO2 assimilation, Cucumis sativus, Photosystem II, phytohormones, Rubisco activity.
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