The ontogeny of leaf spines: progressive versus retrogressive heteroblasty in two New Zealand plant species

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Most plants change shape as they grow. However, ontogenetic changes in morphology (i.e. heteroblasty) can differ markedly among species and the role that heteroblasty might play in plant defence is poorly resolved. We use a model selection approach to characterise heteroblasty in terminal leaf spines produced by two perennial plant species native to New Zealand. Aciphylla aurea (Apiaceae), a perennial herb that inhabits alpine scrublands, exhibited ‘progressive’ heteroblasty. Seedlings produced soft, entire leaves with sharpened tips. However, as plants matured they produced compound leaves with sharpened tips that were far more rigid, giving rise to a strong degree of spinescence at adulthood. By contrast, Podocarpus totara (Podocarpaceae), a tall tree inhabiting cool-temperate forests, exhibited ‘retrogressive’ heteroblasty. The size of terminal leaf spines decreased during development and leaf circularity increased. Furthermore, leaf rigidity peaked at intermediate heights resulting in a peak in terminal leaf spinescence at the sapling stage of development. These results indicate that the mode of spinescence heteroblasty varied between species in ways that appear to facilitate defence at life history stages when plants are most susceptible to attack from large herbivores (i.e. adult plants are better defended in scrublands while saplings are better defended in woodlands). However, results also showed that spinescence was reduced in very young plants, suggesting that the ontogenetic development of spinescence may be constrained at very early ontogenetic stages, perhaps because younger plants lack the energetic resources to structurally reinforce leaf spines.