Evaluation of Murray's law in Psilotum nudum (Psilotaceae), an analogue of ancestral vascular plants

Journal ArticlePrevious work has shown that the xylem of seed plants follows Murray's law when conduits do not provide structural support to the plant. Here, compliance with Murray's law was tested in the stem photosynthesizer Psilotum nudum, a seedless vascular plant. Psilotum nudum was chosen because the central stele does not provide structural support, which means that Murray's law is applicable, and because its simple shoot structure resembles the earliest vascular plants

Traits, properties, and performance: how woody plants combine hydraulic and mechanical functions in a cell, tissue, or whole plant

This review presents a framework for evaluating how cells, tissues, organs, and whole plants perform both hydraulic and mechanical functions. The morphological alterations that affect dual functionality are varied: individual cells can have altered morphology; tissues can have altered partitioning to functions or altered cell alignment; and organs and whole plants can differ in their allocation to different tissues, or in the geometric distribution of the tissues they have. A hierarchical model emphasizes that morphological traits influence the hydraulic or mechanical properties; the properties, combined with the plant unit's environment, then influence the performance of that plant unit. As a special case, we discuss the mechanisms by which the proxy property wood density has strong correlations to performance but without direct causality. Traits and properties influence multiple aspects of performance, and there can be mutual compensations such that similar performance occurs. This compensation emphasizes that natural selection acts on, and a plant's viability is determined by, its performance, rather than its contributing traits and properties. Continued research on the relationships among traits, and on their effects on multiple aspects of performance, will help us better predict, manage, and select plant material for success under multiple stresses in the future.This is the publisher’s final pdf. The published article is copyrighted by The Authors and the New Phytologist Trust. It is published by John Wiley & Sons, Inc. The published article can be found at: www.newphytologist.com The New Phytologist Trust is a not-for-profit organization dedicated to the promotion of plant science, facilitating projects from symposia to free access for our Tansley reviews.Keywords: integration, wind, functional trait, tradeoff, drought, biomechanics, xylem anatomy, multiple stressesKeywords: integration, wind, functional trait, tradeoff, drought, biomechanics, xylem anatomy, multiple stresse

Linking stomatal sensitivity and whole-tree hydraulic architecture

NoneKeywords: Vapor pressure deficit, Forest, Conductance, Transpiration, Diffuse porous trees, Capacity, Water transport, Betula occidentalis, Responses, Cavitatio

A comparison of the hydraulic efficiency of a palm species (Iriartea deltoidea) with other wood types

Palms are an important component of tropical ecosystems, living alongside dicotyledonous trees, even though they have a very different growth pattern and vascular system. As monocots, vessels in palms are located within vascular bundles and, without a vascular cambium that many dicotyledonous trees possess, palms cannot add additional vessels to their vascular system as they get older and taller. This means that hydraulic architecture in palms is more predetermined, which may require a highly efficient hydraulic system. This preset nature, along with the decoupling of hydraulic and mechanical functioning to different cell types, may allow palms to have a more efficient hydraulic system than dicotyledonous trees. Therefore, this study seeks to determine the efficiency of the hydraulic system in the palm Iriartea deltoidea (Ruiz & Pav.) and compare this efficiency with other tree forms. We measured cross-sectional areas of roots, stems and fronds as well as leaf areas of I. deltoidea saplings. Likewise, cross-sections were made and vessel diameters and frequencies measured. This allowed for the calculation of theoretical specific-conductivity (KS, calc), theoretical leaf-specific conductivity (KL, calc), and vessel diameter and vessel number ratios between distal and proximal locations in the palms. I. deltoidea palms were found to have the largest, least frequent vessels that diverged most from the square packing limit (maximum number of vessels that fit into a given area) compared with other major tree forms, and they therefore invested the least space and carbon into water transport structures. Likewise, conduits tapered by approximately one third between ranks (root, bole, petiole), which represents an efficient ratio with regard to the trade-offs between safety and efficiency of the conducting system. Conduits also exhibited a high conservation of the sum of the conduit radii cubed (Σr³) across ranks, thereby approximating Murray’s Law patterning. Therefore, our results indicate that the palm, I. deltoidea, has a very efficient hydraulic system in terms of maintaining a large conducting capacity with a minimal vascular investment. This efficiency may allow palms to compete well with dicotyledonous trees in tropical and subtropical climates but other developmental factors largely restrict palms from regions that experience prolonged freezing temperatures.This is an author's peer-reviewed final manuscript, as accepted by the publisher. The published article is copyrighted by Oxford University Press and can be found at: http://treephys.oxfordjournals.org/.Keywords: Hydraulic architecture, Murray’s law, Palms, Conduit tapering, Vascular anatom

An Annual Pattern of Native Embolism in Upper Branches of Four Tall Conifer Species

Premise of the study: The Pacific Northwest of North America experiences relatively mild winters and dry summers. For the tall coniferous trees that grow in this region, we predicted that loss in the hydraulic conductivity of uppermost branches would be avoided because of difficulty reversing accumulated emboli in xylem that is always under negative pressure. • Methods: To test this hypothesis, we measured native percent loss in hydraulic conductivity (PLC; the decrease of in situ hydraulic conductivity relative to the maximum) monthly throughout 2009 in branches at the tops (~50 m) of four species in an old growth forest in southern Washington. • Key results: Contrary to our prediction, freeze – thaw cycles resulted in considerable native PLC. Branches showed hydraulic recovery in the spring and after a moderate increase in native embolism that was observed after an unusually hot period in August. The September recovery occurred despite decreases in the leaf and stem water potentials compared to August values. • Conclusions: Recoveries in branches of these trees could not have occurred by raising the water potential enough to dissolve bubbles simply by transporting water from roots and must have occurred either through water absorption through needles and/ or refilling under negative pressure. Excluding the August value, native embolism values correlated strongly with air temperature of the preceding 10 d. For three species, we found that branches with lower wood density had higher specific conductivity, but not greater native PLC than branches with higher wood density, which calls into question whether there is any hydraulic benefit to higher wood density in small branches in those species.Keywords: Tsuga heterophylla, Abies grandis, wood density, hydraulic conductivity, Thuja plicata, Pseudotsuga menziesi

Evidence for xylem embolism as a primary factor in dehydration-induced declines in leaf hydraulic conductance

Hydraulic conductance of leaves (K[subscript leaf]) typically decreases with increasing water stress and recent studies have proposed different mechanisms responsible for decreasing K[subscript leaf]. We measured K[subscript leaf] concurrently with ultrasonic acoustic emissions (UAEs) in dehydrating leaves of several species to determine whether declining K[subscript leaf] was associated with xylem embolism. In addition, we performed experiments in which the surface tension of water in the leaf xylem was reduced by using a surfactant solution. Finally, we compared the hydraulic vulnerability of entire leaves with the leaf lamina in three species. Leaf hydraulic vulnerability based on rehydration kinetics and UAE was very similar, except in Quercus garryana. However, water potentials corresponding to the initial decline in K[subscript leaf] and the onset of UAE in Q. garryana were similar. In all species tested, reducing the surface tension of water caused K[subscript leaf] to decline at less negative water potentials compared with leaves supplied with water. Microscopy revealed that as the fraction of embolized xylem increased, K[subscript leaf] declined sharply in Q. garryana. Measurements on leaf discs revealed that reductions in lamina hydraulic conductance with dehydration were not as great as those observed in intact leaves, suggesting that embolism was the primary mechanism for reductions in K[subscript leaf] during dehydration

The dynamic pipeline: hydraulic capacitance and xylem hydraulic safety in four tall conifer species

Recent work has suggested that plants differ in their relative reliance on structural avoidance of embolism versus maintenance of the xylem water column through dynamic traits such as capacitance, but we still know little about how and why species differ along this continuum. It is even less clear how or if different parts of a plant vary along this spectrum. Here we examined how traits such as hydraulic conductivity or conductance, xylem vulnerability curves, and capacitance differ in trunks, large- and small-diameter branches, and foliated shoots of four species of co-occurring conifers. We found striking similarities among species in most traits, but large differences among plant parts. Vulnerability to embolism was high in shoots, low in small- and large-diameter branches, and high again in the trunks. Safety margins, defined as the pressure causing 50% loss of hydraulic conductivity or conductance minus the midday water potential, were large in small-diameter branches, small in trunks and negative in shoots. Sapwood capacitance increased with stem diameter, and was correlated with stem vulnerability, wood density and latewood proportion. Capacitive release of water is a dynamic aspect of plant hydraulics that is integral to maintenance of long-distance water transport.This is the publisher’s final pdf. The published article is copyrighted by John Wiley & Sons Ltd. and can be found at: http://onlinelibrary.wiley.com/journal/10.1111/%28ISSN%291365-3040Keywords: vulnerability curves, safety margins, hydraulic conductance, conifers, capacitanc

Murray's law, the ‘Yarrum’ optimum, and the hydraulic architecture of compound leaves

• There are two optima for maximizing hydraulic conductance per vasculature volume in plants. Murray's law (ML) predicts the optimal conduit taper for a fixed change in conduit number across branch ranks. The opposite, the Yarrum optimum (YO), predicts the optimal change in conduit number for a fixed taper. • We derived the solution for YO and then evaluated compliance with both optima within the xylem of compound leaves, where conduits should have a minimal mechanical role. We sampled leaves from temperate ferns, and tropical and temperate angiosperms. • Leaf vasculature exhibited greater agreement with ML than YO. Of the 14 comparisons in 13 species, 12 conformed to ML. The clear tendency towards ML indicates that taper is optimized for a constrained conduit number. Conduit number may be constrained by leaflet number, safety requirements, and the fact that the number of conduits is established before their diameter during development. • Within a leaf, ML compliance requires leaf‐specific conductivity to decrease from petiole to petiolule with the decrease in leaf area supplied. A similar scaling applied across species, indicating lower leaf‐specific petiole conductivity in smaller leaves. Small leaf size should offset lower conductivity, and petiole conductance (conductivity/length) may be independent of leaf size.Keywords: hydraulic efficiency, wood anatomy, network, leaf specific conductivit

Do Ray Cells Provide a Pathway for Radial Water Movement in the Stems of Conifer Trees?

• Premise of the study: The pathway of radial water movement in tree stems presents an unknown with respect to whole-tree hydraulics. Radial profi les have shown substantial axial sap fl ow in deeper layers of sapwood (that may lack direct connection to transpiring leaves), which suggests the existence of a radial pathway for water movement. Rays in tree stems include ray tracheids and/or ray parenchyma cells and may offer such a pathway for radial water transport. This study investigated relationships between radial hydraulic conductivity (k[subscript s-rad]) and ray anatomical and stem morphological characteristics in the stems of three conifer species whose distributions span a natural aridity gradient across the Cascade Mountain range in Oregon, United States. • Methods: The k [subscript s-rad] was measured with a high-pressure fl ow meter. Ray tracheid and ray parenchyma characteristics and water transport properties were visualized using autofl uorescence or confocal microscopy. • Key results: The k[subscript s-rad] did not vary predictably with sapwood depth among species and populations. Dye tracer did not infi ltrate ray tracheids, and infi ltration into ray parenchyma was limited. Regression analyses revealed inconsistent relationships between k[subscript s-rad] and selected anatomical or growth characteristics when ecotypes were analyzed individually and weak relationships between k[subscript s-rad] and these characteristics when data were pooled by tree species. • Conclusions: The lack of signifi cant relationships between k[subscript s-rad] and the ray and stem morphologies we studied, combined with the absence of dye tracer in ray tracheid and limited movement of dye into ray parenchyma suggests that rays may not facilitate radial water transport in the three conifer species studied.Keywords: Ray parenchyma, Hydraulic architecture, Xylem anatomy, Ray tracheids, Drought, Hydraulic conductivity, Conifers, Radial conductivit

A comparison of daily water use estimates derived from constant-heat sap-flow probe values and gravimetric measurements in pot-grown saplings.

Use of Granier-style heat dissipation sensors to measure sap flow is common in plant physiology, ecology and hydrology. There has been concern that any change to the original Granier design invalidates the empirical relationship between sap flux density and the temperature difference between the probes. Here, we compared daily water use estimates from gravimetric measurements with values from variable length heat dissipation sensors, which are a relatively new design. Values recorded during a one-week period were compared for three large pot-grown saplings of each of the tropical trees Pseudobombax septenatum (Jacq.) Dugand and Calophyllum longifolium Willd. For five of the six individuals, P values from paired t-tests comparing the two methods ranged from 0.12 to 0.43 and differences in estimates of total daily water use over the week of the experiment averaged < 3%. In one P. septenatum sapling, the sap flow sensors underestimated water use relative to the gravimetric measurements. This discrepancy could have been associated with naturally occurring gradients in temperature that reduced the difference in temperature between the probes, which would have caused the sensor method to underestimate water use. Our results indicate that substitution of variable length heat dissipation probes for probes of the original Granier design did not invalidate the empirical relationship determined by Granier between sap flux density and the temperature difference between probes.Keywords: validation, variable-length probeKeywords: validation, variable-length prob