While biochar amendment enhances plant productivity and water-use efficiency (WUE), particularly under water-limited conditions, the specific mechanisms driving these benefits remain unclear. Thus, the present study aims to elucidate the synergistic effects of biochar and reduced irrigation on maize (Zea mays L.) plants, focusing on xylem composition, root-to-shoot signaling, stomatal behavior, and WUE. Maize plants were cultivated in split-root pots filled with clay loam soil, amended by either wheat-straw biochar (WSB) or softwood biochar (SWB) at 2% (w/w). Plants received full irrigation (FI), deficit irrigation (DI), or partial root-zone drying irrigation (PRD) from the 4-leaf to the grain-filling stage. Our results revealed that the WSB amendment significantly enhanced plant water status, biomass accumulation, and WUE under reduced irrigation, particularly when combined with PRD. Although reduced irrigation inhibited photosynthesis, it enhanced WUE by modulating stomatal morphology and conductance. Biochar amendment combined with reduced irrigation significantly increased xylem K⁺, Ca²⁺, Mg²⁺, NO₃^–, Cl^–, PO₄^3–, and SO₄^2– but decreased Na⁺, which in turn lowered xylem pH. Moreover, biochar amendment and especially WSB amendment further increased abscisic acid (ABA) contents in both leaf and xylem sap under reduced irrigation conditions due to changes in xylem ionic constituents and pH. The synergistic interactions between xylem components and ABA led to refined adjustments in stomatal size and density, thereby affecting stomatal conductance and ultimately improving the WUE of maize plants at different scales. The combined application of WSB and PRD can, therefore, emerge as a promising approach for improving the overall plant performance of maize plants with increased stomatal adaptations and WUE, especially under water-limited conditions.

Soil salinity hampers plant performance. Elevated atmospheric CO₂ (e[CO₂]) could alleviate the detrimental effect of salinity on plants but whether abscisic acid (ABA) is involved in this process is unclear. To address this issue, three tomato (Solanum lycopersicum) genotypes with varying endogenous ABA concentrations (wild-type AC, ABA-deficient mutant flacca and ABA-overproduction line SP5) were grown in pots under ambient (400 μmol · mol^–1) or elevated (800 μmol · mol^–1) CO₂ with or without the addition of 100 mmol · L⁻¹sodium chloride (NaCl). The results showed that e[CO₂] favored ion homeostasis by decreasing root-to-shoot delivery of Na⁺, which was mainly attributed to lowered transpiration rate rather than altered xylem-sap Na⁺ concentration. In AC and SP5, the low transpiration rate of e[CO₂]-plants under salinity was accompanied by enhanced endogenous ABA levels, which might play a role in upregulating the abundance of specific transcripts related to Na⁺ homeostasis (i.e., SALT OVERLY SENSITIVE) under salt stress. In flacca, e[CO₂]-induced Na⁺ homeostasis was abolished, which could be ascribed to the low and unaltered ABA levels, albeit the ethylene biosynthesis was enhanced in flacca under salt stress, indicating an antagonistic relationship between ABA and ethylene. Furthermore, e[CO₂] inhibited ethylene biosynthesis under salt stress in all three genotypes. The results enrich our comprehension of the fundamental processes of e[CO₂]-conferred salt tolerance in tomato.