The Origin of Bract Liquid in a Neotropical Heliconia Species

Abstract

The origin of the deep pools of liquid held in the cuplike bracts of Heliconia imbricata (Heliconiaceae), in which flower bases and fruits are submerged, was examined. Strong evidence was obtained that bract fluid is primarily a plant secretion rather than accumulated rainwater. Drained bracts regained their original volumes by the next day regardless of whether they were protected from rain; an average drained bract accumulated over 11 ml of liquid overnight. Newly-opening bracts held on average over 9 ml of fluid, and evidence suggests that fluid continues to be secreted into bracts for weeks after they have opened. The average pH of the accumulated fluid was basic (averaging 7.8) relative to rainwater (pH 5.0). Bract fluid may function in defending flowers and fruits from herbivores, although physical factors of the bracts appear to be important as well. HELICONIA PLANTS (HELICONIACEAE) ARE ABUNDANT and conspicuous throughout the neotropics. All species have thick bracts that surround the flowers and developing fruits, although the arrangement and orientation of the bracts vary greatly within the group (Daniels & Stiles 1979). Many Heliconia species have erect inflorescences consisting of a series of brightly colored, alternately-arranged bracts, many of which at any time contain flowers, buds, and developing fruits. New bracts arise at the top of the rachis every 3-10 days. These erect, cuplike or spoutlike bracts often hold pools of liquid. Several researchers have suggested that the liquid is trapped rainwater, which protects the submerged fruits and flower bases from herbivory (e.g., Skutch 1933, Stiles 1979), although Seifert (1982) did state that in some Heliconia species part of the liquid is actively transported into the bracts. Neither the origin of the fluid nor its protective function have ever been verified experimentally. Here I report on the origin and nature of the bract fluid of Heliconia imbricata (Kuntze) Baker. Heliconia imbricata is one of the most common species of Heliconia in lowland Costa Rica. During its flowering period its dark-red, cuplike bracts hold large volumes of foul-smelling liquid known to contain a diverse microfauna (Vandermeer et al. 1972). Also Heliconia imbricata inflorescences support a small community of aquatic and semiaquatic insects, primarily fly and beetle larvae, that feeds in and around the liquid-filled bracts (Seifert 1975, Seifert & Seifert 1976a, b). Typical of most Heliconia species in Costa Rica, H. imbricata flowers and fruits most heavily during the wetseason months of July and August (Daniels & Stiles 1979). Dry spells are frequent, however, and small standing pools of water probably evaporate quickly. If the risks of floral and fruit herbivory were great and submergence did act ' Received 29 January 1985, accepted 20 June 1985. as protection, then H. imbricata might be expected to actively and regularly secrete, rather than passively and opportunistically trap, fluid in its bracts. Therefore I predicted that if H. imbricata bracts accumulated only rainwater or throughfall liquid, then bagged, drained bracts would remain dry, while exposed, drained ones would accumulate new fluid, and newly-opening bracts would contain no fluid. Additionally, I expected that if the liquid were rainwater, its pH would be equal to or more acidic than rain (since organic decay acidifies standing water). Finally, I examined patterns of liquid volume held in