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Jiro Harada

Faculty of Agriculture, Saga University, Saga 840, Japan

Root system is an important organ for the growth of plants because of its function to absorb water and nutrients. Therefore, for rice plant cultivation, it is generally suggested that the development of large root system and its maintainance is one of the essential factors for the increase in grain yield. Also, in the analyses of high-yield rice plants, it is pointed out that their root systems are characterized by large number, length and diameter of primary root axes with well developed lateral roots. However, causal relationship is still uncertain. I would like to discuss here about ideotype of rice root system from the viewpoint of root system development with enphasis on measures that should be taken to understand the ideotype.

Although there is a great gap between root system and yield, close correlation is reported in many cases. For example, number of elongated primary roots showed positive correlation with the weight of milled brown rice of a hill (Harada et al., 1984). Since the total length of a elongated root is from 10 to 40m, increase of the number of elongated roots reflects to that of total length of roots of a hill. However, this phenomenon does not lead directly to the conclusion that development of large amount of root system, whatever the practice used may be, always contribute to the increase of yield. When we examine the case in which the number of culm is increased, the number of primary root axes will increase, but as the number of culm exceed a certain number, the number of non-productive tiller will increase, average number of florets per head and percentage of ripened grains decrease, resulting in the limit or the decrease of yeild (Harada et al., 1986). Therefore, for analysing the ideotype of root system, it is necessary to introduce the concept of rate or efficiency of roots to shoot growth. In fact, in the case of high- yield rice plants treated by Inabenfide, the rate of shoot dry weight per tolal length of roots was significantly less than that of control plants, although difference in shoot-root ratio in dry weight was not clear (Tanaka et al., 1992).

From the studies on Japonica-Indica hybrid cultivers, it was found that the total number, length and surface area of a crown root varied depending mainly on those of tertiary and thin secondary roots. On the contrary, the total volume, and weight, probably, varied with those of primary axis (Harada et al., 1988). Moreover, factors affect- ing the variation in total length of a hill were thought to be different by varieties; i.e. in the case of Suweon 258, the factor being the number of primary root axes per phytomer. On the contrary, in the case of Milyang 23, the root length of a hill was thought to vary with the total length per crown root (Harada et al., 1994). Consequently, the essential measure for the realization of a high level of grain yield of Japonica-Indica hybrid cultivers is the increase of total root length of a hill irrespective of means of variation.

Harada, J., K. Yamazaki, T. Nakamoto, A. Miyake and T. Umeda 1984. The relationship between the number of primary roots and yield components in rice plant grown on farmers' paddy fields. 1. Analyses per hill and per plant. Jpn. J. Crop Sci. 53: 307-312.

Harada, J., T. Maeda and K. Yamazaki 1986. Relationship between the number and the distribution of primary roots and characteristics of ears of rice plants under varying densities of of planting. Jpn. J. Crop Sci. 55(Extra issue 1) : 66-67.

Harada, J., S. -Y. Kang and K. Yamazaki 1988. Estimations of total root surface area and volume per adventitious root of rice plants and their difference among varieties. Jpn. J. Crop Sci. 57(Extra issue 2) : 89-90.

Harada, J., S. -Y. Kang and K. Yamazaki 1994. Root system development of Japonica-Indica hybrid rice cultivers. 1.Number and elongation direction of primary roots. Jpn. J. Crop Sci. 63: 423-429.

Tanaka, N., J. Harada, S. Arima and S. Sakai 1992. Effects of Inaben- fide on the growth of roots in rice plants (Oryza sativa L.). Jpn. J. Crop Sci. 31: 56-61.