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

Abstract

Rising atmospheric CO₂ concentration ([CO₂]) is believed to mitigate the drought stress on plant, and is regulated by soil nitrogen availability. Nonetheless, effects of elevated [CO₂] (e[CO₂]) 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 C₃ model and crop plant) cultivated at two CO₂ concentrations (400 and 800 μmol mol ⁻¹) were exposed to progressive drought stress after budding-topping. In this study, ¹⁵N 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[CO₂] could alleviate the soil drought and nutrient effects on biomass decreasing in upper-leaf. During progressive drought, e[CO₂]-plants are able to maintain higher root hydraulic conductivity (K_r) than their a[CO₂]-counterparts, the higher K_r induced by e[CO₂] 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 [CO₂] 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[CO₂] 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.