When Ros Gleadow opened the airlock to the greenhouse at The Australian National University, she stepped into the atmosphere of the future. The air was thick with carbon dioxide—700 parts per million, to be precise—which matches the concentration predicted 90 years from now. While evaluating the responses of crops to the altered atmosphere in the summer of 2008, she found that the cotton, sorghum, soybean and cassava plants she’d planted 9 months earlier grew higher, a little woodier, and with more stems and smaller leaves than normal—all of which she’d expected. But when she dug the cassavas out of their pots, the tubers, which usually grow as large as yams, looked like stunted fingers.

Her cassavas of the future had produced 80 percent less food. “It came completely unexpectedly because plants normally grow bigger under higher CO2,” says Gleadow, a plant physiologist at Monash University in Melbourne. Her immediate thought went to the millions of people living in the tropics, where cassava is the third largest source of dietary carbohydrates. “If the yield decreases, there’s going to be a lot of hungry people.”

That wasn’t the only problem. The cassava plants themselves had become poisonous. Like 60 percent of all our staple crops, cassava produces chemicals called cyanogenic glycosides to deter grazing animals, which, when chewed, release cyanide gas. In small quantities, the cyanide tastes like bitter cherries, enough to ward off animals. But the high-CO2 cassavas produced three times the cyanide of today’s plant. (The poison largely shows up in the leaves, which most people avoid, although some in African countries eat the leaves as a protein supplement.) Gleadow hypothesizes that her cassavas may have poisoned themselves, meaning the extra cyanide shrank the tubers (Plant Biology, published online August 6, 2009).

Until recently, modelers saw CO2’s effect on plant life as the silver lining of climate change. They thought increases in the gas would act as fertilizer, making crops grow bigger and more lush. After all, CO2 is one of the main components of photosynthesis. In the late 1980s, experimenters projected as much as 30 percent increases by 2050 in yield for staples like wheat and soy. But recent experiments under open-air conditions showed half that rate of growth (Science, 312: 1918–21, 2006).

The effects of higher CO2 tend to be more nuanced than first projected and, in cases like cassava, species-specific. The plants did devote the extra energy from higher CO2 to their carbon skeletons, growing taller, thicker stems and branches as well as fewer leaves. In cases such as cassava’s, though, the plant devoted part of the added bounty to developing defenses, i.e., producing cyanide. “You’re affecting plants at the heart of their metabolism, so a lot of things change about them, including their chemistry,” says Daniel Taub, a biologist at Southwestern University in Georgetown, Texas.

At the root of the change is a series of microscopic stomata, lip-shaped pores on leaves that plants use to absorb CO2. Under higher levels of CO2, stomata partially shut in C3 plants (plants that convert CO2 into three carbon molecules) like rice, wheat and cassava. Stomata have a secondary function: they emit vapor so that plants can siphon water and nutrients from the soil into their branches, a process called transpiration. Because the stomata contract, plants transpire less and use less water, thus drawing fewer nutrients from the soil. As a result, C3 crops exposed to more CO2 show deficits by up to 15 percent in calcium, magnesium, phosphorus, and—most important—protein (Glob Chang Biol 14: 565–75, 2008).

In November, Gleadow heads off to Mozambique on a grant from the Australian government to examine the effects of elevated CO2 on yams and taro. She and other scientists are also investigating which crops might fare best in the air of the next century, and are breeding them. “I think I know what I’m going to be doing for the next 15 years,” Gleadow says.

The take home message in all of this is that changing the composition of the atmosphere is a risky business that may have unexpected consequences. Increasing concentrations of carbon dioxide can affect plants in unexpected ways and the best way to deal with that is to reduce carbon emissions. Now.

It is well established that leaves and grains are less nutritious when gown at twice-ambient concentrations of CO2 world (e.g. Taub et al. 2007, Glob. Change Biol. 14: 111). We set out to test whether the tubers of cassava would have less protein and more toxic cyanogens, as we had observed in the leaves of several other species. The good news was that the tubers were not more cyanogenic. The big surprise was the decrease in yield. That was an observation, and is consistent with photosynthetic gas exchange data collected by John Evans who is highly respected in this field. Other crop plants (cotton, soy bean, sorghum) growing under the same conditions grew exactly as expected.

There is no doubt about what we found. Cassava tubers were smaller and fewer in number, and the leaves were more toxic. Why that was so is another question. To quote Isaac Asimov ?The most exciting phrase to hear in science, the one that heralds new discoveries, is not 'Eureka!' (I've found it!), but 'That's funny...?. Well, something funny is going on here. It could be an artefact, but unlikely. This is a pretty big difference and all the other plants grown at the same time behaved as expected.

To address a few specific points that have been raised:
(1) We used 40 litre pots to minimise pot effects. Note that we had a root and soil specialist on the team too (Tim Cavagnaro).
(2) Both controlled environment and field studies are used in physiological experiments. There are limitations and benefits in each. Temperature, for example, is better controlled in glasshouses studies. FACE studies typically only use 500-600 ppm (we used up to 710 ppm). A review of long term data from chamber and FACE (free-air CO2 enrichment) studies concluded that the results were broadly similar, although there were some important quantitative differences, especially with trees (Ainsworth & Long 2005, New Phytologist 165: 351?372). The point is that the differences were a matter of magnitude, not direction.
(3) At the time, there were no open-air CO2 facilities anywhere in Australia where cassava could grow outside. This experiment was done on a shoe-string from a donation from a philanthropic organisation. This experiment definitely needs to be repeated. We have recently been offered the opportunity to repeat it in a FACE study in 2010 which is excellent.
(4) This was never seen as the last word on cassava and CO2. That?s the way science works. You don?t dismiss data just because it doesn?t fit with your paradigm. You put it out there for discussion. Then you repeat it. And repeat it again. And tweak the experiment and do it again.

Whatever, let's just hope that the world manages to get the carbon pollution problem under control sometime soon.

-Ros Gleadow

I am not a scientist, I am a curious retired truck driver, but I did look at the pictures and wonder if the only variable between the 350 and 700 examples was the atmosphere.

If both plants were in the same size pot with the same nutrient levels in the soil, the example labeled 350 grew far more tubers that the one marked 700.

Doesn't that answer the question of the "unnatural" growing conditions? I believe the question is rooted in my total ignorance of both the method and the meaning of the experiment.

In my previous comment below (2nd from bottom),I pointed out the negetative effects of growing cassava shrub in small pots left indoors on storage root development, and consequently on whole plant responses to elevated CO2. Here below a published research illustrating an unequivocal evidence of such negative effect in cotton plant(also woody and originally shrub in nature). In a review in Photosynthetica,43:161-176, 2005, I pointed out the danger of extrapolating results of research inappropriately conducted on potted plants grown indoors, to what may take place under field conditions. It is mandatory to get rid of such mediocracy in science,,,Mabrouk A. El-Sharkawy.
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Plant Physiology 96:627-634 (1991)
ﾩ 1991 American Society of Plant Biologists

Environmental and Stress Physiology

Root Restriction as a Factor in Photosynthetic Acclimation of Cotton Seedlings Grown in Elevated Carbon Dioxide 1
Richard B. Thomas and Boyd R. Strain
Botany Department, Duke University, Durham, North Carolina 27706


Interactive effects of root restriction and atmospheric CO2 enrichment on plant growth, photosynthetic capacity, and carbohydrate partitioning were studied in cotton seedlings (Gossypium hirsutum L.) grown for 28 days in three atmospheric CO2 partial pressures (270, 350, and 650 microbars) and two pot sizes (0.38 and 1.75 liters). Some plants were transplanted from small pots into large pots after 20 days. Reduction of root biomass resulting from growth in small pots was accompanied by decreased shoot biomass and leaf area. When root growth was less restricted, plants exposed to higher CO2 partial pressures produced more shoot and root biomass than plants exposed to lower levels of CO2. In small pots, whole plant biomass and leaf area of plants grown in 270 and 350 microbars of CO2 were not significantly different. Plants grown in small pots in 650 microbars of CO2 produced greater total biomass than plants grown in 350 microbars, but the dry weight gain was found to be primarily an accumulation of leaf starch. Reduced photosynthetic capacity of plants grown at elevated levels of CO2 was clearly associated with inadequate rooting volume. Reductions in net photosynthesis were not associated with decreased stomatal conductance. Reduced carboxylation efficiency in response to CO2 enrichment occurred only when root growth was restricted suggesting that ribulose-1,5-bisphosphate carboxylase/oxygenase activity may be responsive to plant source-sink balance rather than to CO2 concentration as a single factor. When root-restricted plants were transplanted into large pots, carboxylation efficiency and ribulose-1,5-bisphosphate regeneration capacity increased indicating that acclimation of photosynthesis was reversible. Reductions in photosynthetic capacity as root growth was progressively restricted suggest sink-limited feedback inhibition as a possible mechanism for regulating net photosynthesis of plants grown in elevated CO2.



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1 Research supported by U.S. Department of Energy, CO2 Research Division, contract DE-FG05-87ER60575.





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The fact that the Cassava increased its toxicity (investing more resources in herbivore deterrence) as well as producing relatively tiny tubers while grown under higher concentrations of carbon dioxide, suggests that Cassavas might be using higher CO2 as a marker of the presence of animals which might be expected to eat them.

Perhaps, then, the tiny tubers reflect the fact that big tubers would be a more dubious investment in the presence of such animals rather than an 'unintended' self-poisoning.

The results prompt me to ask:

What were the changes in the soil pH over time for both groups?

Would the next generation growing in the high CO2 environment have the same effects?

Was there a decrease in the above ground mass proportional to the below ground mass of the plant?

mabrouk el-sharkawy's comments lead me to recall a recurring concern of mine in journal publications. Whether the opposite sides of the discussion are correct or incorrect can be refined by a simple replication of the experiment under a control, and high CO2 ambient atmospheres matching the conditions of Ms. Gleadow and Mr El-Sharkaway's contentions. In spite of the replicability feature of the scientific method, journals publish "original" information, and that is certainly interesting, but a replication under the published conditions by another scientist is the stuff of science that will be "accepted". Debate is useful, but the proof of the pudding is in replication. In my opinion, a section of journals should address experiments that are replications in order to add veracity to original material.

Cassava is a shrub or tree in nature (can reach over two meters in height) that needs a large volume of soil in order to normally develop its storage roots.Thus, growing cassava in pots is invalid experimental system and the wrong approach. Under this artificial system it is not a surprise that the plants exposed to elevated carbon dioxide are not developing normal storage roots.Higher than ambient carbon dioxide enhances plant growth with increasing leaf photosynthetic rate in both C3 and C4 species, with lesser dgree in the latter, as compared to plants exposed to ambient air CO2.The extra carbohydrates that result from enhanced photosynthesis needs a strong root sink to receive such materials.In the current studies such sinks (storage roots) are not normally developing due to physical constraints imposed by the small pots where the cassava plants were grown. Thus, the responses observed from these studies are erroneous and invalid.Field research should have been conducted using the Free-air CO2 enrichment system (FACE experiments)and/or open top champer. I hope the author of this research repeats the experiment under field conditions before generalizing any unwarranted conclusion from unworthy experiments; the same should be said with The Scientist reporter.Furthermore, there are scientific reports in literature that indicate positive responses in cassava exposed to higher than ambient CO2, both in photosynthesis and dry matter accumulation. It happenned that I sent my crtitical comments to the author upon receiving a pdf reprint copy months ago.

I'm looking forward to seeing more about these studies. We should have known better than to imagine a direct "more CO2 = good for plants" situation... Some will certainly thrive, but I'm sure most of them alive today are adapted to the types of CO2 levels we've had in recent geologic time. Surely even those similar plants from the carboniferous era were different species from their relatives today. All of life will have to adapt to the new conditions, like it always does -- and fast environmental changes have happened more than once before. The only real question is whether we ourselves will end up on the losing side along with so many other species -- or be able to keep up using technology...