Evidence suggests plagiotropic clonal species have evolved a branching physiology emphasizing regulation by nodal roots

Abstract

In the plagiotropic nodally rooting clonal herb, Trifolium repens,the development of branches on stems is primarily controlled by the presence of nodal roots, and apical dominance is of secondary importance; only six to ten branches form distal to the youngest nodal root on a horizontal stem. We assessed the hypothesis that this phenomenon is general for clonal herbs with prostrate nodally rooting stems, and that they all have the same physiological system regulating branching, by testing a selection of species from diverse angiosperm families that exhibit either ‘phalanx’ (Leptinella (Asteraceae), Hydrocotyle (Apiaceae), Acaena (Rosaceae)) or ‘guerilla’ (Vinca (Apocynaceae), Glechoma and Lamiastrum (Lamiaceae)) growth strategies. In all these species the establishment of a single nodal root on a prostrate stem, otherwise prevented from nodally rooting, induced the outgrowth of a limited number of axillary buds (the number of which was species specific) at the nodes immediately distal to the newly established root, thereby indicating a phenotypic response similar to that in T. repens. Furthermore, their branching responses to manipulative treatments were also similar to those of T. repens, indicating that their regulatory physiology of axillary bud outgrowth from their prostrate stems is similar. We conclude that, for the group of prostrate nodally rooting clonal herbs as a whole, the apical dominance phenotype arises predominantly from variation in the supply of resources from nodal roots rather than from repression of axillary buds by apical tissues (apical dominance). We suggest that evolution of such a physiological mechanism enhances the exploration for patchily distributed favourable nodal rooting sites by regulating shoot development so as to efficiently utilise the diminishing intra-plant availability of root-supplied resources. For the species examined, inter-specific variation in intensity of branching response to a nodal root is shown to be linked to a trade off in foraging strategy, with the allocation of resources primarily to explorative growth (long internodes, few branches) in ‘guerilla’ species or to exploitive growth (short internodes, many branches) in ‘phalanx’ species.