Adaptive variation in physiological traits underpinning stem elongation responses among nodally-rooting stoloniferous herbs

Abstract

For nodally-rooting stoloniferous herbs a close physiological link exists between the growth of nodal roots and development of shoot tissues of phytomers distal to them. Furthermore, in Trifolium, presence of nodal roots at phytomers proximal to the youngest nodal root on a stem influences the allocation of carbon along the stem when that stem encounters a patch preventing further formation of nodal roots. In this study six further phylogenetically and morphologically diverse stoloniferous species, selected so that they varied along the phalanx–guerrilla continuum (Acaena (Rosaceae), Leptinella (Asteraceae), Mentha (Lamiaceae), Vinca (Apocynaceae), Lamiastrum (Lamiaceae) and Glechoma (Lamiaceae)), were used to test the null hypothesis: that in all stoloniferous species the proportional allocation of carbon to the youngest nodal root decreases if proximally located roots are present when their stems extend into an environment preventing further nodal rooting. As Glechoma and Lamiastrum failed to change their proportional allocation to the youngest nodal root whilst the other five species varied their proportional allocation, with the species with shortest internodes (Acaena and Leptinella) changing the most, we reject this hypothesis. In addition, upon the transition to a dry environment, internode lengths in species with longer internodes (Glechoma and Lamiastrum) were markedly increased (doubled) whilst those in species with shorter internodes showed no response. We suggest this response is driven by apical bud tissues sensing changes in the humidity of their micro-environment and then modifying the length of internodes developing proximal to them. The extent of both these physiological responses correlated linearly with internode length of species (or position along the phalanx–guerrilla continuum), suggesting that these three traits were co-evolving, possibly in response to variation in grain of environmental heterogeneity, in such a way that the trade-off between the explorative and exploitive components of growth was underpinned and the performance of species in particular niches optimised as a result.