In an attempt to downplay the many benefits of atmospheric CO₂ enrichment on plant growth and development, climate alarmists frequently contend that the positive responses induced by rising levels of CO₂ will be negated by the negative impacts of rising levels of ozone (O₃) pollution. However, as evidenced by the many studies we have previously reviewed on this topic (see, for example, those reviews posted under the various subheadings of Ozone: Effects on Plants, listed here), elevated CO₂ reduces, and nearly always completely overrides, the negative effects of O₃ pollution on plant photosynthesis, growth and yield.

The latest study to illustrate as much comes from the work of Liu et al. (2016), who examined the impact of elevated CO₂ and O₃, both alone and in combination, on Chinese pine (Pinus tabulaeformis). As the dominant species of the coniferous forest in northern China, covering close to 3 million square kilometers, this tree species is of significant regional ecological and economic importance.

The work of the five Chinese researchers was conducted in open top chambers at the Shenyang Botanical Garden of the Chinese Academy of Sciences, where they grew four-year-old P. tabulaeformis seedlings under four different treatment conditions of ambient (370 ppm) or elevated (721 ppm) CO₂ and/or ambient (41.5 nmol mol^-1) or elevated (82.6 nmol mol^-1) O₃ for a period of 90 days. At various intervals during their short-term experiment, the scientists measured the net photosynthetic rates of the plants. They also measured parameters pertaining to plant dry weight, cell membrane damage and oxidative stress. And what did their measurements reveal?

As shown in the figure below, Liu et al. report that "after 90 days exposure to elevated CO₂, dry weight showed a significant increased trend, indicating that the elevated concentration of CO₂ improved plant growth." Not surprisingly, tree seedlings exposed to elevated O₃ experienced dry weight declines. Under the combined elevated CO₂ and O₃ treatment, however, the benefits of CO₂ on growth overpowered the negative effects of O₃, producing dry weight values that were not significantly different from those under elevated CO₂ conditions alone.

Similar CO₂-induced enhancements were reported for net photosynthesis and in measurements of antioxidative enzymes that gauge O₃ cellular damage. These findings led the authors to conclude that the results of their analysis "demonstrated that growth, photosynthesis and antioxidative system of the plant were positively affected by elevated CO₂" and that elevated CO₂ helps to "ameliorate oxidative stress and reduce injury by O₃." Thus, we have yet another example of how the positive growth enhancements of elevated CO₂ are sufficient to negate and overpower the negative growth declines of elevated ozone (Center for the Study of Carbon Dioxide and Global Change).

Figure 1.  Effect of elevated CO₂ and O₃, alone and in combination, on biomass dry weight (left panel) and net photosynthetic rate (right panel) of P. tabulaeformis. Abbreviations for treatment conditions include CK for control, EC for elevated CO₂, EO for elevated O₃ and EC + EO for elevated CO₂ and O₃. Adapted from Liu et al. (2016) (Photo by LIU Zhouli). 

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Full text URL: Regulatory Effects of Elevated Carbon Dioxide on Growth and Biochemical Responses to Ozone Stress in Chinese Pine (Pinus tabulaeformis Carr.)

Abstract: This study examined whether carbon dioxide (CO₂) might alleviate ozone (O₃) injury to the dominant coniferous forest species of northern China, Pinus tabulaeformis Carr. After 90 days O₃ exposure, biomass and net photosynthetic rate (Pn) decreased significantly by 24.44 % and 42.89 % compared with the control. A significant increase in malondialdehyde (MDA) was shown, suggesting cell membrane damage and oxidative stress. However, the positive responses of biomass dry weight, antioxidative enzymes and soluble sugar contents under elevated CO₂ alone and the combination of elevated CO₂ and O₃ were observed, indicating that CO₂ could ameliorate O₃-induced injury. The study provided increasing evidence that moderately elevated CO₂ levels may have a beneficial effect on the forest ecosystem to respond to global climate change.

Publication Name: LIU Zhouli, CHEN Wei,  HE Xingyuan,  FU Shilei,  LU Tao.

Email: zlliu@iae.ac.cn.