Daytime depression in temperature-normalised stem CO2 efflux in young poplar trees is dominated by low turgor pressure rather than by internal transport of respired CO2.

Daytime decreases in temperature-normalised stem CO2 efflux (EA_D ) are commonly ascribed to internal transport of respired CO2 (FT ) or to an attenuated respiratory activity due to lowered turgor pressure. The two are difficult to separate as they are simultaneously driven by sap flow dynamics. To achieve combined gradients in turgor pressure and FT , sap flow rates in poplar trees were manipulated through severe defoliation, severe drought, moderate defoliation and moderate drought. Turgor pressure was mechanistically modelled using measurements of sap flow, stem diameter variation, and soil and stem water potential. A mass balance approach considering internal and external CO2 fluxes was applied to estimate FT . Under well-watered control conditions, both turgor pressure and sap flow, as a proxy of FT , were reliable predictors of EA_D . After tree manipulation, only turgor pressure was a robust predictor of EA_D . Moreover, FT accounted for < 15% of EA_D . Our results suggest that daytime reductions in turgor pressure and associated constrained growth are the main cause of EA_D in young poplar trees. Turgor pressure is determined by both carbohydrate supply and water availability, and should be considered to improve our widely used but inaccurate temperature-based predictions of woody tissue respiration in global models.

Stem diameter variations as a versatile research tool in ecophysiology.

High-resolution stem diameter variations (SDV) are widely recognized as a useful drought stress indicator and have therefore been used in many irrigation scheduling studies. More recently, SDV have been used in combination with other plant measurements and biophysical modelling to study fundamental mechanisms underlying whole-plant functioning and growth. The present review aims to scrutinize the important insights emerging from these more recent SDV applications to identify trends in ongoing fundamental research. The main mechanism underlying SDV is variation in water content in stem tissues, originating from reversible shrinkage and swelling of dead and living tissues, and irreversible growth. The contribution of different stem tissues to the overall SDV signal is currently under debate and shows variation with species and plant age, but can be investigated by combining SDV with state-of-the-art technology like magnetic resonance imaging. Various physiological mechanisms, such as water and carbon transport, and mechanical properties influence the SDV pattern, making it an extensive source of information on dynamic plant behaviour. To unravel these dynamics and to extract information on plant physiology or plant biophysics from SDV, mechanistic modelling has proved to be valuable. Biophysical models integrate different mechanisms underlying SDV, and help us to explain the resulting SDV signal. Using an elementary modelling approach, we demonstrate the application of SDV as a tool to examine plant water relations, plant hydraulics, plant carbon relations, plant nutrition, freezing effects, plant phenology and dendroclimatology. In the ever-expanding SDV knowledge base we identified two principal research tracks. First, in detailed short-term experiments, SDV measurements are combined with other plant measurements and modelling to discover patterns in phloem turgor, phloem osmotic concentrations, root pressure and plant endogenous control. Second, long-term SDV time series covering many different species, regions and climates provide an expanding amount of phenotypic data of growth, phenology and survival in relation to microclimate, soil water availability, species or genotype, which can be coupled with genetic information to support ecological and breeding research under on-going global change. This under-exploited source of information has now encouraged research groups to set up coordinated initiatives to explore this data pool via global analysis techniques and data-mining.

Variable hydraulic resistances and their impact on plant drought response modelling.

Plant drought responses are still not fully understood. Improved knowledge on drought responses is, however, crucial to better predict their impact on individual plant and ecosystem functioning. Mechanistic models in combination with plant measurements are promising for obtaining information on plant water status and can assist us in understanding the effect of limiting soil water availability and drought stress. While existing models are reliable under sufficient soil water availability, they generally fail under dry conditions as not all appropriate mechanisms seem yet to have been implemented. We therefore aimed at identifying mechanisms underlying plant drought responses, and in particular investigated the behaviour of hydraulic resistances encountered in the soil and xylem for grapevine (Vitis vinifera L.) and oak (Quercus robur L.). A variable hydraulic soil-to-stem resistance was necessary to describe plant drought responses. In addition, implementation of a variable soil-to-stem hydraulic resistance enabled us to generate an in situ soil-to-stem vulnerability curve, which might be an alternative to the conventionally used vulnerability curves. Furthermore, a daily recalibration of the model revealed a drought-induced increase in radial hydraulic resistance between xylem and elastic living tissues. Accurate information on plant hydraulic resistances and simulation of plant drought responses can foster important discussions regarding the functioning of plants and ecosystems during droughts.

Phloem transport: a review of mechanisms and controls.

It is generally believed that an osmotically generated pressure gradient drives the phloem mass flow. So far, this widely accepted Münch theory has required remarkably few adaptations, but the debate on alternative and additional hypotheses is still ongoing. Recently, a possible shortcoming of the Münch theory has been pointed out, suggesting that the Münch pressure flow is more suitable for herbs than for trees. Estimation of the phloem resistance indicates that a point might be reached in long sieve tubes where the pressure required to drive the Münch flow cannot be generated. Therefore, the relay hypothesis regained belief as it implies that the sieve tubes are shorter then the plant's axial axis. In the source phloem, three different loading strategies exist which probably result from evolutionary advantages. Passive diffusion seems to be the most primitive one, whereas active loading strategies substantially increase the growth potential. Along the transport phloem, a leakage-retrieval mechanism is observed. Appreciable amounts of carbohydrates are lost from the sieve tubes to feed the lateral sinks, while a part of these lost carbohydrates is subsequently reloaded into the sieve tubes. This mechanism is probably involved to buffer short-term irregularities in phloem turgor and gradient. In the long term, the mechanism controls the replenishment and remobilization of lateral stem storage tissues. As phloem of higher plants has multiple functions in plant development, reproduction, signalling, and growth, the fundamental understanding of the mechanisms behind phloem transport should be elucidated to increase our ability to influence plant growth and development.

(11)C-PET imaging reveals transport dynamics and sectorial plasticity of oak phloem after girdling.

Carbon transport processes in plants can be followed non-invasively by repeated application of the short-lived positron-emitting radioisotope (11)C, a technique which has rarely been used with trees. Recently, positron emission tomography (PET) allowing 3D visualization has been adapted for use with plants. To investigate the effects of stem girdling on the flow of assimilates, leaves on first order branches of two-year-old oak (Quercus robur L.) trees were labeled with (11)C by supplying (11)CO2-gas to a leaf cuvette. Magnetic resonance imaging gave an indication of the plant structure, while PET registered the tracer flow in a stem region downstream from the labeled branches. After repeated pulse labeling, phloem translocation was shown to be sectorial in the stem: leaf orthostichy determined the position of the phloem sieve tubes containing labeled (11)C. The observed pathway remained unchanged for days. Tracer time-series derived from each pulse and analysed with a mechanistic model showed for two adjacent heights in the stem a similar velocity but different loss of recent assimilates. With either complete or partial girdling of bark within the monitored region, transport immediately stopped and then resumed in a new location in the stem cross-section, demonstrating the plasticity of sectoriality. One day after partial girdling, the loss of tracer along the interrupted transport pathway increased, while the velocity was enhanced in a non-girdled sector for several days. These findings suggest that lateral sugar transport was enhanced after wounding by a change in the lateral sugar transport path and the axial transport resumed with the development of new conductive tissue.

MRI links stem water content to stem diameter variations in transpiring trees.

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.

Localized stem chilling alters carbon processes in the adjacent stem and in source leaves.

Transport phloem is no longer associated with impermeable pipes, but is instead considered as a leaky system in which loss and retrieval mechanisms occur. Local stem chilling is often used to study these phenomena. In this study, 5-cm- lengths of stems of 3-year-old oak trees (Quercus robur L.) were locally chilled for 1 week to investigate whether observations at stem and leaf level can be explained by the leakage-retrieval mechanism. The chilling experiment was repeated three times across the growing season. Measurements were made of leaf photosynthesis, carbohydrate concentrations in leaves and bark, stem growth and maximum daily stem shrinkage. Across the growing season, a feedback inhibition in leaf photosynthesis was observed, causing increased dark respiration and starch concentration. This inhibition was attributed to the total phloem resistance which locally increased due to the cold temperatures. It is hypothesized that this higher phloem resistance increased the phloem pressure above the cold block up to the source leaves, inducing feedback inhibition. In addition, an increase in radial stem growth and carbohydrate concentration was observed above the cold block, while the opposite occurred below the block. These observations indicate that net lateral leakage of carbohydrates from the phloem was enhanced above the cold block and that translocation towards regions below the block decreased. This behaviour is probably also attributable to the higher phloem resistance. The chilling effects on radial stem growth and carbohydrate concentration were significant in the middle of the growing season, while they were not at the beginning and near the end of the growing season. Furthermore, maximum daily shrinkages were larger above the cold block during all chilling experiments, indicating an increased resistance in the xylem vessels, also generated by low temperatures. In conclusion, localized stem chilling altered multiple carbon processes in the source leaves and the main stem by changing hydraulic resistances.

Tree girdling responses simulated by a water and carbon transport model.

BACKGROUND AND AIMS: Girdling, or the removal of a strip of bark around a tree's outer circumference, is often used to study carbon relationships, as it triggers several carbon responses which seem to be interrelated. METHODS: An existing plant model describing water and carbon transport in a tree was used to evaluate the mechanisms behind the girdling responses. Therefore, the (un)loading functions of the original model were adapted and became a function of the phloem turgor pressure. KEY RESULTS: The adapted model successfully simulated the measured changes in stem growth induced by girdling. The model indicated that the key driving variables for the girdling responses were changes in turgor pressure due to local changes in sugar concentrations. Information about the local damage to the phloem system was transferred to the other plant parts (crown and roots) by a change in phloem pressure. After girdling, the loading rate was affected and corresponded to the experimentally observed feedback inhibition. In addition, the unloading rate decreased after girdling and even reversed in some instances. The model enabled continuous simulation of changes in starch content, although a slight underestimation was observed compared with measured values. CONCLUSIONS: For the first time a mechanistic plant model enabled simulation of tree girdling responses, which have thus far only been experimentally observed and fragmentally reported in literature. The close agreement between measured and simulated data confirms the underlying mechanisms introduced in the model.

Characterization of soluble microbial products and their fouling impacts in membrane bioreactors.

Membrane bioreactor (MBR) fouling is not only influenced by the soluble microbial products (SMP) concentration but by their characteristics. Experiments of separate producing biomass associated products (BAP) and utilization associated products (UAP) allowed the separation of BAP and UAP effects from sludge water (SW). Thus, filtration of individual SMP components and further characterization becomes possible. Unstirred cell filtration was used to study fouling mechanisms and liquid chromatography--organic carbon detection (LC-OCD) and fluorescence excitation--emission matrix (EEM) were used to characterize the foulant. Generally, the SMP exhibiting characteristics of higher molecular weight, greater hydrophilicity and a more reduced state showed a higher retention percentage. However, the higher retention does not always yield higher fouling effects. The UAP filtration showed the highest specific cake resistance and pore blocking resistance attributed to their higher percentage of low molecular weight molecules, although their retention percentage was lower than the SW and BAP filtration. The UAP produced in the cell proliferation phase appeared to have the highest fouling potential.

Development and verification of a water and sugar transport model using measured stem diameter variations.

In trees, water and sugars are transported by xylem and phloem conduits which are hydraulically linked. A simultaneous study of both flows is interesting, since they concurrently influence important processes such as stomatal regulation and growth. A few mathematical models have already been developed to investigate the influence of both hydraulically coupled flows. However, none of these models has so far been tested using real measured field data. In the present study, a comprehensive whole-tree model is developed that enables simulation of the stem diameter variations driven by both the water and sugar transport. Stem diameter variations are calculated as volume changes of both the xylem and the phloem tissue. These volume changes are dependent on: (i) water transport according to the cohesion-tension theory; (ii) sugar transport according to the Münch hypothesis; (iii) loading and unloading of sugars; and (iv) irreversible turgor-driven growth. The model considers three main compartments (crown, stem, and roots) and is verified by comparison with actual measured stem diameter variations and xylem sap flow rates. These measurements were performed on a young oak (Quercus robur L.) tree in controlled conditions and on an adult beech (Fagus sylvatica L.) tree in a natural forest. A good agreement was found between simulated and measured data. Hence, the model seemed to be a realistic representation of the processes observed in reality. Furthermore, the model is able to simulate several physiological variables which are relatively difficult to measure: phloem turgor, phloem osmotic pressure, and Münch's counterflow. Simulation of these variables revealed daily dynamics in their behaviour which were mainly induced by transpiration. Some of these dynamics are experimentally confirmed in the literature, while others are not.