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Shigenori Morita*
Graduate School of Agricultural and Life Sciences,
The University of Tokyo
1-1-1 Yayoi, Bunkyo-ku, Tokyo 113, Japan
Because rice is one of the most important cereal crops in the world, many agronomic studies have been done mainly on the growth and development of shoot with direct or indirect relations to yield. Information on root system, on the other hand, is relatively limited, partly because root research under field conditions is quite tedious and time-consuming, and partly because the contribution of root system to yield is not always clear. However, the most important management practices in rice cultivation, for example fertilization and water management, are artificial actions to root system through soil. In fact, good rice farmers know from their experiences that improvement of soil conditions for better root growth is at least a certain way to establish sustainable high yield. Therefore, we have to study growth and development of root system with reference to its contribution to yield.
When we study root system in rice cultivation, there are two important problems. One is how can we grow root system to get higher yield. To control root system development as expected, there is need to describe morphological responses of rice with different genetic background to different environmental conditions. One possible reason why root research is behind shoot study is that there is no certain viewpoint to understand root system morphology. To establish such a viewpoint we must answer the following three questions: (1) how can root system morphology be characterized quantitatively ? (2) how can the morphology of a whole root system be determined by growth and development of individual roots ? (3) how might growth and development of individual roots be affected by the formation of phytomers that compose the plant body of rice plant ?
The other and more important problem is what is an ideal root system. At present we don't even know if there is an ideal root system in rice cultivation. A survey of published data by researchers, as well as observations by farmers on root system morphology, has shown that rice plants with high yield often had deep and vigorous root system. Although there were several exceptions, the results of the survey indicated that both total root length (and total root weight) and the distribution pattern of roots in soil are quite important indices to characterize the morphology of root systems. However, an ideal root system should be dependent on environmental condition absolutely. Therefore, we have to study much more relationships between morphology and physiology of root system. When we know what is an ideal root system in rice cultivation, we will be able to control root system development to maximize the yield.