Interactive Effects of CO2 and NaCl on Photosynthesis and Water Use Efficiency in Tobacco (Nicotiana tabacum L.)

This study examined the effects of salt stress and elevated atmospheric CO2 concentration ([CO2]) on plant biomass, stomatal morphology, leaf gas exchange, leaf anatomy, photosynthetic pigments and soluble sugars of tobacco (Nicotiana tabacum). Our results showed that the biomass of tobacco was significantly decreased under mild and severe salt stress at both ambient (a[CO2]) and elevated (e[CO2]) [CO2]. However, elevated [CO2] significantly enhanced the belowground biomass of tobacco by 125% (P < 0.001), 246% (P < 0.001) and 246% (P < 0.001) under control, mild and severe salt stress, respectively. Additionally, we also found that salt stress reduced both the stomatal aperture length and width on the abaxial leaf surface under a[CO2]. Concurrently, stomatal conductance significantly decreased by 34% (P = 0.020) under both mild and severe salt stress compared to the control. This reduction in stomatal conductance is partially contributed to the decline in net photosynthetic rates under salt stress. In contrast, elevated [CO2] increased the net photosynthetic rate of tobacco plants grown under the control, but leaf photosynthesis of tobacco plants under salinity conditions was declined at both a[CO2] and e[CO2] in the current study. Meanwhile, elevated [CO2] also significantly reduced both stomatal conductance and transpiration rate, which leads to an enhancement of water use efficiency under control and salinity conditions. Overall, our results suggested that salt stress reduced leaf photosynthesis, but elevated [CO2] partially alleviated the impacts of salt stress on tobacco plants through the “CO2 fertilization effect”. Our results provided important insights into further understanding the potential mechanisms and key processes of agriculture in response to elevated [CO2] and salt stress in the context of future climate change.