Trunk Xylem Research Articles

Simultaneous in situ monitoring of belowground, trunk and relative canopy hydraulic conductance of grapevine demonstrates a soil texture-specific transpiration control

Assessing the interrelationships between belowground, trunk and canopy hydraulics, under various edaphic conditions, is essential to enhance understanding of how grapevine (Vitis vinifera) responds to drought. This work aimed to quantify and compare in situ belowground and trunk hydraulic conductance of the soil-grapevine system to evaluate their coordination with the transpiration control during drought. We simultaneously monitored soil water potential, trunk xylem water potential, canopy xylem water potential, actual transpiration and atmospheric demand to quantify the evolution of belowground, trunk and relative canopy hydraulic conductance. By comparing stomatal regulation at the canopy scale and soil-grapevine system conductance, we assessed their coordination. Transpiration control was triggered by a decrease of belowground hydraulic conductance, and not by xylem cavitation in the trunk. Although the relation between canopy conductance and soil water potential is soil texture specific, where stomata at the canopy scale started to close at less negative soil water potential in sand than in loam, the onset of stomatal closure at the canopy level was at equivalent belowground hydraulic conductance, independently of the soil texture. These findings prove that in situ grapevines coordinate short-term hydraulic mechanisms (e.g., regulation of canopy hydraulic conductance) and longer-term growth (e.g., root:shoot ratio). These belowground and aboveground adjustments are, therefore, soil-texture specific.

Fusarium and Neocosmospora Species Associated with the Decline of Metasequoia glyptostroboides in China.

During surveys conducted in 2020, severe symptoms associated with death and decline were observed on >30-year-old Metasequoia glyptostroboides (Chinese redwood) trees in the shelter-forests along Yangtze River in Jingzhou city, Hubei province, China. A previous study showed that Phytophthora acerina was one of the causal agents of the decline of the Chinese redwood. In this study, a total of 147 fungal isolates were obtained from the diseased roots and xylem of trunks of declining M. glyptostroboides trees. Through morphology and multi-locus phylogenetic analysis, these isolates were identified as eight species belonging to the genera Fusarium and Neocosmospora including F. fujikuroi, F. irregulare, F. odoratissimum, F. reticulatum， N. falciformis, N. keratoplastica, N. solani, and N. tonkinensis. Single inoculation and co-inoculation with P. acerina assays of these Fusarium and Neocosmospora species were then performed to test pathogenicity on three-year-old seedlings of M. glyptostroboides. Lesions (i.e., on seedling stems) caused by species of the genera Neocosmopora and Fusarium were smaller than those caused by P. acerina. Co-inoculation of F. fujikuroi and P. acerina, as well as the co-inoculation of F. reticulatum and P. acerina caused larger lesions than inoculations with P. acerina alone. All these species of Fusarium and Neocosmospora were shown to have the potential to be pathogenic to M. glyptostroboides. This study provided evidence that the decline of M. glyptostroboides in Jingzhou is a disease complex.

Spatial patterns and determinants of nitrogen composition in the trunk xylem sap of tree species from tropical to temperate forests

Trunk xylem sap nitrogen (N) composition can provide key information for interpreting tree N transport strategies in a changing environment. However, there remains poorly explored to properly depict the spatial patterns and determinants of the xylem sap N composition in different tree species across climate zones. Here, we analyzed the trunk xylem sap N of dominant tree species from tropical to temperate forests in eastern China. The random forest (RF) approach was used to quantify the relative importance of biotic and abiotic variables in driving the spatial patterns of the trunk xylem sap N composition. The concentration ranges of total N (TN), ammonium (NH4+) and nitrate (NO3−) in the trunk xylem sap were 0.37–37.23, 0.01–2.60 and 0.01–0.49 mM, respectively. Dissolved organic nitrogen (DON) was the main component of TN in the xylem sap of forest trees, accounting for 83.4 ± 3.6%, followed by NH4+ (11.8 ± 3.2%) and NO3− (4.8 ± 1.3%). In addition, the TN and NH4+ concentrations increased from the tropical rainforest and then decreased to the northern temperate forest, with a peak in the subtropical deciduous forest in the middle-latitude climatic transition zone. The NO3− concentration in the temperate forest was higher than that in other forests, regardless of coniferous and broad-leaf forests. Tree species was the key factor affecting the spatial TN patterns, whereas soil pH primarily and conversely determined the spatial NH4+ and NO3− patterns. Together, tree characteristics and soil properties were the most important factors affecting the spatial patterns of xylem sap N composition. These findings provide new insights into N cycling within forest trees and have implications for better understanding the adaptation to global changes in forest ecosystems.

Comparison of characteristics of tree trunk xylem vessels among three species of <i>Betula</i> in northeast China and their relationships with climate

Comparison of characteristics of tree trunk xylem vessels among three species of <i>Betula</i> in northeast China and their relationships with climate

Foliar water uptake improves branch water potential and photosynthetic capacity in Calligonum mongolicum

Atmospheric water is among the most important water sources for plants in arid ecosystems and plays an important role in facilitating drought stress survival in plants. However, little is known about the specific physiological benefits of foliar water uptake (FWU) for woody plants inhabiting arid desert areas. The objective of this study was to explore the physiological responses of Calligonum mongolicum to atmospheric water uptake by the assimilating branches, with a special focus on the FWU effect on shoot water status and photosynthesis. To this end, the canopies of natural C. mongolicum plants in a desert region of northwestern China were humidified in a field in-situ misting experiment and a long-term wetting experiment, during which physiological measurements of plant water relations and photosynthetic capacity were performed, alongside the observation of FWU by H218O isotopic tracing. Following the in-situ misting experiment, the water potential of the assimilating branches was significantly higher than that of the secondary branches at midnight, suggesting that FWU resulted in a reverse water potential gradient in C. mongolicum at the branch level. The 18O isotope was markedly enriched in the assimilating branches, secondary branches, and trunk xylem following the foliar uptake of 18O labeled water, indicating that the water uptake from the atmosphere by the assimilating branches could replenish the stem. The relative water content and water potential of the assimilating branches were significantly increased by FWU. The net photosynthetic rate, stomatal conductance, and maximum photochemical efficiency of C. mongolicum plants were markedly increased following the wetting treatment. Our results clearly show that C. mongolicum is capable of FWU, and that the absorbed water could be transported downward through the xylem and improve the branch water status and photosynthetic capacity. These findings provide a new perspective for deepening our understanding of the drought survival mechanism of plants in arid environments.

Transfer of Metal(loid)s from Soil to Leaves and Trunk Xylem Sap of Medicinal Plants and Possible Health Risk Assessment.

The objective of the present study was to investigate metal(loid)s in soils, in the trunk xylem sap and in the leaves of the Dipteryx alata plant located near the highway with high vehicle traffic in agricultural regions and near landfills, and to assess the transfer of metal(loid)s from soil to plant and possible health risk assessment. Trunk xylem sap, leaves and soil samples were collected at three sites near the highway. The analysis of trace elements was carried out using inductively coupled plasma optical emission spectroscopy (ICP OES). In the three soil sampling sites far from the highway edge, 15 elements were quantified. The concentrations of elements in the soil presented in greater proportions in the distance of 5 m in relation to 20 and 35 m. The metal(loid)s content in the study soil was higher than in other countries. The concentrations of Al, Cu, Fe, Mg, Mn, P, Se and Zn in the xylem sap were much higher than the leaves. The values of transfer factor of P, Mg and Mn from soil to the xylem sap and transfer factor of P from soil to leaf were greater than 1, indicating that the specie have a significant phytoremediation and phytoextraction potential. This plant has a tendency to accumulate As, Cd and Cr in its leaf tissues. The chronic hazard index (HI) values recorded in this study were above 1 for adults and adolescents. It is concluded that the soil, the trunk xylem sap and leaves of this plant are contaminated by heavy metals. Ingestion of the trunk xylem sap of this plant can cause toxicity in humans if ingested in large quantities and in the long term; therefore, its consumption should be avoided.

Identification of the diterpenoid biosynthesis genes and their expression status in relation to oleoresin yield of masson pine

Conifer oleoresin is a highly precious raw material for industrial sectors and a precursor for biofuels. Diterpenoid, particularly diterpenoid resin acids (DRAs), is one of the most abundant oleoresin components. Masson pine (Pinus massoniana L.) is one of the most important native species for oleoresin yield in China. To better understand the biosynthesis of diterpenoid in high oleoresin-yield masson pine and develop methods for its production in needle and trunk xylem, we used the combination of SMRT full-length transcriptome and Illumina RNA sequencing technologies to establish a transcriptome dataset. Approximately 20,560 and 17,950 full-length non-redundant transcripts were assembled in trunk xylem and needle, respectively. Of those, 29 and 20 DEGs were identified involving in the Terpenoid Backbone Biosynthesis pathway (TBB) (KO 00900) and the Diterpenoid Biosynthesis pathway (DB) (KO 00904) in trunk xylem, as well as 35 and 12 DEGs in the TBB and the DB in the needle, respectively. Compared with the low oleoresin-yield germplasm, 13 up-regulated DEGs with complete open reading frame (ORF) sequence were obtained by Venn analysis in trunk xylem and needle of a high oleoresin-yield clone. Subsequently, the expression patterns of the 16 diterpenoid biosynthesis-related genes (including three types of DXS genes) were validated by quantitative real-time PCR in both 12-year-old and 1-year-old high and low oleoresin-yield masson pine clones. The results showed that the 13 genes in the higher yield oleoresin trees were proved to be higher expression in comparison with the lower ones irrespectively of the tree age; as well as MCS (2-C-methyl-d-erythritol 2, 4-cyclodiphosphate synthase), HDS (1-hydroxy-2-methyl-2-(E)-butenyl4-diphosphate synthase), and IDI (Isopentenyl-diphosphate Delta-isomerase) of the methylerythritol 4-phosphate (MEP) pathway demonstrated a higher expression compared to that of other TBB genes. Both cytosol-localized MVA and plastid-localized MEP pathways gave significant contributions to diterpenoid accumulation, and the latter played a more dominant role. To clarify the potential relationships between oleoresin yield and the expression patterns of 16 genes from the TBB and DB pathways, the Pearson correlation analysis was performed using three tissues (young needle, old needle, and trunk xylem) from the 60-year-old tree. Remarkably, HMGR (Hydroxy methylglutaryl-CoA reductase) and ent-CDS (Ent-copalyl diphosphate synthase) were positive correlation with oleoresin yield in the new needle, whereas FPPS (Farnesyl pyrophosphate synthase) was a negative correlation with that in the old needle; furthermore, HMGS (Hydroxy methylglutaryl-CoA synthase), IDI and CDS (Copalyl diphosphate synthase) were positively correlated with oleoresin synthesis in the trunk xylem. Moreover, the Pearson correlation analysis was also carried out to determine the potential relationships between the 13 genes of the TBB pathways and ent-CDS, CDS, and LDS ((13E)-labda-7 13-dien-15-ol synthase) from DB pathways within three tissues of the 60-year-old masson pine. Furthermore, the diterpenoid and diterpene resin acids (DRAs) demonstrated highly positive correlations with oleoresin yield, and DRAs were detected as the most abundant diterpenoids in oleoresin. Promisingly, HMGR, MCS, HDS, IDI, ent-CDS, and CDS may be used as the key target genes of the molecular-assisted selection for high-oleoresin-yielding germplasm in this species.

Functional hydraulic sectoring in grapevines as evidenced by sap flow, dye infusion, leaf removal and micro-computed tomography.

The supply of water to a plant canopy is dependent on the xylem pathway connecting roots to leaves. In some plants, sectored xylem pathways can restrict resource distribution, resulting in variable quality of organs in the shoots, yet little is known about the effects of sectoring in crop cultivars. In this study, we combined sap flow measurements and infusion of xylem-specific dyes to document functional conductive area and flow pathways from roots to shoots of 20-year-old Thompson Seedless and 8-year-old Chardonnay grapevines. Sap flow measurements and dye infusion demonstrated that water flowed predominantly in discrete xylem (visually identifiable from the trunk surface) sectors along the trunk axis, each supplying limited portions of the canopy. Functional conductive area in the trunk was proportional to that in the shoots even though sector size varied considerably between vines. Leaf area removal experiments further demonstrated sectoring in grapevines; sap flow decreased by >90 % in trunk sectors connected to excised shoots while it remained constant in trunk sectors supplying intact portions of the canopy. Despite the functional sectoring in grapevines, a high degree of interconnectivity of trunk xylem in the tangential direction was confirmed with synchrotron-based micro-computed tomography (microCT) and dye crossover infusion studies. Fruit attached to dyed canes was also similarly sectored; no clusters exhibited dye on non-dyed canes, while 97 % of clusters attached to dyed canes exhibited dye infusion. The dye travelled down the cluster rachis and appeared to accumulate at the pedicel/berry junction, but only on dyed canes. These findings suggest that xylem in grapevine trunks is integrated anatomically, but functions in a sectored manner due to high axial hydraulic conductivity. The functional sectoring of grapevine xylem documented here has important implications for management practices in vineyards and for fruit cluster uniformity within single grapevine.

ЦИФРОВЫЕ ТЕХНОЛОГИИ ПРИ АНАЛИЗЕ СОСТОЯНИЯ ДРЕВЕСНЫХ РАСТЕНИЙ

The state of woody plants determines the viability of forests and environmental safety in urban environments largely. Therefore, scientific studies of the tree response as a complex biosystem on environmental factors changes are especially relevant due to the growing threat of environmental disasters (environmental pollution, active felling, etc.). Many modern methods for assessing the state of such complex macromolecular systems are verbal in nature due to the lack of non-destructive testing methods with controlled accuracy. The shortage of experimental measurements retards the development of modeling the state of trees and, consequently, the development of methods for predicting their response on the influence of external factors. The development of digital technology can change the current state of affairs in this sphere. In the report, within the framework of a systematic approach, the possibility of using digital technologies at analyzing the response of the state of woody plants on environmental factors changes has analyzed. The results of studying the flow of salt solutions in the xylem of tree trunks are presented to solve the problem. Based on experimental data, models are proposed that can become the basis of calculation systems for analyzing the state parameters of woody plants and predicting the occurrence of environmental disasters.

Variations of electric potential in the xylem of tree trunks associated with water content rhythms.

Instantaneous electrical responses in plants have been widely studied, but the mechanism of spontaneous, periodic electric potential alternations in the xylem of tree trunks remains controversial. The generation of the electric potential can be explained by the electrode potential, which depends on ion concentrations near electrodes. However, several different hypotheses about its periodic variations have been proposed, including streaming potential, ion diffusion, charge transport, and oxygen turnover. Here, we performed long-term measurements on the electric potential and water content in the xylem of trees, and observed changes in the electric potential and transpiration rate in response to varied numbers of leaves, light radiation, temperature, and relative air humidity. The electric potential showed a distinct seasonal trend, combined with daily rhythms, and could be affected by environmental changes. Rapid changes in the electric potential routinely lagged behind those of the transpiration rate, but their ranges of change were proportional. Both annual and diurnal patterns of the electric potential were synchronous with the trees' water content. Moreover, we found potential function relationships between the electric potential and water content. Accordingly, we propose a new perspective, that the variations of the electric potential in tree xylem could be associated with water content rhythms.

Identification and functional study of oleoresin terpenoid biosynthesis-related genes in masson pine (Pinus massoniana L.) based on transcriptome analysis

Masson pine (Pinus massoniana L.) is one of the most important species for oleoresin production worldwide. However, the mechanism of oleoresin synthesis in masson pine remains poorly studied. We revealed the key differentially expressed genes associated with terpenoid biosynthesis in trees with different yields of oleoresin in different tissues (trunk xylem and needle) during different seasons (spring and autumn) based on the combined application of the SMRT full-length transcriptome and Illumina RNA sequencing under field operations. A total of 76,754 full-length non-redundant transcripts were obtained in the study. Transcriptome data were annotated with KEGG and GO, and they have been shown to be of high quality by qRT-PCR. Chloroplastic 2-C-methyl-D-erythritol 4-phosphate (MEP) pathway and cytosolic mevalonate (MVA) pathway genes have different preferences under different conditions. Moreover, the phylogenetic analysis results revealed three classes of DXS genes with different functions. Among the tissue-specific genes, AACT1 was expressed in the trunk xylem at the highest levels, while MCS (1, 4) and HDR2 were significantly upregulated in the needle. Among stage-specific genes, DXS6 and HDS6 were expressed in the trunk xylem in August (autumn) at the highest levels. Furthermore, all of the terpene synthase (TPS) differentially expressed genes (DEGs) were clustered into the gymnosperm-specific TPS-d subfamily, including sesquiterpene synthase and diterpene synthase genes. According to the protein sequence alignment, diTPS1 and diTPS5 with three conserved active sites catalysed the synthesis of diterpene resin acids. In the low-yield oleoresin clone, diTPS (1, 5) and GGPPS (4, 6) were upregulated in the trunk xylem. The integration of the transcriptomic data indicated that the AACT, MCS, HDR, DXS, HDS, and TPS (with two conserved active sites) may play critical roles in the terpenoids biosynthesis of masson pine. The results of our study can be useful for developing an understanding of the coniferous genome and to significantly accelerate confer breeding.

Detection of freeze-thaw episodes in xylem of trunks and branches by measuring variations in xylem diameter, xylem water content, and sap flow velocity

Detection of freeze-thaw episodes in xylem of trunks and branches by measuring variations in xylem diameter, xylem water content, and sap flow velocity

Characteristics of chemical components in the trunk xylem sap of pine trees by means of a centrifugation collection method

Knowledge of the characteristics of chemical components transported in the xylem sap of trunks remains deficient and limited because no appropriate method exists to extract the xylem sap from this part of the tree. We thus explored the differences in xylem sap components extracted by means of centrifugation and water displacement methods and depicted the level and behavior of chemical components in the xylem sap of trunks and branches of different aged trees from a pine forest in northern China. There were no significant differences between the two methods with respect to nitrogen (N) compounds and inorganic ions in the xylem sap. Potassium concentrations obtained by the methods were similar and consistent with the values obtained from earlier publications on woody species. This suggests that contamination of the xylem sap by the centrifugation method is negligible, and this method would be a reliable and robust tool for collection of the trunk xylem sap. Dissolved organic N was the dominant component of total N followed by nitrate (NO3−) and ammonium (NH4+). Potassium and chloride were the predominant cation and anion, respectively, of the xylem sap. The NO3− concentration basically did not change, whereas the NH4+ concentration was larger transported from the trunk to branches for the large tree class during foliage senescence. More inorganic N components (mainly NO3−) were found in young trees than in old trees. Our study contributes to improve the diagnostic assessments of tree physiological processes and growth in mature forest trees under environmental changes.

The possible mechanism for the water transport in the tree trunks in early spring

Modern technologies in the field of forest restoration require the creation of an integrated approach for systematizations all the developed methods and theories. The main difficulty in the concept creating may be an insufficient number of formalized models of processes that ensure the vital cycle of woody plants. The paper suggests a physical model of transport of salt solutions in the xylem of tree trunks in early spring. The flow of mineral ions is carried out in the porous wood of the trunk during this period by the principle of a molecular vacuum pump in which the energy for the movement of the salt solution is consumed from the thermopolarization field of the xylem arising by the change in the ambient temperature. The results of measurements of the potential difference along the tree trunk of the families Betula pendula and Populus tremula were used for interpretation the model. Measurements of the potential difference along the tree trunk were made under natural conditions and the ambient temperature was measured. Analysis of the results showed that the thermopolarization phenomena in the xylem of the trunks can be the basis of the mechanism of sap movement in early spring. Therefore the hydraulic ratios known for ion pumps can be used for the formalized simulation of the process under investigation.

Sex determines xylem anatomy in a dioecious conifer: hydraulic consequences in a drier world.

Increased drought frequency and severity may reshape tree species distribution in arid environments. Dioecious tree species may be more sensitive to climate warming if sex-related vulnerability to drought occurs, since lower performance of one sex may drive differential stress tolerance, sex-related mortality rates and biased sex ratios. We explored the effect of sex and environment on branch hydraulic (hydraulic conductivity and vulnerability to embolism) and trunk anatomical traits in both sexes of the dioecious conifer Juniperus thurifera L. at two sites with contrasting water availability. Additionally, we tested for a trade-off between hydraulic safety (vulnerability to embolism) and efficiency (hydraulic conductivity). Vulnerability to embolism and hydraulic conductivity were unaffected by sex or site at branch level. In contrast, sex played a significant role in xylem anatomy. We found a trade-off between hydraulic safety and efficiency, with larger conductivities related to higher vulnerabilities to embolism. At the anatomical level, females' trunk showed xylem anatomical traits related to greater hydraulic efficiency (higher theoretical hydraulic conductivity) over safety (thinner tracheid walls, lower Mork's Index), whereas males' trunk anatomy followed a more conservative strategy, especially in the drier site. Reconciling the discrepancy between branch hydraulic function and trunk xylem anatomy would require a thorough and integrated understanding of the tree structure-function relationship at the whole-plant level. Nevertheless, lower construction costs and higher efficiency in females' xylem anatomy at trunk level might explain the previously observed higher growth rates in mesic habitats. However, prioritizing efficiency over safety in trunk construction might make females more sensitive to drought, endangering the species' persistence in a drier world.

Insights from “The Hidden Half”: The impact of root-zone oxygen and redox dynamics on the response of avocado to long-term irrigation with treated wastewater in clayey soil

Treated wastewater (TWW) is a major source of water for agriculture in Israel; however, recent reports indicate a marked yield loss in TWW-irrigated avocado and citrus orchards planted in clayey soils. The association of the yield loss with clayey soils rather than sandy soils suggests that it is associated with conditions in the root zone, and specifically poor aeration. A three-year study (2012–2015) was conducted in an avocado orchard planted in clayey soil, comparing the oxygen and redox conditions in the root zone of TWW-irrigated plots with fresh water (FW)-irrigated plots, together with the physiological status of the trees. Soil parameters included: continuous in-situ measurement of soil-water tension (SWT), soil oxygen, and soil redox potential, and periodic measurements of soil solution composition. Physiological parameters included: mineral composition of plant tissue from the leaves, trunk xylem and roots, root growth, yield, fruit setting, plant volume, and yield. TWW-irrigated plots were found to endure longer periods of low SWT indicating higher water content, accompanied by lower oxygen levels and more reduced conditions in comparison to FW-irrigated plots. The differences in these soil parameters between treatments were greater during the irrigation season than during the rainy period. The more reduced conditions in the TWW plots did not lead to significant differences in Fe or Mn concentrations in the soil solution or in plant leaves. TWW soil solution had significantly higher Na levels compared with FW. This did not affect the leaf Na content, but was expressed in substantially higher Na content in the root and trunk xylem, with up to seven times more trunk xylem Na in TWW-irrigated plants compared with FW-irrigated plants. Root growth was significantly hindered in TWW-irrigated plots compared with FW-irrigated plots. A negative correlation was found between root growth and the duration of hypoxic conditions, and similarly between root growth and the Na levels in the roots. TWW-irrigated plants had greater fruitlet numbers at the initial fruit-setting stage, but had a smaller number of fruit and a lower yield at harvest. The yield (kg/tree) negatively correlated with the duration of hypoxic conditions in the root zone but not with the Na levels in the roots or xylem. Our findings point towards a substantial role of oxygen deprivation as a major factor leading to the damage to TWW-irrigated orchards in clayey soils. Based on the assimilation of data, we suggest that a downward cascade is instigated in the TWW-irrigated orchards by increased input of Na into the soil, leading to degradation of soil hydraulic properties and reduced aeration. Impaired physiological functioning of the roots due to limited oxygen supply results in less roots growth, lower water uptake and impaired selectivity against Na uptake, thus imposing a negative feedback to increase soil water content, reduce aeration and root-zone oxygen availability for the roots, and further impair plant resistance to the high Na levels.

Оценка микроструктуры ксилемы стволов деревьев методом математического моделирования

Оценка микроструктуры ксилемы стволов деревьев методом математического моделирования

Термополяризационный механизм сокодвижения в период весеннего равноденствия

This article seeks the answer to the question of how the distribution of the concentration of
 dissolved minerals in the sap after the winter period in the xylem is made, if in the fall due to low
 winter temperatures tree tends to reduce the amount of water and sap in its pores. In search of an
 answer it is invited to consider thermal polarization effects in the xylem of tree trunks during the
 spring equinox. The results of monitoring of daily average temperature showed that during the winter
 the temperature of the day varies in the amount of one and a half times greater than during the
 night, and in February this change is almost the same. Whereas in March the temperature change
 occurs in series, they sometimes are different in three times. Maximum temperature change for the
 day and night is achieved during the spring equinox, on average, it is ten degrees. Such a temperature
 change is accompanied by sharp changes in temperature during the days, which, according to
 the estimates, give rise to an electric field in the xylem along the trunk. Under this approach, in the
 work estimated expression was theoretically derived for the concentration of salt ions in the xylem
 of tree trunks, giving reason to believe that the described mechanism also contributes to the value of
 the osmotic pressure. All these considerations lead to the assumption that intensive spring sap flow
 (in the absence of currents of transpiration) is stimulated by thermal polarization effects in the xylem
 of the trunk, caused by the intense ambient temperature changes during this period.

Tree root systems competing for soil moisture in a 3D soil–plant model

Competition for water among multiple tree rooting systems is investigated using a soil–plant model that accounts for soil moisture dynamics and root water uptake (RWU), whole plant transpiration, and leaf-level photosynthesis. The model is based on a numerical solution to the 3D Richards equation modified to account for a 3D RWU, trunk xylem, and stomatal conductances. The stomatal conductance is determined by combining a conventional biochemical demand formulation for photosynthesis with an optimization hypothesis that selects stomatal aperture so as to maximize carbon gain for a given water loss. Model results compare well with measurements of soil moisture throughout the rooting zone, of total sap flow in the trunk xylem, as well as of leaf water potential collected in a Loblolly pine forest. The model is then used to diagnose plant responses to water stress in the presence of competing rooting systems. Unsurprisingly, the overlap between rooting zones is shown to enhance soil drying. However, the 3D spatial model yielded transpiration-bulk root-zone soil moisture relations that do not deviate appreciably from their proto-typical form commonly assumed in lumped eco-hydrological models. The increased overlap among rooting systems primarily alters the timing at which the point of incipient soil moisture stress is reached by the entire soil–plant system.

Investigating xylem embolism formation, refilling and water storage in tree trunks using frequency domain reflectometry

Trunks of large trees play an important role in whole-plant water balance but technical difficulties have limited most hydraulic research to small stems, leaves, and roots. To investigate the dynamics of water-related processes in tree trunks, such as winter embolism refilling, xylem hydraulic vulnerability, and water storage, volumetric water content (VWC) in the main stem was monitored continuously using frequency domain moisture sensors in adult Betula papyrifera trees from early spring through the beginning of winter. An air injection technique was developed to estimate hydraulic vulnerability of the trunk xylem. Trunk VWC increased in early spring and again in autumn, concurrently with root pressure during both seasons. Diurnal fluctuations and a gradual decrease in trunk VWC through the growing season were observed, which, in combination with VWC increase after significant rainfall events and depletion during periods of high water demand, indicate the importance of stem water storage in both short- and long-term water balance. Comparisons between the trunk air injection results and conventional branch hydraulic vulnerability curves showed no evidence of ‘vulnerability segmentation’ between the main stem and small branches in B. papyrifera. Measurements of VWC following air injection, together with evidence from air injection and xylem dye perfusion, indicate that embolized vessels can be refilled by active root pressure but not in the absence of root pressure. The precise, continuous, and non-destructive measurement of wood water content using frequency domain sensors provides an ideal way to probe many hydraulic processes in large tree trunks that are otherwise difficult to investigate.