Chemical Education Today
Nature;Carbon Sinks and Rising Rice: Climate Change and Our Biosphere Sabine Heinhorst* and Gordon Cannon† Department of Chemistry and Biochemistry, University of Southern Mississippi, Hattiesburg, Mississippi 39406-0543 *
[email protected]; †
[email protected] For this year's Earth Day issue of the Journal of Chemical Education, we have chosen to report on Nature articles that discuss connections between climate change and Earth's biosphere. One article reports on anthropogenic carbon emissions and their effects on the carbon balance of terrestrial ecosystems in China in a period of intense economic growth and energy consumption. A second article documents the effects of permafrost thawing on carbon loss from a boreal Arctic ecosystem: a process that is likely to have a positive feedback effect on global warming. Finally, we summarize the remarkable ability of some rice varieties to outgrow rising floodwaters. The genes that enable rice to respond in this manner have been identified and hold the key for the development of varieties that can be cultivated in a broader range of flood-prone land. The Biosphere as a Carbon Sink... During the past decades, China has experienced unprecedented economic growth. To meet the concomitant increase in energy demand, the country has largely relied on burning fossil fuels. Adding the considerable increase in cement production (a process that generates large amounts of CO2), it is not surprising that since 2006 China has surpassed the United States as the world's largest CO2 emitter. Fortunately, part of the CO2 released into the atmosphere from anthropogenic activities is absorbed by the terrestrial biosphere. To understand how this carbon sink affects the global carbon cycle, it is imperative to estimate the contribution of large landmasses such as China. Piao et al. from Peking University, the Chinese Academy of Sciences, and collaborators from France and Great Britain (1) undertook a comprehensive inventory of China's terrestrial carbon balance through the 1980s and 1990s. The scientists evaluated the contributions of biomass and soil through satellite measurements and verified their results with data derived from ecosystem modeling and from atmospheric CO2 determinations. They found that China's forests, which cover about 14% of the country, have converted approximately 1.65 1015 g of carbon into biomass since the early 1980s. The contribution of vegetation in shrubland, which covers about 20% of China, is approximately one-third that of forests. In addition to biomass, organic matter in soil constitutes roughly 42% of the carbon sequestered from the atmosphere. Combining the similar results the researchers obtained by using three independent methods, they conclude that China's terrestrial biosphere constitutes a net carbon sink of a magnitude comparable to that in Europe and roughly two-thirds that in the continental United States. Its net carbon removal constituted an estimated 28-37% of CO2 emissions in the 1980s and 1990s. The authors warn that this value is likely to decrease because of increased fossil fuel burning and deforestation, which 128
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counteract ongoing efforts in China to reforest and restore vegetation. The “News and Views” commentary by Gurney (2) in the same issue provides an informative comparison of the carbon balance of the United States and China and offers estimates on how that balance may change in China in light of current and projected trends in CO2 emissions and land use. ...and the Effects of Global Warming on This Sink E. A. G. Schuur from the University of Florida and his collaborators from the University of California-Riverside and the University of Alaska have assessed the effect of permafrost thawing in the Alaskan tundra on the soil's ability to retain old carbon stores (3). The researchers collected data over a three-year period at three watershed sites at which the permafrost had thawed minimally, moderately, and extensively, respectively. Measurements of net CO2 exchange between tundra and atmosphere revealed that the moderately and extensively thawed sites, which support more plant growth than the minimally thawed permafrost, served as net carbon sinks during the summer months but released more carbon than the minimally thawed site in fall, winter, and spring. Furthermore, through 14C measurements the researchers found a strong positive correlation between the extent of old carbon loss from deeper soil layers and the degree to which the permafrost had thawed. These results paint a bleak picture of the effects that further global warming will have on the permafrost areas of the vast boreal and Arctic ecosystems: The carbon released into the atmosphere from the soil is predicted to create a positive feedback loop and contribute to the warming trend despite the carbon fixation activity of the increased vegetation in these areas. A SNORKEL for Rice in Deep Water According to the Food and Agriculture Organization (FAO) of the United Nations, rice, maize, and wheat are the three most important food sources for the world's population (4). Among the three, rice (Oryza sativa) is the food staple in many Southeast Asian countries, where it is increasingly being cultivated in low-lying areas prone to frequent flooding. Although rice does not mind getting its feet wet, the high-yielding varieties cannot tolerate complete submersion in water, as illustrated by the effect of recent floods in the Philippines on the rice harvest (5). In contrast, several local (albeit low-yielding) rice varieties have the ability to respond to slowly rising flood waters by growing at the astonishing rate of up to 25 cm per day until their stems have increased in length by several meters and their flowers and upper leaves have emerged from the water (Figure 1). Their long, porous
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Vol. 87 No. 2 February 2010 pubs.acs.org/jchemeduc r 2010 American Chemical Society and Division of Chemical Education, Inc. 10.1021/ed800062x Published on Web 01/12/2010
Chemical Education Today
Figure 1. Remarkable ability of deepwater rice to respond to flooding. (A) Rice keeps its “head” above water as the water level rises. (B) Deepwater rice that has grown to a height of several meters. Images courtesy of M. Ashikari.
stems work like snorkels, facilitating gas exchange and keeping the submerged parts aerated. In a collaborative research tour de force, scientists from Nagoya University, Kyushu University, RIKEN, and the National Institute of Agrobiological Resources in Japan, led by M. Ashikari, have identified two genes, appropriately named SNORKEL1 and SNORKEL2, that are responsible for the flooding response of deepwater rice (6). The researchers combined classical plant breeding with molecular genetic approaches to locate the two genes on chromosome 12 of deepwater rice. They show that transfer of the two SNORKEL genes into a highyielding rice variety incapable of underwater stem elongation produces offspring with deepwater rice characteristics. Production of the proteins encoded by the SNORKEL genes is induced by the plant hormone ethylene, which accumulates in organs that are submerged because of the slower diffusion of the gas in water. The SNORKEL proteins, which are ethylene response factors, contribute in an as-yet unknown way to the increase in the growth hormone gibberellic acid that in turn stimulates cell growth and stem elongation. In an accompanying “News and Views” commentary on pp 959-960 of the same issue, L. A. C. J. Voesenek and J. BaileySerres summarize the previous findings of many other researchers that established connections between ethylene, gibberellic acid, and abscisic acid (another plant hormone) signaling; they link those connections to the effects of the SNORKEL genes (7). The authors also point out that the escape response exhibited by deepwater rice and many other wetland plants is not the only strategy plants use to survive inundation by water. Some rice varieties use a quiescence strategy instead; the proteins encoded by a cluster of aptly named SUBMERGENCE genes suppress ethylene-induced underwater stem elongation and let these plants save their resources until they can resume growth once
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floodwaters retreat. Conversion of high-yielding rice varieties to flood-resistant ones by introducing the escape or quiescence trait should broaden the range of suitable land for the cultivation of rice in countries such as Vietnam, Bangladesh, Myanmar, and Cambodia. Hopefully, this will lead to an increase in rice production required to feed an ever-increasing world population. Literature Cited 1. Piao, S.; Fang, J.; Ciais, P.; Peylin, P.; Huang, Y. Sitch S.; Wang, T. The Carbon Balance of Terrestrial Ecosystems in China. Nature 2009, 458 (April 23), 1009-1013. 2. Gurney, K. R. Global Change: China at the Carbon Crossroads. Nature 2009, 458 (April 23), 977-978. 3. Schuur, E. A. G.; Vogel, J. G.; Crummer, K. G.; Lee, H.; Sickman J. O.; Osterkamp, T. E. The Effect of Permafrost Thaw on Old Carbon Release and Net Carbon Exchange from Tundra. Nature 2009, 459 (May 28), 556-559 4. Food and Agriculture Organization Web Site on Staple Foods: What Do People Eat? http://www.fao.org/docrep/U8480E/U8480E07. htm (accessed Dec 2009). 5. International Rice Research Institute Web Site on How Climate Change Threatens Rice Production. http://beta.irri.org/news/ index.php/press-releases/climate-change-threatens-rice-production. html (accessed Dec 2009). 6. Hattori, Y.; Nagai, K.; Furukawa, S.; Song, X.-J.; Kawano, R.; Sakakibara, H.; Wu, J.; Matsumoto, T.; Yoshimura, A.; Kitano, H.; Matsuoka, M.; Mori, H.; Ashikari, M. The Ethylene Response Factors SNORKEL1 and SNORKEL2 Allow Rice To Adapt to Deep Water. Nature 2009, 460 (August 20), 1026-1030. 7. Voesenek, L. A. C. J.; Bailey-Serres, J. Plant Biology: Genetics of High-Rise Rice. Nature 2009, 460 (August 20), 959-960.
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