Heteroblasty in bromeliads - anatomical, morphological and physiological changes in ontogeny are not related to the...

Heteroblasty is defined as an abrupt change in gross morphology during ontogeny, whereas homoblastic species show no or gradual changes. For Bromeliaceae, there are conflicting reports on a very limited number of species on the functional importance of this step change compared with gradual changes (ontogenetic drift). Studying a large set of species should allow more general conclusions. Seventeen homoblastic and heteroblastic species from Panama were investigated, including the entire size range of most species. Measurements included functionally relevant anatomical (water storage tissue), morphological (stomatal and trichome densities) and physiological parameters (transpiration rates, nutrient uptake rates). Size-related variation in all parameters was common, but evidence for a step change in the studied parameters could not be detected in any of the heteroblastic species. Our results caused us to question the widely held view of the course of the ontogenetic development in heteroblastic bromeliads and their functional implications. These findings suggest that the possible functional relevance of heteroblasty in bromeliads require rethinking and future investigations should employ a comparative approach with both homoblastic and heteroblastic species and including the entire size range to account for ontogenetic drift.