Focus on Water: Uptake of Water via Branches Helps Timberline Conifers Refill Embolized Xylem in Late Winter

Xylem embolism is a limiting factor for woody species worldwide. Conifers at the alpine timberline are exposed to drought and freeze-thaw stress during winter, which induce potentially lethal embolism. Previous studies indicated that timberline trees survive by xylem refilling. In this study on Picea abies, refilling was monitored during winter and spring seasons and analyzed in the laboratory and in situ experiments, based on hydraulic, anatomical, and histochemical methods. Refilling started in late winter, when the soil was frozen and soil water not available for the trees. Xylem embolism caused up to 86.2% ± 3.1% loss of conductivity and was correlated with the ratio of closed pits. Refilling of xylem as well as recovery in shoot conductance started in February and corresponded with starch accumulation in secondary phloem and in the mesophyll of needles, where we also observed increasing aquaporin densities in the phloem and endodermis. This indicates that active, cellular processes play a role for refilling even under winter conditions. As demonstrated by our experiments, water for refilling was thereby taken up via the branches, likely by foliar water uptake. Our results suggest that refilling is based on water shifts to embolized tracheids via intact xylem, phloem, and parenchyma, whereby aquaporins reduce resistances along the symplastic pathway and aspirated pits facilitate isolation of refilling tracheids. Refilling must be taken into account as a key process in plant hydraulics and in estimating future effects of climate change on forests and alpine tree ecosystems.Water transport in plant xylem is based on continuous water columns transmitting tension from leaves to roots and soil. This liquid continuum is metastable because cavitation can disrupt water columns and lead to embolism in the conduit network (Tyree and Zimmermann, 2002). During drought stress, increasing tension (i.e. decreasing water potential) causes air entry into xylem conduits via pits when critical thresholds in water potential (Ψ) are reached. Hydraulic dysfunction may also be caused by freeze-thaw events, a common phenomenon in plants growing in seasonal or high elevation environments (Pittermann and Sperry, 2006). Bubbles formed during the freezing of xylem sap may expand as the sap melts and low Ψs are re-established.The biological significance of xylem embolism is that it blocks sap flow and impairs water supply of distal tissues. Over short periods of time, plants can control Ψ by stomatal adjustment of transpiration (Sperry and Pockman, 1993). Developmentally, plants cope with xylem tensions by investing in xylem structures that provide adequate safety from embolism (Hacke and Sperry, 2001; Sperry, 2003; Choat et al., 2012). When avoidance strategies fail (e.g. in the case of intense drought stress), repair strategies may prevent excessive shoot dieback. Repair of the hydraulic system may be based on the formation of new xylem or on refilling of dysfunctional xylem conduits. Several tree species can facilitate the latter by creation of positive pressures in the roots or the stem (Sperry et al., 1988b; Hacke and Sauter, 1996), but refilling has also been reported to take place at negative Ψ (Zwieniecki and Holbrook, 2009; Nardini et al., 2011; Brodersen and McElrone, 2013; Zwieniecki et al., 2013). Although the underlying mechanism is not yet understood, the following factors have been implicated to be involved (Holbrook and Zwieniecki, 1999; Hacke and Sperry, 2003; Zwieniecki and Holbrook, 2009; Nardini et al., 2011): isolation of refilling conduits from functional ones, a pathway and a driving force allowing water shifts into embolized conduits, a source of water increasing Ψ in conduits, as well as signal transduction networks initiating and integrating the process. At least some parts of this sequence are likely based on cellular activities such as the creation of a driving force by changes in osmotic potential, adjustment of pathway resistances via aquaporins, and cellular signaling. The isolation of conduits is an important prerequisite because water columns under tension, unless disconnected from the refilling conduit, would immediately suck out any water released to the conduit.Vessel refilling in the absence of positive pressure has been reported in angiosperm species, whereby the xylem parenchyma and its interaction with the phloem were hypothesized to play a central role (Bucci et al., 2003; Brodersen et al., 2010; Nardini et al., 2011). Surprisingly, refilling was also observed in conifers, although their stem xylem contains a comparatively small amount of parenchyma tissue. For example, in Pseudotsuga menziesii, a percent loss of conductivity (PLC) of up to 60 in winter was followed by complete xylem repair (McCulloh et al., 2011).Winter embolism was also found in conifers growing at high elevation. Sparks and Black (2000) found approximately 35 PLC in Pinus albicaulis and more than 25 PLC in Larix lyallii at the Rocky Mountains treeline. At the European alpine timberline, conifers can exhibit up to 100 PLC, induced by a combination of drought and freeze-thaw stress (Mayr et al., 2002, 2006a). These trees experienced frost drought, because frozen soil blocked water uptake for months. Moreover, the crown is exposed to numerous freeze-thaw events, with more than 100 frost cycles per winter, causing embolism even in conifers with small and resilient tracheids (Mayr et al., 2003a).There are indications that these conifers survive winter embolism by recovery from hydraulic failure in late winter and spring. In branches of Picea abies trees growing along a transect up to the timberline, a significant increase in Ψ and hydraulic conductivity was observed within 2 weeks in March (Mayr et al., 2002). Seasonal courses in P. abies and other conifers revealed a recovery process that was stepwise and lasted several weeks. It resulted in negligible PLC by April to June (Mayr et al., 2003b, 2006a). It has to be emphasized that the first steps of this recovery occurred when snow cover was still present and, consequently, uptake of soil water was blocked by ice barriers in the soil and/or stem base. This raises the question where the water required for the increase in Ψ and hydraulic conductivity might have come from.Several studies demonstrated that plants can take up water not only via the root system but also via the leaves, under certain circumstances (Burgess and Dawson, 2004; Limm et al., 2009). In Pinus contorta, water uptake via branches during winter probably enabled an increase in Ψ and supported recovery from winter embolism (Sparks et al., 2001). The authors of this study suggested that the source of water was snow melting on branch surfaces. This process may also play a role for trees at the alpine timberline. Branches of evergreen conifers are often covered by snow, which melts in late winter, when the radiation becomes stronger, thereby intensively wetting branches. Foliar water uptake thus may enable an overall increase in Ψ during late winter; however, the question remains regarding whether it also allows refilling.For refilling, water has to be shifted to embolized xylem sections. In conifer needles, water may reach the central cylinder via the apoplast or cellular pathways. It is known that aquaporins facilitate water flow between cells, allowing plants to modulate membrane resistance (Maurel et al., 2008; Gomes et al., 2009; Heinen et al., 2009). Accordingly, aquaporins could also be involved in facilitating radial water movement in needles. The driving force for water shifts may be created by osmotic adjustments as previously suggested (Holbrook and Zwieniecki, 1999; Hacke and Sperry, 2003; Zwieniecki and Holbrook, 2009; Nardini et al., 2011). Degradation of starch to osmotically active carbohydrates may enable local adjustments in Ψ to drive refilling of embolized conduits. However, it is unclear whether conditions in the winter at the alpine timberline allow cellular activities such as synthesis or modification of aquaporins, creation of osmotic gradients, and signaling (which is likely required to coordinate these processes). Even in late winter, nighttime temperatures frequently fall below the freezing point and tissues are exposed to high daily temperature fluctuations (Mayr et al., 2006b). Just the isolation of embolized from functional conduits, which is another prerequisite for refilling (see above), probably does not require metabolic activity. Conifer pits act like valves, which are passively closed by aspiration of the torus to the porus of the pit chamber (Pittermann et al., 2005; Hacke and Jansen, 2009; Plavcová et al., 2014). This mechanism might be especially advantageous to ensure isolation during refilling. To our knowledge, a correlation between PLC and pit closure has not been demonstrated up to now.In this study, we analyzed the hydraulic recovery from winter embolism in P. abies trees growing at the alpine timberline. P. abies is one of the most economically important forest tree species in Europe. Using a combination of field measurements and laboratory experiments, we hypothesized that (1) hydraulic recovery of timberline conifers is based on refilling and not on the formation of new xylem, (2) isolation of embolized tracheids is achieved by reversible pit aspiration, (3) active, cellular processes take place in late winter, and (4) water uptake via the needles supports refilling. Despite low night temperatures and persisting snow cover, changes in aquaporin contents in the needle mesophyll and changes in starch pools were thus expected. Analyses were based on several hydraulic, anatomical, and histochemical methods.     

植物根系吸水机理的研究进展

近年来,植物根系吸水机理在细胞、组织和整体水平上的研究进展非常迅速,对阐明植物抗旱机制及其高效利用有限水资源途径的探讨具有重要意义.本文主要对植物根的复合结构和根系在土壤中的分布、根系中水流性质等方面的最新研究状况进行了概述,特别详细地论述了水通道蛋白的表达及功能与根系中水分运动的关系、以及根系输水的调节和根系吸水过程中的信号传导方面的研究动态,并且评价了根的复合运输模型和根系吸水的数学模型等,最后就其可能生理意义及其应用前景作了评述.     

Chloride in Soils and its Uptake and Movement within the Plant: A Review

Natural inputs of chlorine (Cl) to soils come mainly from rainwater,sea spray, dust and air pollution. In addition, human practices,such as irrigation and fertilization, contribute significantlyto Cl deposition. In the soil solution, Cl occurs predominantlyas the chloride anion (Cl^-). The Cl^-anion does not form complexesreadily, and shows little affinity (or specificity) in its adsorptionto soil components. Thus, Cl^-movement within the soil is largelydetermined by water flows. Chlorine is an essential micronutrientfor higher plants. It is present mainly as Cl^-. Chloride isa major osmotically active solute in the vacuole and is involvedin both turgor- and osmoregulation. In the cytoplasm it mayregulate the activities of key enzymes. In addition, Cl^-alsoacts as a counter anion, and Cl^-fluxes are implicated in thestabilization of membrane potential, regulation of intracellularpH gradients and electrical excitability. Chloride enters plantsthrough the roots, and there is some concern over the uptakeof the long-lived radionuclide³⁶Cl, which enters into the foodchain through plants. Chloride is thought to traverse the rootby a symplastic pathway, and Cl^-fluxes across the plasma membraneand tonoplast of root cells have been estimated. These fluxesare regulated by the Cl^-content of the root. Chloride is mobilewithin the plant. The Cl^-concentrations of xylem and phloemsaps have been determined and Cl^-fluxes through the xylem andphloem have been modelled. Measurements of transmembrane voltagesand Cl^-activities in cellular compartments suggest (1) thatactive Cl^-transport across the plasma membrane dominates Cl^-influxto root cells at low Cl^-concentrations in the soil solutionand that passive Cl^-influx to root cells occurs under more salineconditions, and (2) that both active and passive Cl^-transportoccurs at the tonoplast. Electrophysiological studies have demonstratedthe presence of an electrogenic Cl^-/2H⁺symporter in the plasmamembrane of root-hair cells and Cl^-channels mediating eitherCl^-influx or Cl^-efflux across the plasma membrane. Similarly,there is both biochemical and electrophysiological evidencethat Cl^-channels mediate Cl^-fluxes in either direction acrossthe tonoplast and that a Cl^-/nH⁺antiport mediates Cl^-influxto the vacuole. This article reviews the availability of Cl^-inthe soil, the roles and distribution of Cl^-within the plant,the magnitude of Cl^-fluxes across membranes and between tissues,the mechanisms of Cl^-transport across membranes and the electricalcharacteristics and molecular biology of Cl^-channels. Copyright2001 Annals of Botany Company Review, Arabidopsis thaliana, channel, chloride (Cl^-), influx, phloem, plasma membrane, radiochlorine (³⁶Cl), soil, tonoplast, transport, uptake, xylem     

Rehydration versus Growth-induced Water Uptake in Plant Tissues

Experiments show that the rate of water uptake by living tissues external to mature xylem of cotton stems (Gossypium hirsutum L. Auburn 7-683) is very similar to the corresponding curves for leaf tissue. In both cases one obtains a two-phase curve with phase I corresponding to passive rehydration and phase II pertaining to active growth.     

Root Growth and Water Uptake by Maize Plants in Drying Soil

Sharp, R. E and Da vies, W. J. 1985. Root growth and water uptakeby maize plants in drying soil.— J. exp. Bot. 36: 1441–1456. The influence of soil drying on maize (Zea mays L.) root distributionand use of soil water was examined using plants growing in thegreenhouse in soil columns. The roots of plants which were wateredwell throughout the 18 d experimental period penetrated thesoil profile to a depth of 60 cm while the greatest percentageof total root length was between 20–40 cm. High soil waterdepletion rates corresponded with these high root densities.Withholding water greatly restricted root proliferation in theupper part of the profile, but resulted in deeper penetrationand higher soil water depletion rates at depth, compared withthe well watered columns. The deep roots of the unwatered plantsexhibited very high soil water depletion rates per unit rootlength. Key words: Maize, roots, water deficit, soil water depletion     

Focus on Water: Monitoring Plant Drought Stress Response Using Terahertz Time-Domain Spectroscopy

We present a novel measurement setup for monitoring changes in leaf water status using nondestructive terahertz time-domain spectroscopy (THz-TDS). Previous studies on a variety of plants showed the principal applicability of THz-TDS. In such setups, decreasing leaf water content directly correlates with increasing THz transmission. Our new system allows for continuous, nondestructive monitoring of the water status of multiple individual plants each at the same constant leaf position. It overcomes previous drawbacks, which were mainly due to the necessity of relocating the plants. Using needles of silver fir (Abies alba) seedlings as test subjects, we show that the transmission varies along the main axis of a single needle due to a variation in thickness. Therefore, the relocation of plants during the measuring period, which was necessary in the previous THz-TDS setups, should be avoided. Furthermore, we show a highly significant correlation between gravimetric water content and respective THz transmission. By monitoring the relative change in transmission, we were able to narrow down the permanent wilting point of the seedlings. Thus, we established groups of plants with well-defined levels of water stress that could not be detected visually. This opens up the possibility for a broad range of genetic and physiological experiments.Climate change simulations predict an increase in the occurrence of drought events in the Mediterranean area and in central Europe due to smaller amounts of precipitation, especially during summer periods (IPCC, 2007). With the exception of the boreal zone, this leads to an increase in drought risks for every region on the European continent (Iglesias et al., 2007). Water availability is very important for a variety of plant species. Trees and crops play major roles regarding ecosystem stability and food supply. Forest trees are keystone elements in shaping long-term, regional ecosystem composition and stability and are, like most forest species, highly vulnerable to increases in drought severity (Breshears et al., 2005; Choat et al., 2012). Drought-induced forest die-offs thereby directly reduce ecosystem services such as carbon sequestration and timber supply (Allen et al., 2010). Further research is clearly necessary to elucidate the physiological traits and responses of plants regarding their water status.European silver fir (Abies alba) is an important forest tree species of ecological and economic relevance. This study is embedded in the European project LinkTree, “linking genetic variability with ecological responses to environmental changes: forest trees as model systems.” Our group is concerned with the identification of genes involved in the water stress response of silver fir. This species is of special interest because of its lower water-use efficiency compared with other conifer species (Guehl and Aussenac, 1987; Guehl et al., 1991).For this purpose, monitoring plant water status without inducing other forms of stress is instrumental in order to apply well-defined levels of water stress. Obtaining information regarding the water status of a plant is highly problematic without using invasive and destructive methods that usually only allow a retrospective assessment. These include commonly established methods, such as the gravimetric water content and pressure chamber techniques, most notably Scholander’s pressure bomb (Scholander et al., 1965).Chlorophyll fluorescence, stomatal conductance, and visual assessment are examples of nondestructive and noninvasive measurement techniques. The former two only provide indirect information about the plant stress status and, therefore, the water content via photosynthetic activity (Lichtenthaler and Rinderle, 1988; Tardieu and Davies, 1993). The latter is difficult to standardize and highly dependent on the morphology of the studied plant species. Conifers especially are challenging subjects for visually assessing drought stress. Due to their needle morphology, it is nearly impossible to detect early signs of dehydration.Measurement techniques using electromagnetic radiation in the terahertz (THz) regime have shown promising results, due to the nondestructive nature and high sensitivity of THz waves to water. With THz waves, we refer to frequencies in the electromagnetic spectrum between 0.1 and 1 THz, corresponding to wavelengths between 3 and 0.3 mm, which are located between infrared light (thermal radiation) and microwave radiation (used in common wireless data communication systems). In the last decade, terahertz time-domain spectroscopy (THz-TDS) has proven to be a very strong and accurate tool for characterizing and imaging various materials (for review, see Jepsen et al., 2011). Crucial for our study is the remarkably high absorption coefficient of water in this part of the electromagnetic spectrum. Thus, it is a robust technique hardly affected by physiological concentrations of soluble substances. Using transmission geometry, the resulting absorption by plant tissues directly reflects the quantity of water molecules.Furthermore, THz-TDS does not suffer from the disadvantages of other radiation-based techniques. These are mainly focused on the infrared or microwave spectrum but either lack the sensitivity for small changes in leaf water status or are affected by the plant’s inorganic salt content, leading to significant disturbances (Ulaby and Jedlicka, 1984). Moreover, the applicability of emitting microwave radiation is limited to minimal wavelengths of approximately 2.5 mm. The Abbe diffraction limit, therefore, restricts the minimum diameter of a measurable object to approximately 1.25 mm. In order to measure small leaves, such as coniferous needles, electromagnetic radiation with shorter wavelengths is necessary.Although presenting a useful alternative, THz-TDS was not feasible until recently, due to the difficulty of generating and detecting electromagnetic radiation with wavelengths in the THz spectrum. Despite its promising applicability in plant sciences, until now this relatively novel method relied exclusively on measurement setups that allowed only a single measurement per alternating plant (Hadjiloucas et al., 1999; Jördens et al., 2009; Breitenstein et al., 2012; Castro-Camus et al., 2013; Gente et al., 2013). For the purpose of continuously monitoring multiple plants, these setups are only of limited use, since the plants must be relocated for every measurement. This results in two problems: (1) an increase in possible disturbances (e.g. mechanical), influencing the plant’s stress response, and (2) the necessity to precisely target the same measurement spot on every analyzed plant at every consecutive measurement. The latter is of crucial importance for the exact monitoring of any individual plant’s water status because, as we will show in this study, the transmission varies substantially across the area of plant leaf tissue.We present a novel measurement procedure that overcomes the drawbacks of previously proposed methods. Our approach enables us to precisely monitor changes in the water content of multiple plants simultaneously.In the course of this study, three different experiments were performed. The profile measurement and the rehydration experiment were preliminary investigations to examine the influences of needle and tissue thickness and to define a nonlethal stress level. The main experiment established groups of plants with comparable levels of water stress.     

植物对土壤中铀的吸收与富集

核工业发展导致重金属铀排放和扩散,并造成了地表土壤的污染,对人类的生存环境产生了极其不利的影响。如何修复铀污染土壤成为亟待解决的问题。近年来发展起来的植物修复技术以其成本低廉、安全和环保的特点成为修复铀污染土壤的新选择。寻找理想的铀富集植物是这一技术的基础和关键。该文通过实验模拟铀污染的土壤（土壤中铀的浓度为100 mg.kg–1）,进行一次和二次铀污染土壤的植物修复后,从4个方面对植物修复铀污染土壤效果进行评估,即富集铀的浓度、生物提取量、生物富集系数（BFS）和转运系数（TFS）。实验结果表明：第1次修复时,四季香油麦菜（Lactuca dolichophylla）地上部富集铀的浓度为1.67×103 mg.kg–1,生物富集系数和转移系数均大于3;第2次修复时,麦冬（Ophiopogon japoni-cus）富集铀的浓度与第1次修复相比变化不大,而吊兰（Chlorophytum comosum）、四季豆（Phaseolus vulgaris）和艾蒿（Artemisia lavandulaefolia）富集铀的浓度与第1次修复相比均减少4–8倍;施加土壤改良剂鸡粪肥、海藻肥和柠檬酸后发现海藻肥和柠檬酸能够增强植物对铀污染土壤的修复;对两次修复土壤中铀的形态进行对比分析,发现二次修复时土壤中生物有效态铀的含量降低,造成第2次修复的难度增加。     

Water Uptake and Germination of Leguminous Seeds in Soils of Changing Matric and Osmotic Water Potential

Water uptake and germination rate of chickpea and pea seedswere compared under changing water potentials in sand and soilaggregate columns and osmotic solutions. The final water uptake and germination were the same in allcases for a given water potential, but the rates were lowerfor seeds planted in sand columns, probably due to mechanicalconstraints imposed on the swelling seed by the dense sand,since the capillary conductivity, and the diffusivity to waterof the sand were very high. The area of the seed in contact with soil is not of importanceif soil aggregates are small as compared to the seeds but increasesin importance when the seeds and the soil aggregates are ofthe same size and at low water potentials.     

Focus Issue on Roots: Visualization of Root Water Uptake: Quantification of Deuterated Water Transport in Roots Using Neutron Radiography and Numerical Modeling

Our understanding of soil and plant water relations is limited by the lack of experimental methods to measure water fluxes in soil and plants. Here, we describe a new method to noninvasively quantify water fluxes in roots. To this end, neutron radiography was used to trace the transport of deuterated water (D₂O) into roots. The results showed that (1) the radial transport of D₂O from soil to the roots depended similarly on diffusive and convective transport and (2) the axial transport of D₂O along the root xylem was largely dominated by convection. To quantify the convective fluxes from the radiographs, we introduced a convection-diffusion model to simulate the D₂O transport in roots. The model takes into account different pathways of water across the root tissue, the endodermis as a layer with distinct transport properties, and the axial transport of D₂O in the xylem. The diffusion coefficients of the root tissues were inversely estimated by simulating the experiments at night under the assumption that the convective fluxes were negligible. Inverse modeling of the experiment at day gave the profile of water fluxes into the roots. For a 24-d-old lupine (Lupinus albus) grown in a soil with uniform water content, root water uptake was higher in the proximal parts of lateral roots and decreased toward the distal parts. The method allows the quantification of the root properties and the regions of root water uptake along the root systems.Understanding how and where plant roots extract water from soil remains an open question for both plant and soil scientists. One of the open questions concerns the locations of water uptake along the root system (Frensch and Steudle, 1989; Doussan et al., 1998; Steudle, 2000; Zwieniecki et al., 2003; Javaux et al., 2008). A motivation of these studies is that a better prediction of root water uptake may help to optimize irrigation and identify optimal traits to capture water. Despite its importance, there is little experimental information on the spatiotemporal distribution of the uptake zone along roots growing in soil. The lack of experimental data is largely due to the technical difficulties in measuring water fluxes in soils and roots.Quantitative information on the rate and location of root water uptake along roots growing in soil is needed to better understand the function of roots in extracting water from the soil and tolerating drought events. Such information may show which parts of roots are more involved in water extraction and how root hydraulic properties change during root growth and exposure to water-limiting conditions. For instance, it is not clear how root anatomy and the hydraulic conductivity of roots change as the soil becomes dry or the transpiration demand increases. Quantitative information of the location of root water uptake can be used to estimate the spatial distribution of hydraulic conductivities along roots. This information is needed to parameterize the most recent and advanced models of root water uptake, such as those of Doussan et al. (1998) and Javaux et al. (2008).Most of the experimental information on the spatial distribution of water uptake is limited to roots grown in hydroponic and aeroponic cultures (Frensch and Steudle, 1989; Varney and Canny, 1993; Zwieniecki et al., 2003; Knipfer and Fricke, 2010a). These investigations substantially improved our knowledge of the mechanism of water transport in roots. However, roots grown in hydroponic and aeroponic cultures may have different properties than those of roots grown in soils. As the soil dries, the hydraulic conductivity of roots and of the root-soil interface changes and likely affects the profile of root water uptake (Blizzard and Boyer, 1980; Nobel and Cui, 1992; Huang and Nobel, 1993; McCully, 1995; North and Nobel, 1997; Carminati et al., 2011; Knipfer et al., 2011; McLean et al., 2011; Carminati, 2012).New advances in imaging techniques are opening new avenues for noninvasively studying water uptake by roots in soils (Doussan et al., 1998; Garrigues et al., 2006; Javaux et al., 2008; Pohlmeier et al., 2008; Moradi et al., 2011). Imaging methods such as x-ray computed tomography, light transmission imaging, NMR, and computed neutron radiography allow quantifying the changes of water content in the root zone with different accuracy and spatial resolution. However, due to the concomitant soil water redistribution, the local changes in soil water content are not trivially related to root uptake. Consequently, the estimation of root water uptake requires coupling the imaging methods with the modeling of water flow in the soil, which, in turn, requires accurate information on the hydraulic properties of soil and roots. An additional complexity is represented by the peculiar and only partly understood hydraulic properties of the soil in the vicinity of the roots, the so-called rhizosphere.The hydraulic properties of the rhizosphere are influenced by root and microorganism activity, soil compaction due to root growth, and the formation of air-filled gaps between soil and roots when roots shrink (Nye, 1994; North and Nobel, 1997; Carminati et al., 2010; Aravena et al., 2011; Moradi et al., 2011; Carminati, 2013; Zarebanadkouki and Carminati, 2014). To date, it has been technically difficult to quantify the hydraulic properties of the rhizosphere. Carminati et al. (2011) showed that the hydraulic properties of the first 1 to 2 mm near the root affect the profile of water content and water potential toward the root.Recently, we introduced a novel method to noninvasively trace the flow of water in soil and roots (Zarebanadkouki et al., 2012, 2013). The method combines neutron radiography and the injection of deuterated water (D₂O). Neutron radiography is an imaging technique that allows one to quantify the water distribution in thin soil samples with high accuracy and spatial resolution (Moradi et al., 2008). D₂O is an isotope of normal water. Its chemical and physical properties are similar to those of water, but in contrast to water, it is almost transparent in neutron transmission imaging (Matsushima et al., 2012). This property makes D₂O an excellent tracer for neutron imaging of water flow.In our previous experiments (Zarebanadkouki et al., 2012, 2013), D₂O was injected next to selected roots and its transport was monitored using time-series neutron radiography with a spatial resolution of 150 μm and a temporal resolution of 10 s for a duration of 2 h. We grew lupine (Lupinus albus) in aluminum containers (width of 25 cm, height of 30 cm, and thickness of 1 cm) filled with a sandy soil. The soil was partitioned into different compartments with a 1-cm layer of coarse sand acting as a capillary barrier (three vertical and four horizontal layers placed at regular intervals). The capillary barriers limited the transport of D₂O into a given region of soil and facilitated the quantification of D₂O transport into the roots. Figure 1 shows selected neutron radiographs of D₂O injection during the day and night. This figure is modified from Zarebanadkouki et al. (2013). The radiographs show that (1) the radial transport of D₂O into the roots was faster during the day than during the night and (2) the axial transport of D₂O along the roots was visible only during the day, while it was negligible at night. The differences between nighttime and daytime measurements were caused by the net flow of water induced by transpiration.Open in a separate windowFigure 1.Neutron radiographs of two samples after injection of 4 mL of D₂O during the day (A and B) and during the night (C and D). D₂O was injected in one compartment during the nighttime and in two compartments during the daytime. The images show the differences between the actual radiographs at time t and the radiograph before injection (t = 0). Brighter colors indicate lower neutron attenuation and higher D₂O-water ratio. The images show that (1) the transport of D₂O was faster during the day than during the night and (2) D₂O moved axially beyond the capillary barrier toward the shoot only during the day. Images are closeups of the original field of view of 15.75 × 15.75 cm showing the distribution of D₂O in the soil and root after D₂O injection. Figures are extracted from Zarebanadkouki et al. (2013). (A neutron radiograph of the whole sample used for daytime measurement is given in Figure 9.) [See online article for color version of this figure.]The interpretation of tracing experiments with D₂O in which water and D₂O are mixed is not straightforward (Carminati and Zarebanadkouki, 2013; Warren et al., 2013a, 2013b). To determine the convective fluxes from the radiographs, Zarebanadkouki et al. (2012, 2013) introduced a diffusion-convection model of D₂O transport in roots. The model was solved analytically. The model described the increase of the average D₂O concentration in the root with a double-exponential equation, in which the rate constants of the first and second phases were related to the transport of D₂O into the cortex and the stele of the roots. Although the model included important details of the root structure, such as different pathways of water across the root tissue, the diffusion of D₂O across the root tissue was strongly simplified. In particular, our previous model assumed that as soon as the roots were immersed in D₂O, the apoplastic free space of the root cortex was instantaneously saturated with D₂O. In other words, we assumed that all cortical cells and the root endodermis were simultaneously immersed in an identical concentration of D₂O equal to that of the soil. Additionally, we assumed that D₂O concentration inside the cortical cell and the root stele was uniform (well-stirred compartment).Although the radiographs clearly showed a significant axial transport of D₂O beyond the capillary barrier during the daytime (Fig. 1B), the model of Zarebanadkouki et al. (2013) was not capable of simulating it appropriately. Indeed, our previous model could only simulate the changes in D₂O concentration in the root segments immersed in D₂O. Since the concentration of D₂O in the root segment beyond the capillary barrier carries additional information on the axial and radial fluxes along the roots, we decided to modify our model to include such information.Another approximation of the previous model was the assumption that the radial water flow to the root was uniform along the root segment immersed in D₂O. However, Zarebanadkouki et al. (2013) found significant variations in root water uptake along the roots and suggested that root water uptake should be measured with a better spatial resolution.The objective of this study was to provide an adequate model to interpret tracing experiments with D₂O. We developed two different models to describe the transport of D₂O into roots. (1) In the first model, we described the transport of D₂O into the roots by taking into account the different pathways of water across the root tissue (i.e. the apoplastic and the cell-to-cell pathways). Although this model captures the complexity of the root structure, it requires several parameters, such as the ratio of the water flow in the apoplast over the water flow in the cell-to-cell pathway. We refer to this model as the composite transport model. (2) In the second model, we simplified the root tissue into a homogenous flow domain comprising both pathways. The latter model is a simplification of the complex root anatomy, but it has the advantage of requiring fewer parameters. We refer to this model as the simplified model.In the next sections, we introduce the two modeling approaches and run a sensitivity analysis to test whether the transport of D₂O into roots is sensitive to the parameters of the composite transport model. The question was, do we need the composite transport model to accurately estimate the water flow into the roots based on the experiments with neutron radiography? Or alternatively, can we use the simplified model to estimate the fluxes without the need of introducing several parameters?Our final goal was to develop a numerical procedure to extract quantitative information on the water fluxes and the root hydraulic properties based on the tracing experiments with neutron radiography. Based on the results of the sensitivity analysis, we chose the simplified model to simulate the experiments. By fitting the observed D₂O transport into the roots, we calculated the profiles of water flux across the roots of a 24-d-old lupine as well as the diffusion permeability of its roots.     

Solubilization and Plant Uptake of Zinc and Cadmium from Soils Treated with Elemental Sulfur

In growth chamber experiments we studied the potential use of elemental sulfur (S₈) as an acidifying agent to enhance the uptake of Cd and Zn from three different polluted soils by candidate phytoremediation plants (Brassica juncea, Helianthus annuus, Salix viminalis). Two of the three soils were calcareous, the other slightly acidic. One of the calcareous soils had been contaminated by dust emissions from a nearby brass smelter. The pollution of the other two soils had resulted from sewage sludge applications.

Sulfur was added to soils in quantities of 20 to 400 mmol sulfur kg^-1 soil. Plants were grown under fluorescent light in 1.5 l ($OS 13 cm) pots for 28 d.

Within 700 h soil pH decreased significantly in all soils, depending on S₈ dosage. In the acid soil, pH decreased from pH 6.5 to about 4 at the highest treatment level, while pH in one of the calcareous soils dropped even below pH 4. The effect was smaller in the second calcareous soil.

NaNO₃-extractable Cd and Zn increased up to 26-and 13-fold, respectively, in the acid soil, while in the calcareous soils, maximum increases were 9-and 11-fold, respectively.

Increased NaNO₃-extractable concentrations translated well into shoot concentrations (dry matter) in plants. Shoot Zn concentrations in H. annuus, for example, increased from 930 in the controls to 4300 mg kg^-1 in the highest S₈ treatment. However, effects observed in the plants were generally smaller than in the soils. In addition, in some variants growth was negatively affected, resulting in reduced metal removal from the soils.     

植物根系吸水过程中根系水流阻力的变化特征

以植物根系吸水的人工模拟试验所测得的数据为依据,运用水流的电模拟原理,定理分析了不同土壤水分水平处理下植物根系吸水过程中根系水流阻力各主要分量的大小、变化规律及其相对重要性．结果表明,在同一水分水平处理中,植物根内木质部传导阻力（Ｒｃ）随生长时间的推移而减小,随土层深度的加深而增大,土根接触阻力（Ｒｓｒ）、植物根系吸收阻力（Ｒｒ）随生长时间表现出先下降后上升阶段的动态变化特征;在不同水分水平处理中,Ｒｃ、Ｒｓｒ、Ｒｒ均随土壤湿度减小而大幅度增大;在植物根系水流阻力各分量中,Ｒｒ占根系水流阻力的比例为５５％～９６％,Ｒｓｒ约占根系水流阻力的４％～４５％,而Ｒｃ仅占根系水流阻力的７×１０－６,故Ｒｒ是决定植物根系吸水速率的重要因素     

Focus on Metabolism: Posttranslational Protein Modifications in Plant Metabolism

Posttranslational modifications (PTMs) of proteins greatly expand proteome diversity, increase functionality, and allow for rapid responses, all at relatively low costs for the cell. PTMs play key roles in plants through their impact on signaling, gene expression, protein stability and interactions, and enzyme kinetics. Following a brief discussion of the experimental and bioinformatics challenges of PTM identification, localization, and quantification (occupancy), a concise overview is provided of the major PTMs and their (potential) functional consequences in plants, with emphasis on plant metabolism. Classic examples that illustrate the regulation of plant metabolic enzymes and pathways by PTMs and their cross talk are summarized. Recent large-scale proteomics studies mapped many PTMs to a wide range of metabolic functions. Unraveling of the PTM code, i.e. a predictive understanding of the (combinatorial) consequences of PTMs, is needed to convert this growing wealth of data into an understanding of plant metabolic regulation.The primary amino acid sequence of proteins is defined by the translated mRNA, often followed by N- or C-terminal cleavages for preprocessing, maturation, and/or activation. Proteins can undergo further reversible or irreversible posttranslational modifications (PTMs) of specific amino acid residues. Proteins are directly responsible for the production of plant metabolites because they act as enzymes or as regulators of enzymes. Ultimately, most proteins in a plant cell can affect plant metabolism (e.g. through effects on plant gene expression, cell fate and development, structural support, transport, etc.). Many metabolic enzymes and their regulators undergo a variety of PTMs, possibly resulting in changes in oligomeric state, stabilization/degradation, and (de)activation (Huber and Hardin, 2004), and PTMs can facilitate the optimization of metabolic flux. However, the direct in vivo consequence of a PTM on a metabolic enzyme or pathway is frequently not very clear, in part because it requires measurements of input and output of the reactions, including flux through the enzyme or pathway. This Update will start out with a short overview on the major PTMs observed for each amino acid residue (PTMs, including determination of the localization within proteins (i.e. the specific residues) and occupancy. Challenges in dealing with multiple PTMs per protein and cross talk between PTMs will be briefly outlined. We then describe the major physiological PTMs observed in plants as well as PTMs that are nonenzymatically induced during sample preparation (PTMs, in particular for enzymes in primary metabolism (Calvin cycle, glycolysis, and respiration) and the C4 shuttle accommodating photosynthesis in C4 plants (PTMs observed in plants

Photosynthetic CO2 Uptake and Whole Plant Growth as Related to Plant Water Relations

Morphological features of arid region plant life forms are described and interpreted as adaptations to drought although this cannot be easily quantified. Functional adaptations, however, can be measured, and using the annual crop plant Vigna unguiculata (L.) Walp. responses to drought are described at the leaf and the whole plant level. In the first step of this analysis theoretical criteria are developed to define optimal water use. In the second step experimental data are used to test to what extent Vigna follows a theoretically optimal regulation of water and carbon relations. The analysis indicates that the ecological adaptation of regulatory processes may be quantified at a functional level.     

Plant Uptake and Enhanced Dissipation of Di(2-Ethylhexyl) Phthalate (DEHP) in Spiked Soils by Different Plant Species

This study was conducted to investigate the uptake, accumulation and the enhanced dissipation of di(2-ethylhexyl) phthalate (DEHP) spiked in soil (with a concentration of 117.4 ± 5.2 mg kg^?1) by eleven plants including eight maize ( Zea mays) cultivars and three forage species (alfalfa, ryegrass and teosinte). The results showed that, after 40 days of treatment, the removal rates of DEHP ranged from 66.8% (for the control) to 87.5% (for the maize cultivar of Huanong-1). Higher removal rate was observed during the first 10 days than the following days. Plants enhanced significantly the dissipation of DEHP in soil. Enhanced dissipation amount in planted soil was 13.3–122 mg pot^?1 for DEHP, and a net removal of 2.2%–20.7% of the initial DEHP was obtained compared with non-plant soil. The contribution of plant uptake to the total enhanced dissipation was <0.3%, and the enhanced dissipation of soil DEHP might be derived from plant-promoted biodegradation and sorption stronger to the soil. Nevertheless, the capability in accumulation and enhanced dissipation of DEHP from spiked soils varied within different species and cultivars.     

Focus on Ethylene: Bacterial Modulation of Plant Ethylene Levels

A focus on the mechanisms by which ACC deaminase-containing bacteria facilitate plant growth.Bacteria that produce the enzyme 1-aminocyclopropane-1-carboxylate (ACC) deaminase, when present either on the surface of plant roots (rhizospheric) or within plant tissues (endophytic), play an active role in modulating ethylene levels in plants. This enzyme activity facilitates plant growth especially in the presence of various environmental stresses. Thus, plant growth-promoting bacteria that express ACC deaminase activity protect plants from growth inhibition by flooding and anoxia, drought, high salt, the presence of fungal and bacterial pathogens, nematodes, and the presence of metals and organic contaminants. Bacteria that express ACC deaminase activity also decrease the rate of flower wilting, promote the rooting of cuttings, and facilitate the nodulation of legumes. Here, the mechanisms behind bacterial ACC deaminase facilitation of plant growth and development are discussed, and numerous examples of the use of bacteria with this activity are summarized.Agricultural development policies and practices in the past sixty years have largely been based on external inputs (pesticides and fertilizers) to control soil-borne diseases and increase crop yields. Recently, stimulated by the awareness of potentially serious environmental and human health damage caused by the over use of agricultural chemicals (Alavanja et al., 2004; Leach and Mumford, 2008; Damalas and Eleftherohorinos, 2011), the controversy regarding the use of pesticides and fertilizers has gained prominence. Therefore, worldwide agricultural practice is moving toward a more sustainable and environmentally friendly approach.In 2002, in the European Union, 5.7 million ha were designated as being cultivated organically, and by 2011, this number had increased to 9.6 million ha (http://ec.europa.eu/agriculture/markets-and-prices/more-reports/pdf/organic-2013_en.pdf). In other words, in 10 years, the area devoted to organic agriculture in the European Union increased by approximately 400,000 ha per year. This growth in organic agriculture notwithstanding, the total amount of organically cultivated land represents only 5.4% of the total agricultural land in Europe. In this context, the use of microbial inoculants instead of traditional chemicals is gaining popularity, and a number of new products have been formulated, marketed, and applied successfully.The soil surrounding plant roots (the rhizosphere) is one of the main sources of bacteria expressing plant-beneficial activities ( i.e. plant growth-promoting bacteria [PGPB]; Bashan and Holguin, 1998). Stimulation of growth and protection of different crops from pathogens and abiotic stressors by PGPB is well documented under both controlled conditions and in the field, and a large number of papers on this topic are available (Reed and Glick, 2005, 2013; Thakore, 2006). The positive effects induced by PGPB on plant growth are based on: (1) the improvement of mineral nutrition (nitrogen fixation, phosphate solubilization, and iron sequestration), (2) the enhancement of plant tolerance to biotic and abiotic stress (largely mediated by 1-aminocyclopropane-1-carboxylate [ACC] deaminase), (3) the modification of root development (via phytohormone synthesis), and (4) the suppression of phytopathogens (by antibiotics, competition, lytic enzymes, systemic resistance, etc.; Fig. 1). The current knowledge of microorganisms living in the rhizosphere, their role, and their biotechnological and environmental applications has been summarized in several reviews (Glick, 2012; Hirsch and Mauchline, 2012; Bakker et al., 2013; Mendes et al., 2013; Reed and Glick, 2013). This review focuses on the role of bacterial ACC deaminase in supporting the growth of plants exposed to environmental stress. In addition, the issues of the distribution and phylogeny of ACC deaminase, and the possible role of ACC as a signaling molecule, are addressed.Open in a separate windowFigure 1.Schematic overview of the main mechanisms used by PGPB. Following the release of root exudates, a variety of soil microorganisms are attracted to the root. Some of them can efficiently colonize the root surface while others (endophytes) can penetrate the root tissue and spread inside the plant. Plant growth promotion by beneficial microorganisms may occur by either direct or indirect mechanisms. Direct promotion of plant growth involves the improvement of mineral nutrition via nitrogen fixation, phosphate solubilization, and iron chelation, as well as the modulation of phytohormones levels (auxins, cytokinins, GAs, and ethylene). In addition to the increase of biomass, PGPB can positively affect the nutritional value of fruits and edible seeds. The indirect mechanisms are based on the improvement of plant health via suppression of soil-borne diseases by antibiotics, lytic enzymes, siderophore production, induced systemic resistance involving jasmonate and ethylene signaling within the plant, and other molecules (the O-antigenic side chain of the bacterial outer membrane protein lipopolysaccharide, flagellar fractions, pyoverdine, 2,4-diacetylphloroglucinol, cyclic lipopeptide, surfactants, and salicylic acid) that stimulate the host plant’s resistance to pathogens.