Studies on the efficient use of the function of root system in crop plants

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Following results were obtained from our studies to acquire the fundamental information on the efficient use of the function of root system in crop plants.Total length of branch roots formed along several long and thick lateral roots emerging from the basal part of a tap root was found to be around one third to a half of the total root length of a soybean plant. The growth of whole plant was suppressed in soybean when restricting the soil volume available for the extension of roots. However, it rapidly recovered and the ratio of total root length to leaf area of a plant increased after the restriction was removed. In this case, increase in total root length was remarkable in long and thick lateral roots.The growth of soybean plant and the dry matter accumulation in pods increased with long term subsurface irrigation continued until pod filling stage under field condition compared to surface irrigation. Furthermore, elongation rate of a main stem rose with short term treatment of only seven days subsurface irrigation under pot grown condition. These suggest that unequal distribution of roots or water influenced the growth of a plant.Characteristics of growth with respect to shoot- root relationships in high yield Japonica-Indica hybrid rice varieties were investigated. In the seedling stage, increases of seedling dry weight were larger, lengths of seminal root axes were longer, but S/R ratios were larger than those of Japonica variety. However, the branching habits of seminal roots were similar among varieties. After the seedling stage, although S/R ratios turned to be lower at a comparatively early stage and the total root length of a plant larger in the subsequent stage in hybrid varieties than in Japonica variety, leaf area to total root length ratios were smaller.