Loss and recovery of leaf hydraulic conductance: Root pressure, embolism, and extra-xylary resistance

Vascular networks in plant leaves must provide for the safe and efficient transport of water, nutrients, and energy; however, the conditions whereby these networks lose and regain conductive capacity are still poorly understood. We measured the loss and recovery of leaf hydraulic conductance (Kleaf) in a tropical monocotyledon (Bambusa vulgaris) and dicotyledon species (Bauhinia blakeana) using Rehydration Kinetics Method (RKM) as well as a recently developed optical method. We found that both species lost ca 88% of their maximal Kleaf (measured by RKM) before any embolization was detected in their conductive elements (via optical observation). This suggests that the majority of loss in Kleaf, as measured with RKM, was associated with resistances other than embolization. Furthermore, embolism in B. vulgaris, a species known to generate root pressure, was reversed when rehydrated under positive pressure (120 kPa), but not under atmospheric pressure. In contrast, embolism was not reversed in B. blakeana under either elevated or atmospheric pressure. However, reductions in Kleaf that was not associated with embolization was recovered by this species when rehydrated under atmospheric conditions, whereas B. vulgaris did not exhibit any recovery under the same conditions. We suggest that root pressure is an adaptive mechanism allowing for the reversal of embolism and the recovery of extraxylary conductance. The absence of embolism reversal in B. blakeana suggests that embolization may be permanent without neutral or positive xylem pressure, but that the recovery of extraxylary conductance can be regained routinely in this species in the absence of root pressure.

XIM4 meeting report, Sept. 25-27 2019, Padua (Italy)

The fourth edition of the international xylem meeting was held for the first time outside France. This represents an important step forward for the meeting and attests to the resolutely international dimension of this symposium. The conference was organized by the University of Padua in the green setting of the botanical garden. No less than 140 researchers from more than 21 countries attended these three intense days of seminars and discussions. For logistical reasons, the number of places had been limited by the organizers, which forced us to decline a number of registrations. This tends to prove, if there was any need, that the community of researchers in xylem physiology and plant hydraulic functioning is booming, and that these meetings are becoming an essential biennial event for our discipline. For this edition, no less than a hundred communications were presented, including about fifty plenary talks. It is difficult to summarize all the richness of this work in a few lines, so we propose here to highlight some of the most salient points.

Reverse conductivity for water transport and related anatomy in fine roots of six temperate tree species – a potential limitation for hydraulic redistribution

Hydraulic redistribution (HR), the passive reallocation of water along plant structures following a water potential gradient, is an important mechanism for plant survival under drought. For example, trees with deeper roots reallocate water from deeper moist to shallower, drier soil layers sustaining their upper fine root system. The relevance of HR for temperate forest ecosystems is hardly investigated. Both environmental and tree internal factors limiting the capacity for HR, such as low water potential gradients or root anatomy, respectively, are not well understood. Here we investigate fine root anatomy and related capacity for reverse flow of water of six temperate tree species, i.e. Acer pseudoplatanus, Castanea sativa, Fagus sylvatica, Picea abies, Pseudotsuga menziesii and Quercus robur both in forward and reverse flow direction. Additionally, anatomy of primary and secondary roots was analyzed, to test the hypotheses that root anatomy is similar in primary and secondary roots (H1) and conductivity for forward and reverse flow of water in fine roots is identical (H2). In contrast to the two gymnosperm species, most anatomical parameters, e.g. hydraulic conduit diameter and conduit density, were distinctly different between primary and secondary roots in the angiosperms. Therefore, H1 was rejected for angiosperm trees. The reverse flow of water in fine roots was reduced by approx. 40 % compared to the forward flow in angiosperms, while there was no difference in the gymnosperms. Thus, H2 was rejected for angiosperms. This reduction may be caused by vessel structure (e.g. tapering or secondary thickening elements), or perforation plate and pit architecture (e.g. width of aperture opening). Because of the reduced conductivity of reverse water flow, the ability of angiosperm trees to redistribute water along their root system might be lower than expected.

Quantifying seasonal and diurnal variation of stomatal behavior in a hydraulic-based stomatal optimization model

Plant responses to drought occur across many time-scales, with stomatal closure typically considered to be a critical short-term response. Recent theories of optimal stomatal conductance linked to plant hydraulic transport have shown promise, but it is not known if stomata update their hydraulic “shadow price” of water use (marginal increase in carbon cost with a marginal drop in water potential) over days, seasons, or in response to recent drought. Here, I estimate the hydraulic shadow price in five species – two semi-arid gymnosperms, one temperate and two tropical angiosperms – at daily timescales and in wet and dry periods. I tested whether the shadow prices varies predictably as a function of current and/or lagged drought conditions. Diurnal estimates of the hydraulic shadow price estimated from observed stomatal conductance, while variable, did not vary predictably with environmental variables. Seasonal variation in shadow price was observed in the gymnosperm species, but not the angiosperm species, and did not meaningfully influence prediction accuracy of stomatal conductance. The lack of systematic variation in shadow price and high predictive ability of stomatal conductance when using a single set of parameters further emphasizes the potential of hydraulic-based stomatal optimization theories.

Aerial root hydraulic conductivity increases with plant size for the aroid vine Rhodospatha oblongata (Araceae)

Rhodospatha oblongata (Araceae) is an aroid vine which reaches maturity at trees canopies. The beginning of R. oblongata’s ascension towards the canopy occurs when one of the branches reaches the stem of a host, being able to reach eight to ten meters in height. Throughout this ascendant path R. oblongata develops two types of aerial roots: the anchor roots, which is shorter and adhered to the host, never reaching the soil; and the feeder roots, which is long, also adheres to the host but connects the vine to the forest soil. Both roots are here compared in morpho-physiological aspects related to the efficiency of axial hydraulic conductivity. Two hypotheses are tested: i) both roots present distinct xylem hydraulic conductivity; ii) hydraulic conductivity of both roots vary with plant size. The characterization of the roots was based on crescent R. oblongata individuals divided in five size classes. Thirty specimens of each anchor and feeder roots were analyzed along plant size increase. Both roots gradually increase in number and external diameter while the R. oblongata vertically ascends to reach plant canopies. The stele of both roots increase in diameter, in order to accommodate xylem vessels that became larger. The increase in these morpho-anatomical parameters has a positive influence on the xylem hydraulic conductivity, that also increases along the ascendant way of R. oblongata. Comparative measurements show that in general anchor roots present smaller morpho-anatomical structures and lower hydraulic conductivity in comparison to feeder roots. Xylem diameter distribution is unimodal for anchor roots, but bimodal for feeder ones. While all feeder roots present a great concentration of vessels around 60 mm of diameter, the second peak occurs at xylem diameter values that increase with plant size. These modifications optimize the root water transport while the vegetative body of R. oblongata increases in size, connecting its leaves at canopies to the soil water with elevated hydraulic efficiency

"Listen to the Trees!" A tribute to the father of modern cavitation research, Professor John Milburn, on the 20th anniversary of his untimely death

Over the last 35 years, the study of cavitation in plants has become an accepted and important component of the water stress studies performed by plant physiologists. Although the existence of cavitation had been known since Berthelot’s (1850) pioneering work on the tensile strength of water in glass tubes, the tensions at which it occurred in such systems were far more negative than were considered likely to occur in plants. It is to the late Professor John Milburn’s sharp observational powers, lateral thinking and problem-solving approach — illustrated by his pioneering detection of cavitation in plants — that we owe today’s field of cavitation research. John Milburn was constantly thinking of new ways to approach and solve plant physiological problems. In 1966, Milburn and Johnson published their seminal work on the occurrence of cavitation in plants, using data collected via a record player needle and an amplifier. After the invention of the Scholander pressure chamber (Scholander et al. 1965), it became possible to measure easily the xylem pressures at which plants cavitated. Milburn and McLaughlin (1974) found that such pressures were within the physiological ranges that plants experienced and so the phenomenon of cavitation in plants under stress became a fruitful field of research. Professor John Milburn was tragically killed in a flying accident in 1997. The premature loss of such a great scientist, aged only 60, was felt keenly in the Botany Department of the University of New England, Armidale, Australia, where he had been a Professor for 16 years, and also around the world. This article is a tribute to Professor John Milburn, encompassing several of his key discoveries (a rare recording of the sound of cavitation occurring in the audible range is included in this tribute), as well as some of the many aspects of the man. It is timely, on the 20th anniversary of his death, to remind ourselves that today’s experimental water stress research would be the poorer without John Milburn’s pioneering work.

Testing for ion-mediated enhancement of the hydraulic conductance of the leaf xylem in diverse angiosperms

Replacing ultra-pure water solution with ion solution closer to the composition of natural xylem sap increases stem hydraulic conductance by up to 58%, likely due to changes in electroviscosity in the pit membrane pores. This effect has been proposed to contribute to the control of plant hydraulic and stomatal conductance and potentially to influence on carbon balance during dehydration. However, this effect has never been directly tested for leaf xylem, which constitutes a major bottleneck in the whole plant. We tested for an ion-mediated increase in the hydraulic conductance of the leaf xylem (Kx) for seven species diverse in phylogeny and drought tolerance. Across species, no significant changes in Kx were observed between 0 and 15 mM KCl. We further tested for an effect of ion solution during measurements of Kx vulnerability to dehydration in Quercus agrifolia and found no significant impact. These results for leaf xylem contrast with the often strong ion effect reported for stems, and we suggest several hypotheses to account for the difference, relating to the structure of xylem conduits across vein orders, and the ultrastructure of leaf xylem pores. A negligible ion response in leaves would weaken xylem sap ion-mediated control of plant hydraulic conductance, facilitating modeling of whole plant hydraulic behavior and its influence on productivity.

Mexican conifers differ in their capacity to face climate change

The recent massive dieback of forest trees due to drought stress makes assessment of the variability of physiological traits that might be critical for predicting forest response and adaptation to climate change even more urgent. We investigated xylem vulnerability to cavitation and xylem specific hydraulic conductivity in seven species of three principal conifer genera (Juniperus monticola, Juniperus deppeana, Juniperus flaccida, Pinus pseudostrobus, Pinus leiophylla, Pinus devoniana, and the endangered Picea chihuahuana) of the Mexican mountains in order to identify the species most vulnerable to future warmer and drier climates. Hydraulic traits were examined using the in situ flow centrifuge technique (Cavitron) on branches collected from adult trees of natural populations and seedlings growing in a common garden. We found evidence of significant differences in xylem safety between genera (P50: pressure inducing 50% loss of hydraulic conductance): the three juniper species exhibited low P50 values (ranging from -9.9 to -10.4 MPa), relative to the much more vulnerable pine and spruce species (P50 ranging between - 2.9 to - 3.3 MPa). Our findings also revealed no variation in P50 between adult trees assessed in the field and seedlings growing in a common garden. We therefore propose that if, as projected, climate change makes their natural habitats much warmer and drier, populations of Mexican pines and the studied spruce will be likely to decline severely as a result of drought-stress induced cavitation, while the juniper species will survive.

fitplc - an R package to fit hydraulic vulnerability curves

We describe a toolkit to fit hydraulic vulnerability curves, such as the percent loss of xylem hydraulic conductivity ('PLC curves') as a function of the water potential. The toolkit is implemented as an R package, and is thus free to use and open source. The package fits the Weibull or sigmoidal function to measurements of PLC, conductance or conductivity, at corresponding leaf or stem water potentials. From the fitted curve, estimates of Px (the water potential at which x% conductivity is lost, e.g. the P50), and slope parameter (Sx) are provided together with confidence intervals (CI) around the fitted line. The CIs are estimated with the bootstrap. We also demonstrate the advantages of using mixed-effects models in situations where multiple individuals are measured on a species, as compared to the more traditional approach of fitting curves separately and averaging the parameters. We demonstrate the use of the new package with example data on seven species measured with two different techniques.

Wood density as a proxy for vulnerability to cavitation: Size matters

In this study, vulnerability to cavitation, P50 (i.e. the water potential causing 50 % loss of hydraulic conductivity), of Norway spruce trunkwood at different cambial age was related to wood density. Wood density was calculated from mass in the oven dry state related either to volume at the oven-dry state (dry wood density) or to volume at full saturation (basic wood density). Dry wood density and basic wood density were strongly linearly related (r² = 0.99); there was however a shift from the 1:1 reference line with increasing dry wood density. Dry wood density as well as basic wood density had similar high predictive qualities for P50 (r² = 0.85). The quadratic regression lines took however a quite different course below -4 MPa because volume shrinkage increased with increasing dry wood density. For Norway spruce sapwood with high dry wood density, mixing up different wood density traits would thus result in a predicted overestimation or underestimation of vulnerability to cavitation. Gravimetrically wood density measurements at different moisture contents (starting at full saturation) are easily to achieve on standard size specimens and the conversion curves obtained will be of high value for future ecological studies on other species and across species.