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Measuring and modelling long-distance water and sugar transport in trees

Veerle De Schepper UGent (2011)
abstract
Recently, it is generally recognized in literature that approaching water and sugar transport in an integrated way will broaden our fundamental understanding of how trees, and in general plants, behave. Therefore, this PhD-study aimed to enlarge our insight in sugar transport by simultaneously considering water transport. To this end, experimental and modelling studies were performed in which both transport processes were concomitantly measured or modelled. In literature, detailed information about the water and the sugar transport is given. The water transport is driven by the cohesion-tension theory, whereas the sugar transport is driven by the Münch theory. The original Münch theory is adapted by introducing (1) active-passive (un)loading and (2) a leakage-retrieval process along the transport phloem. More and more evidence suggests that a relay system is present in the phloem to maintain a feasible pressure gradient in the sieve tubes. Besides sugar transport, the phloem tissue is also involved in information transmission between sources and sinks. It is a challenge to study sugar transport due to the protective nature of sieve tubes which makes sampling very difficult. Water and sugar transport are integrated at the whole plant level mainly by water recycling, but also by the exchange of potassium, sugars and hormones. Stem diameter variations are influenced by both the water and carbon status of the stem. Therefore, they are a powerful tool to simultaneously study sugar and water transport. In this PhD-study, the effects of a manipulated sugar flow on both transport processes were investigated by analysing stem diameter variations of young oak trees (Quercus robur L.). The manipulation was obtained by mechanically removing two bands of bark (cf. double girdling) and by local stem chilling (cf. cold girdling). Double girdling changed the sugar flow and influenced as such stem growth: stem increment enhanced above the girdled zones (U) and halted below (L) and in between (M) the two girdled zones. The changed sugar transport also resulted in observed time lags between the times of morning shrinkage at different stem heights. These time lags are probably attributed to different sugar contents in the bark. In addition, the water transport was indirectly affected by the manipulated sugar transport. A feedback inhibition of photosynthesis probably provoked stomatal closure, decreasing as such transpiration and the observed xylem sap flow. The effects observed after cold girdling were similar but less pronounced, because chilling only moderately increased the local resistance in both phloem and xylem. The chilling effects were best observed in the middle of the growing season. In this PhD-study, a comprehensive whole tree model was developed that enabled simulation of stem diameter variations driven by both water and sugar transport. This mechanistic model describes stem diameter variations as volume changes dependent on the water transport according to the cohesion-tension theory, the sugar transport according to the Münch hypothesis and irreversible turgor-driven growth according to the Lockhart equation. Modelled and measured data were compared to verify the implemented mechanisms. The measured data consisted of measurements of stem diameter variations and xylem sap flow rates. These measurements were performed on a young oak tree (Quercus robur L.) and on an adult beech tree (Fagus sylvatica L.). A good correspondence between measured and simulated data proves that the model provides a realistic presentation of the simulated processes. In addition the model is able to simulate some physiological processes which are difficult to measure. Therefore, the model was applied to evaluate the mechanisms behind the girdling responses. When the loading and unloading functions of the original model were adapted, measured and simulated stem diameter variations fitted well. According to these model simulations, changes in the sieve tube pressure seemed to trigger all observed girdling responses. Hence, it seemed that information about local damage to the phloem system was transferred to other plant parts by means of a change in phloem pressure. To measure processes related to sugar transport, techniques which measure these processes in a non-destructive way have a high scientific potential. Therefore, two non-invasive techniques are applied in this PhD-study. The first applied method was the magnetic resonance imaging (MRI) technique. MRI was used to verify whether stem diameter variations are caused by changes in stem water content. This relationship was experimentally confirmed by comparing MRI data with dendrometer data. Hence, the interpretation and application of dendrometers is validated by this MRI-study. In addition, the same MRI-study confirmed that the xylem contribution to the total stem diameter variations was much lower than the bark contribution. The MRI-study also revealed that the bark tissues, containing the highest amount of water (the cambium and the youngest, probably conductive, phloem), contributed the least to the bark diameter variations. Hence, the bark tissues which are less physiologically active contribute the most to the total stem diameter variations. When MRI was applied to study the stem increment above a girdled zone, MRI data illustrated that both irreversible structural growth as reversible swelling took place. The second applied non-invasive technique was labelling with the radio-active 11C isotope. The 11C data were analysed in two ways: three-dimensional (3D) visualization of the 11C flow and analysing typical 11C-time-series by applying an input-output model and a mechanistic tracer model. The obtained 3D-images illustrated the sectorial behaviour of sugar transport and its breakdown after bark manipulation. The model analyses of the 11C-time-series showed that the transport velocity and the pathway loss of the sugar flow changed after bark manipulation. Based on these model results, it is suggested that the breakdown of sectoriality is induced by an active change in the lateral transport path, for example by modifying the conductance of plasmodesmata. In conclusion, the performed experimental and modelling studies demonstrated that water and sugar transport are interrelated. Some of the in the literature proposed hypotheses were tested and confirmed. Despite, many questions remain unanswered: which mechanisms trigger the MRI-measured phloem water flow dynamics, which mechanisms drive the altered sugar leakage after girdling and which mechanisms are responsible for sectorial breakdown.
Please use this url to cite or link to this publication:
author
promoter
UGent
organization
alternative title
Meten en modelleren van het lange-afstandstransport van water en suiker in bomen
year
type
dissertation (monograph)
subject
keyword
Quercus robur, water transport, Modelling, sugar transport, phloem, translocation, girdling
pages
XV, 223 pages
publisher
Ghent University. Faculty of Bioscience Engineering
place of publication
Ghent, Belgium
defense location
Gent : Faculteit Bio-ingenieurswetenschappen (A0.030)
defense date
2011-10-03 16:00
ISBN
9789059894624
language
English
UGent publication?
yes
classification
D1
additional info
dissertation consists of copyrighted material
copyright statement
I have transferred the copyright for this publication to the publisher
id
1908904
handle
http://hdl.handle.net/1854/LU-1908904
date created
2011-09-26 16:00:59
date last changed
2012-06-08 08:41:08
@phdthesis{1908904,
  abstract     = {Recently, it is generally recognized in literature that approaching water and sugar transport in an integrated way will broaden our fundamental understanding of how trees, and in general plants, behave. Therefore, this PhD-study aimed to enlarge our insight in sugar transport by simultaneously considering water transport. To this end, experimental and modelling studies were performed in which both transport processes were concomitantly measured or modelled. 
In literature, detailed information about the water and the sugar transport is given. The water transport is driven by the cohesion-tension theory, whereas the sugar transport is driven by the M{\"u}nch theory. The original M{\"u}nch theory is adapted by introducing (1) active-passive (un)loading and (2) a leakage-retrieval process along the transport phloem. More and more evidence suggests that a relay system is present in the phloem to maintain a feasible pressure gradient in the sieve tubes. Besides sugar transport, the phloem tissue is also involved in information transmission between sources and sinks. It is a challenge to study sugar transport due to the protective nature of sieve tubes which makes sampling very difficult. Water and sugar transport are integrated at the whole plant level mainly by water recycling, but also by the exchange of potassium, sugars and hormones. 
Stem diameter variations are influenced by both the water and carbon status of the stem. Therefore, they are a powerful tool to simultaneously study sugar and water transport. In this PhD-study, the effects of a manipulated sugar flow on both transport processes were investigated by analysing stem diameter variations of young oak trees (Quercus robur L.). The manipulation was obtained by mechanically removing two bands of bark (cf. double girdling) and by local stem chilling (cf. cold girdling). Double girdling changed the sugar flow and influenced as such stem growth: stem increment enhanced above the girdled zones (U) and halted below (L) and in between (M) the two girdled zones. The changed sugar transport also resulted in observed time lags between the times of morning shrinkage at different stem heights. These time lags are probably attributed to different sugar contents in the bark. In addition, the water transport was indirectly affected by the manipulated sugar transport. A feedback inhibition of photosynthesis probably provoked stomatal closure, decreasing as such transpiration and the observed xylem sap flow. The effects observed after cold girdling were similar but less pronounced, because chilling only moderately increased the local resistance in both phloem and xylem. The chilling effects were best observed in the middle of the growing season.
In this PhD-study, a comprehensive whole tree model was developed that enabled simulation of stem diameter variations driven by both water and sugar transport. This mechanistic model describes stem diameter variations as volume changes dependent on the water transport according to the cohesion-tension theory, the sugar transport according to the M{\"u}nch hypothesis and irreversible turgor-driven growth according to the Lockhart equation. Modelled and measured data were compared to verify the implemented mechanisms. The measured data consisted of measurements of stem diameter variations and xylem sap flow rates. These measurements were performed on a young oak tree (Quercus robur L.) and on an adult beech tree (Fagus sylvatica L.). A good correspondence between measured and simulated data proves that the model provides a realistic presentation of the simulated processes. In addition the model is able to simulate some physiological processes which are difficult to measure. Therefore, the model was applied to evaluate the mechanisms behind the girdling responses. When the loading and unloading functions of the original model were adapted, measured and simulated stem diameter variations fitted well. According to these model simulations, changes in the sieve tube pressure seemed to trigger all observed girdling responses. Hence, it seemed that information about local damage to the phloem system was transferred to other plant parts by means of a change in phloem pressure. 
To measure processes related to sugar transport, techniques which measure these processes in a non-destructive way have a high scientific potential. Therefore, two non-invasive techniques are applied in this PhD-study. The first applied method was the magnetic resonance imaging (MRI) technique. MRI was used to verify whether stem diameter variations are caused by changes in stem water content. This relationship was experimentally confirmed by comparing MRI data with dendrometer data. Hence, the interpretation and application of dendrometers is validated by this MRI-study. In addition, the same MRI-study confirmed that the xylem contribution to the total stem diameter variations was much lower than the bark contribution. The MRI-study also revealed that the bark tissues, containing the highest amount of water (the cambium and the youngest, probably conductive, phloem), contributed the least to the bark diameter variations. Hence, the bark tissues which are less physiologically active contribute the most to the total stem diameter variations. When MRI was applied to study the stem increment above a girdled zone, MRI data illustrated that both irreversible structural growth as reversible swelling took place. The second applied non-invasive technique was labelling with the radio-active 11C isotope. The 11C data were analysed in two ways: three-dimensional (3D) visualization of the 11C flow and analysing typical 11C-time-series by applying an input-output model and a mechanistic tracer model. The obtained 3D-images illustrated the sectorial behaviour of sugar transport and its breakdown after bark manipulation. The model analyses of the 11C-time-series showed that the transport velocity and the pathway loss of the sugar flow changed after bark manipulation. Based on these model results, it is suggested that the breakdown of sectoriality is induced by an active change in the lateral transport path, for example by modifying the conductance of plasmodesmata. 
In conclusion, the performed experimental and modelling studies demonstrated that water and sugar transport are interrelated. Some of the in the literature proposed hypotheses were tested and confirmed. Despite, many questions remain unanswered: which mechanisms trigger the MRI-measured phloem water flow dynamics, which mechanisms drive the altered sugar leakage after girdling and which mechanisms are responsible for sectorial breakdown.},
  author       = {De Schepper, Veerle},
  isbn         = {9789059894624},
  keyword      = {Quercus robur,water transport,Modelling,sugar transport,phloem,translocation,girdling},
  language     = {eng},
  pages        = {XV, 223},
  publisher    = {Ghent University. Faculty of Bioscience Engineering},
  school       = {Ghent University},
  title        = {Measuring and modelling long-distance water and sugar transport in trees},
  year         = {2011},
}

Chicago
De Schepper, Veerle. 2011. “Measuring and Modelling Long-distance Water and Sugar Transport in Trees”. Ghent, Belgium: Ghent University. Faculty of Bioscience Engineering.
APA
De Schepper, V. (2011). Measuring and modelling long-distance water and sugar transport in trees. Ghent University. Faculty of Bioscience Engineering, Ghent, Belgium.
Vancouver
1.
De Schepper V. Measuring and modelling long-distance water and sugar transport in trees. [Ghent, Belgium]: Ghent University. Faculty of Bioscience Engineering; 2011.
MLA
De Schepper, Veerle. “Measuring and Modelling Long-distance Water and Sugar Transport in Trees.” 2011 : n. pag. Print.