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  1. Free, publicly-accessible full text available October 16, 2024
  2. Abstract Background

    Recent reports of extreme levels of undersaturation in internal leaf air spaces have called into question one of the foundational assumptions of leaf gas exchange analysis, that leaf air spaces are effectively saturated with water vapour at leaf surface temperature. Historically, inferring the biophysical states controlling assimilation and transpiration from the fluxes directly measured by gas exchange systems has presented a number of challenges, including: (1) a mismatch in scales between the area of flux measurement, the biochemical cellular scale and the meso-scale introduced by the localization of the fluxes to stomatal pores; (2) the inaccessibility of the internal states of CO2 and water vapour required to define conductances; and (3) uncertainties about the pathways these internal fluxes travel. In response, plant physiologists have adopted a set of simplifying assumptions that define phenomenological concepts such as stomatal and mesophyll conductances.


    Investigators have long been concerned that a failure of basic assumptions could be distorting our understanding of these phenomenological conductances, and the biophysical states inside leaves. Here we review these assumptions and historical efforts to test them. We then explore whether artefacts in analysis arising from the averaging of fluxes over macroscopic leaf areas could provide alternative explanations for some part, if not all, of reported extreme states of undersaturation.


    Spatial heterogeneities can, in some cases, create the appearance of undersaturation in the internal air spaces of leaves. Further refinement of experimental approaches will be required to separate undersaturation from the effects of spatial variations in fluxes or conductances. Novel combinations of current and emerging technologies hold promise for meeting this challenge.

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  3. Summary

    A robust understanding of phloem functioning in tall trees evades us because current methods for collecting phloem sap do not lend themselves to measuring actively photosynthesizing canopy leaves.

    We show that Raman spectroscopy can be used as a quantitative tool to assess sucrose concentration in leaf samples. Specifically, we found that Raman spectroscopy can predict physiologically relevant sucrose concentrations (adjustedR2of 0.9) in frozen leaf extract spiked with sucrose.

    We then apply this method to estimate sieve element sucrose concentration in rapidly frozen petioles of canopy red oak (Quercus rubra)trees and found that sucrose concentrations are > 1100 mM at midday and midnight. This concentration is predicted to generate a sieve element turgor pressure high enough to generate bulk flow through the phloem, but is potentially too high to allow for sucrose diffusion from photosynthetic cells.

    Our findings support the Münch hypothesis for phloem transport once the carbon is in the phloem and challenge the passive‐loading hypothesis for carbon movement into the phloem for red oak. This study provides the first ˜in‐situ(frozen in the functioning state) source sieve element sucrose concentration characterization in any plant, opening a new avenue for investigation of phloem functioning.

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  4. Summary

    The hydraulic system of vascular plants and its integrity is essential for plant survival. To transport water under tension, the walls of xylem conduits must approximate rigid pipes. Against this expectation, conduit deformation has been reported in the leaves of a few species and hypothesized to function as a ‘circuit breaker’ against embolism. Experimental evidence is lacking, and its generality is unknown.

    We demonstrated the role of conduit deformation in protecting the upstream xylem from embolism through experiments on three species and surveyed a diverse selection of vascular plants for conduit deformation in leaves.

    Conduit deformation in minor veins occurred before embolism during slow dehydration. When leaves were exposed to transient increases in transpiration, conduit deformation was accompanied by large water potential differences from leaf to stem and minimal embolism in the upstream xylem. In the three species tested, collapsible vein endings provided clear protection of upstream xylem from embolism during transient increases in transpiration.

    We found conduit deformation in diverse vascular plants, including 11 eudicots, ginkgo, a cycad, a fern, a bamboo, and a grass species, but not in two bamboo and a palm species, demonstrating that the potential for ‘circuit breaker’ functionality may be widespread across vascular plants.

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  5. Abstract

    Capparis odoratissimais a tree species native to semi‐arid environments of South America where low soil water availability coexists with frequent night‐time fog. A previous study showed that water applied to leaf surfaces enhanced leaf hydration, photosynthesis and growth, but the mechanisms of foliar water uptake are unknown. Here, we combine detailed anatomical evaluations with water and dye uptake experiments in the laboratory, and use immunolocalization of pectin and arabinogalactan protein epitopes to characterize water uptake pathways in leaves. Abaxially, the leaves ofC. odoratissimaare covered with peltate hairs, while the adaxial surfaces are glabrous. Both surfaces are able to absorb condensed water, but the abaxial surface has higher rates of water uptake. Thousands of idioblasts per cm2, a higher density than stomata, connect the adaxial leaf surface and the abaxial peltate hairs, both of which contain hygroscopic substances such as arabinogalactan proteins and pectins. The highly specialized anatomy of the leaves ofC odoratissimafulfils the dual function of minimizing water loss when stomata are closed, while maintaining the ability to absorb liquid water. Cell‐wall related hygroscopic compounds in the peltate hairs and idioblasts create a network of microchannels that maintain leaf hydration and promote water uptake.

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  6. Premise

    The dimensions of phloem sieve elements have been shown to vary as a function of tree height, decreasing hydraulic resistance as the transport pathway lengthens. However, little is known about ontogenetic patterns of sieve element scaling. Here we examine within a single species (Quercus rubra) how decreases in hydraulic resistance with distance from the plant apex are mediated by overall plant size.


    We sampled and imaged phloem tissue at multiple heights along the main stem and in the live crown of four size classes of trees using fluorescence and scanning electron microscopy. Sieve element length and radius, the number of sieve areas per compound plate, pore number, and pore radius were used to calculate total hydraulic resistance at each sampling location.


    Sieve element length varied with tree size, while sieve element radius, sieve pore radius, and the number of sieve areas per compound plate varied with sampling position. When data from all size classes were aggregated, all four variables followed a power‐law trend with distance from the top of the tree. The net effect of these ontogenetic scalings was to make total hydraulic sieve tube resistance independent of tree height from 0.5 to over 20 m.


    Sieve element development responded to two pieces of information, tree size and distance from the apex, in a manner that conserved total sieve tube resistance across size classes. A further differentiated response between the phloem in the live crown and in the main stem is also suggested.

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  7. Abstract

    Earth system models (ESMs) rely on the calculation of canopy conductance in land surface models (LSMs) to quantify the partitioning of land surface energy, water, andCO2fluxes. This is achieved by scaling stomatal conductance,gw, determined from physiological models developed for leaves. Traditionally, models forgwhave been semi‐empirical, combining physiological functions with empirically determined calibration constants. More recently, optimization theory has been applied to modelgwinLSMs under the premise that it has a stronger grounding in physiological theory and might ultimately lead to improved predictive accuracy. However, this premise has not been thoroughly tested. Using original field data from contrasting forest systems, we compare a widely used empirical type and a more recently developed optimization‐typegwmodel, termedBBandMED, respectively. Overall, we find no difference between the two models when used to simulategwfrom photosynthesis data, or leaf gas exchange from a coupled photosynthesis‐conductance model, or gross primary productivity and evapotranspiration for aFLUXNETtower site with theCLM5 communityLSM. Field measurements reveal that the key fitted parameters forBBandMED,g1Bandg1M,exhibit strong species specificity in magnitude and sensitivity toCO2, andCLM5 simulations reveal that failure to include this sensitivity can result in significant overestimates of evapotranspiration for high‐CO2scenarios. Further, we show thatg1Bandg1Mcan be determined from meanci/ca(ratio of leaf intercellular to ambientCO2concentration). Applying this relationship withci/cavalues derived from a leaf δ13C database, we obtain a global distribution ofg1Bandg1M, and these values correlate significantly with mean annual precipitation. This provides a new methodology for global parameterization of theBBandMEDmodels inLSMs, tied directly to leaf physiology but unconstrained by spatial boundaries separating designated biomes or plant functional types.

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