Endogenous and exogenous control of leaf morphology in Iriartea deltoidea (Palmae)

Like many other palms, Iriartea deltoidea undergoes ontogenetic transitions in leaf morphology. It has been asked whether the transition to adult leaf form in Iriartea was a fixed developmental pattern or a more plastic response to changes in environmental conditions, but studies so far have not resolved the question. We re-examined the question by taking measurements on palms growing under the full range of conditions experienced in the natural environment. Using trunk height as a surrogate variable for ontogenetic stage, we found that individual palms undergo the transition to adult morphology over a wide range of heights from < 10 m to > 20 m. Palms growing in gaps undergo the transition early, whereas those growing under a closed canopy undergo the transition late. Noting this, we asked whether the transition occurred in individuals that were growing faster than a critical minimum rate, and found no correlation between growth and morphological stage. A post-hoc hypothesis, that structural support of the crown (measured as trunk diameter) could limit or delay the transition, was confirmed. Our results indicate that multiple conditions must be satisfied before the transition can be completed. The complex requirements for leaf morphogenesis in Iriartea confer an ability to respond flexibly to spatially and temporally varying environmental conditions.