Interactive effects of CO2 enrichment and brassinosteroid on CO2 assimilation and photosynthetic electron transport in Cucumis sativus

CO2 enrichment and brassinosteroids (BR) both have positive impacts on photosynthesis and plant growth. To examine the interactive effect of CO2 enrichment and BR on photosynthesis and plant growth, CO2 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 CO2 level, foliar BR application alone or in combination. Both CO2 elevation and application of BR increased shoot biomass, leaf area, CO2 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 CO2 level had little effects on quantum efficiency of PSII (ФPSII), it significantly increased the electron flux for photosynthetic carbon reduction [Je(PCR)] but decreased electron flux for photorespiratory carbon oxidation [Je(PCO)]. In contrast, BR treatment increased ФPSII and this increase in ФPSII was associated with increased Je(PCR) and Je(PCO). Furthermore, a combined treatment of CO2 elevation and BR resulted in an additive effect on PSII electron flux. However, alternative electron flux was almost unaltered after CO2 enrichment and BR treatment. Thus, short term CO2 elevation did not induce a down-regulation of photosynthesis and there was an additive effect between BR and CO2 on the enhancement of CO2 assimilation in leaves of young cucumber plants.