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Lignified cell walls appear to be impermeable to solutes with molecular weight >300, so ray cell to xylem vessel transport is restricted to unlignified pit membranes., The functional role of pits between living and dead cells has been inferred from anatomical studies but amassing physiological evidence has been challenging. Centrifugation methods were used to strip water from xylem conduits, permitting a more quantitative gravimetric determination of the water and solid contents of cell walls than is possible by more traditional methods. Quantitative anatomical evidence was used to evaluate the water volume in xylem conduits and the water content of living cells. Quantitative perfusion of stems with polyethylene glycol of different molecular weight was used to determine the solute-free space. We measured the portioning of water and solute-free space among anatomical components in stems and demonstrated that lignin impeded the free movement of solutes with molecular weight >300. Hence, movement of large solutes from living cells to xylem conduits is necessarily confined to pit structures that permit transmembrane solute transport via primary walls without lignin. The functional role of pits was additionally indicated by combining data in this paper with previous studies of unusual osmotic relationships in woody species that lack pits between dead wood fibers and vessels. The absence of pits, combined with the evidence of exclusion of solutes of molecular weight >300, explains the unexpected osmotic properties.

After drought-induced embolism and repair, tree xylem may be weakened against future drought events (cavitation fatigue). As there are few data on cavitation fatigue in conifers available, we quantified vulnerability curves (VCs) after embolism/repair cycles on eight European conifer species. We induced 50% and 100% loss of conductivity (LC) with a cavitron, and analyzed VCs. Embolism repair was obtained by vacuum infiltration. All species demonstrated complete embolism repair and a lack of any cavitation fatigue after 50% LC . After 100% LC, European larch (Larix decidua), stone pine (Pinus cembra), Norway spruce (Picea abies), and silver fir (Abies alba) remained unaffected, while mountain pine (Pinus mugo), yew (Taxus baccata), and common juniper (Juniperus communis) exhibited 0.4–0.9 MPa higher vulnerability to embolism. A small cavitation fatigue observed in Scots pine (Pinus sylvestris) was probably biased by incomplete embolism repair, as indicated by a correlation of vulnerability shifts and conductivity restoration. Our data demonstrate that cavitation fatigue in conifers is species-specific and depends on the intensity of preceding LC. The lack of fatigue effects after moderate LC, and relevant effects in only three species after high LC, indicate that conifers are relatively resistant against cavitation fatigue. This is remarkable considering the complex and delicate conifer pit architecture and may be important considering climate change projections.

Novel features related to turgor loss adjustment and higher ability to modulate leaf-construction costs could improve A. altissima fitness in its invaded range. Phenotypic plasticity has been recently proposed to play an important role in invasion process. More precisely, phenotypic plasticity of alien invasive species could be higher in their exotic rather than native habitat, probably because of the release from biotic or abiotic constraints. In this paper, several plant functional traits were analysed on Ailanthus altissima (Mill.) Swingle, a highly invasive species in Europe, aiming at providing a comparison among key functional traits measured on individuals growing in their native habitat and to test if values of functional traits and their related plasticity are higher in exotic habitats. Our analysis pinpointed that variability of functional traits in the native habitat was mainly driven by different irradiance, temperature and evaporative demand of the studied sites, in accordance to the traits trade-offs in the Leaf Economic Spectrum. Physiological traits related to drought tolerance were different between native and exotic habitats. In the native one, A. altissima relied on osmoregulation processes to adjust leaf turgor loss point (Ψtlp), as the osmotic potential at full turgor (π0) were lower in drier sites. In the exotic habitat, individuals in drier sites had similar π0 but lower wall elasticity (e), suggesting that leaves had thinner cell walls. Moreover, plasticity in specific leaf area and e were higher in the exotic habitat. The novel features related to Ψtlp adjustment and the higher plasticity in traits related to leaf-construction costs could increase A. altissima fitness in response to different environmental conditions in its invasive range.

International audience; An improved way of calculating hydraulic conductance (K) in a Cochard cavitron is described. Usually K is determined by measuring how fast water levels equilibrate between two reservoirs while a stem spins in a centrifuge. A regression of log meniscus position versus time was used to calculate K and this regression method was compared to the old technique that takes the average of discrete values. Results of a hybrid Populus 84K shows that the relative error of the new approach is significantly lower than the old technique by 4~5 times. The new computational method results in a relative error less than 0.5% or 0.3% from 8 or 12 points of measurement, respectively. The improved precision of K measurement also requires accurate assessment of stem temperature because temperature changes K by 2.4% o C -1 . A computational algorithm for estimating stem temperature stability in a cavitron rotor was derived. This algorithm provides information on how long it takes stem temperature to be known to within an error of ±0.1 o C.

The cause of morphological plasticity of leaves within the crowns of tall trees still debated. Whether it is driven by irradiance or hydraulic constraints is inconclusive. In a previous study, we hypothesized that water stress caused between-site and within-tree morphological variability in mature Robinia trees. To test this hypothesis, we designed an experiment to analyze the effect of long-term water stress on leaf growth of Robinia seedlings in a controlled environment. Two treatments were performed: well-watered (midday water potential, Ψ w = −0.45 MPa) and water-stressed (Ψ w = −1.0 Mpa), which resulted in significant differences in physiology, relative growth rate, and the temporal progress of leaf growth. Variation of leaf cell sizes among treatments was comparable to the variability previously observed in the field. However, values of leaf density and leaf mass per unit area tended to be lower in our controlled experiments than in the field, which may reflect differences between mature leaves of juvenile and adult trees. Our tentative conclusion is that leaf water stress may be the primary factor controlling morphological changes observed in the field, but further experiments are needed to document the relative importance of irradiance.

Introduction Root pressure is a potentially important mechanism for dissolving embolisms in previously cavitated vessels, but the occurrence of root pressure is not a universal property in plants.

Molecular and physiological studies in walnut (Juglans regia) are combined to establish the putative role of leaf plasma membrane aquaporins in the response of leaf hydraulic conductance (K leaf) to irradiance. The effects of light and temperature on K leaf are described. Under dark conditions, K leaf was low, but increased by 400% upon exposure to light. In contrast to dark conditions, K leaf values of light-exposed leaves responded to temperature and 0.1 mm cycloheximide treatments. Furthermore, K leaf was not related to stomatal aperture. Data of real-time reverse transcription-polymerase chain reaction showed that K leaf dynamics were tightly correlated with the transcript abundance of two walnut aquaporins (JrPIP2,1 and JrPIP2,2). Low K leaf in the dark was associated with down-regulation, whereas high K leaf in the light was associated with up-regulation of JrPIP2. Light responses of K leaf and aquaporin transcripts were reversible and inhibited by cycloheximide, indicating the importance of de novo protein biosynthesis in this process. Our results indicate that walnut leaves can rapidly change their hydraulic conductance and suggest that these changes can be explained by regulation of plasma membrane aquaporins. Model simulation suggests that variable leaf hydraulic conductance in walnut might enhance leaf gas exchanges while buffering leaf water status in response to ambient light fluctuations.

International audience; Whole plant hydraulic conductance was measured for two co-occuring neotropical rainforest understory shrub species of the genus Piper, which differ in their habitat preference with respect to soil moisture: P. trigonum is restricted to wet habitats, whereas P. cordulatum occurs in drier sites. Plants were grown under four treatments (two light conditions crossed with two watering regimes), representing the extremes of conditions in their natural habitat. The data showed significantly higher total hydraulic conductance and leaf specific conducance in the drought-avoiding species, P. trigonum, than in the drought-tolerant species P. cordulatum. The measured parameters also differed between growth treatments. The species differences in hydraulic conductance could be interpreted as adaptive in the respective species' habitat, and might thus be important in determining habitat preference and distribution of the two species.; Conductance hydraulique de deux arbustes néotropicaux du sous étage ayant des habitats préférentiels. La conductance hydraulique de la plante entière a été mesurée pour deux espèces arbustives du genre Piper du sous étage d'une forêt néotropicale. Ces espèces différent par leur adaptation à l'humidité du sol : P. trigonum est localisé dans les habitats humides, alors que P. cordulatum se rencontre dans les zones sèches. Les plants ont été soumis à quatre traitements (deux conditions d'éclairement combinées avec deux régimes hydriques), représentant les conditions extrêmes de leurs habitats naturels. Les résultats montrent une conductance hydraulique et une conductance spécifique des feuilles significativement plus élevées pour l'espèce évitant la sécheresse, P. trigonum, par rapport à l'espèce tolérant la sécheresse, P. cordulatum. Les paramètres de croissance mesurés diffèrent aussi selon les traitements. Les différences de conductance hydraulique entre espèces peuvent être interprétées comme une adaptation aux conditions respectives de l'habitat, et peuvent être alors un facteur important dans la préférence pour l'habitat et la répartition des deux espèces.

Trees of Juglans regia L. shed leaves when subjected to drought. Before shedding (when leaves are yellow), the petioles have lost 87% of their maximum hydraulic conductivity, but stems have lost only 14% of their conductivity. This is caused by the higher vulnerability of petioles than stems to water-stress induced cavitation. These data are discussed in the context of the plant segmentation hypothesis.

The tempo of rehydration of sunflower (Helianthus animus L.) leaves was measured after dehydration in a pressure bomb down to water potentials of −0.5 to −1.6 MPa. When rehydrated from small water deficits (−0.5 to −0.8 MPa) the plot of log rehydration rate versus time is concave. When rehydration starts from large deficits (−1.2 to −1.6 MPa) the semilog plot has a characteristic shoulder, i.e. a rehydration phase of long half-time is followed by a phase of short half-time. The experimental curves were fitted with parallel and series models of rehydration. In the parallel model two compartments are connected by resistances in parallel with the water source and rehydrate independently. In the series model one compartment is connected with the water source via a resistance and the second compartment is connected in series with the first by another resistance so that water entering the second compartment must pass through the first. Amongst nineteen experiments, ten could be fitted very closely by both the parallel and series models and nine could not be fitted by either model.