Modeling Leaf Hydraulics in “Tank Bromeliads”
Mentor:Gretchen North, Professor of Biology, Occidental College
A. nudicaulis and G. lingulata are two species of tank bromeliads found in the rainforest canopy. A. nudicaulis is found high in the canopy, and does CAM photosynthesis whereas G. lingulata is shade tolerant and performs C3. Both plants’ root systems do little more than anchor them upon the branches of taller trees; the plants gather water from the vase-shaped tank formed by their leaves. Their lack of significant stem or root system makes them a model organism for studying leaf hydraulics. We considered leaf water flow as occurring either axially (through the xylem) or radially (from the xylem to the stomates). These values were then used in a leaky-pipe model, which simplifies the leaf as a semi-permeable soda straw. This model was then used to calculate the leaf’s properties using the Landsberg-Fowkes “Leaky Pipe Model” equation, derived in part from the Hagen Poiseuille flow equation for a porous cylinder. We measured the water potential by pressure bombing, the rate of transpiration of the whole leaf by recording the plant’s changing mass over time on a balance, and rate of water uptake through the leaf tissue. This allowed us to calculate axial conductance, Kh (m^4 s^-1 MPa^-1) and Lp (m s^-1 MPa^-1), which could be used in the Landsberg-Fowkes model to find Lr, the radial conductivity. We calculated Kh and Lp for plants of both nudicaulis and lingulata. We measured both individual leaf transpiration as well as whole leaf transpiration. Additionally, for both the individual leaves as well as the whole plant, we calculated leaf areas, and were able to find an average transpiration per area. Pressure-volume curves also helped find how capacitance affects leaf hydraulics. Differences in water uptake were analyzed and comparisons against other plants for which hydraulic conductances have been measured.