Abstract
Elongation of crown roots in rice plant was studied in relation to the growth stage of the top. Cultivars used were Ginmasari and Toyonishiki, of medium type with respect to the number of leaves on the main stem (15 and 16, respectively). They were grown in submerged pots. Four types of elongation were recognized as follows. The first type was slow elongation, with the final length being not so long. The second type was rapid elongation in the early course, followed by gradually reducing rate, with the final length and the elongating duration being both long. The third type was elongation in which crown roots elongated with approximately constant rate during the entire course, with the final length being as long as that of the second type. The last type was elongation with constant rate, terminating in comparatively shorter length. As to the lower roots of the shoot unit in the main stem, each of these four types was found in the crown roots of the following shoot units or growth stage. The first type was observed only in the crown roots of two or three shoot units which developed just after germination or transplanting. Therefore, it was designated here as the early growth-stage type of crown roots elongation (abbreviated to E G type elongation). The second type was seen in the crown roots which appeared in a few shoot units after E G type elongation. As those shoot units were situated in relatively lower position, this type was referred as the elongation type of the crown roots from the lower shoot unit (L U type elongation). The crown roots which appeared from the couple of shoot units upper than those lower ones showed the third type of elongation. Those shoot units occupied relatively higher position and so this type was called the elongation type of the crown roots from the higher shoot units (H U type elongation). The fourth type was found in the uppermost-positioned crown roots of one or two shoot units (U P type elongation). In general trend, the higher the position of the shoot unit from which crown roots developed became, the greater the elongation rate of its crown roots was, until reaching the maximum in the crown roots of higher-positioned shoot units out of the ones of H U type elongation. In a tiller, a similar transition of elongation types to that in the main stem was observed in advancing sequence of shoot units, except that E G type elongation was not found. But the tiller which appeared from the higher shoot unit of the main stem had less number of shoot units whose crown roots showed L U type elongation. As to the upper roots, the final length was much shorter than that of the lower roots within the same shoot unit. Besides this, several inferiorities were observed in regard to the elongating rate and/or the final length in the upper roots from the upper shoot units in the main stem and tillers or from the higher-positioned tiller. The panicle differentiation stage was around the transition from the appearance of crown roots of L U type elongation to that of H U type, or was the early time of crown roots appearance of H U type elongation. Crown roots of E G type and those of L U type elongation of lower shoot units finished elongation slightly before the panicle differentiation stage and the later the crown roots appeared, the later their elongation terminated. All the crown roots ceased elongation around the heading stage.
