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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

Het bloeden van appelbomen in het tweede teeltjaar kan leiden tot een lagere kwaliteit en zelfs uitval. Bloeden ontstaat rond begin april en wordt o.m. veroorzaakt door een oplopende bodemtemperatuur. De knipbomen staan dan nog niet in blad, de worteldruk neemt toe en de hoofdstam heeft een forse snoeiwond. Door deze combinatie kan het voorkomen dat plantsap a.h.w. door de snoeiwond wordt gedrukt. Dit heeft een negatief effect op de bovenste bladknop(pen). Door het bloeden bestaat het risico op een lagere kwaliteit of uitval. Naast de bodemtemperatuur hebben ook andere factoren zoals het moment van inknippen invloed op het ontstaan van het bloeden.

Het bloeden van appelbomen in het tweede teeltjaar kan leiden tot een lagere kwaliteit en zelfs uitval. Bloeden ontstaat rond begin april en wordt o.m. veroorzaakt door een oplopende bodemtemperatuur. De knipbomen staan dan nog niet in blad, de worteldruk neemt toe en de hoofdstam heeft een forse snoeiwond. Door deze combinatie kan het voorkomen dat plantsap a.h.w. door de snoeiwond wordt gedrukt. Dit heeft een negatief effect op de bovenste bladknop(pen). Door het bloeden bestaat het risico op een lagere kwaliteit of uitval. Naast de bodemtemperatuur hebben ook andere factoren zoals het moment van inknippen invloed op het ontstaan van het bloeden.

Onderzoek naar tabaksblad in kool toont aan dat deze groeiafwijking ontstaat tijdens de teelt. Factoren die tijdens de teelt van sluitkool het optreden van tabaksblad (inwendig rand) in belangrijke mate beinvloeden zijn de groeisnelheid en de waterhuishouding van het gewas

Due to their long life span, changing climatic conditions are of particular importance for trees. Climate changes will affect the water balance, which can become an important limiting factor for photosynthesis and growth. Long-distance water transport in trees is directly related to the transpiration stream and very sensitive to changes in the soil-plant-atmosphere water continuum. Therefore the study of long distance transport gives information about tree response to changing climatic conditions. Here the dynamic behaviour of water transport processes in trees has been studied by the MRI method, which is a direct and non-invasive tool.. MRI flow imaging has been applied to diffuse- and ring-porous trees to study drought stress and the occurrence of xylem vessel cavitations. World-wide unique dedicated MRI hardware is described that allows imaging of sap flow in intact trees with a maximal trunk diameter of 4 cm and height of several meters. This setup is used to investigate xylem and phloem flow in an intact tree quantitatively. Flow is quantified in terms of (averaged) velocity, volume flow (flux) and flow conducting area, either in imaging mode or resolved on the level of annual rings. Results obtained for the same tree, imaged at two different field strengths (0.7 and 3 T), are compared. An overall shortening of observed T2 values is manifest going from 0.7 to 3 T. Although susceptibility artefacts may be present at 3 T, the results are still reliable and the gain in sensitivity due to the higher magnetic field strength results in shorter measurement time (or a better spatial resolution or a higher signal to noise ratio) with respect to the 0.7 T system. By use of such dedicated hardware xylem and phloem flow, and its mutual interaction, can be studied in intact trees in relation to the water balance and in response to environmental (stress) conditions (Chapter 2). To further investigate the effect of susceptibility artefacts on MRI flow imaging by PFG-STE MRI on 3 T, water flow was studied in a number of model porous media with or without surface relaxation, internal magnetic field inhomogeneities (susceptibility artefacts) and exchange with stagnant water pools, mimicking the tree situation (Chapter 3). In such situations a clear dependence of the flow characteristics on the observation time is demonstrated. The most reliable results are obtained at relatively short observation times. This limits the observation of low flow velocities and the discrimination between flowing and non-flowing water. It is shown that correlated displacement-T2 measurements are available to improve the discrimination of flowing and non-flowing water and can be of help to decide about the functional activity of xylem conduits (Chapter 4). A method that reveals exchange between the flowing and stagnant fractions in the system is presented. Further it is demonstrated how this exchange can be quantified (Chapter 3). Xylem flow, flow conducting area and water content in the storage pools of sapwood and cambial zone were investigated simultaneously and non-destructively by MRI in diffuse-porous laurel (Laurus nobilis) and viburnum (Viburnum tinus) trees during a drydown period and recovery after watering (Chapter 4). The development of the drought stress was detected by the decrease in average velocity, volume flow and flow conducting area as observed by MRI flow imaging. A decrease in flow conducting area was observed with a delay of one day in comparison to the observed reduction in average velocity and volume flow. The re-watering of the plants resulted in the fast restore of the flow conducting area to the value observed under well watered conditions, demonstrating that if cavitations had been induced they refilled quite fast. In addition, a significant increase in the average velocity and volume flow was observed, but still lower than the original values. Imaging water content in the cambial zone indicated a gradual decrease of the water content, which speeded up during the drought stress. The rate of decrease was dependent of day/night conditions. Watering resulted in the partial restore of water content in this zone. Water content in sapwood showed a clear diurnal variation. The water storage pool in sapwood depleted quickly upon switching on the light, gradually restoring in the afternoon. Drought stress did not change the character of diurnal variation of water content significantly, but it increased the amplitude of the diurnal variation. Re-watering of the tree resulted in a 10% water loss in sapwood. Thus, for the first time the coupling between water floe in xylem vessels and water content in storage pools was demonstrated. The oldest annual ring was rather inactive in long distance water transport. We found that the transport activity of this ring was not sensitive to any environmental change and that the variation of water content in sapwood was uniform in all annual rings Non-destructive measurements of cavitation were made with MRI to test whether large earlywood vessels of ring-porous xylem are as vulnerable as some standard methods have suggested (Chapter 5). Potted, 3-4 year old Quercus robur L. trees were droughted to water potentials measured with temperature-corrected stem psychrometers. Imaging of (vessel) water content indicated that earlywood cavitation in trunks was not detectible until water potentials dropped below -3 MPa. Most earlywood vessels were cavitated below -4 MPa. Dye perfusions through excised branch segments gave comparable results. Imaging of flow conducting area (FCA) indicated a gradual decline in trunk water conduction that was not solely associated with cavitation, but probably resulted from stomatal closure and too low velocities to be discriminated from non-flowing water. Dye perfusion and FCA indicated a significant portion of earlywood vessels were non-conducting even at the most favorable water potentials. No refilling of embolized vessels was detected in rewatering experiments. Contradictory to the MRI results, standard centrifuge and air-injection methods on Q. robur stem segments indicated complete cavitation at xylem pressures at or below -1 MPa. An artifact in these destructive methods was revealed by experiments on the related species Q. gambelii Nutt. When earlywood vessels became air-filled during collection prior to being refilled in the lab, they became much more vulnerable to cavitation. Residual bubbles left behind in the refilled vessels may be responsible. These results suggest revised protocols for measuring vulnerability curves by destructive methods. An about linear correlation between water potential and decrease of water content in cambial zone of oak (Quercus robur L.) was observed (Chapter 5).

Due to their long life span, changing climatic conditions are of particular importance for trees. Climate changes will affect the water balance, which can become an important limiting factor for photosynthesis and growth. Long-distance water transport in trees is directly related to the transpiration stream and very sensitive to changes in the soil-plant-atmosphere water continuum. Therefore the study of long distance transport gives information about tree response to changing climatic conditions. Here the dynamic behaviour of water transport processes in trees has been studied by the MRI method, which is a direct and non-invasive tool.. MRI flow imaging has been applied to diffuse- and ring-porous trees to study drought stress and the occurrence of xylem vessel cavitations. World-wide unique dedicated MRI hardware is described that allows imaging of sap flow in intact trees with a maximal trunk diameter of 4 cm and height of several meters. This setup is used to investigate xylem and phloem flow in an intact tree quantitatively. Flow is quantified in terms of (averaged) velocity, volume flow (flux) and flow conducting area, either in imaging mode or resolved on the level of annual rings. Results obtained for the same tree, imaged at two different field strengths (0.7 and 3 T), are compared. An overall shortening of observed T2 values is manifest going from 0.7 to 3 T. Although susceptibility artefacts may be present at 3 T, the results are still reliable and the gain in sensitivity due to the higher magnetic field strength results in shorter measurement time (or a better spatial resolution or a higher signal to noise ratio) with respect to the 0.7 T system. By use of such dedicated hardware xylem and phloem flow, and its mutual interaction, can be studied in intact trees in relation to the water balance and in response to environmental (stress) conditions (Chapter 2). To further investigate the effect of susceptibility artefacts on MRI flow imaging by PFG-STE MRI on 3 T, water flow was studied in a number of model porous media with or without surface relaxation, internal magnetic field inhomogeneities (susceptibility artefacts) and exchange with stagnant water pools, mimicking the tree situation (Chapter 3). In such situations a clear dependence of the flow characteristics on the observation time is demonstrated. The most reliable results are obtained at relatively short observation times. This limits the observation of low flow velocities and the discrimination between flowing and non-flowing water. It is shown that correlated displacement-T2 measurements are available to improve the discrimination of flowing and non-flowing water and can be of help to decide about the functional activity of xylem conduits (Chapter 4). A method that reveals exchange between the flowing and stagnant fractions in the system is presented. Further it is demonstrated how this exchange can be quantified (Chapter 3). Xylem flow, flow conducting area and water content in the storage pools of sapwood and cambial zone were investigated simultaneously and non-destructively by MRI in diffuse-porous laurel (Laurus nobilis) and viburnum (Viburnum tinus) trees during a drydown period and recovery after watering (Chapter 4). The development of the drought stress was detected by the decrease in average velocity, volume flow and flow conducting area as observed by MRI flow imaging. A decrease in flow conducting area was observed with a delay of one day in comparison to the observed reduction in average velocity and volume flow. The re-watering of the plants resulted in the fast restore of the flow conducting area to the value observed under well watered conditions, demonstrating that if cavitations had been induced they refilled quite fast. In addition, a significant increase in the average velocity and volume flow was observed, but still lower than the original values. Imaging water content in the cambial zone indicated a gradual decrease of the water content, which speeded up during the drought stress. The rate of decrease was dependent of day/night conditions. Watering resulted in the partial restore of water content in this zone. Water content in sapwood showed a clear diurnal variation. The water storage pool in sapwood depleted quickly upon switching on the light, gradually restoring in the afternoon. Drought stress did not change the character of diurnal variation of water content significantly, but it increased the amplitude of the diurnal variation. Re-watering of the tree resulted in a 10% water loss in sapwood. Thus, for the first time the coupling between water floe in xylem vessels and water content in storage pools was demonstrated. The oldest annual ring was rather inactive in long distance water transport. We found that the transport activity of this ring was not sensitive to any environmental change and that the variation of water content in sapwood was uniform in all annual rings Non-destructive measurements of cavitation were made with MRI to test whether large earlywood vessels of ring-porous xylem are as vulnerable as some standard methods have suggested (Chapter 5). Potted, 3-4 year old Quercus robur L. trees were droughted to water potentials measured with temperature-corrected stem psychrometers. Imaging of (vessel) water content indicated that earlywood cavitation in trunks was not detectible until water potentials dropped below -3 MPa. Most earlywood vessels were cavitated below -4 MPa. Dye perfusions through excised branch segments gave comparable results. Imaging of flow conducting area (FCA) indicated a gradual decline in trunk water conduction that was not solely associated with cavitation, but probably resulted from stomatal closure and too low velocities to be discriminated from non-flowing water. Dye perfusion and FCA indicated a significant portion of earlywood vessels were non-conducting even at the most favorable water potentials. No refilling of embolized vessels was detected in rewatering experiments. Contradictory to the MRI results, standard centrifuge and air-injection methods on Q. robur stem segments indicated complete cavitation at xylem pressures at or below -1 MPa. An artifact in these destructive methods was revealed by experiments on the related species Q. gambelii Nutt. When earlywood vessels became air-filled during collection prior to being refilled in the lab, they became much more vulnerable to cavitation. Residual bubbles left behind in the refilled vessels may be responsible. These results suggest revised protocols for measuring vulnerability curves by destructive methods. An about linear correlation between water potential and decrease of water content in cambial zone of oak (Quercus robur L.) was observed (Chapter 5).

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.

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 De penetratie van planten door bladluisstiletten laat zich goed onderzoeken met het elektropenetratiegram (EPG), een registratiesysteem waarbij plant en insect deel uitmaken van een elektrisch circuit. Een groot aantal details van de plant-bladluisinteracties tijdens stilet penetraties is door EPG-studies opgehelderd

Overzicht van processen in de champignon die met voeding, transport en verdamping te maken hebben

Telers willen meer inzicht in factoren die de groei van de champignon beïnvloeden, de verdamping van het water en het effect van sproeihoeveelheden en tijdstippen. In dit artikel een overzicht van onderzochte thema's, factoren en bijbehorende behandelingen en van teeltomstandigheden waarbij een toename van intern vocht werd waargenomen

Invloed van voedingselementen op het voorkomen van zweten en bladkiepen tijdens tulpenbroei. Gegevens in bijgaande tabel: Bladkiep en CA-gehalte van de plant, bij verschillende behandelingen (teelt op potgrond, teelt op demiwater, bemest, bemest en belucht)

Onderzoek naar tabaksblad in kool toont aan dat deze groeiafwijking ontstaat tijdens de teelt. Factoren die tijdens de teelt van sluitkool het optreden van tabaksblad (inwendig rand) in belangrijke mate beinvloeden zijn de groeisnelheid en de waterhuishouding van het gewas

Telers willen meer inzicht in factoren die de groei van de champignon beïnvloeden, de verdamping van het water en het effect van sproeihoeveelheden en tijdstippen. In dit artikel een overzicht van onderzochte thema's, factoren en bijbehorende behandelingen en van teeltomstandigheden waarbij een toename van intern vocht werd waargenomen

Overzicht van processen in de champignon die met voeding, transport en verdamping te maken hebben

Invloed van voedingselementen op het voorkomen van zweten en bladkiepen tijdens tulpenbroei. Gegevens in bijgaande tabel: Bladkiep en CA-gehalte van de plant, bij verschillende behandelingen (teelt op potgrond, teelt op demiwater, bemest, bemest en belucht)

Nuclear magnetic resonance (NMR) was used to investigate the effects of changes in root temperature, of changes in the area of root in contact with culture solution and of day/night rhythm on the water balance of a cucumber and a gherkin plant. Results are discussed in terms of water potential, flow rate and resistance using a previously presented model of water balance. As long as water uptake alone is varied, flow rate and water content (or potential) will change in the same direction. In contrast, from that model it is predicted that changes in transpiration will affect flow rate and water content in opposite ways. An experimental verification of this prediction was given in the previous paper. Results obtained by the NMR method are compared to those determined using a dendrometer. The results demonstrate that the NMR method is a valuable tool to study plant water balance and that it can serve as a technique for discriminating between changes in plant water balance that are due to changes in water uptake by roots and those due to changes in transpiration.

This Thesis describes the application of a non-destructive pulsed proton NMR method mainly to measure water transport in the xylem vessels of plant stems and in some model systems. The results are equally well applicable to liquid flow in other biological objects than plants, e.g. flow of blood and other body fluids in human and animals (Chapter 8). The method is based on a pulse sequence of equidistantπpulses in combination with a linear magnetic field gradient G.Following a general introduction and a survey of the properties of water in plants (Chapters 1 and 2), the basic NMR theory as well as reviews on the application of pulsed NMR to the determination of flow, diffusion and water content are presented in chapter 3.A mathematical treatment has produced analytical expressions for the shape of the signal S(t), based on a model in which the flowing fluid is thought to receive a ½π-τ-(π-τ-) n pulse train: a ½πpulse upon entering the r.f. coil followed by a sequence of equidistantπpulses until the fluid leaves the coil; simultaneously, this movement. of the fluid along a magnetic field gradient applied in the direction of flow produces a phase shift of the nuclear magnetization with respect to the rotating frame of reference (Chapter 4). Although this model does not lead to perfect agreement between the experimental and theoretical signal shape S(t), it correctly predicts the effects of experimental parameters on S(t) via analytical expressions. The main results from this theoretical treatment in combination with computer simulations, which have been experimentally verified in glass capillary systems, are:- as long as T 2≥ ½T 1 , the mean linear flow velocity v can be found from the time t max at which a maximum appears in the signal shape: v=C/t max , where C is a calibration constant, depending on G,τand the flow profile. If T 2 >T 2 ) or∂[S(t max ) . t max ]/∂t max vs. t max (T 1≈ T 2 ).Based on flow measurements in plant stem segments (Chapter 5) it has been suggested that T 2 strongly depends on the vessel diameter for the narrow xylem capillaries. This behaviour of T 2 can explain negative results in plant stems with small vessel diameter. Under the present experimental conditions the method has been successfully applied to Cucurbitaceae (cucumber, gherkin, pumpkin) and tomato plants.T 2 measurements in wheat leaves have been shown to be insensitive to the presence of cell-bound paramagnetic ions (Chapter 7). The magnitude of T 2 of two separate water fractions (covering -90% of the total water content) has been found to be inversely proportional to water content. Measurements of flow and water content have been combined for an intact gherkin plant (Chapter 5), demonstrating that the combination of both NMR methods results in a powerful non-invasive method to study important parts of the plant water balance simultaneously. The results strongly suggest that the method can be used as an early warning for development of stress phenomena in plants, due to drought and other factors. From the flow measurements it has been shown how in a plant system the values of T 2 and T 1 of the water in the xylem vessels can be determined and estimated, respectively.A comparison between the results obtained with NMR, heat pulse and weight balance flow measurements is presented in Chapter 6. A linear relationship between the linear flow velocity obtained by NMR and the volume flowrate determined by the balance method yields an effective cross-sectional area available for flow of ~50% of the cross-sectional area of the xylem vessels measured by using a microscope. NMR measurements alone yield a slightly lower value of the effective cross-sectional area. Compared with the NMR method, the heat pulse method monitors only relative changes in the flow velocity. A plot of the flow velocity obtained by the heat pulse method versus the volume flowrate obtained by the balance method exhibits some unwanted experimental scatter.Chapter 8 suggests some applications of the pulsed NMR flow method, also to other systems than plants, and defines important instrumental requirements for these applications.

Nuclear magnetic resonance (NMR) was used to investigate the effects of changes in root temperature, of changes in the area of root in contact with culture solution and of day/night rhythm on the water balance of a cucumber and a gherkin plant. Results are discussed in terms of water potential, flow rate and resistance using a previously presented model of water balance. As long as water uptake alone is varied, flow rate and water content (or potential) will change in the same direction. In contrast, from that model it is predicted that changes in transpiration will affect flow rate and water content in opposite ways. An experimental verification of this prediction was given in the previous paper. Results obtained by the NMR method are compared to those determined using a dendrometer. The results demonstrate that the NMR method is a valuable tool to study plant water balance and that it can serve as a technique for discriminating between changes in plant water balance that are due to changes in water uptake by roots and those due to changes in transpiration.

This Thesis describes the application of a non-destructive pulsed proton NMR method mainly to measure water transport in the xylem vessels of plant stems and in some model systems. The results are equally well applicable to liquid flow in other biological objects than plants, e.g. flow of blood and other body fluids in human and animals (Chapter 8). The method is based on a pulse sequence of equidistantπpulses in combination with a linear magnetic field gradient G.Following a general introduction and a survey of the properties of water in plants (Chapters 1 and 2), the basic NMR theory as well as reviews on the application of pulsed NMR to the determination of flow, diffusion and water content are presented in chapter 3.A mathematical treatment has produced analytical expressions for the shape of the signal S(t), based on a model in which the flowing fluid is thought to receive a ½π-τ-(π-τ-) n pulse train: a ½πpulse upon entering the r.f. coil followed by a sequence of equidistantπpulses until the fluid leaves the coil; simultaneously, this movement. of the fluid along a magnetic field gradient applied in the direction of flow produces a phase shift of the nuclear magnetization with respect to the rotating frame of reference (Chapter 4). Although this model does not lead to perfect agreement between the experimental and theoretical signal shape S(t), it correctly predicts the effects of experimental parameters on S(t) via analytical expressions. The main results from this theoretical treatment in combination with computer simulations, which have been experimentally verified in glass capillary systems, are:- as long as T 2≥ ½T 1 , the mean linear flow velocity v can be found from the time t max at which a maximum appears in the signal shape: v=C/t max , where C is a calibration constant, depending on G,τand the flow profile. If T 2 >T 2 ) or∂[S(t max ) . t max ]/∂t max vs. t max (T 1≈ T 2 ).Based on flow measurements in plant stem segments (Chapter 5) it has been suggested that T 2 strongly depends on the vessel diameter for the narrow xylem capillaries. This behaviour of T 2 can explain negative results in plant stems with small vessel diameter. Under the present experimental conditions the method has been successfully applied to Cucurbitaceae (cucumber, gherkin, pumpkin) and tomato plants.T 2 measurements in wheat leaves have been shown to be insensitive to the presence of cell-bound paramagnetic ions (Chapter 7). The magnitude of T 2 of two separate water fractions (covering -90% of the total water content) has been found to be inversely proportional to water content. Measurements of flow and water content have been combined for an intact gherkin plant (Chapter 5), demonstrating that the combination of both NMR methods results in a powerful non-invasive method to study important parts of the plant water balance simultaneously. The results strongly suggest that the method can be used as an early warning for development of stress phenomena in plants, due to drought and other factors. From the flow measurements it has been shown how in a plant system the values of T 2 and T 1 of the water in the xylem vessels can be determined and estimated, respectively.A comparison between the results obtained with NMR, heat pulse and weight balance flow measurements is presented in Chapter 6. A linear relationship between the linear flow velocity obtained by NMR and the volume flowrate determined by the balance method yields an effective cross-sectional area available for flow of ~50% of the cross-sectional area of the xylem vessels measured by using a microscope. NMR measurements alone yield a slightly lower value of the effective cross-sectional area. Compared with the NMR method, the heat pulse method monitors only relative changes in the flow velocity. A plot of the flow velocity obtained by the heat pulse method versus the volume flowrate obtained by the balance method exhibits some unwanted experimental scatter.Chapter 8 suggests some applications of the pulsed NMR flow method, also to other systems than plants, and defines important instrumental requirements for these applications.