On the Development of Crown Roots in Various Growth Stages of the Rice Plant

Abstract

It was reported that a shoot of rice plants is composed of shoot units, each of which consists of an apical leaf, a basal bud and an apical and basal root zones, and that the crown root emergence of a shoot unit is usually synchronized with the emergence of the leaf of the third upper shoot unit. The present study was carried out, with early-sown and late-sown rice plants and rice plants with soil kept in constant temperature during the growing period as materials, in order to clarify the pattern of development of crown roots emerged from shoot units of the main stem, with special reference to the rate of elongation and the diameter of crown roots and the acropetal emergence of secondary roots. The rate of elongation of crown roots increased from lower to upper shoot units with the highest rate in the ninth or the tenth unit, upper than which the rate declined upto the eleventh shoot unit which was the highest one with crown root emerged, in both cases of natural and controlled soil temperature conditions. The elongating rate of rapidly growing crown roots of early-sown rice plants at the stage of the thirteenth leaf emerging was found to be bigger than that of crown roots which emerged on the same day of late-sown rice plants at the stage of the tenth leaf emerging. The diameter of crown roots and the rate of acropetal emergence of secondary roots of the former plants were also bigger than those of the latter plants. As to the diameter, the rate of elongation of the primary roots and the rate of acropetal emergence of the secondary roots as associated with the growth of the primary roots, the crown roots from the upper root zone were not superior to those from the lower root zone of the same shoot unit. Furthermore, a high positive correlation was found between the diameter and the rate of elongation in crown roots. From these results, it may be said that the development of crown roots of each shoot units is affected not only by environmental factors but also by the physiological situations of rice plants which depend on the developmental stages.