Paper Reviewed
Xu, X., Wu, P, Song, H., Zhang, J., Zheng, S., Xing, G., Hou, L. and Li, M. 2020. Identification of candidate genes associated with photosynthesis in eggplant under elevated CO2. Biotechnology & Biotechnological Equipment 34: 1166-1175.

Most informed persons know that CO2 is a necessary substrate for plant photosynthesis and that enriching the air with higher levels of CO2 typically enhances this key plant growth parameter. This well-known fact has been proven in literally thousands of laboratory and field experiments conducted over the past five decades, many of which studies we have reported on here at CO2 Science over the past quarter century.

Given as much, we tend not to review studies that focus solely on the basic photosynthetic response of plants to elevated levels of CO2, choosing instead to simply add them and their findings to our ever-growing Plant Growth Database. However, breaking such tradition, we here review the work of Xu et al. (2020), who studied the photosynthetic response of eggplant (Solanum melongena) at ambient (400 ppm) and elevated (800 ppm) levels of CO2. What makes Xu et al.'s study unique is the daily duration of CO2 enrichment and the corresponding photosynthetic response they measured. Typically, nearly all CO2-enrichment studies apply CO2 enrichment for the full 24 hours of a day. A small fraction, however, will provide such enrichment during daylight hours only. Xu et al., in contrast, only applied elevated CO2 for two hours per day (between 0800 and 1000), and only on sunny days.

So what did their study reveal?

Working in a greenhouse at the College of Horticulture in Shanxi Agricultural University, China, the CO2 treatments were applied for a period of four weeks. At the end of each week the authors measured several growth-related parameters. The results of such measurements revealed that a mere 2 hours per day of CO2 enrichment led to (1) a net photosynthetic enhancement in eggplant of 86.11%, improved (2) light adaptability and (3) light energy conversion efficiency of eggplant leaves, (4) a 27% reduction in dark respiration rate, (5) a 26% decrease in stomatal conductance (6) a 102% increase in leaf intercellular CO2 concentration, and increases in eggplant (7) height, (8) width, and (9) leaf numbers.

Transcriptome sequencing technology was also employed by the authors, which analysis revealed 169 differentially expressed genes, 99 of which were up-regulated and 70 of which were down-regulated by elevated CO2. Pathway analysis of these differences further revealed the differentially expressed genes were related to carbon metabolism, carbon fixation, chlorophyll and porphyrin metabolism.

And so it is, in the words of Xu et al., that "the application of exogenous CO2 in the greenhouse stimulated [eggplant] growth." And it accomplished this feat, mind you, via a doubling of CO2 for only a mere two hours per day on sunny days. That is quite the accomplishment for a colorless, odorless and tasteless atmospheric trace gas that far too many incorrectly label as a pollutant!