Effect of Water Management of Paddy Fields on the Direction of Crown Root Growth and the Lateral Root Formation of Rice Plants

[摘要] Growth and development in the roots of rice plants were observed under the influence of water management techniques such as flooding, midseason drainage, intermittent irrigation and water percolation. Two methods were applied in this observation. One was to dot each site where crown roots and lateral roots appeared through the profiles of 10×30 cm made at depths of 10, 12.5, 15, 20, 25 and 30 cm from the soil surface after having taken a soil block by means of an improved monolith method (Figs. 3, 4 and 6). The other method was to measure the length of each crown root after cutting off vertically by a stainless steel cylinder (diameter: 150 mm) and to judge the directions of the growing crown roots by their length which were classified as either short or long. The rates of growth of the crown roots grown obliquely downwards or vertically, were higher in water-percolated field, as well as in field drained at midseason, and much lower in intermittently irrigated field than in flooded field. It has been already shown by the authors that the crown roots can be classified into the following three types according to the growing direction: all the upper primary roots of "shoot units" grew horizontally; the lower primary roots of "shoot units" grew in various directions; and most primary roots of the prophyll "shoot units" grew vertically irrespective of growth stage. It has also been shown that only the lower primary roots changed their growing direction under water mangement, i.e. all the upper primary roots grew horizontally, and most primary roots of the prophyll "shoot unit" grew obliquely downwards or vertically irrespective of water management. It was found that the crown roots grown horizontally into arable soil formed more secondary roots, with densities of 200-300 per 10 cm of crown root, than did those grown obliquely downwards or vertically into the subsoil, which formed 70-200 secondary roots per 10 cm of crown root, irrespective of water mangaement and growth stage. In the crown roots grown into the subsoil, the density of thick secondary roots, which usually branched out into tertiary roots, was much higher in the field of intermittently irrigated field as well as that drained at midseason; and higher in the percolated field than in flooded field. The formation of additional lateral roots was also shown in the following ways: tertiary roots developed in fields flooded or water percolated irrespective of the direction of crown root growth; fourth roots on the crown roots grew into the arable soil and tertiary roots on the crown roots grew into the subsoil in the field drained at midseason; fifth roots on the crown roots grew into the arable soil and fourth roots on the crown roots grew into the subsoil in intermittently irrigated field.

[学科分类] 农业科学（综合）