Elevated CO₂ and high endogenous ABA level alleviate PEG-induced short-term osmotic stress in tomato plants

Elevated CO2 concentration (e[CO2]) alleviates the impact of drought stress on plants where abscisic acid (ABA) is involved. To explore the mechanisms by which tomato plants respond to short-term osmotic stress, Solanum lycopersicum cv. Ailsa Craig (AC), a transgenic line overproducing ABA (sp5), and an ABA-deficient mutant (flacca) were hydroponically grown under ambient CO2 (400 ppm) and e[CO2] (800 ppm) and then exposed to 10% or 15% (w/v) polyethylene glycol (PEG) 6000 for 24 h before transferring to PEG-free nutrient solution for 24 h. Under non-stress condition, e[CO2] decreased root hydraulic conductance (K-root), which was overridden by high endogenous ABA in sp5 through increasing specific leaf area and root branching intensity. Basically, e[CO2] improved stress resistance through enhanced water status. PEG stress decreased stomatal conductance and osmotic potential in AC but these effects were less pronounced in sp5, which exhibited a stronger osmotic adjustment (OA) and improved plant fitness. A greater flexibility of hydraulic system and a reduced sensitivity of K-root to ABA might confer sp5 a great ability to recover from PEG stress. On the contrary, high stomatal density, size and pore aperture offlacca rendered plants suffering severe stress. Moreover, the premise that PEG stress could mimic soil water deficit was the sufficient achievement of OA. Our results indicate that e[CO2] and high endogenous ABA level could improve osmotic stress resistance in tomato plants via osmotic and hydraulic adjustments.