Studies on Root System Formation in Leguminous Crop Plants : 1. Three types of root system formation in main roots

Abstract

In researches conducted so far on formation of root system of leguminous plants, stress has been placed, in most cases, on study of how roots spread underground. Consequently, characteristics of root systems have been defined by a mere term of the shallow-root or the deep-root type. This study is different from previous researches. That is, study attaches importance not so much to spreading condition of root system as to clarification of characteristics observed in developing process of branch roots. The purpose of study is thus to find out a law of nature governing growth of root system, and, on this basis, to classify root system in a new way. The experiment was conducted during period from autumn of 1960 to summer of 1963. The varieties used in experiment were as follows: Alfalfa (Atlantic) Common vetch Red clover Broad bean Ladino clover Chinese milk vetch Egyptian clover Soy bean (Hogyoku) Sweet clover Lupine (Yellow flower) Cowpea The results obtained were as follows: 1) As for root system formation, it was observed that main root began to grow first, and then branches of first order started to grow from its base and extended toward root apex (see Fig. 1). This phenomenon is common to all specimen plants. 2) It was made clear that, as plants grew, their root systems transformed from uniform standard type to following three different types according to their varieties: a. Alfalfa type b. Vetch type c. Intermediate type 3) Alfalfa type Fig. 2 is a schematic diagram showing process of this type of root system. The main root (0) grows first, and first-order branches (a1, a2, ……a8) appears from its base and extend toward root apex one after another (Standard type). But subsequently, second group of first-order branches (b1, b2, ……b6) begin to develop. Following their development, te subsequent groups of (c1, c2, ……c4) and (d1, d2) begin to take shape. This process of development is repeated at regular intervals. a) In case of alfalfa, development of first-order branches took place 4 to 5 times by time when plant began to blossom. b) The type of characteristics observed during development of first-order branches were observable also in case of branches of second and third orders. c) It was interesting to note that primodium of subsequent first-order branches (such as b1, c1 and d1) appeared, as a rule, side by side with initial branches (a). d) These characteristics were observed in case of alfalfe, Chinese milk vetch, red clover, ladino clover and like. e) These characteristics were not affected by soil conditions or reaping methods. 4. Vetch type This type of plant is far simpler than alfalfa type in characteristics. That is, once first-order branches grow at base of main root and extend root apex, no similar branches appear any more. Common vetches belonging to vicia group, broad beans and cowpeas presented this phenomenon. (See Fig. 4) 5. Intermediate type This type can be regarded as standing halfway between alfalfa and vetch types. It is also conceivable, however, that this type originally belongs to alfalfa or vetch type but none less presents characteristics peculiar to intermediate depending on conditions surrounding roots. In author's experiment, soy beans and yellow lupines presented such characteristics.