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Developmental initiation of plant vascular tissue, including xylem and phloem, from the vascular cambium depends on environmental factors, such as temperature and precipitation. Proper formation of vascular tissue is critical for the transpiration stream, along with photosynthesis as a whole. While effects of individual environmental factors on the transpiration stream are well studied, interactive effects of multiple stress factors are underrepresented. As expected, climate change will result in plants experiencing multiple co-occurring environmental stress factors, which require further studies. Also, the effects of the main climate change components (carbon dioxide, temperature, and drought) on vascular cambium are not well understood. This review aims at synthesizing current knowledge regarding the effects of the main climate change components on the initiation and differentiation of vascular cambium, the transpiration stream, and photosynthesis. We predict that combined environmental factors will result in increased diameter and density of xylem vessels or tracheids in the absence of water stress. However, drought may decrease the density of xylem vessels or tracheids. All interactive combinations are expected to increase vascular cell wall thickness, and therefore increase carbon allocation to these tissues. A comprehensive study of the effects of multiple environmental factors on plant vascular tissue and water regulation should help us understand plant responses to climate change.

Four tree species in the Kostelec n. Č. l. arboretum (Czech Republic) have been repeatedly damaged by Dendrocopos medius. The unique aspect of this otherwise common behavior called girdling consists in regularly visiting the same trees every spring, although there are more than 1.200 tree species within the arboretum. We monitored transpiration, leaf phenology and the chemical composition of the xylem sap of girdled and nongirdled trees. Spectral analysis revealed slightly higher amounts of sugars, especially saccharose, in Cladrastis Raf. as the most regularly girdled tree among other conditions, comparing girdled to non-girdled trees. Higher transpiration rates were not confirmed in connection with girdling-quite the opposite- Cladrastis Raf. as the most highly favored tree for girdling showed the lowest transpiration rates (in average 6 kg water per day within spring months) compared to other non-girdled trees. We presume that the birds do not choose a particular tree on the basis of any visible or chemical traits but they examine many trees within their territory. Afterwards they probably remember the position of trees whose xylem sap starts to flow early in the spring compared to other trees, as their transpiration stream is enriched with sweet organic substances that represent an advantage for the forthcoming nesting period. [ABSTRACT FROM AUTHOR]

Water movement from the xylem to stomata is poorly understood. There is still no consensus about whether apoplastic or symplastic pathways are more important, and recent work suggests vapour diffusion may also play a role. The objective of this study was to estimate the proportions of hydraulic conductance outside the bundle sheath contributed by apoplastic, symplastic and gas phase pathways, using a novel analytical framework based on measurable anatomical and biophysical parameters. The calculations presented here suggest that apoplastic pathways provide the majority of conductance outside the bundle sheath under most conditions, whereas symplastic pathways contribute only a small proportion. The contributions of apoplastic and gas phase pathways vary depending on several critical but poorly known or highly variable parameters namely, the effective Poiseuille radius for apoplastic bulk flow, the thickness of cell walls and vertical temperature gradients within the leaf. The gas phase conductance should increase strongly as the leaf centre becomes warmer than the epidermis - providing up to 44% of vertical water transport for a temperature gradient of 0.2 K. These results may help to explain how leaf water transport is influenced by light absorption, temperature and differences in leaf anatomy among species. [ABSTRACT FROM AUTHOR]

Understanding the regulation of calcium uptake, xylem transport and its impacts on growth and leaf gas exchange is a subject that has received insufficient recent attention. Calcium (Ca) is unique within the group of key elements required for plant growth in that it also has a role in cellular signalling via regulation of changes in its cytoplasmic concentration. Its mobility, within the plant, is however somewhat constricted by its chemistry and cellular signalling role, and its adsorptive capacity within the aopoplast and the xylem. Supply and demand for Ca is achieved by a homeostatic balance which if perturbed can cause a number of distinctive physiological conditions, often related to Ca deficiency. In this issue Rothwell and Dodd present experiments with bean ( Phaseolus vulgaris) and pea ( Pisum sativum) plants grown in a field soil exposed to the processes of soil liming (application of Ca carbonate (CaCO). Given that there is evidence of free Ca in the xylem sap altering stomatal conductance it is reasonable to ask the question does liming elevate Ca in the transpiration stream which may explain the observed reduced growth which they hypothesise is due to Ca-induced stomatal closure. They show that liming doubled soil exchangeable Ca, reduced stomatal conductance and shoot biomass in both species compared with unlimed controls. However, xylem sap Ca concentration increased only in bean. Interestingly, the same was not true for the pea where the root xylem sap concentration remained unchanged despite an increase in soil available Ca. Given that stomatal conductance decreased in both species, but in response to a lime-induced increase in xylem sap Ca in only one; this questions the role of Ca in inducing stomatal closure. They propose that their data suggest that as yet unidentified antitranspirant causes stomatal closure in both species not the increase in xylem sap Ca per se. [ABSTRACT FROM AUTHOR]

Abstract: In this paper, we present and discuss a mathematical model of ion uptake and transport in roots of plants. The underlying physical model of transport is based on the mechanisms of forced diffusion and convection. The model can take account of local variations in effective ion and water permeabilities across the major tissue regions of plant roots, represented through a discretized coupled system of governing equations including mass balance, forced diffusion, convection and electric potential. We present simulation results of an exploration of the consequent enormous parameter space. Among our findings we identify the electric potential as a major factor affecting ion transport across, and accumulation in, root tissues. We also find that under conditions of a constant but realistic level of bulk soil salt concentration and plant–soil hydraulic pressure, diffusion plays a significant role even when convection by the water transpiration stream is operating. [Copyright &y& Elsevier]

Throughout their lifetimes, plants must coordinate the regulation of various facets of growth and development. Previous evidence has suggested that the Arabidopsis thaliana R2R3-MYB, AtMYB61, might function as a coordinate regulator of multiple aspects of plant resource allocation., Using a combination of cell biology, transcriptome analysis and biochemistry, in conjunction with gain-of-function and loss-of-function genetics, the role of AtMYB61 in conditioning resource allocation throughout the plant life cycle was explored., In keeping with its role as a regulator of resource allocation, AtMYB61 is expressed in sink tissues, notably xylem, roots and developing seeds. Loss of AtMYB61 function decreases xylem formation, induces qualitative changes in xylem cell structure and decreases lateral root formation; in contrast, gain of AtMYB61 function has the opposite effect on these traits. AtMYB61 coordinates a small network of downstream target genes, which contain a motif in their upstream regulatory regions that is bound by AtMYB61, and AtMYB61 activates transcription from this same motif. Loss-of-function analysis supports the hypothesis that AtMYB61 targets play roles in shaping subsets of AtMYB61-related phenotypes., Taken together, these findings suggest that AtMYB61 links the transcriptional control of multiple aspects of plant resource allocation. [ABSTRACT FROM AUTHOR]

Aquaporins are multifunctional membrane channels which belong to the family of major intrinsic proteins (MIPs) and are best known for their ability to facilitate the movement of water. In the present study, earlier results from microarray experiments were followed up. These experiments had suggested that, in barley (Hordeum vulgare L.), aquaporin family members are expressed in distinct patterns during leaf development. Real-time PCR and in situ hybridization were used to analyse the level and tissue-distribution of expression of candidate aquaporins, focusing on plasma membrane and tonoplast intrinsic proteins (PIPs, TIPs). Water channel function of seven aquaporins, whose transcripts were the most abundant and the most variable, was tested through expression in yeast and, in part, through expression in oocytes. All PIP1 and PIP2 subfamily members changed in expression during leaf development, with expression being much higher or lower in growing compared with mature tissue. The same applied to those TIPs which were expressed at detectable levels. Specific roles during leaf development are proposed for particular aquaporins. [ABSTRACT FROM PUBLISHER]

Leaf hydraulics of Aesculus hippocastanum L. were measured over the growing season and during extensive leaf mining by the larvae of an invasive moth ( Cameraria ohridella Deschka et Dimic) that specifically destroy the palisade tissue. Leaves showed seasonal changes in hydraulic resistance ( Rlamina) which were related to ontogeny. After leaf expansion was complete, the hydraulic resistance of leaves and the partitioning of resistances between vascular and extra-vascular compartments remained unchanged despite extensive disruption of the palisade by leafminers (up to 50%). This finding suggests that water flow from the petiole to the evaporation sites might not directly involve the palisade cells. The analysis of the temperature dependence of Rlamina in terms of Q10 revealed that at least one transmembrane step was involved in water transport outside the leaf vasculature. Anatomical analysis suggested that this symplastic step may be located at the bundle sheath where the apoplast is interrupted by hydrophobic thickening of cell walls. Our findings offer some support to the view of a compartmentalization of leaves into well-organized water pools so that the transpiration stream would involve veins, bundle sheath and spongy parenchyma, while the palisade tissue would be largely by-passed with the possible advantage of protecting cells from short-term fluctuations in water status. [ABSTRACT FROM AUTHOR]

We examined the leaf hydraulic design in 10 species based on their rehydration kinetics. In all cases, a biphasic response described the temporal pattern of water uptake, with time constants of ∼30 to 800 s and ∼800 to 8000 s. The time constants of the fast phase were significantly shorter in the six angiosperms (30 to 110 s) compared with the two single-veined conifer species (>400 s) examined, while the two multi-veined gymnosperm species, Gnetum gnemon and Ginkgo biloba, had time constants for the fast phase of ∼150 s. Among angiosperm species, the fast phase constituted 50–90% of the total water absorbed, whereas in gymnosperms 70–90% of the water uptake could be assigned to the slow phase. In the four gymnosperms, the relative water uptake corresponding to the fast phase matched to a good degree the relative volume of the venation and bundle sheath extension; whereas in the angiosperm species, the relatively larger water influx during the fast phase was similar in relative volume to the combined venation, bundle sheath extension, epidermis and (in four species) the spongy mesophyll. This suggests a general trend from a design in which the epidermis is weakly connected to the veins (all four gymnosperms), to a design with good hydraulic connection between epidermis and veins that largely bypasses the mesophyll (four of six angiosperms), to a design in which almost the entire leaf appears to function as a single pool. [ABSTRACT FROM AUTHOR]

Feeding dynamics in the spittlebug Philaenus spumarius were analysed to determine the food source. Evidence from previous reports indicates that this species and similar insects feed from the xylem. However, it is not clear whether they extract sap from the main transpiration stream, or from some subsidiary compartment of xylem, such as immature or cavitated vessels. In feeding from such subsidiary compartments, the insects could avoid the large hydraulic tensions thought to prevail in xylem on the main transpiration stream, Philaenus, for example, is shown here to initiate feeding, and to feed rapidly, from plants in which the full xylem tension is estimated at - 10 bar, From measurements of feeding rates and feeding patterns in stressed and unstressed plants, it is shown that Philaenus cannot feed from subsidiary xylem compartments. It is concluded that this insect feeds directly from the main transpiration stream, and at the full hydraulic tension which prevails there. The anatomy of the feeding pump in Philaenus appears consistent with this conclusion. Spittlebugs therefore offer an experimental tool for quasi- non-invasive sampling of the transpiration stream. [ABSTRACT FROM AUTHOR]

Zinc accumulation in spinach (Spinacea oleracea L. cv. Verbeterd Breedblad), especially in the leaves, has been studied in plants grown on nutrient solutions. The extent of accumulation depends on, but is not proportional to the external zinc supply. Zinc uptake by water flow does not account for the concentrations observed in the various plant parts. The relative accumulation rate is related to the relative growth rate, but depends also on leaf position and leaf age. The data suggest that the midribs act as sinks for the nutrient solution, whilst the leaf blades in turn act as sinks for the midribs. We suggest that strong binding of zinc occurs to specific organic compounds in the cell, thus, diminishing the concentration of the free zinc ions in the biological tissue. [ABSTRACT FROM AUTHOR]

Changes of view on the course of the transpiration stream beyond the veins in leaves are followed from the imbibition theory of Sachs, through the (symplastic) endosmotic theory of Pfeffer (which prevailed almost unquestioned until the late 1930's), to Strugger's experiments with fluorescent dye tracers and the epifluorescence microscope. This latter work persuaded many to return to the apoplastic-(wall)-path viewpoint which, despite early and late criticisms that were never rebutted, is still widely held. Tracer experiments of the same kind are still frequently published without consideration of the evidence that they do not reveal the paths of water movement. Experiments on rehydration kinetics of leaves have not produced unequivocal evidence for either path. The detailed destinies of the solutes that reach the leaf in the transpiration stream have received little attention. Consideration of physical principles governing flow and evaporation in a transpiring leaf emphasizes that: (1) Diffusion over interveinal distances at the rates in water will account for substantial solute movement in a few minutes, even in the absence of flow. (2) Diffusion can occur also against opposing flow. (3) Volume fluxes in veins are determined by the diameter of the largest vessels, and all the leaves examined contain high conductance supply veins which are tapped into by low-conductance distributing veins. (4) Edges and teeth of leaves will be places of especially rapid evaporation, and they often have high-conductance veins leading to them. (5) Solutes in the stream will tend to accumulate at leaf margins. On the basis of recent work, the view is maintained that the water of the stream enters the symplast through cell membranes very close to tracheary elements. Also, that this occurs locally over a small area of membrane. Many solutes in the stream are left outside in the apoplast. [ABSTRACT FROM AUTHOR]

Vascular plants have respiring tissues which are perfused by the transpiration stream, allowing solubilization of respiratory CO2 in the xylem sap. The transpiration stream could provide a conduit for the internal delivery of respiratory CO2 to leaves. Trees have large amounts of respiring tissues in the root systems and stems, and may have elevated levels of CO2 in the xylem sap which could be delivered to and refixed by the leaves. Xylem sap from the shoots of three Populus deltoides trees had mean dissolved inorganic carbon concentrations (CO2+H2CO3+HCO−3) ranging from 0. 5 to 0. 9 mM. When excised leaves were allowed to transpire 1 mM[14C]NaHCO3, 99. 6% of the label was fixed in the light. Seventy‐seven percent of the label was fixed in major veins and the remainder was fixed in the minor veins. Autoradiography confirmed that label was confined to the vasculature. In the dark, approximately 80% of the transpired label escaped the leaf, the remainder was fixed in the major veins, slightly elevating dark respiration measurements. This indicates that the vascular tissue in P. deltoides leaves is supplied with a carbon source distinct from the atmospheric source fixed by interveinal lamina. However, the contribution of CO2 delivered to the leaves in the transpiration stream and fixed in the veins was only 0. 5% of atmospheric CO2 uptake. In the light 90% of the label was found in sugar, starch and protein, a pattern similar to that found for atmospheric uptake of[14C]CO2. Compared with leaves labelled in the light, leaves labelled in the dark had more label in organic acid, amino acid and protein and less label in sugar and starch. After a 5‐s pulse the majority of the label fed to petioles in both the light and the dark was found in malate. The majority of the label was found in malate at 120 s in the dark; only 2% of the label was found in phosphorylated compounds at 120 s. The proportion of label found in phosphorylated compounds increased from 17% at 5 s to 80% at 120 s in the light. This suggests that CO2 delivered to leaves in the light via the transpiration stream is fixed in the veins, a small portion through dark fixation into malate, the remainder by C‐3 photosynthesis. [ABSTRACT FROM AUTHOR]

Acetaldehyde and ethanol are usually thought to be produced in plant tissues as a mechanism to tolerate hypoxic conditions. We have found acetaldehyde and ethanol to be common in the vascular cambium and in the transpiration stream of trees. In nonflooded trees, acetaldehyde and ethanol concentrations averaged 130 and 40 µM in the cambium and 130 and 50 µM in the xylem sap, respectively. Ethanol concentrations in the transpiration stream and the cambium increased to as much as 5 mM upon flooding. Ethanol concentrations in the vascular cambium of Populus deltoides could not be eliminated by placing logs from nonflooded trees in a pure oxygen environment for as long as 96 h, but increased by almost 3 orders of magnitude when logs were exposed to low external partial pressures of O2. These results suggest that the vascular cambium was not hypoxic, despite the presence of acetaldehyde, ethanol and the enzymes for their synthesis. [ABSTRACT FROM AUTHOR]

The fluorescent probe sulphorhodamine G (SR) has been previously used as an indicator of low extra-cellular pH and, by inference, of proton extrusion activity in living leaves. In legumes the SR uptake and proton extrusion was characteristic of the extended bundle sheath system (EBS) or paraveinal mesophyll, composed of bundle sheath cells and the related network of bridging cells between veins. This system has been identified as a site of temporary storage of amino carbon in soybean. A tree species, Populus deltoides Bartr. ex Marsh, was known both to have the EBS system in its leaves and to carry organic nitrogen in its xylem sap. It is now shown that P. deltoides also accumulates the SR probe in the EBS system. This association has been explored in 8 other broad-leaved tree species. Seven of the 8 species have EBS systems and accumulate SR in them in early summer. The 8th species, Tilia americana L. has no EBS system and shows weak SR accumulation. The capacity to accumulate SR (and by inference to scavenge solutes from the transpiration stream) disappeared in all species at various stages in late summer. In two species, in addition, SR accumulation is interrupted for several weeks during fruit growth. It is proposed that EBS systems wilt be found in many dicotyledonous leaves, and will be found to scavenge solutes, especially organic nitrogen, from the xylem sap. [ABSTRACT FROM AUTHOR]

Perfluoroalkyl acids (PFAAs) bioaccumulate in crops, with uptake being particularly high for short-chain PFAAs that are constantly transported with transpiration water to aerial plant parts. Due to their amphiphilic surfactant nature and ionized state at environmental pH, predicting the partitioning behavior of PFAAs is difficult and subject to considerable uncertainty, making experimental data highly desirable. Here, we applied a plant uptake model that combines advective flux with measured partition coefficients to reproduce the set of empirically derived plant uptake and soil-partitioning data for nine PFAAs in red chicory, in order to improve the mechanistic understanding and provide new insights into the complex uptake processes. We introduced a new parameter for retarded uptake (R) to explain the slow transfer of PFAA across biomembranes of the root epidermis, which has led to low transpiration stream concentration factors (TSCF s) presented in literature so far. We estimated R values for PFAAs using experimental data derived for red chicory and used the modified plant uptake model to simulate uptake of PFAA into other crops. Results show that this semi-empirical model predicted PFAAs transport to shoots and fruits with good accuracy based on experimental root to soil concentration factors (RCF dw) and soil to water partition coefficients (K d) as well as estimated R values and plant-specific data for growth and transpiration. It can be concluded that the combination of rather low K d with high RCF dw and the absence of any relevant loss are the reason for the observed excellent plant uptake of PFAAs. Image 1 • Short-chain perfluoroalkyl acids (PFAAs) are highly bioaccumulative in edible crops. • Plant uptake model extended to interpret the uptake of nine PFAAs into red chicory. • New parameter R for slow transfer through root membranes estimated for red chicory. • The newly developed semi-empirical model was successfully applied for other crops. • High PFAA uptake in plants is the combination of persistence, low K d and high RCF. [ABSTRACT FROM AUTHOR]

Developmental initiation of plant vascular tissue, including xylem and phloem, from the vascular cambium depends on environmental factors, such as temperature and precipitation. Proper formation of vascular tissue is critical for the transpiration stream, along with photosynthesis as a whole. While effects of individual environmental factors on the transpiration stream are well studied, interactive effects of multiple stress factors are underrepresented. As expected, climate change will result in plants experiencing multiple co-occurring environmental stress factors, which require further studies. Also, the effects of the main climate change components (carbon dioxide, temperature, and drought) on vascular cambium are not well understood. This review aims at synthesizing current knowledge regarding the effects of the main climate change components on the initiation and differentiation of vascular cambium, the transpiration stream, and photosynthesis. We predict that combined environmental factors will result in increased diameter and density of xylem vessels or tracheids in the absence of water stress. However, drought may decrease the density of xylem vessels or tracheids. All interactive combinations are expected to increase vascular cell wall thickness, and therefore increase carbon allocation to these tissues. A comprehensive study of the effects of multiple environmental factors on plant vascular tissue and water regulation should help us understand plant responses to climate change. [ABSTRACT FROM AUTHOR]