Concluding Remarks

Among cereal crops, rice is unique in that it is planted in the paddy field. The submerged condition of the root system confers it with a tolerance to anoxia. However, an enhanced growth of the root system during the tillering stage by draining water will bring about an enhanced growth of the root, which in turn enhances the grain yield. The importance of a well grown root system is also demonstrated in the practice of transplanting rice from nursery to paddy field. Besides establishing an arrangement of rice plants to optimize the growing space and achieve a better condition for crop management, the practice also confers the seedling with the vigor for growth. In the process of transplanting, the extended roots on seedlings are cut, but the new growth of abundant roots ensures a better colonization of the paddy soil. These facts show that the cultured rice is a highly domesticated cereal plant and its optimum life cycling by attentive farming is needed to obtain a high grain yield from a unit land area. Besides selecting cultivars suitable for a certain agroecosystem, such as the use of dwarf cultivars that tolerate strong wind better in areas where typhoon conditions may prevail, attentive farming with appropriate technology is always the key to assure a high productivity.

The genetic, biochemical and molecular biological elements that may influence the grain yield, or more precisely, the grain filling with starch, were mentioned where appropriate in this paper. The starch molecule is composed of only one type of hexose, glucose, and only two types of glucosidic linkages. However, the biosynthesis of a seemingly simple starch molecule is a complex process, and we know only fragmented knowledge for the time being. The modern biochemistry and molecular biology have revealed the fact that, even one type of catalytic reaction may be catalyzed by multiple forms of isoenzymes specifically in certain tissue or organ at certain time frame of plant growth. In the fermentation technology, it has been known that breeding of a microbe harboring multiple copies of a gene, or a high gene dose, may bring about the enhancement of the gene activity. However, as exemplified in the silencing of endosperm Wx by a transgenic Wx, microbial experiences may not apply to higher plants always. Whether the molecular breeding of rice may surpass the brilliant outcomes that the traditional rice breeders worldwide have achieved is a big challenge to the new breed of rice breeders. The writer has the feeling that no remarkable achievements may be forthcoming in the near future even though much effort is being dedicated to achieve such goal.

A new environmental concern on the rice productivity is the effect of increasing carbon dioxide in the atmosphere, the emission from the paddy field of green house gases other than carbon dioxide, and the global warming. Work on the monitoring of green house gas emission under different agroenvironments has been carried out intensively. The obtained data may be applicable to modify the rice cropping practices to make the rice cultivation more environment-friendly. The effects of increased carbon dioxide concentration or temperature were studied. Rice cultivars were grown under the atmospheric concentrations of carbon dioxide at 340 and 650 \xL L"1 (52), or 350 and 700 |iL L"1 (53) levels. The midday canopy photosynthetic rate was enhanced by 18 to 34% under a normal water supply, and 5 to 12% at a water deficit condition in the higher carbon dioxide concentration. However, the enrichment of carbon dioxide enhanced neither starch accumulation nor the activities of ADPG-PPase, UDPG-PPase, SuS and SS in the seed. An interesting observation is that, when rice was cultivated under 4, 18, 28 and 35 °C, only the 18 °C treated one had a higher Wx activity, and hence an increase of amylose content (54). A transgenic study by transforming rice with a chimeric construct of Wx promoter and GUS reporter revealed that the response was due to the response of the promoter.

The rice biology has entered a new era of deciphering the molecular mechanisms of very richly developed biological phenomena. Undoubtedly, all of the outcome of such research will contribute to the development of plant science in general and the crop science in particular.

0 0

Post a comment