Cassava about-FACE: Greater than expected yield stimulation of cassava (Manihot esculenta) by future CO2 levels

Background/Question/Methods

Cassava is the fifth most important crop overall in terms of human caloric intake and has long been vital to food security in Sub-Saharan Africa.  Cassava’s importance as a food security crop is paradoxically hindered by inherent nutritional limitations as it has the lowest carbohydrate to protein ratio of the ten most cultivated crops. Moreover, cassava contains cyanogenic glycosides. Climate change is expected to have its most severe impact on crops in food insecure regions yet little is known about how cassava productivity will respond to climate change. The most important driver of climate change is globally increasing atmospheric CO₂ concentration [CO₂].  Yet, the potential for cassava to enhance food security in an elevated CO₂ world is uncertain as greenhouse and open top chamber (OTC) study reports are ambiguous with respect to the effect of elevated [CO₂] on yield and cyanide content.  Theory suggests that stimulation by elevated [CO₂] should benefit C₃ plants with large vegetative sinks, which fill throughout the growing season (i.e. tuberous roots such as cassava), more than other C₃ crops whose sinks are reproductive.  Studies have yielded misleading results in the past regarding the effect of elevated [CO₂] on crop productivity, particularly in cases where pots restricted sink growth. To resolve these conflicting results, we compare the response of cassava to growth at ambient (ca. 385 ppm) and elevated [CO₂] (585 ppm) under field conditions and fully open air CO₂elevation (FACE).

Results/Conclusions

After three and half months of growth at elevated [CO₂], above ground biomass was 30% greater and cassava root tuber dry mass increased over 100% (89% fresh weight).  High photosynthetic rates and photosynthetic stimulation, larger canopies (LAI), and a large sink capacity all contributed to cassava’s substantial growth and yield stimulation. Cassava exhibited photosynthetic acclimation via decreased Rubisco capacity (V_c,max) but only early in the season prior to root tuber initiation when sink capacity was smaller. Finally, and in contrast to a recent greenhouse study, we found no evidence of increased cyanide concentration in elevated CO₂.  All of our results are consistent with theoretical expectation; however, the magnitude of the yield increase reported here surpasses all other C₃ crops and thus exceeds expectations.