Flooding tolerance of rice

• Rice Research Institute, Thailand

Flooding causes serious problems in many rice lands, restricting yields for a number of reasons. Tiller death and lodging during grain fill and maturation mean direct loss of grain, while adverse effects on the timing of tillering, flowering and grain fill can reduce yield potential. Traditional farmers in flood-affected areas face marginal returns and considerable uncertainty, and so consider that they cannot afford adequate levels of nitrogen fertiliser input or pest control. Similarly, they continue to use tall-strawed varieties, because flood liability poses too great a threat to the higher-yielding semi-dwarf varieties. Huge regions have potential for stable agriculture if these limitations can be alleviated.
The problem can be divided into short-term flooding lasting between a day and several weeks and long-term flooding, in co-operation with scientists in Thailand and the International Rice Research Institute. Thailand and IRRI already have a collaborative research program on varietal improvement and agronomy in areas of rice subjected to flooding, which is now producing varieties with greater flood tolerance.
To complement that work, the Australian interdisciplinary team will investigate the physiological mechanisms responsible for such tolerance. The research will consist of four closely integrated components: environmental measurements on the flood waters; crop physiological studies in the field in Thailand and whole-plant studies in Australia; physiological experiments on the biophysical and biochemical mechanisms responsible for tolerance; and tissue culture.
The physiological studies will emphasise comparisons between varieties with differences in tolerance, thus enhancing their usefulness in the plant breeding and agronomic program.
Experiments will concentrate on the period during the fast rise of flood waters. This rise induces rapid elongation in flood-tolerant varieties, which may increase in length by 1 metre within 5-10 days. Team members will attempt to define the factors determining the rate of elongation, beginning with environment measurements, by Thais and Australians, in deep-water rice areas to provide essential data for use in glasshouse and laboratory studies. Experiments both in the field in Thailand and under controlled conditions in Australia will evaluate the source of reduced carbon and nitrogen required for elongation of the plant above the flood waters an determine the longevity of submerged parts. Concurrently, the team in Australia will study the mechanisms for rapid elongation by measuring changes in cell wall characteristics, and evaluating the possible roles of hormones in initiating such changes. Other experiments will measure turgor pressure of internodes, including the use of isolated internodes.
Better definition of the problem will lead to formulation of more specific objectives for research into those physiological phenomena shown to be of key importance to the response of the plant as a whole during the period of rapid elongation.
Tissue culture will form an integral part of the work. In addition to its value for experiments with isolated internodes or cells, it will provide new lines. These new lines, while important for the breeding programs, have even more value in the present project for the physiological studies because they allow the scientists to compare closely related genotypes differing in a particular characteristic. Such comparisons can very efficiently elucidate mechanisms involved in adaptation to adverse conditions.
Participation in research during exchange visits, and workshops held at intervals of 12-18 months, will help in co-ordinating the project as a whole and incorporating the findings into the plant breeding and agronomic work.