ROOT NODULES IN THE GENUS COLLETIA

Abstract

Following the earlier reported detection of root nodules on a species of Colletia growing in the Glasgow Botanic Gardens, the authors' enquiries have revealed that Colletia species are nodulated in some other botanic gardens, both in Europe and elsewhere. Cytological study of the nodules under the electron microscope shows that the endophyte is a procaryotic, actinomycete‐like organism consisting of septate, branching hyphae which are 0.37 to 0.53 μm in diameter and form terminal spherical vesicles 2.0 to 2.5 μm in diameter. The vesicles are internally sub‐divided and lie peripherally within the host cell. The endophyte is enclosed within a thin layer (or capsule) of material which judging by previous studies of other non‐legume nodules consists of carbohydrate‐related substances laid down by the host cytoplasm. Data from long‐term growth trials and acetylene assays show that Colletia nodules are nitrogen‐fixing and capable of satisfying the nitrogen‐needs of the host plant. Of the five genera of the Rhamnaceae that have so far been found to be nodule‐bearing, four (namely Colletia, Discaria, Trevoa and Talguenea) are in the tribe Colletieae, while the fifth genus (Ceanothus) is placed elsewhere in the family by taxonomists. A very similar situation exists in the Rosaceae as regards the distribution of nodulating genera and species. It is concluded that in these families the habit of symbiosis with a nitrogen‐fixing actinomycete probably arose, in some instances, prior to the establishment of now‐existing genera, but in others at a later stage. This conclusion is consistent with the belief that nodule symbioses were initiated in an earlier era when plants in general were subject to nitrogen stress. Copyright © 1982, Wiley Blackwell. All rights reserved