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

Influence, of habitat on physiological and structural characteristics was i nvestigated for broad-leaved tropical monocotyledons in the genus Heliconia (Heliconiaceae). Sever? species were selected from three different light r egimes, enabling an analysis of the extent to which this genus has adapted its photosynthetic strategies and morphological characteristics to differen t daily photon flux densities (PFD). Predictably, light response curves sho wed a clear gradient with respect to light saturation and rates of maximum net assimilation (A(max)). Heliconia latispatha, an open site species, show ed saturation at higher PFD (1400 mu mol m(-2) s(-1)) and higher A(max) (14 -16 mu mol m(-2) s(-1)) than N. mathiasiae, a forest edge species (PFD 1000 mu mol m(-2) s(-1); A(max) 7.5-8.5 mu mol m(-2) s(-1)) and H. irrasa of de ep-shade forest understorey (PFD 250 mu mol m(-2) s(-1); A(max) 3.5 mol m(- 2) s(-1)). Leaf blade areas were largest in open sites, and leaf specific m ass 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 stoma tal densities. These rapidly growing herbaceous perennials appear to alloca te 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 trad e-off between economy and protection against tissue loss from falling branc hes. Presence of below-ground rhizomes in Heliconia may provide the key to this difference.