Elevated CO₂ Modulates N Uptake and N Use Efficiency of Tobacco (<i>Nicotiana tabacum</i> L.) Response to Soil Progressive Drought at Topping Stage

ABSTRACT Rising atmospheric CO 2 concentration ([CO 2 ]) is believed to mitigate the drought stress on plant, and is regulated by soil nitrogen availability. Nonetheless, effects of elevated [CO 2 ] ( e [CO 2 ]) on plant response to drought and their interactive effects on plant N uptake and utilisation remain critical unknown. In this study, three‐month‐old potted tobacco plants (a C 3 model and crop plant) cultivated at two CO 2 concentrations (400 and 800 μmol mol −1 ) were exposed to progressive drought stress after budding‐topping. In this study, 15 N isotopic technique used for analysing the fertiliser‐N transformation in soil and plant organs. Results shown drought generally decreased tobacco biomass, and soil nutrient supply significantly intensify this leaf biomass decreasing induced by drought, while e [CO 2 ] could alleviate the soil drought and nutrient effects on biomass decreasing in upper‐leaf. During progressive drought, e [CO 2 ]‐plants are able to maintain higher root hydraulic conductivity ( K r ) than their a [CO 2 ]‐counterparts, the higher K r induced by e [CO 2 ] were correlated with leaf stomatal conductance ( g s ) decrease and xylem sap ABA ([ABA] xylem ) increase. In other words, soil nutrient supply enhanced the [ABA] xylem of drought‐stressed plants with 3–7 folds higher than none nutrient supply counterparts. Elevated [CO 2 ] induced more fertiliser‐N accumulated in tobacco leaves, especially under well‐watered conditions, which in turn yielded higher photosynthetic nitrogen use efficiency in leaves but low in whole plant. These results suggested that e [CO 2 ] could alleviate drought‐induced photosynthesis limitation by improving root biomass (mainly fine root) and K r , but the alleviation varied with soil nutrient conditions, thereby mitigating the drought‐negative effects on upper‐leaf growth, finally improved tobacco water use efficiency and tobacco production.