Root Nodule Formation on the Garden Bean, Studied by a Technique of Tissue Culture

Abstract

1. Excised roots and etiolated plants of the black wax bean, a variety of Phaseolus vulgaris L., have been raised in artificial culture by an adaptation of the Wilson technique. Growth of the roots is admittedly not normal, in amount at least, as compared with the standard for root development of field plants. (Weaver and Bruner [19] state with reference to beans: "Plants only in the cotyledon stage, but grown in warm, mellow soil, often have well-branched tap roots twelve inches long.") In the present experiments the roots usually attained about half that length, occasionally two-thirds. They were also more brittle than normal but no regular morphological abnormality could be found. 2. Nodulation (a) of excised roots (grown alone or in combination with etiolated whole plants or tops with adventitious roots), (b) of etiolated tops having adventitious roots only, and (c) of etiolated whole plants of the bean has been accomplished. 3. The ratio of nodulated roots of the various types to the corresponding non-nodulated is low, and indicates that some factor, as yet unknown, is depressing nodule development. It appears further that sugar is essential to nodulation in these experiments but does not entirely overcome the effect of absence of light. The excised roots are distinctly less susceptible to infection and development of nodules than are the roots of etiolated whole plants or of tops rerooted. 4. The nodules studied were histologically normal but lacked the usual starch of the bean nodule. Possibly this lack indicates merely that no excess was absorbed and stored from the 0.5 per cent sucrose in the nutrient. There was no sign of that parasitism noted by Thornton (17) and attributed to lack of carbohydrate food in his darkened nodules. 5. Study of the results of these experiments for their bearing on the question of a root secretion stimulatory to infection shows little either way. Nodulation of entirely isolated excised roots is positive proof of infection unaided by any hypothetical substance originating in the tops, as late as that reported by Thornton at least. The increase in nodulation of excised roots grown with whole plants is slight and certainly insufficient to indicate the working of a root secretion. The considerably increased nodulation of etiolated whole plants or tops re-rooted might suggest the presence of a substance derived from the tops and favorable to nodulation. But there is no guaranty of the normality of the excised root. Some pathological condition, depressing to nodulation, may account for the apparent difference between nodulation of the excised root and of the etiolated whole plant.