Cool rice for a warmer world

Rice breeders throughout the world are being challenged by climate change to develop cool rice varieties for a warmer world, as current varieties may no longer be productive under rising temperature.

In a recent international workshop in Wuhan, Peoples Republic of China, rice breeders acknowledged that to secure grain yield and quality in a warming world, they must adopt new tools and identify genetic strategies to overcome the effects of high temperature on sterility and grain-filling. They must also develop tools to be able to select for high yield and high grain quality in a warmer world.

No doubt, the earth’s climate is changing in an unprecedented manner in the past 400,000 years, said Dr. Reiner Wiesmann and Dr. Achim Dobermann, both of the International Rice Research Institute, in a paper presented during the workshop.

They said that as shown in the con-clusions of the recent Fourth Assessment Report of the Intergovernmental Panel on Climate Change (IPCC), the mean planet-wide surface temperature will rise by 1.4 to 5.8 degrees Centigrade by year 2100.

Warming of the climate system is now even evident from a large range of observations of temperature increases in various parts of the world. Of the 12 years from 1995 to 2006, 11 ranked among the warmest years since 1850. What’s more, they said that the earth system is warming at an accelerating speed in contrast to the total temperature increase of 0.76 degrees C since the industrial revolution.

Citing the IPCC report, the two IRRI scientists said cold days, cold nights and frost have become less frequent, while hot days, hot nights, and heat waves have become frequent over the last century. And it is almost certain that these trends will continue. In addition, extreme temperatures, such as warm spells and heat waves, will also become more frequent.

The scientists in the workshop recognized that high temperature reduces crop yields. For rice, high temperature during flowering is detrimental. Exposure to temperatures higher than 35°C for a few hours can greatly increase spikelet sterility by reducing pollen viability and, hence, can cause irreversible yield loss. Studies at IRRI indicated that grain yields in the dry season can decline by 10 percent for every 1°C increase in the minimum night temperature during the growing season. High temperature also affects grain quality by significantly reducing head rice yields and grain size.

In a paper presented during the work-shop, Dr. Norvie L. Manigbas and Dr. Leo-cadio S. Sebastian, both of the Philippine Rice Research Institute, said the Philippines will be among the most vulnerable to the effects of climate change because of its limited resources, topological characteris-tics, and existence of varying agricultural lands in coastal, upland and low-lying areas.

Wiesmann and Dobermann said inten-sive rice systems will have to adjust through some generic response strategies to cope with the new challenges of climate change. For instance, changes in germ-plasm, planting dates, tillage, crop estab-lishment, water management, pest management and nutrient management must occur to cope with climate change and increase resource use efficiency. Particularly important are increased resiliency of rice to high temperature stress and water scarcity (by utilizing water-saving irrigation techniques), and increased nitrogen use efficiency.

Shifts in food consumption will likely trigger increasing diversification of rice-based cropping systems, they added. Where it is possible, a shift from double or triple monoculture to rice-upland crop systems such as rice-corn, rice-rice-corn or rice-corn-corn will occur. Rice monoculture in double- or triple-crop systems may remain the preferred choice in lowlands with heavy clay soils and other constraints to production of upland crops.

Climate change may also move the boundaries for double- and triple-cropping that will require judicious use of fragile water and soil resources. In some areas like southern China, high costs, the need to conserve water and the demand for feedstuffs may cause a change from double-rice cropping to single cropping with longer growth duration or even a complete conversion of rice systems to feed-producing systems such as corn-soybean.

The new systems must also be more resilient to climatic stress arising from global climate change and/or local air pollution.

Likewise, even as agricultural produc-tion may only be a small polluter compared to other sectors, Asian agriculture in the near future will be targeted by various regulations regarding environmental impacts, the IRRI scientists said.

At present, there is hardly an explicit climate policy in Asia, but this is likely to change in the near future, drawing more and more attention to the issue of curtailing greenhouse gas emissions across the various sectors of the economy including agricultural production. In fact, the agricultural sector in Asia offers a variety of options to reduce emissions without compromising development targets or even by fostering rural development.

These trends will occur simultaneously and they will evolve significant changes in the cycling of energy, water, carbon, nitrogen and other elements at field to regional and global-scale, the IRRI scientists concluded.