Photosynthate allocations in rice plants: food production or atmospheric methane?

A tmospheric methane is recognized as one of the most important greenhouse gases. Sources of atmospheric methane are about 1/3 natural and 2/3 human-caused (1). Its concentration has roughly doubled in the past 100 years. This increase and any future continuation of it can affect Earth’s climate through global radiative forcing (increased forcing to date is between 0.5 and 0.7 W m 2) (2). Thus, methane is the second most important anthropogenic greenhouse gas after CO2, whose radiative forcing to Year 2000 is 1.4 W m 2. Flooded rice fields are a significant source of atmospheric methane. Worldwide emission from rice has been extrapolated from reports from China, India, Vietnam, Korea, and the Philippines to be from 21 to 30 teragrams per year (1 teragram 1012 g) (3). These values are less than several estimates since 1981, but still represent a globally significant source. Rice is the staple food for nearly 50% of the world’s peoples, many in Asia. The world per capita rice consumption in 1990 was 58 kg yr 1 of milled rice. This represents 23% of the average world per capita caloric intake and 16% of the protein intake (4). Because rice is such an important food source for much of the world, it is imperative to develop ways of reducing the impact of rice agriculture on the global atmosphere and subsequent climate change. An important finding that may lead to such a reduction while also increasing rice production is presented in the article appearing in this issue of PNAS by Denier van der Gon et al. (5). These researchers demonstrated that an inverse relationship exists between a rice plant’s capacity to store photosynthetically fixed carbon as grain and seasonally emitted methane. Under a common set of climatic and agricultural conditions, lower methane emissions are observed from plots that contain rice plants with higher numbers of filled grain spikelets, indicating that plants that more closely approach their potential yield limit emit less methane to the atmosphere. These data support the hypothesis that higher methane emissions observed in the tropical wet season as opposed to the dry season (6) are associated with lower harvest index values resulting in excess carbon that could not be allocated to rice grain. This excess carbon is then available to soil bacteria for the production and emission of additional methane. To meet the increased rice demand of a growing global population, rice cultivation must continue to expand at or beyond its current rate. If current population trends continue, by 2020, 1.2 billion new humans will be added to Asia alone. It is projected that the world’s annual rough rice production must increase from the 1990 value of 473 million tons to 781 million tons by 2020, and over a billion tons by the next century. Because arable land is limited in major rice growing areas because of increased urbanization, increased production has to be achieved mainly by intensifying cropping (i.e., two or three crops per year) and developing new higher yielding rice cultivars rather than expanding the area of rice cultivation. Irrigated rice will continue to dominate production. Irrigated rice land now comprises about half of the total harvested area, but contributes more than two-thirds of the total grain production (7). As we move into the future, rice grain production must increase to feed an increasing population, while at the same time, methane emissions from irrigated rice agriculture need to be reduced to help stabilize the global climate. Thus, the relationship between rice grain yield and the emission of methane from irrigated rice fields emerges as a major scientific and policy issue. Methane emission from rice fields is the result of a complex array of soil processes involving plant microbe interactions. Flooding rice fields promotes anaerobic fermentation of carbon sources supplied by the rice plants and other incorporated organic substrates. Methane emission is the net result of opposing bacterial processes—production in anaerobic microenvironments, and consumption and oxidation in aerobic microenvironments, both of which can be found side by side in flooded rice soil. This process is diagrammed in Fig. 1. Major substrates for the methanogenic bacteria are derived from root exudates, lysates, and dead organic material derived from senescent rice plants and incorporated vegetation (8, 9). Several specific low molecular weight organic substrates are produced in the process of mineralization that are in turn converted to methane by methanogenic bacteria (10, 11). Thus, variations in the rate of production (and emission) of methane, which is the terminal product of this plant-microbe system, ultimately depend on variations in parameters that determine the physiological state of the rice plant, such as nutrient supply, temperature, sunlight, and water. Under ideal cropping conditions, where climate factors and the availability of carbon substrates from sources other than rice plants are similar, the level of observed methane emission is positively correlated with the rice plant above ground and below ground biomass (8). This observation can be interpreted to mean that photosynthetically produced carbon is allocated within the rice plant in a fairly uniform fashion. The ratio of carbon allocated to above ground biomass to that allocated to root processes and subsequent methane production remains essentially constant over time. Different levels of photosynthetic activity can be experienced in temperate and subtropical regions by varying the rice planting date (9). If the source of organic substrate