Tiny tubers
By Daniel Grushkin
Volume 23 | Issue 10 | Page 17
the-scientist.com
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.”
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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 tubers, which usually grow as large as yams, looked like stunted fingers.
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.
Comments on this article
A Method to Their Meagreness
by James Morgan
[Comment posted 2009-10-30 17:03:19]
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.
comment:
pH, next gen
by Brian Lee
[Comment posted 2009-10-30 15:46:18]
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?
comment:
Tubers and CO2
by Daniel Jones
[Comment posted 2009-10-30 14:52:40]
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.
comment:
To grow a root-crop shrub in pot in a greenhouse is a wrong way
by mabrouk el-sharkawy
[Comment posted 2009-10-30 14:06:16]
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.
comment:
Nothing biological is ever simple...
by Cheryl Scott
[Comment posted 2009-10-30 13:13:49]
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... |
