Effect of CO2 Elevation on Tomato Gas Exchange, Root Morphology and Water Use Efficiency under Two N-Fertigation Levels

Atmospheric elevated CO₂ concentration (<i>e</i>[CO₂]) decreases plant nitrogen (N) concentration while increasing water use efficiency (WUE), fertigation increases crop nutrition and WUE in crop; yet the interactive effects of <i>e</i>[CO₂] coupled with two N-fertigation levels during deficit irrigation on plant gas exchange, root morphology and WUE remain largely elusive. The objective of this study was to explore the physiological and growth responses of ambient [CO₂] (<i>a</i>[CO₂], 400 ppm) and <i>e</i>[CO₂] (800 ppm) tomato plant exposed to two N-fertigation regimes: (1) full irrigation during N-fertigation (FIN); (2) deficit irrigation during N-fertigation (DIN) under two N fertilizer levels (reduced N (N1, 0.5 g pot^-1) and adequate N (N2, 1.0 g pot^-1). The results indicated that <i>e</i>[CO₂] associated with DIN regime induced the lower N2 plant water use (7.28 L plant^-1), maintained leaf water potential (-5.07 MPa) and hydraulic conductivity (0.49 mol m^-2 s^-1 MPa^-1), greater tomato growth in terms of leaf area (7152.75 cm²), specific leaf area (223.61 cm² g^-1), stem and total dry matter (19.54 g and 55.48 g). Specific root length and specific root surface area were increased under N1 fertilization, and root tissue density was promoted in both <i>e</i>[CO₂] and DIN environments. Moreover, a smaller and denser leaf stomata (4.96 µm² and 5.37 mm^-2) of N1 plant was obtained at <i>e</i>[CO₂] integrated with DIN strategy. Meanwhile, this combination would simultaneously reduce stomatal conductance (0.13 mol m^-2 s^-1) and transpiration rate (1.91 mmol m^-2 s^-1), enhance leaf ABA concentration (133.05 ng g^-1 FW), contributing to an improvement in WUE from stomatal to whole-plant scale under each N level, especially for applying N1 fertilization (125.95 µmol mol^-1, 8.41 µmol mmol^-1 and 7.15 g L^-1). These findings provide valuable information to optimize water and nitrogen fertilizer management and improve plant water use efficiency, responding to the potential resource-limited and CO₂-enriched scenario.