Engineering, feasibility, and safety of force-controlled oropharyngeal swabs with a 3D-printed feedback system FCCSS (force controlled COVID-19 swab study) - a preliminary study.

Errors in laboratory diagnostics of viral infections primarily occur during the preanalytical phase, which is especially observed in sample collection. Hitherto, no efforts have been made to optimize oropharyngeal smears. An accurate method to analyze the necessary conditions for a valid oropharyngeal smear test is required, especially to avoid false negative results, which can lead to promotion of the spread of viruses such as SARS-CoV-2. In this study, a maximum-force failure analysis was performed on a swab, and the highest tolerable force was then measured on 20 healthy volunteers to obtain the dimensions of the possible force to be applied on a swab. Subsequently, a device which can validate and reproducibly indicate this force during swab collection was developed. The study demonstrated that swabs generally fail at a maximum force of 5 N. Furthermore, an average force of 2.4±1.0 N was observed for the 20 volunteers. Lastly, this study described the development of a device which presents the selected force with a mean accuracy of 0.05 N (Force applied by Device 1: 0.46±0.05 N, Device 2: 1.55±0.11 N, Device 3: 2.57±0.18 N) and provides feedback via haptic and acoustic clicks as well as with a visual indicator. In the future, the swab will be analyzed for the presence of viral pathogens to determine its diagnostic performance corresponding to the force (German Clinical Trials Register Number 00024455).

Minimal impacts of invasive Scaevola taccada on Scaevola plumieri via pollinator competition in Puerto Rico.

Introduction: Scaevola taccada and Scaevola plumieri co-occur on shorelines of the Caribbean. Scaevola taccada is introduced in this habitat and directly competes with native dune vegetation, including S. plumieri, a species listed as locally endangered and threatened in Caribbean locations. This study addresses whether the invasive S. taccada also impacts the native S. plumieri indirectly by competing for pollinators and represents the first comparative study of insect visitation between these species. Methods: Insect visitation rates were measured at sites where species co-occur and where only the native occurs. Where species cooccur, insect visitors were captured, identified and analyzed for the pollen they carry. Pollen found on open-pollinated flowers was analyzed to assess pollen movement between the two species. We also compared floral nectar from each species by measuring volume, sugar content, and presence and proportions of amine group containing constituents (AGCCs). Results: Our results demonstrate that both species share insect visitors providing the context for possible pollinator competition, yet significant differences in visitation frequency were not found. We found evidence of asymmetrical heterospecific pollen deposition in the native species, suggesting a possible reproductive impact. Insect visitation rates for the native were not significantly different between invaded and uninvaded sites, suggesting that the invasive S. taccada does not limit pollinator visits to S. plumieri. Comparisons of nectar rewards from the invasive and the native reveal similar volumes and sugar concentrations, but significant differences in some amine group containing constituents that may enhance pollinator attraction. Conclusion: Our analysis finds no evidence for pollination competition and therefore S. taccada's main impacts on S. plumieri are through competitive displacement and possibly through reproductive impacts as a consequence of heterospecific pollen deposition.

Assessment of micromotion at the bone-bone interface after coracoid and scapular-spine bone-block augmentation for the reconstruction of critical anterior glenoid bone loss-a biomechanical cadaver study.

BACKGROUND: Glenoid bone loss is among the most important risk factors for recurrent anterior shoulder instability, and a bony reconstruction is recommended in cases of critical bone loss (> 15%). The commonly used surgical techniques, including coracoid transfer, are associated with considerable complications. The aim of this study was to assess the motion at the glenoid-bone-block interface after coracoid and spina-scapula bone-block reconstruction of the anterior glenoid. METHODS: Twelve cadaveric shoulders were tested. A 20% bone defect of the anterior glenoid was created, and the specimens were randomly assigned for glenoid augmentation using a coracoid bone block (n = 6) or a scapular spine bone block (n = 6). The glenoid-bone interface was cyclically loaded for 5000 cycles with a force of 170 N. The micromotion was tracked using an optical measurement system (GOM ARMIS) and was evaluated with the GOM Correlate Pro software. RESULTS: The most dominant motion component was medial irreversible displacement for the spina-scapula (1.87 mm; SD: 1.11 mm) and coracoid bone blocks (0.91 mm; SD: 0.29 mm) (n.s.). The most medial irreversible displacement took place during the first nine cycles. The inferior reversible displacement was significantly greater for spina-scapula bone blocks (0.28 mm, SD: 0.16 mm) compared to coracoid bone blocks (0.06 mm, SD: 0.10 mm) (p = 0.02). CONCLUSIONS: The medial irreversible displacement is the dominant motion component in a bone-block reconstruction after a critical bone loss of the anterior glenoid. The spina-scapula and coracoid bone blocks are comparable in terms of primary stability and extent of motion. Thus, spina-scapula bone blocks may serve as alternatives in bony glenoid reconstruction from a biomechanical point of view.

Shoulder-specific Outcome after Proximal Humerus Fracture Influences Medium-term Overall Quality of Life. / Das schulterspezifische Outcome nach proximaler Humerusfraktur hat Einfluss auf die mittelfristige allgemeine Lebensqualität.

BACKGROUND: Surgical treatment of proximal humerus fracture is an established procedure. Postoperative complications have been shown to have a significant impact on shoulder-specific outcome. Little is known to date about an influence on injury-independent quality of life. AIM OF THE WORK: The aim of this retrospective study is to analyse whether patients with a poor functional outcome after surgically treated proximal humerus fracture also show a reduced general quality of life in the medium term. Emphasis is placed on the analysis of patients with poor functional outcome due to postsurgical complications. MATERIAL AND METHODS: Evaluation of all patients operated at one level 1 trauma centre with a proximal humerus fracture in the period 01.01.2005 to 31.12.2015 and follow-up using validated scores (Constant-Murley Score [CMS], EQ-5D). Two groups, group A with good outcome (∆CMS ≤ 15P.) and group B with poor outcome (∆CMS ≥ 16P.), were defined. Furthermore, descriptive variables including definition of a complication were defined in advance. RESULTS: 138 patients were included in the study (group A: 91, group B: 47). The mean follow-up was 93.86 months ± 37.33 (36-167). Group B had significantly more complications. Furthermore, patients with poor outcome regarding the shoulder (group B) were also found to have significantly lower EQ-VAS (78.9 ± 19.2 [20.0-100] vs. 70.4 ± 19.5 [15.0-98.0]; p = 0.008) and EQ index (0.91 ± 0.14 [0.19-1.00] vs. 0.82 ± 0.17 [0.18-1.00]; p < 0.001). DISCUSSION: In conclusion, in the present study, the patients with poor outcome of shoulder function in CMS have significantly lower overall quality of life after a mean of more than 6 years of follow-up. The poor outcome was due to a significantly higher postoperative complication rate. This was independent of the fracture morphology present and the surgical procedure used.

[Combined intrapulmonary/intramediastinal K-wire position after K-wire osteosynthesis of the clavicle : A case report]. / Kombinierte intrapulmonale/intramediastinale K-Draht-Lage nach K-Draht-Osteosynthese an der Clavicula : Ein Case Report.

BACKGROUND: Lateral clavicle fractures can be treated both conservatively and surgically depending on the fracture classification. Different surgical techniques have been described for the operative treatment. The selection of the appropriate technique is decisive for the functional outcome and healing process without complications. CASE: We report on a patient with a secondary dislocation of two Kirschner wires after Kirschner wire osteosynthesis. The secondary dislocation caused one of the wires to migrate into the mediastinum and pulmonary tissue, directly under the aortic arch. To prevent further migration with potential damage to surrounding structures, a uniportal video-assisted thoracoscopy was performed to retrieve the wire. CONCLUSION: The treatment of lateral clavicle fractures should be performed with bent Kirschner wires as they can otherwise lead to severe complications including the occurrence of pseudarthrosis or secondary migration of the material. Safe and stable surgical techniques (plate osteosynthesis, hybrid treatment) should be preferred if they are available.

Hydraulic integrity of plant organs during drought stress and recovery in herbaceous and woody plant species.

The relationship between root, stem, and leaf hydraulic status and stomatal conductance during drought (field capacities: 100-25%) and drought recovery was studied in Helianthus annuus and five tree species (Populus×canadensis, Acer saccharum, A. saccharinum, Picea glauca, and Tsuga canadensis). Measurements of stomatal conductance (gs), organ water potential, and vessel embolism were performed and the following was observed: (i) cavitation only occurred in the petioles and not the roots or stems of tree species regardless of drought stress; (ii) in contrast, all H. annuus organs exhibited cavitation to an increasing degree from root to petiole; and (iii) all species initiated stomatal closure before cavitation events occurred or the expected turgor loss point was reached. After rewatering: (i) cavitated vessels in petioles of Acer species recovered whereas those of P. ×canadensis did not and leaves were shed; (ii) in H. annuus, cavitated xylem vessels were refilled in roots and petioles, but not in stems; and (iii) despite refilled embolisms in petioles of some species during drought recovery, gs never returned to pre-drought conditions. Conclusions are drawn with respect to the hydraulic segmentation hypothesis for above- and below-ground organs, and the timeline of embolism occurrence and repair is discussed.

Online consultation in an orthopedic trauma surgery outpatient clinic: is there a learning curve?

BACKGROUND: In the context of the German contact restrictions due to the COVID-19 pandemic of March 2020, an online-based consultation system was established in our university orthopedic outpatient department to maintain patient care. As a basis for contact-minimizing communication, this was continued after the contact restrictions were lifted. The aim of this prospective pilot study was to assess the effectiveness, technical feasibility, and patient flow in this system under lockdown conditions and in the period afterwards. METHODS: The evaluation took place from the beginning of the first lockdown on March 13, 2020, until May 31, 2021. For each patient encounter, the quality of the sound and video connections was documented. The outcomes of the consultations were recorded. Four categories were distinguished: 1) no follow-up necessary, 2) follow-up via online consultation, 3) referral for surgical therapy, and 4) follow-up in the outpatient clinic for physical examination. A comparison was made between an early cohort right after implementation of the online consultation and and a late cohort after establishment of the consultation. RESULTS: There were 408 patient encounters via online consultation. A total of 360 (88%) consultations were uninterrupted. Initial presentations accounted for 124 (30%) consultations. In 75 (18%) patients, no further follow-up was necessary. Follow-up via online consultation was scheduled in 82 (20%) patients, direct referral for surgery was made in 86 (21%) patients, and a follow-up for physical examination was arranged in 165 (40%) patients. When comparing the early and late cohort, there was no difference in the duration of the conversation (p = 0.23). A significant difference was found in the type of further treatment. In the late cohort, conservative therapy was used more often (p < 0.01), resulting in a lower number of follow-up visits for clinical examination (p < 0.01). CONCLUSION: While a definite decision for further procedure was possible solely by online consultation in a large percentage of cases, 40% of patients still needed an additional in-person consultation for physical examination. A learning curve could be observed regarding the selection of patients suited for online consultation. Overall, online consultation is a useful measure to manage patient volume and to visibly support direct doctor-patient contact.

[Video consultation in an orthopedic trauma surgery outpatient clinic : Effective adjunctive interventions in lockdown and post-lockdown scenarios-a prospective pilot study]. / Die Videosprechstunde in einer unfallchirurgisch/orthopädischen Spezialsprechstunde : Effektive ergänzende Maßnahme in Lockdown- und Post-Lockdown-Szenarien ­ eine prospektive Pilotstudie.

BACKGROUND: With the regulation of the Saxon State Government and the Saxon State Ministry for Social Affairs and Social Responsibility on the modification of the Infection Protection Act of March 2020 coming into force, a video-based outpatient consultation was implemented to maintain patient care. In order to allow communication with minimized contact, this was continued after the lockdown. AIM OF THE WORK: The aim of this prospective pilot study was to assess the effectiveness of a video-based outpatient consultation service, technical feasibility and control of patient flow under both lockdown and post-lockdown conditions. MATERIAL AND METHODS: The initial evaluation was conducted up to 14 December 2020 when the second restrictive measures were implemented by the state government. The quality of the connections regarding sound and image was documented. Furthermore, the consequences of the conversations were documented. Distinctions were made in four categories: 1. no follow-up visit, 2. follow-up via video consultation, 3. operative intervention and 4. in-person follow-up visit for clinical examination. RESULTS: There were 236 video-based outpatient consultations, 182 (82%) consultations were without restrictions and 47 (21%) consultations were initial presentations. There were no follow-up consultations in 41 (18%) patients. Video-based follow-up was scheduled in 36 (16%) patients, direct referral for surgery in 36 (16%) patients, and in-person follow-up in 105 (47%) patients. DISCUSSION: In 40% of the patients a definite decision could be made by the initial video-based consultation alone. On the other hand, 47% of the patients needed in-person follow-up for a clinical examination. Thus, video consultation is a very useful measure to manage patient volume and visibly support direct doctor-patient contact.

Podcasts as a teaching tool in orthopaedic surgery : Is it beneficial or more an exemption card from attending lectures?

OBJECTIVE: The aim of this study was to evaluate the influence of the introduction of online podcasts as part of the main lecture series in orthopaedics on the number of lecture attendees, the examination results and the assessment of teaching by the students. Additionally, we evaluated the use of other media for examination preparation. METHODOLOGY: At the beginning and end of the lecture series questionnaires were handed out to the students to evaluate their attitudes towards attending lectures, the use of video podcasts and examination preparation. In addition, the number of lecture attendees and podcast usage during the semester were counted and the statements of the students in the evaluation assessments of orthopaedic teaching were evaluated. The examination results were correlated in a statistical analysis with the learning materials provided by the students for examination preparation. RESULTS: At the end of the lecture series, 284 students stated that they used the lecture podcast about twice as often as attending lectures; however, for the majority of the students the provision of a video podcast was no reason not to attend the lecture. For example, 37.2% stated that they never and 26.8% stated that they rarely had not attended the lecture by providing the podcasts. Of the students 91-95% considered the availability of lecture podcasts to be a rather meaningful or very meaningful supplement to the lecture visit. Students increasingly used digital media to prepare for examinations instead of using traditional analogue methods. None of the learning methods or materials examined showed a statistically significant advantage in examination results. CONCLUSION: Students in the age of digitalization use a variety of learning materials and are no longer bound to classical analog teaching methods. The use of online podcasts had no negative impact on examination performance. Most students perceived lecture podcasts as a useful supplement to lecture attendance. The students praised the expansion of the teaching curriculum to include additional digital offers with positive comments in the evaluations, but without achieving an improvement in these student evaluations.

Signal coordination before, during and after stomatal closure in response to drought stress.

Signal coordination in response to changes in water availability remains unclear, as does the role of embolism events in signaling drought stress. Sunflowers were exposed to two drought treatments of varying intensity while simultaneously monitoring changes in stomatal conductance, acoustic emissions (AE), turgor pressure, surface-level electrical potential, organ-level water potential and leaf abscisic acid (ABA) concentration. Leaf, stem and root xylem vulnerability to embolism were measured with the single vessel injection technique. In both drought treatments, it was found that AE events and turgor changes preceded the onset of stomatal closure, whereas electrical surface potentials shifted concurrently with stomatal closure. Leaf-level ABA concentration did not change until after stomata were closed. Roots and petioles were equally vulnerable to drought stress based on the single vessel injection technique. However, anatomical analysis of the xylem indicated that the increased AE events were not a result of xylem embolism formation. Additionally, roots and stems never reached a xylem pressure threshold that would initiate runaway embolism throughout the entire experiment. It is concluded that stomatal closure was not embolism-driven, but, rather, that onset of stomatal closure was most closely correlated with the hydraulic signal from changes in leaf turgor.

[Clinical-practical Examination in Orthopaedic Teaching: Who is the "Ideal" Examiner?] / Klinisch-praktische Prüfungen in der orthopädischen Lehre: Wer ist der "ideale" Prüfer?

Background Medical curricula require an increasing amount of integrated clinical practical skills education. German medical faculties are increasingly implementing the OSCE (objective structured clinical examination) to test students' practical as well as communication skills. Teaching and assessment tools for practical skills include simulators and simulated patient scenarios, where peer-assisted teaching (PAT) is applied. Trained student tutors support this process. This study examines for the first time the use of PAT student tutors as examiners in an orthopaedic OSCE unit and the existence of possible differences in rating between faculty and PAT student tutors. Material and Methods N = 94 students (out of n = 324) in their first clinical year were randomly assigned to the OSCE unit "spine and pelvis examination" in March/April 2016 and data were collected over the three-day OSCE period. A structured checklist was developed and the two examiners (one university professor from the Department of Orthopaedics, one PAT student tutor) received identical preparatory training. The checklist was completed by both examiners independently and data were collected for statistical analysis. The analysis focused on the overall evaluation in points, as well as separate checklist sections focusing on different competences and comparison of the three consecutive examination days. Analysis was conducted by SPSS. Results The average number of points in the overall evaluation was 19.5 (out of 25) points. The student tutor evaluated the students with an average of 19.1 points, the professor with 19.9 points. A significant difference was observed between the two examiner evaluations of day one (p < 0.001) and in the overall analysis (p < 0.001). On OSCE examination days 2 and 3, no significant differences in evaluation were observed. Conclusion The analysis shows only one significant evaluation difference between the two assessors, with the PAT student tutor's evaluation being stricter. The authors conclude that a point difference of 0.8 out of 25 in the average overall evaluation and an interrater reliability of 0.95 in an orthopaedic OSCE station examining practical skills as well as communication skills justifies employing PAT student tutors as assessors in this context. The obligatory requirements for this are a detailed PAT student tutor training, the development of a well-structured and valid checklist, as well as continuous quality assurance of the overall OSCE.

Peer-assisted teaching student tutors as examiners in an orthopedic surgery OSCE station - pros and cons.

BACKGROUND: The OSCE (objective structured clinical examination) is composed of oral and practical examination in order to examine students' abilities to imply clinical examination techniques and to interact with patients. The examiners for this procedure can be either lecturers or peers. The aim of this work is to evaluate the peer-assisted teaching student tutors as examiners in an orthopedic surgery OSCE station. METHODS: We analyzed the OSCE data from 2013 to 2015. During this period over 300 medical students were examined each year. An evaluation was conducted at an orthopedic station and examined by peer students to assess the advantages and disadvantages of peer-assisted teaching student tutors as examiners. RESULTS: We have noticed that student peers are more flexible regarding their schedule and they have been well trained for OSCE. Concerning the economic aspects, student peers are clearly of major economic advantage. Disadvantages were not reported in our study probably because peers were well trained and the checklists are monitored regularly. CONCLUSION: Student peers in OSCE are of major advantage due to their flexible time schedule and relatively low costs. They must be well trained and the checklists are to be monitored regularly. Our study shows that peer tutor examiners conducted the examination as competent as lecture examiners. However, legal restrictions on the employment of students should be considered.

Plant hydraulics as a central hub integrating plant and ecosystem function: meeting report for 'Emerging Frontiers in Plant Hydraulics' (Washington, DC, May 2015).

Water plays a central role in plant biology and the efficiency of water transport throughout the plant affects both photosynthetic rate and growth, an influence that scales up deterministically to the productivity of terrestrial ecosystems. Moreover, hydraulic traits mediate the ways in which plants interact with their abiotic and biotic environment. At landscape to global scale, plant hydraulic traits are important in describing the function of ecological communities and ecosystems. Plant hydraulics is increasingly recognized as a central hub within a network by which plant biology is connected to palaeobiology, agronomy, climatology, forestry, community and ecosystem ecology and earth-system science. Such grand challenges as anticipating and mitigating the impacts of climate change, and improving the security and sustainability of our food supply rely on our fundamental knowledge of how water behaves in the cells, tissues, organs, bodies and diverse communities of plants. A workshop, 'Emerging Frontiers in Plant Hydraulics' supported by the National Science Foundation, was held in Washington DC, 2015 to promote open discussion of new ideas, controversies regarding measurements and analyses, and especially, the potential for expansion of up-scaled and down-scaled inter-disciplinary research, and the strengthening of connections between plant hydraulic research, allied fields and global modelling efforts.

Functional analysis of embolism induced by air injection in Acer rubrum and Salix nigra.

The goal of this study was to assess the effect of induced embolism with air injection treatments on the function of xylem in Acer rubrum L. and Salix nigra Marsh. Measurements made on mature trees of A. rubrum showed that pneumatic pressurization treatments that created a pressure gradient of 5.5 MPa across pit membranes (ΔP pit) had no effect on stomatal conductance or on branch-level sap flow. The same air injection treatments made on 3-year-old potted A. rubrum plants also had no effect on whole plant transpiration. A separate study made on mature A. rubrum trees showed that 3.0 and 5.5 MPa of ΔP pit values resulted in an immediate 100% loss in hydraulic conductance (PLC) in petioles. However, the observed change in PLC was short lived, and significant hydraulic recovery occurred within 5-10 min post air-pressurization treatments. Similar experiments conducted on S. nigra plants exposed to ΔP pit of 3 MPa resulted in a rapid decline in whole plant transpiration followed by leaf wilting and eventual plant death, showing that this species lacks the ability to recover from induced embolism. A survey that measured the effect of air-pressurization treatments on seven other species showed that some species are very sensitive to induction of embolism resulting in leaf wilting and branch death while others show minimal to no effect despite that in each case, the applied ΔP pit of 5.5 MPa significantly exceeded any native stress that these plants would experience naturally.

Analysis of spatial and temporal dynamics of xylem refilling in Acer rubrum L. using magnetic resonance imaging.

We report results of an analysis of embolism formation and subsequent refilling observed in stems of Acer rubrum L. using magnetic resonance imaging (MRI). MRI is one of the very few techniques that can provide direct non-destructive observations of the water content within opaque biological materials at a micrometer resolution. Thus, it has been used to determine temporal dynamics and water distributions within xylem tissue. In this study, we found good agreement between MRI measures of pixel brightness to assess xylem liquid water content and the percent loss in hydraulic conductivity (PLC) in response to water stress (P50 values of 2.51 and 2.70 for MRI and PLC, respectively). These data provide strong support that pixel brightness is well correlated to PLC and can be used as a proxy of PLC even when single vessels cannot be resolved on the image. Pressure induced embolism in moderately stressed plants resulted in initial drop of pixel brightness. This drop was followed by brightness gain over 100 min following pressure application suggesting that plants can restore water content in stem after induced embolism. This recovery was limited only to current-year wood ring; older wood did not show signs of recovery within the length of experiment (16 h). In vivo MRI observations of the xylem of moderately stressed (~-0.5 MPa) A. rubrum stems revealed evidence of a spontaneous embolism formation followed by rapid refilling (~30 min). Spontaneous (not induced) embolism formation was observed only once, despite over 60 h of continuous MRI observations made on several plants. Thus this observation provide evidence for the presence of naturally occurring embolism-refilling cycle in A. rubrum, but it is impossible to infer any conclusions in relation to its frequency in nature.

How strong is intracanopy leaf plasticity in temperate deciduous trees?

Intracanopy plasticity in tree leaf form is a major determinant of whole-plant function and potentially of forest understory ecology. However, there exists little systematic information for the full extent of intracanopy plasticity, whether it is linked with height and exposure, or its variation across species. For arboretum-grown trees of six temperate deciduous species averaging 13-18 m in height, we quantified intracanopy plasticity for 11 leaf traits across three canopy locations (basal-interior, basal-exterior, and top). Plasticity was pronounced across the canopy, and maximum likelihood analyses indicated that plasticity was primarily linked with irradiance, regardless of height. Intracanopy plasticity (the quotient of values for top and basal-interior leaves) was often similar across species and statistically indistinguishable across species for several key traits. At canopy tops, the area of individual leaves was on average 0.5-0.6 times that at basal-interior, stomatal density 1.1-1.5 times higher, sapwood cross-sectional area up to 1.7 times higher, and leaf mass per area 1.5-2.2 times higher; guard cell and stomatal pore lengths were invariant across the canopy. Species differed in intracanopy plasticity for the mass of individual leaves, leaf margin dissection, ratio of leaf to sapwood areas, and stomatal pore area per leaf area; plasticity quotients ranged only up to ≈2. Across the six species, trait plasticities were uncorrelated and independent of the magnitude of the canopy gradient in irradiance or height and of the species' light requirements for regeneration. This convergence across species indicates general optimization or constraints in development, resulting in a bounded plasticity that improves canopy performance.

A potential role for xylem-phloem interactions in the hydraulic architecture of trees: effects of phloem girdling on xylem hydraulic conductance.

We investigated phloem-xylem interactions in Acer rubrum L. and Acer saccharum Marsh. Our experimental method allowed us to determine xylem conductance of an intact branch by measuring the flow rate of water supplied at two delivery pressures to the cut end of a small side branch. We found that removal of bark tissue (phloem girdling) upstream of the point at which deionized water was delivered to the branch resulted in a decrease (24% for A. rubrum and 15% for A. saccharum) in branch xylem hydraulic conductance. Declines in hydraulic conductance with girdling were accompanied by a decrease in the osmotic concentration of xylem sap. The decrease in xylem sap concentration following phloem girdling suggests that ion redistribution from the phloem was responsible for the observed decline in hydraulic conductance. When the same measurements were made on branches perfused with KCl solution (approximately 140 mOsm kg(-1)), phloem girdling had no effect on xylem hydraulic conductance. These results suggest a functional link between phloem and xylem hydraulic systems that is mediated by changes in the ionic content of the cell sap.

Vulnerability of xylem vessels to cavitation in sugar maple. Scaling from individual vessels to whole branches.

The relation between xylem vessel age and vulnerability to cavitation of sugar maple (Acer saccharum Marsh.) was quantified by measuring the pressure required to force air across bordered pit membranes separating individual xylem vessels. We found that the bordered pit membranes of vessels located in current year xylem could withstand greater applied gas pressures (3.8 MPa) compared with bordered pit membranes in vessels located in older annular rings (2.0 MPa). A longitudinal transect along 6-year-old branches indicated that the pressure required to push gas across bordered pit membranes of current year xylem did not vary with distance from the growing tip. To understand the contribution of age-related changes in vulnerability to the overall resistance to cavitation, we combined data on the pressure thresholds of individual xylem vessels with measurements of the relative flow rate through each annual ring. The annual ring of the current year contributed only 16% of the total flow measured on 10-cm-long segments cut from 6-year-old branches, but it contributed more than 70% of the total flow when measured through 6-year-old branches to the point of leaf attachment. The vulnerability curve calculated using relative flow rates measured on branch segments were similar to vulnerability curves measured on 6-year-old branches (pressure that reduces hydraulic conductance by 50% = 1.6-2.4 MPa), whereas the vulnerability curve calculated using relative flow rates measured on 6-year-old branches were similar to ones measured on the extension growth of the current year (pressure that reduces hydraulic conductance by 50% = 3.8 MPa). These data suggest that, in sugar maple, the xylem of the current year can withstand larger xylem tensions than older wood and dominates water delivery to leaves.

Ionic control of the lateral exchange of water between vascular bundles in tomato.

Ions can enhance water flow through the xylem via changes in the hydraulic resistance at border pit membranes. Because flow between adjacent xylem vessels occurs primarily via bordered pit fields, it is hypothesized that xylem sap ion concentrations would affect lateral movement of water more than longitudinal flow. Using tomato as a model system, evidence is presented for ion-mediated changes in xylem hydraulic resistance and the lateral transport of water. Water flow between adjacent xylem bundles increased by approximately 50% in the presence of ions while longitudinal flow only increased by approximately 20%. However, the enhancement of lateral exchange due to ions was magnified by the presence of a pressure difference between vascular bundles. These results indicate that the degree of nutrient-sharing among sectors of a plant may depend on both nutrient concentration and the availability of water in the root zone.