Zotz 2015 (Conference Paper) Heteroblasty

Heteroblasty in bromeliads

Author(s):—G. Zotz in Benko-Iseppon, A.M.; Alves, M. & Louzada, R. (2015) An overview and abstracts of the First World Congress on Bromeliaceae Evolution. Rodriguésia 66(2): A1-A66.

Publication:— (2015).

Abstract:—More than a century ago, the term heteroblasty was coined by Carl Goebel to describe abrupt changes in form in the ontogeny of a range of vascular plants with diverse taxonomic affiliations and representing different life forms (e.g. herbs, vines, trees). Today, in 2015, we are still struggling with terminological issues, with unambiguous definitions of heteroblastic vs. homoblastic species, and are still far from an understanding of the functional and evolutionary consequences of these sudden changes for any of the groups with heteroblasticmembers. Considering the diversity of ecologies it is probably futile to expect one single explanation. Hence, I will focus on heteroblasty in one particular group with a common phylogenic background and a relative similarity in ecology: epiphytic bromeliads. Considering that water scarcity is arguably the most limiting abiotic factor in the epiphytic habitat, it seemed straightforward to interpret the observed ontogenetic shift from juveniles with many narrow leaves to later stages with broad leaves (tank-forming individuals) as an adaptation to intermittent water supply. This appeared particularly obvious because the early stages resemble so-called atmospheric bromeliad species, which are typically associated with drier habitats. Although early experimental studies provided some support for this notion, extensive work in the last years challenged this view. For example, it could be shown that most of the changes were actually continuous changes with plant size (ontogenetic drift). Moreover, heteroblastic species are as common in the understory of very wet forest as they are in more seasonal forest. Thus, the functional role of heteroblasty in bromeliads is still an open question ? we are currently testing alternative hypotheses, e.g. the role of leaf form in the avoidance of self-shading in the understory. Reviewing the current state of knowledge demonstrates that heteroblastic changes are a fascinating phenomenon in many ways and an underutilized resource in the study of ecological and evolutionary processes.

Keywords:—Water Relations; Ecophysiology; Ontogeny.