1963 Volume 32 Issue 2 Pages 163-180
Shoots of rice plants were designated as "shoot units", each with an apical leaf, a basal bud and upper and lower root zones (see Evans and Grover) and Sharman and Fig. 5). The main shoot of the rice cultivar 'Norin No. 41' consists of 15 such shoot units and of one cotyledonary unit. The cotyledonary unit has one seminal root and one coleoptile (Fig. 9). Five primary roots appear in the first shoot unit after emergence of the seminal root (Fig. 3A). Upper and lower primary roots on a shoot unit emerge essentially at the same time. The time interval for root emergence and leaf emergence of successive shoot units is 3 to 9 days. Leaf emergence is defined as the first appearance of a new leaf from the leaf sheath below. Beginning with the fifth shoot unit, leaf emergence is usually synchronized with the emergence of roots three shoot units below. Lower (basal) roots of a shoot unit are usually larger in diameter than upper (apical) ones. In young plants, full-grown roots of earlier shoot units are not as long as roots of later shoot units. The longest ones are found among lower roots of the ninth shoot unit and they continue to grow for about 30 days, from the time of twelfth leaf emergence to the time of ear heading (Fig. 10). Thereafter, roots on successive shoot units become gradually shorter (Figs. 12, 13). Roots of the eleventh and twelfth shoot unit are shorter, and also produce abundant secondary and tertiary roots which may be aberrant (Figs. 14, 15). It is of interest that growth behavior of upper and lower roots in every shoot unit is so different, suggesting different physiological functions. The process of root system formation in a paddy is as follows. For the first twenty days after seedlings are transplanted, most of primary roots (initiated from earlier shoot units) concentrate above the plow sole (Fig. 1A). Subsequently, roots initiated from the eighth and ninth shoot units penetrate through the plow sole into deeper soil, although the majority remain in the upper 15 cm (Fig. 1C). Furthermore, primary roots appear and elongate until the flowering stage, about 85 days after transplanting. Roots of the eleventh and twelfth shoot units emerge at about the time of flowering and early ripening, and they concentrate near the soil surface. These are called super-ficial roots and produce a net with many secondary and tertiary roots. Throughout the development of the rice plant, the shape of the root system in the longitudinal plane (Fig. 1A-E) is usually elliptical, but the size varies with plant age. For instance, the size during early growth is comparatively small and it increases with time, although it retains the same shape. Roots of the third to sixth shoot units of rapidly growing plants develop horizontally or upward near the soil surface, whereas roots of the seventh to ninth shoot units are positively geotropic and penetrate into deep soils. The roots of the tenth and eleventh shoot unit also grow horizontally or upward near the soil surface. Thus roots of younger and older shoot units tend to be ageotropic. The nature of this phenomenon is the subject of a continuing investigation.
