NSF Award
1258499
Tank bromeliads are plants that grow on trees in the forest canopy in the Neotropics, deriving sustenance from the water and nutrients collected in the tanks formed at the base of their leaves. In addition to the ecological importance of tank bromeliads due to the rich communities sustained in and by the tank water, they are excellent case studies for investigating the role of environmental variables such as light in governing leaf water uptake. Many bromeliad species can occur at several positions in the canopy, thus even a single host tree can provide a range of plants from different light exposures. The capacity of leaves to transport water, or leaf hydraulic conductance, can determine a plant's ability to succeed in its environment, and light can determine that capacity due to long-term effects on leaf anatomy and morphology and short-term effects on leaf physiology. <br/><br/>Because tank bromeliads virtually lack absorptive roots and stems, they also provide excellent models in which to investigate external and internal regulators of leaf hydraulic conductance. The relatively simple strap-shape of the leaves as well as their parallel venation facilitate measurement and modeling of leaf hydraulic conductance for bromeliads, and their use of tanks as reservoirs allows manipulation of the water supply to investigate the effects of environmental and internal regulators. Specifically, light and pH are known to affect leaf hydraulic conductance at least in part via their effects on aquaporins, proteins that regulate water movement across membranes. <br/><br/>This research is truly integrative in that the regulation of leaf hydraulic conductance will be assessed in the field at the level of plant responses to light variation within the forest canopy as well as in the laboratory, using experimental manipulations of light, tank-water pH, and aquaporin inhibitors. Aquaporin gene expression in leaves from different light exposures and in different leaf regions will also be measured and compared. For plants in both field and laboratory, anatomical and morphological traits will be investigated and incorporated into a model of leaf hydraulic conductance that will be further informed and tested by microscopic examination of the pathways indicated by tracer and diagnostic staining. Because the leaves of tank bromeliads are the organs of both supply and demand, understanding how their leaf hydraulic conductance is regulated is fundamental to understanding their ecological function and how they might respond to changes that are rapidly occurring in tropical forests. The role of tank bromeliads and other plants that grow in the branches of the tropical tree canopy is predicted to become even more important as primary forest becomes more fragmented and is replaced by secondary forest.<br/><br/>The research will be carried out entirely by the principal investigator and undergraduate students, both in the field at La Selva Biological Station in Costa Rica and in the laboratory at Occidental College. A key component of the proposed research is its use of a field station in Costa Rica, in part because Occidental College's location in Los Angeles attracts many students with personal and intellectual ties to Latin America. One major goal of the proposed research is to strengthen those ties and to develop an international experience for undergraduates that will demonstrate the role of science in understanding how people and other organisms are linked by global processes such as water use and changes in forest structure.
