Cassava (Manihot esculenta Cranz.) is a shrub-like plant native to South America which produces tubers rich in carbohydrates. It is now widely cultivated across the southern hemisphere and the tubers are an important food staple for an estimated 750 million people. In addition, the plant leaves are also consumed in some areas.

The scare
In 2009, the Australian biologists Dr. Roslyn M. Gleadow of Monash University, Victoria, Australia, and coworkers published a research paper entitled “Growth and nutritive value of cassava (Manihot esculenta Cranz.) are reduced when grown in elevated CO2” [1]. The abstract of that paper also is clear in describing the (potential) significance of the findings in glowing terms, it includes the following statement: “The responses to CO2 shown here point to the possibility that there could be severe food shortages in the coming decades unless CO2 emissions are dramatically reduced, or alternative cultivars or crops are developed.”

To add fuel to the fire, the co-editors of this special issue of Plant Biology (which represents the proceedings of a symposium on Plant Functioning in a Changing Global Environment, held at Creswick, Australia, in Dec. 2008, with M. Tausz as chairman), say in their introductory editorial [2]: “Given that cassava tubers are a staple food in many of the poorest regions of our planet, and that leaves are often eaten as a protein supplement, this is an alarming result.”

Furthermore, the study by Gleadow et al. also found a substantial increase in cyanogenic glycoside concentrations of the cassava plants when grown under elevated CO2 levels. Cassava leaves and roots both contain such glycosides that break down to release toxic hydrogen cyanide when chewed or crushed, potentially another reason for concern.

Not surprisingly then, this paper created quite a stir. Believers of the “CO2-climate change connection” were ecstatic and took it as evidence for a supposedly undesirable effect of rising CO2 levels in the atmosphere.

The facts
What was not reported though, was the fact that Gleadow’s findings were entirely opposite to previous work of the same kind. For example, Imai et al. [3] observed a fourfold increase in cassava biomass when growing the plants in soil with additional fertilizer and in an atmosphere of 700 ppm CO2, relative to 350 ppm CO2. In contrast, Gleadow et al. reported a severe reduction in plant growth with an almost identical increase of CO2 in the atmosphere and with cassava grown in a synthetic nutrient broth.

Gleadow’s three sets of experiments used CO2 concentrations in the air of 360 (more or less ambient conditions), 550 (elevated CO2), and 710 ppm (highly elevated CO2). The plants were grown in a synthetic nutrient broth, called Hewitt’s solution, with two levels of nitrate ions, namely at one mM (nitrate deficient) and 12 mM (a standard level) nitrate. As Gleadow et al. mention, the type of results to be expected when plants are grown under elevated CO2 levels, namely an increasing plant yield, was observed for soybeans and cotton plants grown in the same greenhouse.

Of course, there are many variables determining the optimal nutrient and other conditions affecting plant growth. In addition to the main nutrients (nitrogen, phosphorus, and potassium salts), there are also micronutrients (such as iron and other metal salts), which are required for plant growth, particularly when grown under hydroponic conditions. It is not clear if such micronutrients were supplied. Furthermore, cassava is known to have other requirements, unlikely to have been fulfilled in Gleadow’s experimental setup. For example, Leihner [4] states that “for healthy growth and good yield, cassava depends strongly on mycorrhizal symbiosis.” It is also unlikely that Gleadow would not have been aware of that.

Another severe shortcoming of the paper is that the results by Imai et al. [3] with a very similar CO2 regime have not been referenced. It is unlikely that Gleadow would not have been aware of that paper. In any event, Gleadow’s omission to cite Imai’s work also raises a question about the peer review standard for this supplemental issue of Plant Biology.

Any serious peer review would have picked up on that omission too and would have demanded the paper by Imai et al. to be referenced and the results of Gleadow’s own work to be discussed vis-à-vis the results of Imai et al. Therefore, it would appear that the “rigorous, independent peer-review system” claimed by the journal, had not been followed for this special issue.

In 2010, Timothy Wells, a free-lance TV producer interviewed Gleadow to shed more light on the relevance of her study. As John O’Sullivan [5] reports, though she had agreed to the interview in advance, Gleadow abruptly ended the interview when Wells asked about details of her study, to the point of calling campus security.

What is at issue here? Very simply, the question is whether or not Gleadow’s experiments were employing realistic (natural) growth conditions for the cassava plants to prosper in the first place. The results by Imai, and other work, make that unlikely. Gleadow’s refusal to discuss such critical questions speaks volumes by itself.

Gleadow’s study appears to have been designed, perhaps inadvertently, in order to produce some spectacular results rather than to assess the effects of elevated CO2 on cassava growth under realistic and common natural soil conditions.

While it may appear to be a good piece of scientific work on first sight, on closer inspection, Gleadow’s et al. work has the hallmark of junk science. In fact, it looks like “cassava-gate” to me.

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