Tomato plants with different endogenous ABA levels show unique phenotypic and phytohormones response to elevated CO₂ and progressive soil drying

Increasing atmospheric CO₂ concentrations ([CO₂]) accompanied by drought stress have a significant impact on plant growth and development. Our previous study showed that elevated [CO₂] (e[CO₂]) can stimulate net photosynthetic rate (A_n) and depress stomatal conductance (g_s) in tomato (Solanum lycopersicum). Here, we further studied the interactive effect of e[CO₂] and endogenous abscisic acid (ABA) on plants response to drought stress. Tomato plants with different endogenous ABA levels including Ailsa Craig (AC), its ABA-deficient mutant (flacca) and transgenic tomato with high ABA concentration (SP5) were grown in ambient [CO₂] (a[CO₂], 400 ppm) and e[CO₂] (800 ppm) and subjected to progressive soil drying. Our results indicated that at onset of drought, e[CO₂] increased the A_n in flacca and SP5, but not in AC. Compared to a[CO₂], e[CO₂] decreased g_s in AC and SP5, and retarded stomatal closure only in SP5 during soil drying. Under drought, e[CO₂] improved leaf water use efficiency in all genotypes and relatively better maintained leaf water potential except in the flacca. Drought stress induced significant accumulation of ABA in leaf, especially in SP5, but relatively lower in flacca. However, compared to AC and SP5, flacca accumulated large amounts of ethylene under drought, suggesting that in plants with ABA deficiency, ethylene may play a compensatory role in inducing stomatal closure during soil drying. These findings improve our understanding of plant performances to a future drier and CO₂-enriched environment.