Morphological and Physiological Studis on Nodule Formation in Leguminous Crops : VI. On the factors detemining the location of nodule formation on root system in kidney bean and sword bean.

Abstract

In a previous paper (KAMATA 1958a), the author reported that eighteen leguminous crops might be classified into two groups which belonged to the following types ; i. e. I-A type--nodules formed on both the primary and all the lateral roots, and I-B type--nodulation took place only on the lateral roots. This experiment was carried out to know the mechanism of nodule formation in I-B type using kidney bean (Phaseolus vulgaris L.) and sword bean (Canavalia gladiata D. C.) as materials. By the histological study, it was clearly seen that the number of the cells in primary root, which were infected with penetration of Rhzobium were similar in both cases, I-B type (kidney bean and sword bean) and I-A type (soybean and peanut), even though the nodule scarcely formed afterwards on the primary root in the former. The weak staining with HEIDENHAIN'S iron-alum hematoxylin, when examined microscopically, proved no proliferation of nodule bacterium in the first infection cells of I-B type plants. Cutting off a part of cotyledon from seed before sowing resulted to stop the development of nodule on a primery root in I-B type crops until flowering season. The nodule bacterium however, succeeded to grow, when it was cultured on RAGGIO'S complete cultural medium added with the different amount of the extract obtained by the systematic extraction method from primary, secondary and tertiary roots of I-B type plants. The fact proved that the extract contained no inhibitor, which might be a toxic or promotive substance for the growth of bacterium in primary roots of I-B type plants. In comparison of the amount of reducing sugar, non-reducing sugar and crude starch in root and rootlets of I-A type plant (soybean) and I-B type plants (kidney bean and sword bean) measured by Somogi's method, a complete parallelism was found between the concentration of available sugar, which was carbon source for growth of nodule bacterium in vitro, and the degree of nodule development. The amount of available sugar in the primary root of I-B type plants was lower and the amount of unavailable carbohydrate such as starch was higher as compared with those contained in other rootlets. Therefore, the nodule bacterium in the first infection cell might be inhibited to grow by the lack of available sugar in the primary roots in I-B type plants. If the above mentioned supposition is appropriate, nodules must appear on primary root of I-B type crops, when unavailable carbohydrate in the root is changed into available sugar. The wilting treatment was thought to be one of the useful methods for changing carbohydrate to available sugar, and such results were obtained after treatments of wilting given three times during seedling stage as incease of available sugar, decrease of crude starch and the development of nodules on the primary roots in both the case of kidney bean and sword bean.