Paper Reviewed
Bourgault, M., James, A.T. and Dreccer, M.F. 2017. Pot size matters revisited: does container size affect the response to elevated CO2 and our ability to detect genotypic variability in this response in wheat? Functional Plant Biology 44: 52-61.

Introducing their recent publication, Bourgault et al. (2017) write that although many studies have investigated the impacts of rising atmospheric CO2 on wheat, much remains to be learned with respect to how those impacts are modulated by genotypic variability. And considering projections that the global population will exceed nine billion persons in just a few short decades from now, it is indeed important that scientists identify which plant genotypes are most responsive to elevated levels of CO2 so that farmers can take advantage of the growth-enhancing benefits of carbon dioxide and increase yields to feed the ever-growing population of the planet.

As their contribution to the topic, Bourgault et al. grew four wheat genotypes (SB062, SB139, SB169 and Attila) from seed to harvest in controlled-environment glasshouse bays under either ambient (420 ppm) or elevated (700 ppm) CO2 concentrations at a day/night temperature of 24/15 °C. A third treatment consisted of plants also grown at an elevated CO2 concentration of 700 ppm, but at a warmer day/night temperature of 28/22 °C. Concerned that the size of the container in which the plants were grown might affect the genotypic response, the authors grew half their plants in each treatment in 1.4 L pots and half in 7.5 L columns. Thereafter, multiple measurements were made on various plant traits at different stages of development, including fourth leaf emergence, first node development, spike development, anthesis and harvest.

Results of the analysis revealed that the container size influenced the response to elevated CO2 and/or elevated CO2 at the higher temperature. Biomass accumulation in potted plants, for example, was enhanced under elevated CO2 in both temperature regimes during the early stages of growth regardless of container size. However, by anthesis, they had reduced to the point of marginal significance, and they further declined to a level of insignificant difference (from the ambient CO2 treatment) at plant maturity. In contrast, a CO2-induced increase in biomass was observed at all stages of plant growth in plants growing in the larger 7.5 L columns (see figure below).

Figure 1. Biomass accumulation at anthesis for four wheat genotypes grown in pots (left panel) and in columns (right panel) under ambient CO2 (aCO2), elevated CO2 (eCO2) and elevated CO2 with high temperature (eCO2HT). Adapted from Bourgault et al. (2017).

Additionally, Bourgault et al. report that they found a lack of consistency in 14 of the 23 growth-related parameters they measured between the plants growing in pots versus columns, leading them to conclude that "growth restriction [occurred in the 1.4 L pots] within 30 days of sowing even with regular and abundant water and fertilization," which finding, they conclude, "is a concern considering that a good reason to use controlled environment chambers and glasshouses is to investigate the traits that underpin biomass accumulation without the climatic and soil variability inherent with field studies." Furthermore, they note that given their observations, if they had only grown the four wheat genotypes in pots instead of columns, or stopped the experiment short at anthesis instead of maturity, they "would not have been able to [correctly] identify the most responsive cultivar" and "would have reached different conclusions about the effect of elevated CO2 and/or elevated CO2 and high temperature on wheat."

Concluding their remarks, the three Australian researchers say their study is intended as "a message of caution to controlled environment experimenters: using small containers can artificially create conditions that could either hide or overly express genotypic variability in some traits in response to elevated CO2 compared with what might be expected in larger containers." And that is good advice worth following.