One adaptation of plants to cope with drought or frost stress is to develop wood that is able to withstand the formation and distribution of air bubbles (emboli) in its water conducting xylem cells under negative pressure. The ultrastructure of interconduit pits strongly affects drought-induced embolism resistance, but also mechanical properties of the xylem are involved. The first experimental evidence for a lower embolism resistance in stems of herbaceous plants compared to stems of their secondarily woody descendants further supports this mechanical-functional trade-off. An integrative approach combining (ultra)structural observations of the xylem, safety-efficiency aspects of the hydraulic pipeline, and xylem–phloem interactions will shed more light on the multiple adaptive strategies of embolism resistance in plants

Aims Various correlations have been identified between anatomical features of bordered pits in angiosperm xylem and vulnerability to cavitation, suggesting that the mechanical behaviour of the pits may play a role. Theoretical modelling of the membrane behaviour has been undertaken, but it requires input of parameters at the nanoscale level. However, to date, no experimental data have indicated clearly that pit membranes experience strain at high levels during cavitation events. † Methods Transmission electron microscopy (TEM) was used in order to quantify the pit micromorphology of four tree species that show contrasting differences in vulnerability to cavitation, namely Sorbus aria, Carpinus betulus, Fagus sylvatica and Populus tremula. This allowed anatomical characters to be included in a mechanical model that was based on the Kirchhoff -Love thin plate theory. A mechanistic model was developed that included the geometric features of the pits that could be measured, with the purpose of evaluating the pit membrane strain that results from a pressure difference being applied across the membrane. This approach allowed an assessment to be made of the impact of the geometry of a pit on its mechanical behaviour, and provided an estimate of the impact on airseeding resistance. † Key Results The TEM observations showed evidence of residual strains on the pit membranes, thus demonstrating that this membrane may experience a large degree of strain during cavitation. The mechanical modelling revealed the interspecific variability of the strains experienced by the pit membrane, which varied according to the pit geometry and the pressure experienced. The modelling output combined with the TEM observations suggests that cavitation occurs after the pit membrane has been deflected against the pit border. Interspecific variability of the strains experienced was correlated with vulnerability to cavitation. Assuming that air-seeding occurs at a given pit membrane strain, the pressure predicted by the model to achieve this mechanical state corresponds to experimental values of cavitation sensitivity (P 50 ). † Conclusions The results provide a functional understanding of the importance of pit geometry and pit membrane structure in air-seeding, and thus in vulnerability to cavitation.

Woody perennials' reliance on nonstructural carbohydrates (NSC) reserves for the resumption of spring growth necessitates an accumulation of NSC prior to dormancy. It is assumed that during dormancy temperature-regulated biological activities gauge the progression of winter and affect the metabolic rates and physiology of NSC reserves. Thus, changes in temperature signal the arrival of spring and determine the amount of reserves available for growth resumption. As woody perennials are dependent on dispersed storage of NSC during spring, they need an integrated remobilization and redistribution for synchronous and effective development of photosynthetic and reproductive organs. However, it is not known how storage compartments interact at the whole plant level, when NSC reserves are mobilized, or how local and distal storage compartments influence the biology of spring growth resumption. The goal of this mini-review is to shift the focus of winter biology from bud-centric to the whole plant. We discuss winter NSC management in the context of climate change with a special emphasis on how projected mild winters may affect the carbon budget, transport, and allocation during winter. We look at three aspects of NSC regulation underlying dormancy (I) the molecular regulation of dormancy (II) temperature dependent winter NSC metabolism, and (III) spring NSC remobilization and redistribution processes.

Nonstructural carbohydrate (NSC) storage plays a critical role in tree function and survival, but understanding and predicting local NSC storage dynamics is challenging because NSC storage pools are dispersed throughout the complex architecture of trees and continuously exchange carbon between source and sink organs at different time scales. To address these knowledge gaps, characterization and understanding of NSC diel variation are necessary. Here, we analyzed diurnal NSC dynamics in the overall architecture of almond (Prunus dulcis) trees. We also analyzed the allocation of newly assimilated carbon using isotopic labeling. We show that both components of NSC (i.e. soluble carbohydrates and starch) are highly dynamic at the diurnal time scale and that these trends are influenced by tissue type, age, and/or position within the canopy. In leaves, starch reserves can be depleted completely during the night, while woody tissue starch levels may vary by more than 50% over a daily cycle. Recently assimilated carbon showed a dispersed downward allocation across the entire tree. NSC diurnal fluctuations within the tree's structure in combination with dispersed carbon allocation patterns provide evidence for the presence of vertical mixing and suggest that the xylem acts as a secondary NSC redistribution pathway.

During spring, bud growth relies on long-distance transport of remotely stored carbohydrates. A new hypothesis suggests this transport is achieved by the interplay of xylem and phloem. During the spring, carbohydrate demand of developing buds often exceeds locally available storage, thus requiring the translocation of sugars from distant locations like limbs, stems and roots. Both the phloem and xylem have the capacity for such long-distance transport, but their functional contribution is unclear. To address this ambiguity, the spatial and temporal dynamics of carbohydrate availability in extension shoots of Juglans regia L. were analyzed. A significant loss of extension shoot carbohydrates in remote locations was observed while carbohydrate availability near the buds remained unaffected. This pattern of depletion of carbohydrate reserves supports the notion of long-distance translocation. Girdling and dye perfusion experiments were performed to assess the role of phloem and xylem in the transport of carbohydrate and water towards the buds. Girdling caused a decrease in non-structural carbohydrate concentration above the point of girdling and an unexpected concurrent increase in water content associated with impeded xylem transport. Based on experimental observations and modeling, we propose a novel mechanism for maintenance of spring carbohydrate translocation in trees where xylem transports carbohydrates and this transport is maintained with the recirculation of water by phloem Münch flow. Phloem Münch flow acts as a pump for generating water flux in xylem and allows for transport and mobilization of sugars from distal locations prior to leaves photosynthetic independence and in the absence of transpiration.

While Arabidopsis thaliana has been proposed as a model species for wood development, the potential of this tiny herb for studying xylem hydraulics remains unexplored and anticipated by scepticism. Inflorescence stems of A. thaliana were used to measure hydraulic conductivity and cavitation resistance, whereas light and electron microscopy allowed observations of vessels. In wild-type plants, measured and theoretical conductivity showed a significant correlation (R 2 = 0.80, P < 0.01). Moreover, scaling of vessel dimensions and intervessel pit structure of A. thaliana were consistent with structure–function relationships of woody plants. The reliability and resolution of the hydraulic methods applied to measure vulnerability to cavitation were addressed by comparing plants grown under different photoperiods or different mutant lines. Sigmoid vulnerability curves of A. thaliana indicated a pressure corresponding to 50% loss of hydraulic conductance (P 50) between –3 and –2.5MPa for short-day and long-day plants, respectively. Polygalacturonase mutants showed a higher P 50 value (–2.25MPa), suggesting a role for pectins in vulnerability to cavitation. The application of A. thaliana as a model species for xylem hydraulics provides exciting possibilities for (1) exploring the molecular basis of xylem anatomical features and (2) understanding genetic mechanisms behind xylem functional traits such as cavitation resistance. Compared to perennial woody species, however, the lesser amount of xylem in A. thaliana has its limitations.

Wood provides water transport and mechanical support of trees. Sap is transported under negative pressure in plant xylem conduits, which can be subject to embolism during severe drought. Typically, denser woods show thicker cell walls and stronger mechanical properties. Ten transgenic poplar lines modified for expression of genes involved in lignin metabolism were produced from the female clone 717-1B4 of Populus tremula × Populus alba to test the hypothesis of a possible trade-off between hydraulic and mechanical functions. Poplar lines underexpressed genes encoding for cinnamoyl alcohol dehydrogenase (CAD), cinnamoyl CoA reductase (CCR) and caffeic acid 3-O- methyltransferase (COMT), while new poplar lines underexpressed the CAD genes or overexpressed the MYB308 gene, encoding for a transcription factor repressing the phenylpropanoid metabolism. To maximize the contrast between line behaviors, these plants were grown under two different water regimes, and the impact on their hydraulic traits and xylem properties was analyzed to test for a link between water condition and mechanical and hydraulic properties. Our results show that the resistance to xylem cavitation was lower for the transgenic lines than for the control line 717-1b4 while they show neither a positive nor a negative tendency for the longitudinal Young's modulus between the transgenic lines and the control line. ASOMT10b and ASOMT2b, which possessed a down-regulated expression for all the genes, showed a lower value of the resistance to implosion index (t/b)2. No difference for xylem hydraulic conductivity between the lines was found. The changes in lignin metabolism in these transgenic lines did not affect the water transport, despite the change in the lignin content. Our data on the transgenic poplar lines do not therefore support the mechanical vs. hydraulic trade-off hypothesis and we point out that angiosperm trees have numerous ways to acclimate their internal structure in order to adjust their mechanical properties without hydraulic coupling. Moreover, we observed an acclimation to water stress for P50 but not for the Young's modulus. MYB308-25.1 showed better mechanical properties and vulnerability to cavitation than the control line 717-1b4. Finally, we present evidence that lignins are involved in the vulnerability to cavitation, probably through modifications of pit structure and behavior

Trees experience two distinct environments: thermally-variable air and thermally-buffered soil. This generates intra-tree temperature gradients, which can affect carbon metabolism and water transport. In this study, we investigated whether carbohydrate allocation within trees is assisted by temperature gradients. We studied pistachio (Pistacia integerrima) to determine: (1) temperature-induced variation in xylem sugar concentration in excised branches; (2) changes in carbon allocation in young trees under simulated spring and fall conditions; and (3) seasonal variability of starch levels in mature orchard trees under field conditions. We found that warm branches had less sugar in perfused sap than cold branches due to increasing parenchyma storage. Simulated spring conditions promoted allocation of carbohydrates from cold roots to warm canopy and explained why starch levels surged in canopies of orchard trees during early spring. This driving force of sugar transport is interrupted in fall when canopies are colder than roots and carbohydrate redistribution is compartmentalized. On the basis of these findings, we propose a new mechanistic model of temperature-assisted carbohydrate allocation that links environmental cues and tree phenology. This data-enabled model provides insights into thermal “fine-tuning” of carbohydrate metabolism and a warning that the physiological performance of trees might be impaired by climatic changes.