Paper Reviewed
Li, S., Li, Y., He, X., Li, Q., Liu, B. and Ai, X. 2019. Response of water balance and nitrogen assimilation in cucumber seedlings to CO2 enrichment and salt stress. Plant Physiology and Biochemistry 139: 256-263.

High soil salinity is an abiotic stress that limits plant growth in many parts of the world. Among other things, it induces various physiological changes, including ion imbalance and/or ion toxicity, reduced photosynthesis, metabolism and, ultimately, growth.

In recent years, multiple studies have revealed that the detrimental effects of salinization are often reduced or fully ameliorated under elevated CO2 concentrations, yet few researchers have investigated the potential physiological mechanisms involved in this alleviation. And so, in an attempt to provide some clarification in this regard, Li et al. (2019) recently investigated the effects of elevated CO2 on the water balance and nitrogen assimilation of cucumber seedlings exposed to salt stress.

The work was conducted in a controlled-environment greenhouse at Shandong Agricultural University in China. Cucumber (Cucumis sativus, cv. Jinyou No. 35) seedlings were grown hydroponically at ambient (400 ppm) or elevated (800 ppm) CO2 concentrations under normal (nutrient solution + 0 mmol L^-1 NaCl) or salt stressed (nutrient solution + 80 mmol L^-1 NaCl) conditions. After 7 days of treatment the authors performed a series of analyses on the plant leaf and roots.

Results of the analysis revealed that elevated CO2 activated enzymes in the plant roots under salinity stress that ensured the function of proton pumps, which "improves root activity and root hydraulic conductivity and ameliorates root growth status and water absorption." Moreover, the authors note that elevated CO2 "promotes both the efflux/regionalization of the dilution of Na⁺ within plants under the action of membrane proton pumps and water, thereby reducing ion toxicity."

In commenting on their findings, Li et al. say that "maintaining a good water state under stress conditions and diluting salt within a whole plant to reduce ion toxicity are very important for improving the salt tolerance of plants." And the fact that their findings showed that elevated CO2 increased root activity and hydraulic conductance, which enhanced root water absorption, suggests that this may be "one of the mans for plants to maintain water balance under salt stress."

Such results are encouraging news to those who presently farm on salt-stressed soils. They indicate that rising atmospheric CO2 in the years and decades ahead will greatly assist in the growth and development of plants presently plagued by this abiotic stress.