Spatial patterns of root branching and actinorhizal nodulation in Discariatrinervis seedlings

The spatial patterns of root branching (topology) and actinorhizal nodulati on of Discaria trinervis seedlings under natural and controlled conditions were analysed. The link, i.e. the root segment between either two branching points, or between a meristem and a branching point, was used as a structu ral unit of roots. Nodulated field plants displayed a random model of root topology and experimental plants without root symbionts a herringbone model (Fitter, 1991). The occurrence of these different root topologies would su ggest that root symbionts influence root model architecture. The frequency distribution of the number of nodules per link on the main roots, for field collected plants, fitted a negative binomial model. Nodules were mainly si tuated near the main root's proximal end. The position (number of links cou nted from the main root's proximal end to the root apex) of lateral nodulat ed roots coincided with the nodulated area of the main root. A nodulation a ssay showed that the spatial pattern of root nodules was not a consequence of heterogeneous distribution of infective units of Frankia in the soil. Th e similar pattern of nodule position in field plants and experimental plant s suggested that a regulation of nodulation might be also working in plants in their natural environment.