Chunshuo Liu

This study aimed to explore the effect of doubled CO₂ concentration (d[CO₂]) on the modulation of root morphological structure, leaf potassium (K)/sodium (Na) ratio, and nutrient stoichiometry, as well as water use efficiency (WUE) of a C₄ maize (Zea mays L.) in response to soil drought and salinity. C₄ maize was grown in two atmospheric CO₂ concentrations of 400 and 800 ppm (a[CO₂] and d[CO₂]), subjected to two soil water regimes (well-watered and drought stress) and two soil salinity levels (0 and 100 mM NaCl pot⁻¹ (non-salt and salt stress)). The results indicated that soil drought increased maize root tissue density and specific root length. Both d[CO₂] and salt stress reduced leaf phosphorus (P) and K concentrations; conversely, drought stress enhanced leaf nitrogen (N) and K concentrations. The lower specific leaf area, but greater specific leaf N and N/K under soil drought, was amplified by salt stress. In contrast, d[CO₂] promoted leaf carbon (C)/N and C/K. Notably, d[CO₂] combined with soil drought enhanced leaf K/Na under salt stress. Moreover, d[CO₂] ameliorated the adverse impacts of soil drought and salinity on root morphology in terms of enlarged root length and root surface area, contributing to superior leaf C, N, and K use efficiency and consequently improved C₄ maize plant dry mass and WUE. These findings would provide essential knowledge to elevate salt tolerance and achieve optimal nutrient homeostasis and WUE in C₄ maize, adapting to future drier and more saline soils under a CO₂-enriched scenario.

Atmospheric elevated CO₂ concentration (e[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 e[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] (a[CO₂], 400 ppm) and e[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 e[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 e[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 e[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.