Doubling CO2 Modulates Root Morphology to Enhance Maize Elemental Stoichiometry and Water Use Efficiency Under Soil Drought and Salinity

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.