Elevated CO₂ concentration in the air (e[CO₂]) decreases stomatal density (SD) and stomatal conductance (g_s) where abscisic acid (ABA) may play a role, yet the underlying mechanism remains largely elusive. We investigated the effects of e[CO₂] (800 ppm) on leaf gas exchange and water relations of two tomato (Solanum lycopersicum) genotypes, Ailsa Craig (WT) and its ABA-deficient mutant (flacca). Compared to plants grown at ambient CO₂ (400 ppm), e[CO₂] stimulated photosynthetic rate in both genotypes, while depressed the g_s only in WT. SD showed a similar response to e[CO₂] as g_s, although the change was not significant. e[CO₂] increased leaf and xylem ABA concentrations and xylem sap pH, where the increases were larger in WT than in flacca. Although leaf water potential was unaffected by CO₂ growth environment, e[CO₂] lowered osmotic potential, hence tended to increase turgor pressure particularly for WT. e[CO₂] reduced hydraulic conductance of leaf and root in WT but not in flacca, which was associated with downregulation of gene expression of aquaporins. It is concluded that ABA-mediated regulation of g_s, SD, and gene expression of aquaporins coordinates the whole-plant hydraulics of tomato grown at different CO₂ environments.

The interactive effects of CO₂ elevation, N fertilization, and reduced irrigation regimes on fruit yield (FY) and quality in tomato (Solanum lycopersicum L.) were investigated in a split-root pot experiment. The plants were grown in two separate climate-controlled greenhouse cells at atmospheric [CO₂] of 400 and 800 ppm, respectively. In each cell, the plants were fertilized at either 100 or 200 mg N kg^-1 soil and were either irrigated to full water holding capacity [i.e., a volumetric soil water content of 18%; full irrigation (FI)], or using 70% water of FI to the whole pot [deficit irrigation (DI)] or alternately to only half of the pot [partial root-zone irrigation (PRI)]. The yield and fruit quality attributes mainly from sugars (sucrose, fructose, and glucose) and organic acids (OAs; citric acid and malic acid) to various ionic (NH₄⁺, K⁺, Mg²⁺, Ca²⁺, NO₃^-, SO₄^2-, and PO₄^3-) concentrations in fruit juice were determined. The results indicated that lower N supply reduced fruit number and yield, whereas it enhanced some of the quality attributes of fruit as indicated by greater firmness and higher concentrations of sugars and OAs. Elevated [CO₂] (e[CO₂]) attenuated the negative influence of reduced irrigation (DI and PRI) on FY. Principal component analysis revealed that the reduced irrigation regimes, especially PRI, in combination with e[CO₂] could synergistically improve the comprehensive quality of tomato fruits at high N supply. These findings provide useful knowledge for sustaining tomato FY and quality in a future drier and CO₂-enriched environment.