Effects of Exogenous Spermidine and Elevated CO2 on Physiological and Biochemical Changes in Tomato Plants Under Iso-osmotic Salt Stress

Carbon dioxide (CO2) enrichment is used to boost crop yield in greenhouse vegetable production that often exposes vegetables to the simultaneous occurrence of elevated CO2 and salinity due to frequent irrigation and fertilization in facility horticulture. The beneficial effects of exogenous spermidine (Spd, a kind of polyamine) on plant growth and development under salt stress have been widely reported; however, little information is available on the effects of Spd on the combined treatment of CO2 enrichment and iso-osmotic salt stress. In this work, the effects of exogenous Spd (0.25 mM) on plant growth, chlorophyll content, water status, osmotic adjustment, and the antioxidant system were investigated under CO2 enrichment (800 ppm) and iso-osmotic salt stress [150 mmol/L NaCl and 100 mmol/L Ca(NO3)2] in tomato (Solanum lycopersicum L.). The results showed that iso-osmotic salt stress significantly decreased fresh and dry weights of plants, relative water content, total root length, total root surface area, total root volume, and average root diameter in tomato plants. However, either elevated CO2 or exogenous Spd both attenuated iso-osmotic salt stress-induced reductions in growth parameters and the combined treatment of elevated CO2 and Spd showed a more profound effect, leading to the enhanced tolerance to salt stress in tomato plants. Elevated CO2 and/or exogenous Spd-induced alleviation of iso-osmotic salt stress was associated with efficient osmotic adjustment and antioxidant defense, which minimized the salt stress-induced oxidative stress as well. Therefore, Spd application can be advocated to mitigate secondary salinization in protected vegetable production where elevated CO2 is simultaneously used to boost crop yield.