Staff Publications

Staff Publications

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    'Staff publications' is the digital repository of Wageningen University & Research

    'Staff publications' contains references to publications authored by Wageningen University staff from 1976 onward.

    Publications authored by the staff of the Research Institutes are available from 1995 onwards.

    Full text documents are added when available. The database is updated daily and currently holds about 240,000 items, of which 72,000 in open access.

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    Scots pine trees react to drought by increasing xylem and phloem conductivities
    Kiorapostolou, Natasa ; Camarero, J.J. ; Carrer, Marco ; Sterck, Frank ; Brigita, Brigita ; Sangüesa-Barreda, Gabriel ; Petit, Giai - \ 2020
    Tree Physiology 40 (2020)6. - ISSN 0829-318X - p. 774 - 781.
    Pinus sylvestris - forest dieback - hydraulic failure - phenotypic plasticity - phloem - tree mortality - wood anatomy - xylem

    Drought limits the long-distance transport of water in the xylem due to the reduced leaf-to-soil water potential difference and possible embolism-related losses of conductance and of sugars in the phloem due to the higher viscosity of the dehydrated sugary solution. This condition can have cascading effects in water and carbon (C) fluxes that may ultimately cause tree death. We hypothesize that the maintenance of xylem and phloem conductances is fundamental for survival also under reduced resource availability, when trees may produce effective and low C cost anatomical adjustments in the xylem and phloem close to the treetop where most of the hydraulic resistance is concentrated. We analyzed the treetop xylem and phloem anatomical characteristics in coexisting Scots pine trees, symptomatic and non-symptomatic of drought-induced dieback. We selected the topmost 55 cm of the main stem and selected several sampling positions at different distances from the stem apex to test for differences in the axial patterns between the two groups of trees. We measured the annual ring area, the tracheid hydraulic diameter (Dh) and cell wall thickness (CWT), the conductive phloem area and the average lumen diameter of the 20 largest phloem sieve cells (Dph). Declining trees grew less than the non-declining ones, and despite the similar axial scaling of anatomical traits, had larger Dh and lower CWT. Moreover, declining trees had wider Dph. Our results demonstrate that even under drought stress, maintenance of xylem and phloem efficiencies is of primary importance for survival, even if producing fewer larger tracheids may lead to a xylem more vulnerable to embolism formation.

    Defenses against Virus and Vector: A Phloem-Biological Perspective on RTM- and SLI1-Mediated Resistance to Potyviruses and Aphids
    Kloth, Karen J. ; Kormelink, Richard - \ 2020
    Viruses 12 (2020)2. - ISSN 1999-4915
    aphids - phloem - plant resistance - potyviruses

    Combining plant resistance against virus and vector presents an attractive approach to reduce virus transmission and virus proliferation in crops. RestrictedTobacco-etch virus Movement (RTM) genes confer resistance to potyviruses by limiting their long-distance transport. Recently, a close homologue of one of the RTM genes, SLI1, has been discovered but this gene instead confers resistance to Myzus persicae aphids, a vector of potyviruses. The functional connection between resistance to potyviruses and aphids, raises the question whether plants have a basic defense system in the phloem against biotic intruders. This paper provides an overview on restricted potyvirus phloem transport and restricted aphid phloem feeding and their possible interplay, followed by a discussion on various ways in which viruses and aphids gain access to the phloem sap. From a phloem-biological perspective, hypotheses are proposed on the underlying mechanisms of RTM- and SLI1-mediated resistance, and their possible efficacy to defend against systemic viruses and phloem-feeding vectors.

    Light on phloem transport (an MRI approach)
    Prusova, Alena - \ 2016
    Wageningen University. Promotor(en): Herbert van Amerongen, co-promotor(en): Henk van As. - Wageningen : Wageningen University - ISBN 9789462579156 - 130
    solanum lycopersicum - phloem - light - flow - photoperiod - nuclear magnetic resonance - biophysics - magnetic resonance imaging - solanum lycopersicum - floëem - licht - stroming - fotoperiode - kernmagnetische resonantie - biofysica - kernspintomografie

    This thesis (Light on phloem transport – an MRI approach) aims to answer the question whether phloem transport can be a limiting factor for photosynthesis efficiency (and ultimately causing a bottleneck towards achieving higher yields). To answer this key question, we manipulated the source: sink ratio within tomato (Solanum lycopersicum L.) while measuring phloem transport with magnetic resonance imaging (MRI) flowmetry. Additionally we compared phloem flow characteristics of two potato plants (Solanum tuberosum L.) which differed in source : sink ratio. In Chapter 2, the source strength was manipulated by varying the light intensity. An increase in phloem sap volume flow under higher light intensities was observed. However, under all light intensities applied, the phloem flow velocity was found to be constant (as has previously been suggested in other studies) although a clear diurnal pattern was observed. This finding does not fit in current models to describe the mechanism of phloem transport and a different mechanism must be at play. The results of this chapter demonstrate that increased levels of photo-assimilates are transported in sieve tubes, which are activated when needed by the plant. This is the first study which shows that plants activate individual sieve tubes when more photo-assimilates are available, yet maintain constant velocity. Those observations were in a tomato plant with pruned fruit trusses (i.e., in a simplified system). In Chapter 3, we investigated whether tomato plants still exhibit constant phloem flow velocity (with a diurnal pattern) under normal conditions, i.e., with strong sinks (tomato fruits) still attached. This was tested for both a long and short photoperiod by measuring flow characteristics with MRI flowmetry. We simultaneously monitored other plant processes like xylem flow rates with a heat balance sensor, net photosynthesis with gas exchange and stem diameter changes with a linear motion potentiometer. With this integrated approach, we revealed a correlation between night phloem volume flow, dark respiration and stem growth. We also conclusively showed that phloem volume flow performs a diurnal pattern under a variety of source-sink ratios which appears to be a normal behaviour for tomato plants growing under moderately-high light conditions. In chapters 2 and 3 we learned that under higher source strength a greater amount of phloem sap is transported, but the changes in flow were not accompanied by changes in velocity. To further our understanding of the mechanisms driving phloem transport, it is of interest to know how the sucrose concentration in phloem sap relates to phloem flow. In Chapter 4 we used an average T2 relaxation time in the phloem vascular tissue region to reveal the plant’s phloem carbon status under source manipulation. In this chapter we demonstrated that T2 relaxation time, when measured in parallel with phloem flow, can provide additional information about phloem region carbon status, i.e., changes in the T2 relaxation time are correlated with changes in sucrose concentration in the whole phloem region.

    When studying phloem transport in plants with magnetic resonance imaging (MRI) flowmetry, plants which are relatively easy to manipulate (e.g. fruit pruning) like tomato have so far been used. However, tomato plants (used in all three previous chapters) have relatively low sink strength beneath the MRI measurement site. A potentially preferable approach is to work with plants with strong sinks beneath the measurement site. In Chapter 5 we studied potato as a potentially better test subject for MRI flowmetry as it possesses strong sink below the MRI measurement site (i.e., developing tubers). For that purpose we used two potato plants (cv. Desiree) both with several developing tubers. One of the plants overexpressed the StSWEET gene (35S:StSWEET) which appears to have altered its source : sink ratio. As a result, the 35S:StSWEET plant transported 60% more phloem sap than Desiree WT. Strikingly, the average phloem flow velocity in both plants was the same and the greater amount of transported phloem sap in the 35S:StSWEET plant was accommodated by more sieve tubes than in Desiree WT. This finding agrees with the hypothesis about the conserved nature of phloem flow velocity, where volume flow is regulated by the number of active sieve tubes (Chapter 2 and 3). In this chapter we also demonstrate that a potato plant with developing tubers represents a good subject to study phloem transport with MRI flowmetry. We concluded that under optimal conditions (which are commonly met in greenhouses) phloem transport is likely to reach its maximum capacity and therefore photosynthesis could be limited by the export and transport of photo-assimilates because of the finite number of sieve tubes and constant flow velocity.

    Understanding the effect of carbon status on stem diameter variations
    Swaef, T. de; Driever, S.M. ; Meulebroek, L. van; Vanhaecke, L. ; Marcelis, L.F.M. ; Steppe, K. - \ 2013
    Annals of Botany 111 (2013)1. - ISSN 0305-7364 - p. 31 - 46.
    daily trunk shrinkage - fruit-growth - tomato leaves - matter production - transport model - sugar-transport - sap flow - plant - water - phloem
    Background Carbon assimilation and leaf-to-fruit sugar transport are, along with plant water status, the driving mechanisms for fruit growth. An integrated comprehension of the plant water and carbon relationships is therefore essential to better understand water and dry matter accumulation. Variations in stem diameter result from an integrated response to plant water and carbon status and are as such a valuable source of information. Methods A mechanistic water flow and storage model was used to relate variations in stem diameter to phloem sugar loading and sugar concentration dynamics in tomato. The simulation results were compared with an independent model, simulating phloem sucrose loading at the leaf level based on photosynthesis and sugar metabolism kinetics and enabled a mechanistic interpretation of the ‘one common assimilate pool’ concept for tomato. Key Results Combining stem diameter variation measurements and mechanistic modelling allowed us to distinguish instantaneous dynamics in the plant water relations and gradual variations in plant carbon status. Additionally, the model combined with stem diameter measurements enabled prediction of dynamic variables which are difficult to measure in a continuous and non-destructive way, such as xylem water potential and phloem hydrostatic potential. Finally, dynamics in phloem sugar loading and sugar concentration were distilled from stem diameter variations. Conclusions Stem diameter variations, when used in mechanistic models, have great potential to continuously monitor and interpret plant water and carbon relations under natural growing conditions
    MRI links stem water content to stem diameter variations in transpiring trees
    Schepper, V. De; Dusschoten, D. van; Copini, P. ; Jahnke, S. ; Steppe, K. - \ 2012
    Journal of Experimental Botany 63 (2012)7. - ISSN 0022-0957 - p. 2645 - 2653.
    sap flow dynamics - sugar-transport - diurnal changes - radius changes - beech tree - xylem flow - phloem - long - anatomy - storage
    In trees, stem diameter variations are related to changes in stem water content, because internally stored water is depleted and replenished over a day. To confirm this relationship, non-invasive magnetic resonance imaging (MRI) was combined with point dendrometer measurements in three actively transpiring oak (Quercus robur L.) trees. Two of these oak trees were girdled to study the stem increment above the girdling zone. MRI images and micrographs of stem cross-sections revealed a close link between the water distribution and the anatomical features of the stem. Stem tissues with the highest amount of water were physiologically the most active ones, being the youngest differentiating xylem cells, the cambium and the youngest differentiating and conductive phloem cells. Daily changes in stem diameter corresponded well with the simultaneously MRI-measured amount of water, confirming their strong interdependence. MRI images also revealed that the amount of water in the elastic bark tissues, excluding cambium and the youngest phloem, contributed most to the daily stem diameter changes. After bark removal, an additional increase in stem diameter was measured above the girdle. This increase was attributed not only to the cambial production of new cells, but also to swelling of existing bark cells. In conclusion, the comparison of MRI and dendrometer measurements confirmed previous interpretations and applications of dendrometers and illustrates the additional and complementary information MRI can reveal regarding water relations in plants.
    Temporal dynamics in wheat grain zinc distribution: is sink limitation the key?
    Stomph, T.J. ; Choi, E.Y. ; Stangoulis, J.C.R. - \ 2011
    Annals of Botany 107 (2011)6. - ISSN 0305-7364 - p. 927 - 937.
    developing seeds - transport - biofortification - endosperm - phloem - barley - zn - remobilization - deposition - manganese
    Background and Aims - Enhancing the zinc (Zn) concentration in wheat (Triticum aestivum) grain is a breeding objective in order to improve human Zn nutrition. At enhanced plant Zn uptake, grain Zn levels do not increase proportionally and within the grain the endosperm Zn levels remain below grain Zn levels. This study analysed the temporal dynamics of Zn concentrations in grain tissues during grain filling to find major bottlenecks. Methods - Plants of two cultivars were grown at 1 and 5 mg Zn kg-1 soil. Individual panicles were harvested 7, 14, 24 or 34 d after their flowering or at maturity and seeds were dissected into constituting tissues, which were analysed for Zn and other minerals. Key Results - The Zn concentration of the crease was found to increase five- to nine-fold between 7 and 34 d after anthesis, while that of the endosperm decreased by 7 and 45 % when grown at 1 or 5 mg Zn kg-1, respectively. The Zn turnover rate (d-1) in the crease tissues was either independent of the Zn application level or higher at the lower Zn application level, and the Zn concentration increased in the crease tissues with time during grain filling while the turnover rate gradually decreased. Conclusions - There is significant within-seed control over Zn entering the seed endosperm. While the seed crease Zn concentration can be raised to very high levels by increasing external Zn supply, the endosperm Zn concentrations will not increase correspondingly. The limited transfer of Zn beyond the crease requires more research to provide further insight into the rate-determining processes and their location along the pathway from crease to the deeper endosperm
    The genome of the cucumber, Cucumis sativus L.
    Huang, S.W. ; Li, R.Q. ; Vossen, E.A.G. van der - \ 2009
    Nature Genetics 41 (2009)12. - ISSN 1061-4036 - p. 1275 - 1281.
    rice genome - ethylene biosynthesis - arabidopsis-thaliana - gene - sequence - resistance - plants - phloem - expression - diversity
    Cucumber is an economically important crop as well as a model system for sex determination studies and plant vascular biology. Here we report the draft genome sequence of Cucumis sativus var. sativus L., assembled using a novel combination of traditional Sanger and next-generation Illumina GA sequencing technologies to obtain 72.2-fold genome coverage. The absence of recent whole-genome duplication, along with the presence of few tandem duplications, explains the small number of genes in the cucumber. Our study establishes that five of the cucumber's seven chromosomes arose from fusions of ten ancestral chromosomes after divergence from Cucumis melo. The sequenced cucumber genome affords insight into traits such as its sex expression, disease resistance, biosynthesis of cucurbitacin and 'fresh green' odor. We also identify 686 gene clusters related to phloem function. The cucumber genome provides a valuable resource for developing elite cultivars and for studying the evolution and function of the plant vascular system
    Aphid watery saliva counteracts sieve-tube occlusion: a universal phenomenon?
    Will, T. ; Kornemann, S.R. ; Furch, A.C.U. ; Tjallingii, W.F. ; Bel, A.J.E. van - \ 2009
    Journal of Experimental Biology 212 (2009). - ISSN 0022-0949 - p. 3305 - 3312.
    companion cell - phloem - element - plant - pressure - proteins - translocation - penetration - acquisition - acceptance
    Ca2+-binding proteins in the watery saliva of Megoura viciae counteract Ca2+-dependent occlusion of sieve plates in Vicia faba and so prevent the shut-down of food supply in response to stylet penetration. The question arises whether this interaction between aphid saliva and sieve-element proteins is a universal phenomenon as inferred by the coincidence between sieve-tube occlusion and salivation. For this purpose, leaf tips were burnt in a number of plant species from four different families to induce remote sieve-plate occlusion. Resultant sieve-plate occlusion in these plant species was counteracted by an abrupt switch of aphid behaviour. Each of the seven aphid species tested interrupted its feeding behaviour and started secreting watery saliva. The protein composition of watery saliva appeared strikingly different between aphid species with less than 50% overlap. Secretion of watery saliva seems to be a universal means to suppress sieve-plate occlusion, although the protein composition of watery saliva seems to diverge between species.
    MRI of intact plants
    As, H. van; Scheenen, T. ; Vergeldt, F.J. - \ 2009
    Photosynthesis Research 102 (2009)2-3. - ISSN 0166-8595 - p. 213 - 222.
    nuclear-magnetic-resonance - distance water transport - membrane-permeability - diffusion constants - spin relaxation - nmr microscopy - sap flow - photosynthesis - phloem - xylem
    Nuclear magnetic resonance imaging (MRI) is a non-destructive and non-invasive technique that can be used to acquire two- or even three-dimensional images of intact plants. The information within the images can be manipulated and used to study the dynamics of plant water relations and water transport in the stem, e.g., as a function of environmental (stress) conditions. Non-spatially resolved portable NMR is becoming available to study leaf water content and distribution of water in different (sub-cellular) compartments. These parameters directly relate to stomatal water conductance, CO2 uptake, and photosynthesis. MRI applied on plants is not a straight forward extension of the methods discussed for (bio)medical MRI. This educational review explains the basic physical principles of plant MRI, with a focus on the spatial resolution, factors that determine the spatial resolution, and its unique information for applications in plant water relations that directly relate to plant photosynthetic activity
    Comparison of xylem flow velocities determined by MRI and a non-invasive heat pulse technique in Golden Alder and Silver Birch
    Helfter, C. ; Hand, D. ; Windt, C.W. ; As, H. van; Mencuccini, M. - \ 2007
    Comparative Biochemistry and Physiology. A, Molecular and Integrative Physiology 146 (2007)4, suppl.1. - ISSN 1095-6433 - p. S65 - S66.
    Molecular sabotage of plant defense by aphid saliva
    Will, T. ; Tjallingii, W.F. ; Thönnessen, A. ; Bel, A.J.E. van - \ 2007
    Proceedings of the National Academy of Sciences of the United States of America 104 (2007)25. - ISSN 0027-8424 - p. 10536 - 10541.
    calcium-binding proteins - sieve tubes - gel-electrophoresis - polyacrylamide gels - phloem - penetration - potentials - resistance - membrane - gossypii
    Aphids, which constitute one of the most important groups of agricultural pests, ingest nutrients from sieve tubes, the photoassimilate transport conduits in plants. Aphids are able to successfully puncture sieve tubes with their piercing mouthparts (stylets) and ingest phloem sap without eliciting the sieve tubes' normal occlusion response to injury. Occlusion mechanisms are calcium-triggered and may be prevented by chemical constituents in aphid saliva injected into sieve tubes before and during feeding. We recorded aphid feeding behavior with the electrical penetration graph (EPG) technique and then experimentally induced sieve tube plugging. Initiation of sieve tube occlusion caused a change in aphid behavior from phloem sap ingestion to secretion of watery saliva. Direct proof of "unplugging" properties of aphid saliva was provided by the effect of aphid saliva on forisomes. Forisomes are proteinaceous inclusions in sieve tubes of legumes that show calcium-regulated changes in conformation between a contracted state (below calcium threshold) that does not occlude the sieve tubes and a dispersed state (above calcium threshold) that occludes the sieve tubes. We demonstrated in vitro that aphid saliva induces dispersed forisomes to revert back to the nonplugging contracted state. Labeling Western-blotted saliva proteins with 45Ca2+ or ruthenium red inferred the presence of calcium-binding domains. These results demonstrate that aphid saliva has the ability to prevent sieve tube plugging by molecular interactions between salivary proteins and calcium. This provides aphids with access to a continuous flow of phloem sap and is a critical adaptation instrumental in the evolutionary success of aphids.
    Local and systemic responses induced by aphids in Solanum tuberosum plants
    Dugravot, S. ; Brunissen, L. ; Létocart, E. ; Tjallingii, W.F. ; Vincent, C. ; Giordanengo, Ph. ; Cherqui, A. - \ 2007
    Entomologia Experimentalis et Applicata 123 (2007)3. - ISSN 0013-8703 - p. 271 - 277.
    electrical penetration graphs - induced resistance - feeding-behavior - myzus-persicae - host-plants - macrosiphum-euphorbia - potato aphid - phloem - infestation - herbivores
    The aphids Macrosiphum euphorbiae (Thomas) and Myzus persicae (Sulzer) (Homoptera: Aphididae) are serious pests of potato (Solanum tuberosum L.) (Solanaceae), notably in transmitting several plant viruses. Heterospecific interactions may occur between these two species as they are often seen at the same time on the same potato plant in the field. As aphid infestation is known to induce both local and systemic changes, we conducted experiments to determine the effect of previous infestation on probing behaviour and feeding-related parameters. We used the DC electrical penetration graph technique to characterize the influence of previous infestation by conspecific M. persicae or by heterospecific Ma. euphorbiae on M. persicae feeding behaviour at both local and systemic levels, i.e., on previously infested leaves and on non-previously infested leaves of infested plants, respectively. Conspecific and heterospecific infestation led to similar modification of M. persicae feeding activities. However, the effects of previous infestation occurring at the local level were opposite to those observed at the systemic level. Myzus persicae food acceptance was slightly enhanced on previously infested leaves, whereas it was inhibited on non-infested leaves of infested plants, which indicated an induced resistance mechanism. Our results advance the understanding of the mechanisms involved in aphid-host plant acceptance and colonization processes on potato plants in conspecific and heterospecific situations.
    Nuclear magnetic response imaging of sap flow in plants
    Windt, C.W. - \ 2007
    Wageningen University. Promotor(en): Herbert van Amerongen; T.J. Schaafsma, co-promotor(en): Henk van As. - [S.l.] : S.n. - ISBN 9789085047292 - 172
    sapstroom - kernmagnetische resonantie - transport over water - floëem - ricinus communis - sap flow - nuclear magnetic resonance - water transport - phloem - ricinus communis

    This thesis deals with Nuclear Magnetic Resonance (NMR) imaging of long distance transport in plants. Long distance transport in plants is an enigmatic process. The theoretical framework that describes its basic properties has been in place for almost a century, yet at the same time only little is known about the dynamics of long distance transport inside the living plant. The latter is caused by the fact that the two pathways in which transport takes place, the xylem and the phloem, are virtually inaccessible to invasive experimentation. As a result a wide range of questions about the dynamics of long distance transport have yet to be answered. Examples of such questions, as addressed in this study, are: how fast does phloem sap move; how variable is the phloem sap flow velocity between species and over the diurnal cycle; what percentage of the potential flow conducting area in xylem tissue is functional. Or with regard to fruits: what percentage of the influx to fruits occurs through the xylem, what percentage through the phloem; does the xylem remain functional throughout fruit development; and does backflow from the fruit to the plant occur. Here, we show that NMR flow imaging provides a non-invasive and quantitative means to answer these intriguing questions.
    In order to be able to compare the results from different plants, the flow imaging data need to be independent from anatomical characteristics such as conduit diameter. To quantify flow, the signal from a known quantity of water in a reference object is compared with the signal from flowing water in the plant. When the NMR signals arising from the flowing water in the plant and that of the reference object have different relaxation rates, quantification problems may arise. In porous materials relaxation is influenced by pore diameter. Correspondingly, the relaxation of flowing water in plants will be influenced by conduit diameter. We developed a T2 resolved flow imaging method to measure the T2 relaxation behaviour of the flowing water (chapter 2), and used it to determine how xylem conduit diameter affects T2 relaxation. Furthermore, we investigated whether conduit diameter dependent T2 changes need to be corrected for when quantifying results of NMR flow imaging (chapter 3). We found that conduit diameter indeed affected the T2 of the flowing water. However, because the effects were relatively small, T2 resolved flow imaging was not needed to correct for conduit diameter induced changes in T2. Standard, non flow resolved T2 imaging sufficed. The accuracy of the quantification of volume flow in the xylem in all cases was ±10% or better. The T2 resolved flow imaging sequence that was developed will be of interest for research that deals with liquids moving through microscopic conduits, such as porous media, bioreactors, and biomats.
    With regard to the dynamics and basic properties of xylem and phloem transport, three subjects are taken into account: xylem transport, phloem transport, and long distance transport to fruits.
    For the xylem, one of the questions to be solved is how much of the potential flow conducting area that is present in the xylem anatomy, is in reality used to conduct flow. Xylem tissue consists of many conduits of varying diameters, ranging from small to large. We investigated the relationship between xylem conduit diameter distribution and flow conducting area in stems of various plant species (chapter 3), and the relation between the diurnal dynamics of xylem sap flow and the flow conducting area (chapter 4). We found that in the stems with the widest conduits only a small proportion of the potential flow conducting area conducted flow (as low as 31%). In stems that only possessed narrow conduits, a much larger part of the total xylem conduit cross sectional area conducted flow (up to 86%). We conclude that when wide conduits are present and are functional, the role of the narrowest conduits in terms of the conductance of water is almost negligible. Secondly, we found that the flow conducting area does not stay constant throughout the diurnal cycle. Decreases in xylem flux at night were accompanied by a decrease in velocity, but also by a decrease in flow-conducting area. It is well known that plant stems exhibit a diurnal pattern of shrinkage and expansion due to changes in xylem pressure. However, the diurnal changes in the flow conducting area were opposite to the changes in stem diameter and thus could not be explained by pressure dependent elastic changes in conduit diameter.
    With regard to the phloem we investigated the following questions: how fast does phloem sap move, how variable is the sap flow velocity between species and over the diurnal cycle; and what percentage of the shoot-bound xylem sap is returned to the root system by means of the phloem (chapter 4). Furthermore we investigated how phloem volume flow responds to local cooling (chapter 6). We compared the diurnal phloem and xylem flow dynamics in poplar, tomato, castor bean, and tobacco. In contrast to the highly variable sap flow velocities in the xylem, the sap flow velocities in the phloem remained very constant throughout the diurnal cycle. The differences in the average phloem flow velocity between the four species also were remarkably small (0.25 - 0.40 mm/s). We hypothesize that upper and lower bounds for phloem flow velocity may exist: when phloem flow velocity is too high, wall bound (parietal) organelles may be stripped away from sieve tube walls; when sap flow is too slow or is highly variable, phloem borne signalling could become unpredictable. The phloem to xylem volume flow ratio reflects the amount of xylem water that within the plant is (re)used for phloem transport. It may be indicative for the water use efficiency of a plant. This ratio was surprisingly large at night for poplar, castor bean and tobacco (ranging from 0.19 for poplar to 0.55 in tobacco), but as low as 0.04 in tomato.
    With regard to long distance transport to fruits we investigated three long-standing questions: how much of the influx into the fruit occurs by means of the xylem, and how much by means of the phloem; does the xylem remain functional throughout fruit development; and does backflow from the fruit to the plant occur. As a model system we used a tomato truss. We found that xylem transport into the truss remained functional throughout the full 8 weeks of truss growth. During that period at least 75% of the net influx occurred through the xylem, and about 25% through a region that contains both internal phloem and internal xylem (perimedullary region). These results contradict earlier estimates that were made on the basis of indirect measurements. Halfway during truss development a xylem backflow to the plant appeared. However, the influx volume always was larger, implying that there was no net loss of water from the truss to the plant. Interestingly a circulation of xylem sap in the truss stalk remained even after the fruits were removed, probably caused by pressure gradients originating from the main stem. During the experiment about half of the cumulative net influx into the truss was lost to the air due to evaporation.
    Because of the extreme sensitivity of xylem and phloem to invasive experimentation, only little is known about the dynamics of long distance transport in the living plant. The fact that the exploratory NMR flow imaging experiments in this study swiftly turned up a number of surprises with regard to the dynamics of flow underscores this observation. We found indications that phloem flow velocity is much more constant in nature than previously assumed, over the course of a day as well as between species. We observed that, depending on the species, at night a significant amount of xylem water can be recycled by means of the phloem, thus helping to maintain xylem circulation during periods of low transpiration. With regard to long distance transport to fruits, we found that during truss growth in tomato the majority of water influx does not occur by means of the phloem, but through the xylem. These results illustrate that the dynamics of xylem and phloem sap flow in the living plant are far from being understood, but also that NMR flow imaging provides an excellent non-invasive tool to help elucidate it.

    Intact plant magnetic resonance imaging to study dynamics in long-distance sap flow and flow-conducting surface area
    Scheenen, T.W.J. ; Vergeldt, F.J. ; Heemskerk, A.M. ; As, H. van - \ 2007
    Plant Physiology 144 (2007)2. - ISSN 0032-0889 - p. 1157 - 1165.
    embolism repair - water transport - vessel contents - xylem vessel - phloem - nmr - microscopy - tissues - stems - mri
    Due to the fragile pressure gradients present in the xylem and phloem, methods to study sap flow must be minimally invasive. Magnetic resonance imaging (MRI) meets this condition. A dedicated MRI method to study sap flow has been applied to quantify long-distance xylem flow and hydraulics in an intact cucumber (Cucumis sativus) plant. The accuracy of this MRI method to quantify sap flow and effective flow-conducting area is demonstrated by measuring the flow characteristics of the water in a virtual slice through the stem and comparing the results with water uptake data and microscopy. The in-plane image resolution of 120 x 120 µm was high enough to distinguish large individual xylem vessels. Cooling the roots of the plant severely inhibited water uptake by the roots and increased the hydraulic resistance of the plant stem. This increase is at least partially due to the formation of embolisms in the xylem vessels. Refilling the larger vessels seems to be a lengthy process. Refilling started in the night after root cooling and continued while neighboring vessels at a distance of not more than 0.4 mm transported an equal amount of water as before root cooling. Relative differences in volume flow in different vascular bundles suggest differences in xylem tension for different vascular bundles. The amount of data and detail that are presented for this single plant demonstrates new possibilities for using MRI in studying the dynamics of long-distance transport in plants.
    Stiletpenetratie door bladluizen
    Tjallingii, W.F. - \ 2003
    Entomologische Berichten 63 (2003)5. - ISSN 0013-8827 - p. 110 - 116.
    aphidoidea - voedingsgewoonten - floëem - sapstroom - plantenweefsels - aphidoidea - feeding habits - phloem - sap flow - plant tissues
    Stylet penetration by aphids can be studied well by the electrical penetration graph (EPG) technique, a recording system which incorporates both plant and insect in an electrical circuit. A large number of details of plant-aphid interactions during stylet penetration has been elucidated by EPG-studies
    Functional imaging of plants: A nuclear magnetic resonance study of a cucumber plant
    Scheenen, T. ; Heemskerk, A. ; Jager, A. de; Vergeldt, F.J. ; As, H. van - \ 2002
    Biophysical Journal 82 (2002)1. - ISSN 0006-3495 - p. 481 - 492.
    nmr-microscopy - noninvasive measurement - water transport - sap ascent - flow - relaxation - xylem - tissues - h-1-nmr - phloem
    Functional magnetic resonance imaging was used to study transients of biophysical parameters in a cucumber plant in response to environmental changes. Detailed flow imaging experiments showed the location of xylem and phloem in the stem and the response of the following flow characteristics to the imposed environmental changes: the total amount of water, the amount of stationary and flowing water, the linear velocity of the flowing water, and the volume flow. The total measured volume flow through the plant stem was in good agreement with the independently measured water uptake by the roots. A separate analysis of the flow characteristics for two vascular bundles revealed that changes in volume flow of the xylem sap were accounted for by a change in linear-flow velocities in the xylem vessels. Multiple-spin echo experiments revealed two water fractions for different tissues in the plant stem; the spin-spin relaxation time of the larger fraction of parenchyma tissue in the center of the stem and the vascular tissue was down by 17% in the period after cooling the roots of the plant. This could point to an increased water permeability of the tonoplast membrane of the observed cells in this period of quick recovery from severe water loss
    Microscopic imaging of slow flow and diffusion: a pulsed field gradient stimulated echo sequence combined with turbo spin echo imaging
    Scheenen, T.W.J. ; Vergeldt, F.J. ; Windt, C.W. ; Jager, P.A. de; As, H. van - \ 2001
    Journal of Magnetic Resonance 151 (2001)1. - ISSN 1090-7807 - p. 94 - 100.
    water-flow - nmr - plants - transport - phloem - fluid - rare
    In this paper we present a pulse sequence that combines a displacement-encoded stimulated echo with rapid sampling of k-space by means of turbo spin echo imaging. The stimulated echo enables the use of long observation times between the two pulsed field gradients that sample q-space completely. Propagators, constructed with long observation times, could discriminate slowly flowing protons from diffusing protons, as shown in a phantom in which a plug flow with linear velocity of 50¿m/s could clearly be distinguished from stationary water. As a biological application the apparent diffusion constant in longitudinal direction of a transverse image of a maize plant stem had been measured as a function of observation time. Increasing contrast in the apparent diffusion constant image with increasing observation times were caused by differences in plant tissue: although the plant stem did not take up any water, the vascular bundles, concentrated in the outer ring of the stem, could still be discerned because of their longer unrestricted diffusional pathways for water in the longitudinal direction compared to cells in the parenchymal tissue. In the xylem region of a tomato pedicel flowing water could be distinguished from a large amount of stationary water. Linear flow velocities up to 0.67 mm/s were measured with an observation time of 180 ms.
    Bark anatomy of some Sarcolaenaceae and Rhopalocarpaceae and their systematic position
    Outer, R.W. den; Vooren, A.P. - \ 1980
    Wageningen : Veenman (Mededelingen Landbouwhogeschool 80-6) - 15 p.
    phloem - malvaceae - ochnaceae - forestry - wood - bark - wood properties
    Tentative determination key to 600 trees, shrubs and climbers from the Ivory Coast, Africa, mainly based on characters of the living bark, besides the rhytidome and the leaf
    Outer, R.W. den - \ 1972
    Wageningen : Veenman (Mededelingen Landbouwhogeschool 72-18,19,20,21) - 4
    planten - identificatie - bedektzadigen - bosbouw - bomen - floëem - schors, bomen - plantenmorfologie - ivoorkust - bladeren - plantenfysiologie - plants - identification - angiosperms - forestry - trees - phloem - bark - plant morphology - cote d'ivoire - leaves - plant physiology
    Movement of 45Ca in the phloem of Yucca
    Wiersum, L.K. ; Vonk, C.A. ; Tammes, P.M.L. - \ 1971
    Wageningen : [s.n.] (Publication / Plant physiological research centre no. 104) - 2
    floëem - liliaceae - yucca - calcium - voedingsstoffen - metabolisme - plantenvoeding - assimilatie - phloem - liliaceae - yucca - calcium - nutrients - metabolism - plant nutrition - assimilation
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