Characterizing Tank Bromeliads: Relations between Leaf Hydraulics and Tank pH
Authors:John Dawson, Franklin Maharaj
Mentor:Gretchen North, Professor of Biology, Occidental College
Bromeliads are a family of flowering plants found in the tropics. The hydraulic conductances of their leaves can be quantified with a modified version of the Landsberg-Fowkes equation for viscous fluids in a porous cylinder. Evidence indicates that these theoretical approximations for water movement are dictated by symplastic flow through aquaporins. These membrane channel proteins are notoriously pH sensitive; however, the tanks that provide the bromeliads with water are frequently found to be acidic, and pH values under 4 are not uncommon. It is suspected cellular respiration creates carbonic acid in the tank, which lowers the pH. We considered two explanations as to why the aquaporins appear to be operating despite their acidic environment. The first is that the aquaporins are pH insensitive, as a small number of variations of the protein found in other organisms are capable of working under acidic conditions. The second is that there exists a pH neutralizing mechanism near the trichomes, just as the water is entering the plant. To better understand how aquaporins facilitate water movements despite the acidic environment, a variety of different experiments were performed on both Guzmania lingulata and Guzmania monostachia, chosen in part due to their low capacitance. Hydraulic conductances were calculated using measurements taken from plants exposed to varying levels of photosynthetic light, pH, and known aquaporin inhibitors, such as mercury(II) chloride. As the tanks of bromeliads in the rainforest contain a host of microorganisms, which alter the conductances, field measurements were taken at the La Selva Biological Station in Costa Rica to compare the hydraulic conductances to the bromeliads grown at Occidental. While our preliminary results indicated multiple interconnected relationships between pH, leaf hydraulic conductances, and photosynthetic activity, many more experiments will need to be performed to establish a concrete theory of aquaporin mechanism .