Structural and physiological adaptation to light environments in neotropical Heliconia (Heliconiaceae)

Influence of habitat on physiological and structural characteristics was investigated for broad-leaved tropical monocotyledons in the genus Heliconia (Heliconiaceae). Seven species were selected from three different light regimes, enabling an analysis of the extent to which this genus has adapted its photosynthetic strategies and morphological characteristics to different daily photon flux densities (PFD). Predictably, light response curves showed a clear gradient with respect to light saturation and rates of maximum net assimilation (Amax). Heliconia latispatha, an open site species, showed saturation at higher PFD (1400 μmol m−2 s−1) and higher Amax (14–16 μmol m−2 s−1) than H. mathiasiae, a forest edge species (PFD 1000 μmol m−2 s−1; Amax 7.5–8.5 μmol m−2 s−1) and H. irrasa of deep-shade forest understorey (PFD 250 μmol m−2 s−1; Amax 3.5 mol m−2 s−1). Leaf blade areas were largest in open sites, and leaf specific mass was also significantly higher, but leaf support efficiency was highest in understorey species. Species in open sites had thicker leaves with more chlorenchyma, whereas deep-shade species had very thin leaves and low stomatal densities. These rapidly growing herbaceous perennials appear to allocate much of their above-ground biomass to leaf tissues and have a relatively low investment in support tissues. This contrasts with understorey palms, in which leaf form and structural investment has been interpreted as a trade-off between economy and protection against tissue loss from falling branches. Presence of below-ground rhizomes in Heliconia may provide the key to this difference.