Biology subjects

Kaack, L.

Publications and source records attributed to Kaack, L..

9 recordsLinked to original sources

bioRxiv2026

Beyond air-seeding: Dynamic, multiphase interactionsreveal a two-step mechanism of embolism propagation inangiosperm xylem

BackgroundThe mechanism underlying drought-induced embolism in angiosperm xylem has been attributed to air-seeding. This concept describes the bulk flow of gas from embolised to neighbouring conduits through the penetration of gas-liquid menisci across pores in interconduit pit membranes. While there is compelling evidence for the spatial propagation of embolism, air-seeding rests on various simplifying assumptions. Among others, air-seeding presumes that xylem sap has physical properties comparable to pure water, that pit membranes can be approximated as structures with simple pores, and that embolism occurs whenever a gas-liquid interface crosses a pit membrane. ScopeRecent experimental and theoretical work demonstrates that the biophysical conditions and processes governing gas-liquid interactions at interconduit pit membranes are fundamentally more dynamic and complex than assumed by air-seeding. These phenomena include: (1) gas movement through constriction pore networks, (2) insoluble, polar lipids at conduit surfaces and interfaces, (3) dynamic surface tension of xylem sap that depends on the local packing density of interfacial lipids, (4) bubble snap-off dynamics within pit membranes, (5) surfactant-stabilized nanobubbles in sap that is oversaturated with dissolved gas, and (6) electrostatic interactions between charged interfaces. Importantly, embolism propagation involves bubble generation and embolism formation as distinct, temporarily and spatially separated processes. Embolism formation occurs when nanobubbles become unstable, whereas nanobubbles below critical stability thresholds can remain stable in sap-filled conduits. ConclusionsTogether, these findings reconfirm that pit membranes function as safety valves enabling water transport according to the cohesion-tension theory, and provide mechanistic insights into embolism propagation. They address the question why plants do not suffer constant embolism formation despite negative xylem pressures. We conclude that a revised framework explicitly accounting for the 3D structure of pit membranes, and multiphase, dynamic processes operating within them are required to explain the biophysics underlying water transport and embolism resistance in angiosperm xylem.

plant biology↗

bioRxiv2026

Interconduit pit membranes of temperate angiosperms undergo changes in pit membrane thickness and electron density within the final growth ring

O_LIPit membranes play a crucial role in water transport between neighbouring xylem conduits by providing hydraulic safety and flow resistance. While effects of pit membranes thickness on embolism resistance and temporal changes in the ultrastructure of pit membranes have been documented between sapwood and heartwood, these changes are largely unknown within conduits of the current-year. C_LIO_LIWe studied interconduit pit membranes of branches of eight angiosperm species by sampling wood from a temperate forest over four consecutive seasons, focusing on the latest growth ring. We quantified the pit membrane thickness and greyscale intensity (as a proxy for electron density) using transmission electron microscopy and image analysis. C_LIO_LIOur observations showed considerable interspecific variation in changes to pit membrane thickness and electron density. Several species exhibited the thinnest pit membranes and highest electron density at the end of a growing season, while others showed minimal variation over time. Intra-tree variation showed that changes in pit membrane shrinkage and electron density were associated with conduit diameter: pit membranes in wide conduits showed larger modification over time than narrow ones. C_LIO_LIOur results indicate seasonal changes in the structure and chemistry of angiosperm pit membranes, even within the latest growth ring. While their shrinkage might increase resistance to flow, the occurrence of darker pit membranes indicates coating and penetration by polar lipids, affecting the behaviour of gas-liquid interfaces. We speculate that the degree of modification that pit membranes undergo is mechanistically driven by the sap flow rate, and possibly conduit dimensions. C_LI

plant biology↗

bioRxiv2023

Gas diffusion kinetics drive embolism spread in angiosperm xylem: evidence from flow-centrifuge experiments and modelling

Understanding xylem embolism formation is challenging due to dynamic changes and multiphase interactions in conduits. If embolism spread involves gas movement in xylem, we hypothesise that it is affected by time. We measured hydraulic conductivity (Kh) in flow-centrifuge experiments over one hour at a given pressure and temperature for stem samples of three angiosperm species. Temporal changes in Kh at 5, 22, and 35{degrees}C, and at various pressures were compared to modelled gas pressure changes in a recently embolised vessel in the centre of a centrifuge sample. Temporal changes in Kh at 22{degrees}C showed maximum relative increases between 6% and 40%, and maximum decreases between 41% and 61% at low and high centrifugal speed, respectively. Logarithmic changes in Kh were species-specific, and most pronounced during the first 15 minutes. Embolism formation started near the edges of centrifuge samples and gradually increased at the centre. Moreover, measured decreases in Kh strongly correlated with modelled increases in gas concentration in a recently embolised vessel. Although embolism is mostly pressure-driven, our experimental and modelled data indicate that time, conduit characteristics, and temperature are involved due to their role in gas diffusion. Gas diffusion, however, does not cover the entire process of embolism spread.

plant biology↗

bioRxiv2022

Dynamic changes in gas solubility of xylem sap reiterate the enigma of plant water transport under negative pressure

Despite a long research history, we do not fully understand why plants are able to transport xylem sap under negative pressure without constant failure. Microbubble formation via direct gas entry is assumed to cause hydraulic failure, while the concentration of gas dissolved in xylem sap is traditionally supposed to be constant, following Henrys law. Here, the concentration of soluble gas in xylem sap was estimated in vivo using well-watered Citrus plants under varying levels of air temperature and photoperiodic exposure, and compared to modelled data. The gas concentration in xylem sap showed non-equilibrium curves, with a minimum over- or undersaturation of 5% compared to gas solubility based on Henrys law. A similar diurnal pattern was obtained from the gas concentration in the cut-open conduits and discharge tube, and oversolubility was strongly associated with decreasing xylem water potentials during transpiration. Although our model did not explain the daily changes in gas solubility for an anisobaric situation, oversolubility characterises nanoconfined liquids, such as sap inside cell walls. Thus, plants are able to transport sap under negative pressure with relatively high amounts of dissolved gas, providing them with a buffering capacity to prevent hydraulic failure, despite diurnal changes in pressure and temperature.

plant biology↗

bioRxiv2021

Polar lipids are linked to nanoparticles in xylem sap of temperate angiosperm species

Xylem sap of angiosperm species has been found to include low concentrations of polar lipids and nanoparticles, including surfactant-coated nanobubbles. Although the nanoparticles have been suggested to consist of polar lipids, no attempt has been made to determine if nanoparticle and lipid concentrations are related. Here, we examined concentrations of nanoparticles and lipids in xylem sap and contamination control samples of six temperate angiosperm species with a NanoSight device and based on mass spectrometry. We found (1) that the concentration of nanoparticles and lipids were both diluted when an increasing amount of sap was extracted, (2) that their concentrations were significantly correlated in three species, (3) that their concentrations were affected by vessel anatomy, and (4) that concentrations of nanoparticles and lipids were very low in contamination-control samples. Moreover, there was little seasonal difference, no freezing-thawing effect on nanoparticles, and little seasonal variation in lipid composition. These findings indicate that lipids and nanoparticles are related to each other, and largely do not pass interconduit pit membranes. Further research is needed to examine the formation and stability of nanoparticles in xylem sap in relation to lipid composition, and the complicated interactions among the gas, liquid, and solid phases in xylem conduits.

plant biology↗

bioRxiv2021

A Unit Pipe Pneumatic model to simulate gas kinetics during measurements of embolism in excised angiosperm xylem

O_LIThe Pneumatic method has been introduced to quantify embolism resistance in plant xylem of various organs. Despite striking similarity in vulnerability curves between the Pneumatic and hydraulic methods, a modeling approach is highly needed to demonstrate that xylem embolism resistance can be accurately quantified based on gas diffusion kinetics. C_LIO_LIA Unit Pipe Pneumatic (UPPn) model was developed to estimate gas diffusion from intact conduits, which were axially interconnected by interconduit pit membranes. The physical laws used included Ficks law for diffusion, Henrys law for gas concentration partitioning between liquid and gas phases at equilibrium, and the ideal gas law. C_LIO_LIThe UPPn model showed that 91% of the extracted gas came from the first two series of embolized, intact conduits, and only 9% from the aqueous phase after 15 s of simulation. Embolism resistance measured with a Pneumatic apparatus was systematically overestimated by 2 to 17%, corresponding to a typical measuring error of 0.11 MPa for P50 (the water potential equivalent to 50% of the maximum amount of gas extracted). C_LIO_LIBecause results from the UPPn model are supported by experimental evidence, there is a good theoretical and experimental basis for applying the pneumatic method to research on embolism resistance of angiosperms. C_LI

plant biology↗

bioRxiv2020

Pore constrictions in intervessel pit membranes reduce the risk of embolism spreading in angiosperm xylem

O_LIEmbolism spreading in angiosperm xylem occurs via mesoporous pit membranes between vessels. Here, we investigate how the size of pore constrictions in pit membranes is related to pit membrane thickness and embolism resistance. C_LIO_LIIn three models, pit membranes are modelled as multiple layers to investigate how pit membrane thickness and the number of intervessel pits per vessel determine pore constriction sizes, the probability of encountering large pores, and air-seeding. These estimations were complemented by measurements of pit membrane thickness, embolism resistance, and number of intervessel pits per vessel (n = 31, 31, and 20 species, respectively). C_LIO_LIConstriction sizes in pores decreased with increasing pit membrane thickness, which agreed with the measured relationship between pit membrane thickness and embolism resistance. The number of pits per vessel affected constriction size and embolism resistance much less than pit membrane thickness. A strong relationship between estimated air-seeding pressures and measured embolism resistance was observed. C_LIO_LIPore constrictions provide a mechanistic explanation why pit membrane thickness determines embolism resistance, and suggest that hydraulic safety can be uncoupled from hydraulic efficiency. Although embolism spreading remains puzzling and encompasses more than pore constriction sizes, angiosperms are unlikely to have leaky pit membranes, which enables tensile transport of water. C_LI

plant biology↗

bioRxiv2020

A semi-automated method for measuring xylem vessel length distribution

Knowledge about the length of xylem vessels is essential to understand water transport in plants because these multicellular units show a 100-fold variation, from less than a centimeter to many meters. However, the available methods to estimate vessel length distribution (VLD) are excessively time consuming and do not allow large and in-depth surveys. Here, we describe a semi-automated method to measure VLD using an automated Pneumatron device. Gas conductivity of a xylem tissue with a certain length is estimated in a straightforward and precise way with the Pneumatron in a way theoretically similar to the air-injection method. The method presented enables fast and easy measurements using multiple devices simultaneously (>50 samples day-1), which is a significant advantage. Here, the apparatus is described in detail as well as how measurements are taken. We also present the software and an R-script for data analysis. The method described represents an important contribution to studies on plant hydraulic architecture and can improve our understanding about the role of VLD in plant performance under varying water availability.

plant biology↗

bioRxiv2019

Lipids in xylem sap of woody plants across the angiosperm phylogeny

Lipids have been observed attached to lumen-facing surfaces of mature xylem conduits of several plant species, but there has been little research on their functions or effects on water transport, and only one lipidomic study of the xylem apoplast. Therefore, we conducted lipidomic analyses of xylem sap from woody stems of seven plants representing six major angiosperm clades, including basal magnoliids, monocots, and eudicots, to characterize and quantify phospholipids, galactolipids, and sulfolipids in sap using mass spectrometry. Locations of lipids in vessels of Laurus nobilis were imaged using TEM and confocal microscopy. Xylem sap contained the galactolipids di- and mono-galactosyldiacylglycerol (DGDG and MGDG), as well as all common plant phospholipids, but only traces of sulfolipids, with total lipid concentrations in extracted sap ranging from 0.18 to 0.63 nmol / mL across all seven species. Contamination of extracted sap from lipids in cut living cells was found to be negligible. Lipid composition of sap was compared to wood in two species and was largely similar, suggesting that sap lipids, including galactolipids, originate from cell content of living vessels. Seasonal changes in lipid composition of sap were observed for one species. Lipid layers coated all lumen-facing vessel surfaces of Laurus nobilis, and lipids were highly concentrated in inter-vessel pits. The findings suggest that apoplastic, amphiphilic xylem lipids are a universal feature of angiosperms. The findings require a reinterpretation of the cohesion-tension theory of water transport to account for the effects of apoplastic lipids on dynamic surface tension and hydraulic conductance in xylem.

plant biology↗