To furcate or not to furcate: the dance between vessel number and diameter in leaves

Stretched sapwood, ultra-widening permeability and ditching da Vinci: revising models of plant form and function

BACKGROUND

The mechanisms leading to dieback and death of trees under drought remain unclear. To gain an understanding of these mechanisms, addressing major empirical gaps regarding tree structure-function relations remains essential.

SCOPE

We give reasons to think that a central factor shaping plant form and function is selection simultaneously favouring constant leaf-specific conductance with height growth and isometric (1:1) scaling between leaf area and the volume of metabolically active sink tissues ('sapwood'). Sapwood volume-leaf area isometry implies that per-leaf area sapwood volumes become transversely narrower with height growth; we call this 'stretching'. Stretching means that selection must favour increases in permeability above and beyond that afforded by tip-to-base conduit widening ("ultra-widening permeability"), via fewer and wider vessels or tracheids with larger pits or larger margo openings. Leaf area-metabolically active sink tissue isometry would mean that it is unlikely that larger trees die during drought because of carbon starvation due to greater sink-source relationships as compared to shorter plants. Instead, an increase in permeability is most plausibly associated with greater risk of embolism, and this seems a more probable explanation of the preferential vulnerability of larger trees to climate change-induced drought. Other implications of selection favouring constant per-leaf area sapwood construction and maintenance costs are departure from the da Vinci rule expectation of similar sapwood areas across branching orders, and that extensive conduit furcation in the stem seems unlikely.

CONCLUSIONS

Because all these considerations impact the likelihood of vulnerability to hydraulic failure versus carbon starvation, both implicated as key suspects in forest mortality, we suggest that these predictions represent essential priorities for empirical testing.

Vascular optimality dictates plant morphology away from Leonardo's rule

Metabolic scaling theory (MST) provides an understanding of scaling in organismal morphology. Empirical data on the apparently universal pattern of tip-to-base conduit widening across vascular plants motivate a set of generalized MST (gMST) relationships allowing for variable rates of conduit coalescence and taper and a transition between transport and diffusive domains. Our model, with coalescence limited to the distalmost part of the conductive system, reconciles previous MST-based models and extends their applicability to the entire plant. We derive an inverse relationship between stem volume taper and conduit widening, which implies that plant morphology is dictated by vascular optimality and not the assumption of constant sapwood area across all branching levels, contradicting Leonardo's rule. Thus, energy efficiency controls conduit coalescence rate, lowering the carbon cost needed to sustain the vascular network. Our model shows that as a plant grows taller, it must increase conduit widening and coalescence, which may make it more vulnerable to drought. We calculated how our gMST model implies a lower carbon cost to sustain a similar network compared to previous MST-based models. We also show that gMST predicts the cross-sectional area of vessels and their frequency along the relative length better than previous MST models for a range of plant types. We encourage further research obtaining data that would allow testing other gMST predictions that remain unconfirmed empirically, such as conduit coalescence rate in stems. The premise of energy efficiency can potentially become instrumental to our understanding of plant carbon allocation.

Designing a solar interfacial evaporator based on tree structures for great coordination of water transport and salt rejection

Solar interfacial evaporation has been receiving increasing attention but it is still a huge challenge to achieve excellent coordination between efficient water transport and salt rejection. Here, unlike the common wood-inspired evaporators with equal-diameter directional pores, we have constructed an integrated structure with highly connected gradient pores that mimic the xylem vessels and phloem sieve tubes found in trees. The bio-inspired structure can reduce the resistance of water transport and salt rejection in the same channel. The average transport speed of the 6.5 cm high (2 cm in diameter) porous structure reached 1.504 g s^-1, and water was transported 16 cm after 100 seconds. Using multilayer graphene oxide as the photothermal conversion material, the evaporators with different heights can work for more than 9 hours under the condition of 1 sun illumination and 23 wt% brine without any salt crystallization, and the evaporation rates range from 3.28 to 4.51 kg m^-2 h^-1, with the highest energy utilization efficiency of about 80%. When used in heavy metal treatment, the rejection was greater than 99.99%. This research provides a simple but innovative design idea for evaporators and is expected to further expand the application of solar interfacial evaporation.

Comparative anatomy of leaf petioles in temperate trees and shrubs: the role of plant size, environment and phylogeny

BACKGROUND AND AIMS

Petioles are important plant organs connecting stems with leaf blades and affecting light-harvesting ability of the leaf as well as transport of water, nutrients and biochemical signals. Despite the high diversity in petiole size, shape and anatomy, little information is available regarding their structural adaptations across evolutionary lineages and environmental conditions. To fill this knowledge gap, we investigated the variation of petiole morphology and anatomy of mainly European woody species to better understand the drivers of internal and external constraints in an evolutionary context.

METHODS

We studied how petiole anatomical features differed according to whole-plant size, leaf traits, thermal and hydrological conditions, and taxonomic origin in 95 shrubs and trees using phylogenetic distance-based generalized least squares models.

KEY RESULTS

Two major axes of variation were related to leaf area and plant size. Larger and softer leaves are found in taller trees of more productive habitats. Their petioles are longer, with a circular outline and are anatomically characterized by the predominance of sclerenchyma, larger vessels, interfascicular areas with fibres and indistinct phloem rays. In contrast, smaller and tougher leaves are found in shorter trees and shrubs of colder or drier habitats. Their petioles have a terete outline, phloem composed of small cells and radially arranged vessels, fibreless xylem and lamellar collenchyma. Individual anatomical traits were linked to different internal and external drivers. Petiole length and vessel diameter increase with increasing leaf blade area. Collenchyma becomes absent with increasing temperature, and petiole outline becomes polygonal with increasing precipitation.

CONCLUSIONS

We conclude that species' temperature and precipitation optima, plant height, and leaf area and thickness exerted a significant control on petiole anatomical and morphological structures not confounded by phylogenetic inertia. Species with different evolutionary histories but similar thermal and hydrological requirements have converged to similar petiole anatomical structures.

The hydraulic architecture of an arborescent monocot: ontogeny-related adjustments in vessel size and leaf area compensate for increased resistance

Bamboos are arborescent monocotyledons that have no secondary growth, but can continually produce conduits with diameters appropriate to the current size of the plant. Here, we studied bamboo hydraulic architecture to address the mechanisms involved in compensating for the increase in hydraulic resistance during ontogeny. We measured the hydraulic weighted vessel diameters (D_h ) at different distances from the apex along the stem of Bambusa textilis. The hydraulic resistance of different components and individuals of different heights were quantified using the high-pressure flowmeter method. The D_h showed tip-to-base widening with a scaling exponent in the range of those reported for trees. Although theoretical hydraulic conductivity decreased from base-to-tip, leaf-specific conductivity did not change. Leaves contributed the most to the whole-shoot hydraulic resistance, followed by the leaf-bearing branches. Roots contributed c. 13% to whole-plant resistance. Interestingly, taller individuals showed lower whole-shoot resistance owing to an increased number of resistances in parallel (side-branches), while leaf-specific resistance was independent of plant size. Tip-to-base vessel widening and height-independent constant leaf-specific conductance could be mechanisms for hydraulic optimization in B. textilis. Similar patterns have also been found in woody plants with secondary growth, but this bamboo exhibits them without secondary growth.

The Widened Pipe Model of plant hydraulic evolution

Shaping global water and carbon cycles, plants lift water from roots to leaves through xylem conduits. The importance of xylem water conduction makes it crucial to understand how natural selection deploys conduit diameters within and across plants. Wider conduits transport more water but are likely more vulnerable to conduction-blocking gas embolisms and cost more for a plant to build, a tension necessarily shaping xylem conduit diameters along plant stems. We build on this expectation to present the Widened Pipe Model (WPM) of plant hydraulic evolution, testing it against a global dataset. The WPM predicts that xylem conduits should be narrowest at the stem tips, widening quickly before plateauing toward the stem base. This universal profile emerges from Pareto modeling of a trade-off between just two competing vectors of natural selection: one favoring rapid widening of conduits tip to base, minimizing hydraulic resistance, and another favoring slow widening of conduits, minimizing carbon cost and embolism risk. Our data spanning terrestrial plant orders, life forms, habitats, and sizes conform closely to WPM predictions. The WPM highlights carbon economy as a powerful vector of natural selection shaping plant function. It further implies that factors that cause resistance in plant conductive systems, such as conduit pit membrane resistance, should scale in exact harmony with tip-to-base conduit widening. Furthermore, the WPM implies that alterations in the environments of individual plants should lead to changes in plant height, for example, shedding terminal branches and resprouting at lower height under drier climates, thus achieving narrower and potentially more embolism-resistant conduits.

From Carlquist's ecological wood anatomy to Carlquist's Law: why comparative anatomy is crucial for functional xylem biology

All students of xylem structure-function relations need to be familiar with the work of Sherwin Carlquist. He studies xylem through the lens of the comparative method, which uses the appearance of similar anatomical features under similar conditions of natural selection to infer function. "Function" in biology implies adaptation; maximally supported adaptation inferences require experimental and comparative xylem scientists to work with one another. Engaging with comparative inferences of xylem function will, more likely sooner rather than later, bring one to the work of Sherwin Carlquist. To mark his 90th birthday, I highlight just a few examples of his extraordinarily perceptive and general comparative insights. One is "Carlquist's Law", the pervasive tendency for vessels to be solitary when background cells are conductive. I cover his pioneering of "ecological" wood anatomy, viewing xylem variation as reflecting the effects of selection across climate and habit variation. Another is the embolism vulnerability-conduit diameter relationship, one of the most widely invoked structure-function relationships in xylem biology. I discuss the inferential richness within the notion of Carlquistian paedomorphosis, including detailed functional inferences regarding ray cell orientation. My final example comes from his very recent work offering the first satisfactory hypothesis accounting for the geographical and histological distribution of scalariform perforation plates as an adaptation, including "Carlquist's Ratchet", why scalariform plates are adaptive but do not re-evolve once lost. This extraordinarily rich production over six decades is filled with comparative inferences that should keep students of xylem function busy testing for decades to come.

Allometric co-variation of xylem and stomata across diverse woody seedlings

Leaf stomatal density is known to co-vary with leaf vein density. However, the functional underpinning of this relation, and how it scales to whole-plant water transport anatomy, is still unresolved. We hypothesized that the balance of water exchange between the vapour phase (in stomata) and liquid phase (in vessels) depends on the consistent scaling between the summed stomatal areas and xylem cross-sectional areas, both at the whole-plant and single-leaf level. This predicted size co-variation should be driven by the co-variation of numbers of stomata and terminal vessels. We examined the relationships of stomatal traits and xylem anatomical traits from the entire plant to individual leaves across seedlings of 53 European woody angiosperm species. There was strong and convergent scaling between total stomatal area and stem xylem area per plant and between leaf total stomatal area and midvein xylem area per leaf across all the species, irrespective of variation in leaf habit, growth-form or relative growth rate. Moreover, strong scaling was found between stomatal number and terminal vessel number, whereas not in their respective average areas. Our findings have broad implications for integrating xylem architecture and stomatal distribution and deepen our understanding of the design rules of plants' water transport network.