Paper Reviewed
Forbes, S.J., Cernusak, L.A., Northfield, T.D., Gleadow, R.M., Lambert, S. and Cheesman, A.W. 2020. Elevated temperature and carbon dioxide alter resource allocation to growth, storage and defense in cassava (Manihot esculenta). Environmental and Experimental Botany 173: 103997.

Cassava (Manihot esculenta) is an important food crop known for its edible roots. According to Forbes et al. (2020), it is “one of the most efficient producers of carbohydrates and energy among all food crops [and] a vital source of calories for more than a billion people in developing and tropical countries.” In sub-Saharan Africa alone it accounts for “almost two-thirds of direct caloric intake.” Given the important role of cassava with respect to the food security of millions of persons, Forbes et al. write that “exploration of the responses of cassava to predicted climatic changes is vital.” And thus they set out to investigate the interactive effects of rising temperature and atmospheric CO2 concentrations on the growth, productivity, physiology and chemical defense of a high-yielding and low hydrogen cyanide-producing cultivar, MAus7.

The experiment was conducted in a climate controlled greenhouse facility divided into three treatments, (1) ambient CO2 (400 ppm) and ambient temperature, (2) ambient CO2 and elevated temperature (3.6 °C above ambient) and (3) elevated CO2 (800 ppm) and elevated temperature (3.6 °C above ambient). Ambient CO2 values were set to 400 ppm whereas elevated values were maintained at 800 ppm. Ambient treatment temperatures were set to track the ambient conditions of Cairns, North Queensland, Australia, and elevated temperatures were kept at 3.6 °C above ambient. Twenty days after planting, eighteen similarly sized cassava plants were placed into each treatment chamber and maintained under their respective treatments for an additional 74 days.

Based on data presented in the authors’ Tables A2 and A3, raising the temperature alone caused a 12% reduction in plant photosynthesis. However, this reduction was fully mitigated in the elevated CO2 and elevated temperature treatment, which stimulated photosynthesis by 33% relative to the control treatment. Other growth-related parameters also benefited from the elevated levels of CO2 and temperature, including (relative to the control treatment) plant height (+19.4%), relative growth rate (+15.0%), aboveground biomass (+72.6%), belowground biomass (+58.4%), leaf number (+39.1%), leaf mass (+65.9%), fine root mass (+103.5%), tuber number (+30.2%) and tuber mass (+39.1%).

Forbes et al. also report the average tuber cyanide concentration declined under elevated temperature conditions (compared to control) but remained similar in the elevated CO2 and elevated temperature treatment. With respect to cassava leaf tissues, the cyanide concentration in this portion of the plant was highest in the control treatment. Both the elevated temperature and combined elevated temperature and CO2 treatments had concentrations that were 29% lower. The authors also report that there was “no alteration in the quantity or diversity of leaf phenolic compounds” and “no significant differences in leaf nitrogen content per unit leaf area.” Lastly, plant water use efficiency in the combined elevated temperature and CO2 treatment was 38.7% greater than that reported under control conditions.

The above findings in general agree with that observed by previous researchers highlighting the growth-related benefits of atmospheric CO2 on cassava (Rosenthal et al., 2012; Cruz et al., 2016; Cruz et al., 2018). Furthermore, they demonstrate there is no cause for health-related concerns from future cassava consumption, as Forbes et al. write “in future climate scenarios where increases in air temperature are expected to be accompanied by concomitant increases in atmospheric CO2, the results suggest an increase in the quality of cassava leaves for human (and perhaps livestock) consumption by decreased cyanide concentrations and possible reduced dependence upon processing before human consumption.” Continuing, they add “this is important as cassava leaves are increasingly an important source of proteins, vitamins and micronutrients, especially within food-insecure regions.”

Thus, considering all of the above, it is abundantly clear that rising atmospheric CO2 and temperature will benefit the production of cassava and help enhance food security in developing regions. Furthermore, this study puts to rest alarmist concerns that cassava plants will contain greater (and potentially deadly) concentrations of cyanide in the future because of rising atmospheric CO2.

References
Cruz, J.L., Alves, A.A.C., LeCain, D.R., Ellis, D.D. and Morgan, J.A. 2016. Elevated CO2 concentrations alleviate the inhibitory effect of drought on physiology and growth of cassava plants. Scientia Horticulturae 210: 122-129.

Cruz, J.L., LeCain, D.R., Alves, A.A.C., Filho, M.A.C. and Coelho, E.F. 2018. Elevated CO2 reduces whole transpiration and substantially improves root production of cassava grown under water deficit. Archives of Agronomy and Soil Science 64: 1623-1634.

Rosenthal, D.M., Slattery, R.A., Miller, R.E., Grennan, A.K., Cavagnaro, T.R., Fauquet, C.M., Gleadow, R.M. and Ort, D.R. 2012. Cassava about-FACE: Greater than expected yield stimulation of cassava (Manihot esculenta) by future CO2 levels. Global Change Biology 18: 2661-2675.

This article appeared on the CO2 Science website at http://www.co2science.org/articles/V23/jun/a11.php

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