Abstract
Epiphytic plants in the Bromeliaceae known as tank bromeliads essentially lack stems and absorptive roots and instead take up water from reservoirs formed by their overlapping leaf bases. For such plants, leaf hydraulic conductance is plant hydraulic conductance. Their simple strap-shaped leaves and parallel venation make them suitable for modeling leaf hydraulic conductance based on vasculature and other anatomical and morphological traits. Plants of the tank bromeliad Guzmania lingulata were investigated in a lowland tropical forest in Costa Rica and a shaded glasshouse in Los Angeles, CA, USA. Stomatal conductance to water vapor and leaf anatomical variables related to hydraulic conductance were measured for both groups. Tracheid diameters and numbers of vascular bundles (veins) were used with the Hagen–Poiseuille equation to calculate axial hydraulic conductance. Measurements of leaf hydraulic conductance using the evaporative flux method were also made for glasshouse plants. Values for axial conductance and leaf hydraulic conductance were used in a model based on leaky cable theory to estimate the conductance of the radial pathway from the vein to the leaf surface and to assess the relative contributions of both axial and radial pathways. In keeping with low stomatal conductance, low stomatal density, low vein density, and narrow tracheid diameters, leaf hydraulic conductance for G. lingulata was quite low in comparison with most other angiosperms. Using the predicted axial conductance in the leaky cable model, the radial resistance across the leaf mesophyll was predicted to predominate; lower, more realistic values of axial conductance resulted in predicted radial resistances that were closer to axial resistance in their impact on total leaf resistance. Tracer dyes suggested that water uptake through the tank region of the leaf was not limiting. Both dye movement and the leaky cable model indicated that the leaf blade of G. lingulata was structurally and hydraulically well-suited to conserve water.
Highlights
Tank bromeliads have a captive water supply, held in reservoirs formed by overlapping leaf bases
The radial pathways of water into the leaf from the tank and out of the leaf vasculature through the mesophyll and other extravascular tissues are more complex than the axial pathway through the veins, but an overall picture of leaf hydraulic conductance can be developed through a combination of physiological and anatomical measurements and mathematical modeling
G. lingulata is conservative with respect to photosynthesis and water use
Summary
Tank bromeliads have a captive water supply, held in reservoirs formed by overlapping leaf bases. For these species, the leaf is the organ of both supply and demand: the leaf base absorbs water and nutrients captured in the tank and delivers them to the leaf blade. The uptake, delivery, and use of water are solely leaf processes; for tank bromeliads, plant hydraulic conductance is the collective hydraulic conductance of its leaves. The hydraulic system for tank bromeliads is simplified because absorptive roots and stems are lacking and because the leaves themselves are classically monocotyledonous: simple, entire, and largely strapshaped. Water that is absorbed through the leaf base is transpired by the leaf blade after traveling through a vascular system that consists largely of parallel veins. The goal of this study is to analyze leaf hydraulics for a tank bromeliad, because such plants are hydraulically intriguing and ecologically important in the forests of the Neotropics (Nadkarni, 1984), and because they represent a sizable group of plants that has been largely neglected with respect to leaf hydraulics: non-grass monocots
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