Interactive effects of CO 2 enrichment and brassinosteroid on CO 2 assimilation and photosynthetic electron transport in Cucumis sativus

Abstract

CO 2 enrichment and brassinosteroids (BR) both have positive impacts on photosynthesis and plant growth. To examine the interactive effect of CO 2 enrichment and BR on photosynthesis and plant growth, CO 2 assimilation, chlorophyll fluorescence quenching, carbohydrate metabolism, photosynthetic gene transcript and enzyme activity were analyzed in leaves of young plants of cucumber ( Cucumis sativus L.) in response to a doubling of growth CO 2 level, foliar BR application alone or in combination. Both CO 2 elevation and application of BR increased shoot biomass, leaf area, CO 2 assimilation, total soluble sugar and starch contents, transcript for photosynthetic gene and activity for enzymes involved in Benson–Calvin cycle but a combination of the two treatments resulted in a more significant effect. Although an elevation of CO 2 level had little effects on quantum efficiency of PSII ( Ф PSII), it significantly increased the electron flux for photosynthetic carbon reduction [ J e(PCR)] but decreased electron flux for photorespiratory carbon oxidation [ J e(PCO)]. In contrast, BR treatment increased Ф PSII and this increase in Ф PSII was associated with increased J e(PCR) and J e(PCO). Furthermore, a combined treatment of CO 2 elevation and BR resulted in an additive effect on PSII electron flux. However, alternative electron flux was almost unaltered after CO 2 enrichment and BR treatment. Thus, short term CO 2 elevation did not induce a down-regulation of photosynthesis and there was an additive effect between BR and CO 2 on the enhancement of CO 2 assimilation in leaves of young cucumber plants.