Correlations among Size of Root Apex, Root Apex-Lateral Distance, and Elongation Rate in Rice Roots

Abstract

The root apex-lateral distance, i.e. the distance from the root apex including root cap to the first lateral root, has been shown to be the indicator of the activity of nutrient absorption in rice roots). In this study, the interrelations among root apex-lateral distance, root elongation rate, and size of the root apex of rice plants were investigated. The roots used for observation were seminal and crown roots of rice plant var. Norin No. 29 grown in agar medium with or without mineral nutrients and roots in culture of isolated seminal root provided with scutellum. First, a significant positive correlation was established between root apex-lateral distance and elongation rate of roots, meaning that the higher the elongation rate, the longer the apex-lateral distance. The linear regression of Y= 10+1.6X was obtained, whereby Y is root apex-lateral distance in mm and X is root clongation rate in mm per day. Secondly, the correlation between root apex-lateral distance and diameter or length of the root apex was shown to be highly significant in roots growing normally. This means that the larger the root apex, the thicker the root and the longer the root apex-lateral distance. However, in the culture of excised seminal roots fed with fructose, the abnormal "lion-tailed" roots were obtained, wherein the root apex-lateral distance was relatively short in proportion to the diameter of root apex. This abnormal formation may be the sequence of the suppression of cell multiplication in root apices followed by the maturation of subsisting meristem. Finally, the correlation between size of the root apex and elongation rate was analyzed histologically using the method of Burstrom. It was shown that the larger the root apex, the longer the zone of cell elongation as well as the zone of cell multiplication, and that the size of root apex was related to the activities of cell elongation (assessed by the number of epidermal cells produced per day). Based on these results, the significance of the root apex-lateral distance as the indicator of root elongation rate and of the activity of nutrient absorption was discussed in relation to the morphogenesis of roots.