Diurnal variations in hydraulic conductivity and root pressure can be correlated with the expression of putative aquaporins in the roots of lotus japonicus

Searching for mechanisms driving root pressure in Zea mays-a transcriptomic approach

While there are many theories and a variety of innovative datasets contributing to our understanding of the mechanism generating root pressure in vascular plants, we are still unable to produce a specific cellular mechanism for any species. To discover these mechanisms, we used RNA-Seq to explore differentially expressed genes in three different tissues between individual Zea mays plants expressing root pressure and those producing none. Working from the perspective that roots cells are utililizing a combination of osmotic exudation and hydraulic pressure mechanisms to generate positively-pressured flow of water into the xylem from the soil, we hypothesized that differential expression analysis would yield candidate genes coding for membrane transporters, ion channels, ATPases, and hormones with clear relevance to root pressure generation. In basal stem and coarse root tissue, we observed these classes of differentially expressed genes and more, including a strong cytoskeletal remodeling response. Fine roots displayed remarkably little differential expression relevant to root pressure, leading us to conclude that they either do not contribute to root pressure generation or are constitutively expressing root pressure mechanisms regardless of soil water content.

Detection of plant cadmium toxicity by monitoring dielectric response of intact root systems on a fine timescale

The root dielectric response was measured on a minute scale to assess its efficiency for monitoring short-term cadmium (Cd) toxicity non-destructively. Electrical capacitance (CR), dissipation factor (DR) and electrical conductance (GR) were detected during the 24 to 168 h after Cd treatment (0, 20, 50 mg Cd2+ kg-1 substrate) in potted maize, cucumber and pea. Stress was also evaluated by measuring leaf chlorophyll content, Fv/Fm and stomatal conductance (gs) in situ, and shoot and root mass and total root length after harvest. CR showed a clear diurnal pattern, reflecting the water uptake rate, and decreased significantly in response to excessive Cd due to impeded root growth, the reduced tissue permittivity caused by accelerated lignification, and root ageing. Cd exposure markedly increased DR, indicating greater conductive energy loss due to oxidative membrane damage and enhanced electrolyte leakage. GR, which was coupled with root hydraulic conductance and varied diurnally, was increased transiently by Cd toxicity due to enhanced membrane permeability, but declined thereafter owing to stress-induced leaf senescence and transpiration loss. The time series of impedance components indicated the comparatively high Cd tolerance of the applied maize and the sensitivity of pea cultivar, which was confirmed by visible shoot symptoms, repeated physiological investigations and biomass measurements. The results demonstrated the potential of single-frequency dielectric measurements to follow certain aspects of the stress response of different species on a fine timescale without plant injury. The approach can be combined with widely used plant physiological methods and could contribute to breeding crop genotypes with improved stress tolerance.

Monitoring of plant water uptake by measuring root dielectric properties on a fine timescale: diurnal changes and response to leaf excision

BACKGROUND

The measurement of root dielectric response is a useful non-destructive method to evaluate root growth and function. Previous studies tracked root development throughout the plant growing cycle by single-time electrical measurements taken repeatedly. However, it is known that root conductivity and uptake activity can change rapidly, coupled with the day/night cycles of photosynthetic and transpiration rate. Therefore, the low-frequency dielectric monitoring of intact root-substrate systems at minute-scale temporal resolution was tested using a customized impedance measurement system in a laboratory environment. Electrical capacitance (CR) and conductance (GR) and the dissipation factor (DR) were detected for 144 h in potted maize, cucumber and pea grown under various light/dark and temperature conditions, or subjected to progressive leaf excision or decapitation. Photosynthetic parameters and stomatal conductance were also measured to evaluate the stress response.

RESULTS

The CR and GR data series showed significant 24-h seasonality associated with the light/dark and temperature cycles applied. This was attributed to the diurnal patterns in whole-plant transpiration (detected via stomatal conductance), which is strongly linked to the root water uptake rate. CR and GR decreased during the 6-day dark treatment, and dropped proportionally with increasing defoliation levels, likely due to the loss of canopy transpiration caused by dark-induced senescence or removal of leaves. DR showed a decreasing trend for plants exposed to 6-day darkness, whereas it was increased markedly by decapitation, indicating altered root membrane structure and permeability, and a modified ratio of apoplastic to cell-to-cell water and current pathways.

CONCLUSIONS

Dynamic, in situ impedance measurement of the intact root system was an efficient way of following integrated root water uptake, including diurnal cycles, and stress-induced changes. It was also demonstrated that the dielectric response mainly originated from root tissue polarization and current conduction, and was influenced by the actual physiological activity of the root system. Dielectric measurement on fine timescale, as a diagnostic tool for monitoring root physiological status and environmental response, deserves future attention.

An analytical complete model of root pressure generation: Theoretical bases for studying hydraulics of bamboo

To better understand the dynamics and functional roles of root pressure, we represent a novel and the first complete analytical model for root pressure, which can be applied to complex roots/shoots. The osmotic volume of a single root is equal to that of the vessel lumen in fine roots and adjacent apoplastic spaces. Water uptake occurs via passive osmosis and active solute uptake (J s * , osmol s-1 ), resulting in the osmolyte concentration Cr (mol·kg-1 of water) at a fixed osmotic volume. Solute loss occurs via two passive processes: radial diffusion of solute Km (Cr - Csoil ) from fine roots to soil, where Km is the diffusional constant and Csoil is the soil-solute concentration, and the mass flow of solute and water into the plant from the fine roots. The proposed model predicts the quadratic function of root pressure (Pr ),P r 2 + b P r + c = 0 , where b and c are the functions of plant hydraulic resistance, soil water potential, solute flux and gravitational potential. The model demonstrates the root pressure-mediated distribution of water through the hydraulic architecture of a 6.8-m-tall bamboo shoot. This model provides a theoretical basis to test the functional roles of root pressure in shaping the hydraulic architecture and refilling potential xylem embolisms.

Straining the root on and off triggers local calcium signalling

A fundamental function of an organ is the ability to perceive mechanical cues. Yet, how this is accomplished is not fully understood, particularly in plant roots. In plants, the majority of studies dealing with the effects of mechanical stress have investigated the aerial parts. However, in natural conditions roots are also subjected to mechanical cues, for example when the root encounters a hard obstacle during its growth or when the soil settles. To investigate root cellular responses to root compression, we developed a microfluidic system associated with a microvalve allowing the delivery of controlled and reproducible mechanical stimulations to the root. In this study, examining plants expressing the R-GECO1-mTurquoise calcium reporter, we addressed the root cell deformation and calcium increase induced by the mechanical stimulation. Lateral pressure applied on the root induced a moderate elastic deformation of root cortical cells and elicited a multicomponent calcium signal at the onset of the pressure pulse, followed by a second one at the release of the pressure. This indicates that straining rather than stressing of tissues is relevant to trigger the calcium signal. Although the intensity of the calcium response increases with the pressure applied, successive pressure stimuli led to a remarkable attenuation of the calcium signal. The calcium elevation was restricted to the tissue under pressure and did not propagate. Strain sensing, spatial restriction and habituation to repetitive stimulation represent the fundamental properties of root signalling in response to local mechanical stimulation. These data linking mechanical properties of root cells to calcium elevation contribute to elucidating the pathway allowing the root to adapt to the mechanical cues generated by the soil.

Coordination of hydraulic and leaf-level gas exchange traits during water-deficit acclimation in apple rootstocks

Water deficit episodes impact apple (Malus domestica) productivity through challenging the trees' water status, the influence of extreme high temperature climate has become increasingly prominent in recent years. Rootstocks can bestow specific properties on the fruit trees such as the resistance to drought stress. However, the related hydraulic mechanisms in response to water deficit have not been fully understood. Herein, five rootstocks (SH6, GM256, M9, M26, and MM106) were examined under water limitation. The hydraulic conductance of root (Kroot), shoots (Kshoot), and stems (Kstem-shoot) in the five rootstocks reduced slightly during drought stress. Whereas the leaf water potential and photosynthesis of five rootstocks decreased dramatically when they were exposed to drought stress. Additionally, the Kshoot and Kstem-shoot were strongly correlated with the total plant leaf area. Aquaporins (AQPs) involved in the symplastic water transport pathway, the PIP2:1, TIP1:1, and TIP2:2 mRNA levels of all genotypic rootstocks showed significant regulation under drought stress. We examined the relationships among photosynthesis, apoplastic, and symplastic water movement pathways to achieve a comprehensive understanding of rootstocks' hydraulic strategy for improving drought adaptation. The PIP2:1 and TIP2:1 in leaves were more sensitive to root hydraulic conductance in response to drought stress. Furthermore, the coordinated relationship existed in leaf-specific conductance of shoot (Kl -shoot) and transpiration rate (Tr) under drought stress in the rootstocks. Overall, the drought resistance in the five dwarfing rootstocks is associated with the rapid re-establishment of water-related traits, and the effect of the canopy on the drought resistance in apple rootstocks merits much more attention.

Changes in root hydraulic conductivity in wheat (Triticum aestivum L.) in response to salt stress and day/night can best be explained through altered activity of aquaporins

Salt stress reduces plant water flow during day and night. It is not known to which extent root hydraulic properties change in parallel. To test this idea, hydroponically grown wheat plants were grown at four levels of salt stress (50, 100, 150 and 200 mM NaCl) for 5-8d before harvest (d14-18) and subjected to a range of analyses to determine diurnal changes in hydraulic conductivity (Lp) at cell, root and plant level. Cell pressure probe analyses showed that the Lp of cortex cells was differentially affected by salt stress during day and night, and that the response to salt stress differed between the main axis of roots and lateral roots. The Aquaporin (AQP) inhibitor H₂ O₂ reduced Lp to a common, across treatments, level as observed in salt-stressed plants during the night. Analyses of transpiring plants and exuding root systems provided values of root Lp which were in the same range as values modeled based on cell-Lp. The results can best be explained through a change in root Lp in response to salt stress and day/night, which results from an altered activity of AQPs. qPCR gene expression analyses point to possible candidate AQP isoforms.

Exposure to high nitrogen triggered a genotype-dependent modulation of cell and root hydraulics, which can involve aquaporin regulation

Root nitrogen acquisition has been proposed to be regulated by mass flow, a process by which water flow brings nutrients to the root surface, depending on a concerted regulation of the root hydraulic properties and stomatal conductance. As aquaporins play an important role in regulating transcellular water flow, we aimed at evaluating the short-term effect of high nitrogen (HN) availability on the dynamics of hydraulic parameters at both the root and cell level and the regulation of aquaporins. The effect of short-term HN (8 mM NO₃ ^- ) treatment was investigated on 12 diverse 15-day-old maize genotypes. Root exposure to HN triggered a rapid (<4 h) increase in the root hydraulic conductivity (L_pr ) in seven genotypes while no L_pr variation was recorded for the others, allowing the separation of the genotypes into two groups (HN-responsive and HN-nonresponsive). A remarkable correlation between L_pr and the cortex cell hydraulic conductivity (L_pc ) was observed. However, while differences in gas exchange parameters were also observed, the variations were genotype-specific and not always correlated with the root hydraulic parameters. We then investigated whether HN-induced L_pr variations were linked to the activity and regulation of plasma membrane PIP aquaporins. While some changes in PIP mRNA levels were detected, this was not correlated with the protein levels. On the other hand, the rapid variation in L_pr observed in the B73 genotype was correlated with the PIP protein abundance in the plasma membrane, highlighting PIP posttranslational mechanisms in the short-term regulation of root hydraulic parameters in response to HN treatment.

Hormonal and environmental signaling pathways target membrane water transport

Plant water transport and its molecular components including aquaporins are responsive, across diverse time scales, to an extremely wide array of environmental and hormonal signals. These include water deficit and abscisic acid (ABA) but also more recently identified stimuli such as peptide hormones or bacterial elicitors. The present review makes an inventory of corresponding signalling pathways. It identifies some main principles, such as the central signalling role of ROS, with a dual function of aquaporins in water and hydrogen peroxide transport, the importance of aquaporin phosphorylation that is targeted by multiple classes of protein kinases, and the emerging role of lipid signalling. More studies including systems biology approaches are now needed to comprehend how plant water transport can be adjusted in response to combined stresses.

Root hydraulics adjustment is governed by a dominant cell-to-cell pathway in Beta vulgaris seedlings exposed to salt stress

Soil salinity reduces root hydraulic conductivity (L_pr) of several plant species. However, how cellular signaling and root hydraulic properties are linked in plants that can cope with water restriction remains unclear. In this work, we exposed the halotolerant species red beet (Beta vulgaris) to increasing concentrations of NaCl to determine the components that might be critical to sustaining the capacity to adjust root hydraulics. Our strategy was to use both hydraulic and cellular approaches in hydroponically grown seedlings during the first osmotic phase of salt stress. Interestingly, L_pr presented a bimodal profile response apart from the magnitude of the imposed salt stress. As well as L_pr, the PIP2-aquaporin profile follows an unphosphorylated/phosphorylated pattern when increasing NaCl concentration while PIP1 aquaporins remain constant. L_pr also shows high sensitivity to cycloheximide. In low NaCl concentrations, L_pr was high and 70 % of its capacity could be attributed to the CHX-inhibited cell-to-cell pathway. More interestingly, roots can maintain a constant spontaneous exudated flow that is independent of the applied NaCl concentration. In conclusion, Beta vulgaris root hydraulic adjustment completely lies in a dominant cell-to-cell pathway that contributes to satisfying plant water demands.

Positive pressure in xylem and its role in hydraulic function

Although transpiration-driven transport of xylem sap is well known to operate under absolute negative pressure, many terrestrial, vascular plants show positive xylem pressure above atmospheric pressure on a seasonal or daily basis, or during early developmental stages. The actual location and mechanisms behind positive xylem pressure remain largely unknown, both in plants that show seasonal xylem pressure before leaf flushing, and those that show a diurnal periodicity of bleeding and guttation. Available evidence shows that positive xylem pressure can be driven based on purely physical forces, osmotic exudation into xylem conduits, or hydraulic pressure in parenchyma cells associated with conduits. The latter two mechanisms may not be mutually exclusive and can be understood based on a similar modelling scenario. Given the renewed interest in positive xylem pressure, this review aims to provide a constructive way forward by discussing similarities and differences of mechanistic models, evaluating available evidence for hydraulic functions, such as rehydration of tissues, refilling of water stores, and embolism repair under positive pressure, and providing recommendations for future research, including methods that avoid or minimise cutting artefacts.

Linking root and stem hydraulic traits to leaf physiological parameters in Coffea canephora clones with contrasting drought tolerance

Knowing the key hydraulic traits of different genotypes at early seedling stages can potentially provide crucial information and save time for breeding programs. In the current study we investigated: (1) how root, stem and whole plant conductivities are linked to xylem traits, and (2) how the integrated hydraulic system impacts leaf water potential, gas exchange, chlorophyll a fluorescence and the growth of three coffee cultivars (clones of Coffea canephora Pierre ex Froehner cv. Conilon) with known differences in drought tolerance. The Conilon clones CL 14, CL 5 V and CL 109A, classified as tolerant, moderately tolerant, and sensitive to drought respectively, were grown under non-limiting soil-water supply but high atmospheric demand (i.e., high VPD_air). CL 14 and CL 5 V displayed higher root and stem hydraulic conductance and conductivity, and higher whole plant conductivity than CL 109A, and these differences were associated with higher root growth traits. In addition, CL 109A exhibited a non-significant trend towards wider vessels. Collectively, these responses likely contributed to reduce leaf water potential in CL 109A, and in turn, reduced leaf gas exchange, especially during elevated VPD_air. Even when grown under well-watered conditions, the elevated VPD_air observed during this study resulted in key differences in the hydraulic traits between the cultivars corresponding to differences in plant water status, gas exchange, and photochemical activity. Together these results suggest that coffee hydraulic traits, even when grown under non-water stress conditions, can be considered in breeding programs targeting more productive and efficient genotypes under drought and high atmospheric demand.

Drought-Induced Root Pressure in Sorghum bicolor

Root pressure, also manifested as profusive sap flowing from cut stems, is a phenomenon in some species that has perplexed biologists for much of the last century. It is associated with increased crop production under drought, but its function and regulation remain largely unknown. In this study, we investigated the initiation, mechanisms, and possible adaptive function of root pressure in six genotypes of Sorghum bicolor during a drought experiment in the greenhouse. We observed that root pressure was induced in plants exposed to drought followed by re-watering but possibly inhibited by 100% re-watering in some genotypes. We found that root pressure in drought stressed and re-watered plants was associated with greater ratio of fine: coarse root length and shoot biomass production, indicating a possible role of root allocation in creating root pressure and adaptive benefit of root pressure for shoot biomass production. Using RNA-Seq, we identified gene transcripts that were up- and down-regulated in plants with root pressure expression, focusing on genes for aquaporins, membrane transporters, and ATPases that could regulate inter- and intra-cellular transport of water and ions to generate positive xylem pressure in root tissue.

Genotypic variability in radial resistance to water flow in olive roots and its response to temperature variations

As radial root resistance (Rp) represents one of the key components of the soil-plant-atmosphere continuum resistance catena modulating water transport, understanding its control is essential for physiologists, modelers and breeders. Reports of Rp, however, are still scarce and scattered in the scientific literature. In this study, we assessed genetic variability in Rp and its dependence on temperature in five widely used olive cultivars. In a first experiment, cultivar differences in Rp at 25 °C were evaluated from flow-pressure measurements in excised roots and subsequent analysis of root traits. In a second experiment, similar determinations were performed continually over a 5-h period in which temperature was gradually increased from 12 to 32 °C, enabling the assessment of Rp response to changing temperature. Despite some variability, our results did not show statistical differences in Rp among cultivars in the first experiment. In the second, cultivar differences in Rp were not significant at 12 °C, but they became so as temperature increased. Furthermore, the changes in Rp between 12 and 32 °C were higher than those expected by the temperature-driven decrease in water viscosity, with the degree of that change differing among cultivars. Also, Rp at 25 °C reached momentarily in the second experiment was consistently higher than in the first at that same, but fixed, temperature. Overall, our results suggest that there is limited variability in Rp among the studied cultivars when plants have been exposed to a given temperature for sufficient time. Temperature-induced variation in Rp might thus be partly explained by changes in membrane permeability that occur slowly, which explains why our values at 25 °C differed between experiments. The observed cultivar differences in Rp with warming also indicate faster acclimation of Rp to temperature changes in some cultivars than others.

Element content and expression of genes of interest in guard cells are connected to spatiotemporal variations in stomatal conductance

Element content and expression of genes of interest on single cell types, such as stomata, provide valuable insights into their specific physiology, improving our understanding of leaf gas exchange regulation. We investigated how far differences in stomatal conductance (g_s ) can be ascribed to changes in guard cells functioning in amphistomateous leaves. g_s was measured during the day on both leaf sides, on well-watered and drought-stressed trees (two Populus euramericana Moench and two Populus nigra L. genotypes). In parallel, guard cells were dissected for element content and gene expressions analyses. Both were strongly arranged according to genotype, and drought had the lowest impact overall. Normalizing the data by genotype highlighted a structure on the basis of leaf sides and time of day both for element content and gene expression. Guard cells magnesium, phosphorus, and chlorine were the most abundant on the abaxial side in the morning, where g_s was at the highest. In contrast, genes encoding H⁺ -ATPase and aquaporins were usually more abundant in the afternoon, whereas genes encoding Ca²⁺ -vacuolar antiporters, K⁺ channels, and ABA-related genes were in general more abundant on the adaxial side. Our work highlights the unique physiology of each leaf side and their analogous rhythmicity through the day.

Hypernodulating soybean mutant line nod4 lacking 'Autoregulation of Nodulation' (AON) has limited root-to-shoot water transport capacity

BACKGROUND AND AIMS

Although hypernodulating phenotype mutants of legumes, such as soybean, possess a high leaf N content, the large number of root nodules decreases carbohydrate availability for plant growth and seed yield. In addition, under conditions of high air vapour pressure deficit (VPD), hypernodulating plants show a limited capacity to replace water losses through transpiration, resulting in stomatal closure, and therefore decreased net photosynthetic rates. Here, we used hypernodulating (nod4) (282.33 ± 28.56 nodules per plant) and non-nodulating (nod139) (0 nodules per plant) soybean mutant lines to determine explicitly whether a large number of nodules reduces root hydraulic capacity, resulting in decreased stomatal conductance and net photosynthetic rates under high air VPD conditions.

METHODS

Plants were either inoculated or not inoculated with Bradyrhizobium diazoefficiens (strain BR 85, SEMIA 5080) to induce nitrogen-fixing root nodules (where possible). Absolute root conductance and root conductivity, plant growth, leaf water potential, gas exchange, chlorophyll a fluorescence, leaf 'greenness' [Soil Plant Analysis Development (SPAD) reading] and nitrogen content were measured 37 days after sowing.

KEY RESULTS

Besides the reduced growth of hypernodulating soybean mutant nod4, such plants showed decreased root capacity to supply leaf water demand as a consequence of their reduced root dry mass and root volume, which resulted in limited absolute root conductance and root conductivity normalized by leaf area. Thereby, reduced leaf water potential at 1300 h was observed, which contributed to depression of photosynthesis at midday associated with both stomatal and non-stomatal limitations.

CONCLUSIONS

Hypernodulated plants were more vulnerable to VPD increases due to their limited root-to-shoot water transport capacity. However, greater CO2 uptake caused by the high N content can be partly compensated by the stomatal limitation imposed by increased VPD conditions.

Variable effects of arbuscular mycorrhizal fungal inoculation on physiological and molecular measures of root and stomatal conductance of diverse Medicago truncatula accessions

Association with arbuscular mycorrhizal fungi (AMF) can impact on plant water relations; mycorrhizal plants can exhibit increased stomatal conductance (g_s ) and root hydraulic conductance (normalized to root dry weight, L_o ), and altered expression of aquaporins (AQP). Many factors regulate such responses; however, plant intraspecific diversity effects have yet to be explored. Twenty geographically diverse accessions of Medicago truncatula were inoculated with the AMF Funneliformis mosseae or mock-inoculated, and grown under well-watered conditions. Biomass, g_s , shoot nutrient concentrations and mycorrhizal colonization were measured in all accessions, and L_o and gene expression in five accessions. The diverse accessions varied in physiology and gene expression; some accessions were also larger or had higher g_s when colonized by F. mosseae. In the five accessions, L_o was higher in two accessions when colonized by AMF and also maintained within a much smaller range than the mock-inoculated plants. Expression of MtPIP1 correlated with both g_s and L_o , and when plants were more than 3% colonized, mycorrhizal colonization correlated with L_o . Accession and AMF treatments had profound effects on M. truncatula, including several measures of plant water relations. Correlations between response variables, especially between molecular and physiological variables, across genotypes, highlight the findings of this study.

The Relationship between Turgor Pressure Change and Cell Hydraulics of Midrib Parenchyma Cells in the Leaves of Zea mays

Leaf dehydration decreases water potential and cell turgor pressure. Therefore, changes in cell turgor pressure may regulate water transport across plant cell membranes. Using a cell pressure probe, the hydraulic properties of parenchyma cells in the midrib of maize (Zea mays L.) leaves were measured (half time of water exchange in cells as a measure of hydraulic conductivity Lp). Using intact plants with root systems encased in a pressure chamber, the root systems were pressurized and the turgor pressure in leaf cells increased by increments up to 0.3 MPa. However, the increase in the cell turgor did not increase but stabilized values. Increased water potential in leaf cells seemed to have stabilizing effects on the probably due to enhanced water availability. When the cell turgor decreased by 0.1 MPa to 0.3 MPa with releasing the pressure in the pressure chamber, was temporarily increased to a large degree,a factor of up to 13 within 30 min.

Composite Transport Model and Water and Solute Transport across Plant Roots: An Update

The present review examines recent experimental findings in root transport phenomena in terms of the composite transport model (CTM). It has been a well-accepted conceptual model to explain the complex water and solute flows across the root that has been related to the composite anatomical structure. There are three parallel pathways involved in the transport of water and solutes in roots - apoplast, symplast, and transcellular paths. The role of aquaporins (AQPs), which facilitate water flows through the transcellular path, and root apoplast is examined in terms of the CTM. The contribution of the plasma membrane bound AQPs for the overall water transport in the whole plant level was varying depending on the plant species, age of roots with varying developmental stages of apoplastic barriers, and driving forces (hydrostatic vs. osmotic). Many studies have demonstrated that the apoplastic barriers, such as Casparian bands in the primary anticlinal walls and suberin lamellae in the secondary cell walls, in the endo- and exodermis are not perfect barriers and unable to completely block the transport of water and some solute transport into the stele. Recent research on water and solute transport of roots with and without exodermis triggered the importance of the extension of conventional CTM adding resistances that arrange in series (epidermis, exodermis, mid-cortex, endodermis, and pericycle). The extension of the model may answer current questions about the applicability of CTM for composite water and solute transport of roots that contain complex anatomical structures with heterogeneous cell layers.

Hydraulic redistribution: limitations for plants in saline soils

Hydraulic redistribution (HR), the movement of water from wet to dry patches in the soil via roots, occurs in different ecosystems and plant species. By extension of the principle that HR is driven by gradients in soil water potential, HR has been proposed to occur for plants in saline soils. Despite the inherent spatial patchiness and salinity gradients in these soils, the lack of direct evidence of HR in response to osmotic gradients prompted us to ask the question: are there physical or physiological constraints to HR for plants in saline environments? We propose that build-up of ions in the root xylem sap and in the leaf apoplast, with the latter resulting in a large predawn disequilibrium of water potential in shoots compared with roots and soil, would both impede HR. We present a conceptual model that illustrates how processes in root systems in heterogeneous salinity with water potential gradients, even if equal to those in non-saline soils, will experience a dampened magnitude of water potential gradients in the soil-plant continuum, minimizing or preventing HR. Finally, we provide an outlook for understanding the relevance of HR for plants in saline environments by addressing key research questions on plant salinity tolerance.

Rapid changes in root hydraulic conductivity and aquaporin expression in rice (Oryza sativa L.) in response to shoot removal - xylem tension as a possible signal

Background and Aims It is not clear how plants adjust the rate of root water uptake to that of shoot water loss. The aim of this study on rice was to test the idea that root aquaporins (AQPs) and xylem tension play a role in this adjustment. Methods Three-week-old rice (Oryza sativa L.) plants, which were grown hydroponically, had their entire shoot system removed, and root hydraulic conductivity (exudation analyses) and gene expression (quantitative real-time PCR) of root plasma membrane intrinsic aquaporin proteins (PIPs) was followed within 60 min after shoot excision. Key Results All three PIP1 genes (OsPIP1;1, OsPIP1;2 and OsPIP1;3) and three of the six PIP2 genes tested (OsPIP2;1, OsPIP2;4 and OsPIP2;5) showed a rapid (5 min) and lasting (60 min) decrease in gene expression. Expression decreased by up to 85 % within 60 min. The other three PIP2 genes tested (OsPIP2;2, OsPIP2;3 and OsPIP2;6) showed a varied response, with expression decreasing either only initially (5 min) or after 60 min, or not changing at all. In a follow-up experiment, plants had their shoot system removed and the detached root system immediately connected to a vacuum pump through which some tension (80 kPa) was applied. This application of tension prevented any significant decrease in PIP expression. Conclusions Shoot removal leads to a rapid decrease in expression of all PIP1s and some PIP2s in roots of rice. Xylem tension plays some role in this process.

An Integrated View of Whole-Tree Hydraulic Architecture. Does Stomatal or Hydraulic Conductance Determine Whole Tree Transpiration?

Hydraulic conductance exerts a strong influence on many aspects of plant physiology, namely: transpiration, CO₂ assimilation, growth, productivity or stress response. However we lack full understanding of the contribution of root or shoot water transport capacity to the total water balance, something which is difficult to study in trees. Here we tested the hypothesis that whole plant hydraulic conductance modulates plant transpiration using two different seedlings of citrus rootstocks, Poncirus trifoliata (L.) Raf. and Cleopatra mandarin (Citrus reshni Hort ex Tan.). The two genotypes presented important differences in their root or shoot hydraulic conductance contribution to whole plant hydraulic conductance but, even so, water balance proved highly dependent on whole plant conductance. Further, we propose there is a possible equilibrium between root and shoot hydraulic conductance, similar to that between shoot and root biomass production, which could be related with xylem anatomy.

Effect of soil temperature on root resistance: implications for different trees under Mediterranean conditions

The effect of temperature on radial root hydraulic specific resistance (Rp) is a known phenomenon; however, the impact ofRpvariations expected from soil temperature changes over the tree root system is unknown. The present article analyses the relations hip ofRpwith temperature in olive 'Picual' and a hybrid rootstock, GF677, at five different temperatures, showing that a variation of 3- and 4.5-folds exists for olive 'Picual' and GF677 in the range from 10 to 20 °C. The functions obtained were scaled up to show the theoretical changes of total radial root system resistance in a common tree orchard in a Mediterranean climate at a daily and seasonal scale, using recorded soil temperature values: a difference between summer and winter of 3.5-fold for olive 'Picual' and 9-fold for GF677 was observed. Nevertheless,Rpchanges are not only related to temperature, as cavitation or circadian rhythms in aquaporin expression may also play a role. The results obtained from an experiment with the two cultivars submitted to constant pressure and temperature during several hours exhibited a variation inRp, but this was of lower magnitude than that observed due to temperature changes. Finally, a comparison ofRpat 25 °C between GF677 and GN15 (another rootstock obtained from the same parental as GF677) showed significant differences. According to our results, diurnal and seasonal changes inRpdue to temperature variations are of significant importance, and it would therefore be advisable to assess them explicitly into soil-plant-atmosphere continuum models.

Drought tolerance strategies highlighted by two Sorghum bicolor races in a dry-down experiment

Drought stress is the major environmental stress that affects more and more frequently plant growth and productivity due to the current climate change scenario. Unravelling the physiological mechanism underlying the response of plants to water stress and discover traits related to drought tolerance provide new and powerful tools for the selection in breeding programmes. Four genotypes of Sorghum bicolor (L.) Moench were screened in a dry-down experiment using different approaches to discover physiological and molecular indicators of drought tolerance. Different strategies were identified in response to drought among the four genotypes and the two Sorghum race allowing to state the tolerance of durra race compared to the caudatum one and, within the durra race, the drought tolerance of the genotype IS22330. It retained high biomass production and high tolerance index, it had a low threshold of fraction of transpirable soil water and high capacity to recover leaf apparatus after drought stress. Furthermore in this study, the expression levels of four genes highlighted that they could be used as proxy for drought tolerance. Dehdrine (DHN) could be used for screening drought tolerance both in durra and in caudatum races. NADP-Malic Enzyme, Carbonic Anhydrase (CA) and Plasma membrane Intrinsic Protein (PIP2-5), being up-regulated by drought stress only in durra race, have a more limited, though nonetheless useful application. In the tolerant durra genotype IS22330 in particular, the regulation of stomatal openings was strongly related to NADP-Malic Enzyme expression.

Root hydraulic conductivity and adjustments in stomatal conductance: hydraulic strategy in response to salt stress in a halotolerant species

Recent advances at the molecular level are introducing a new scenario that needs to be integrated into the analysis of plant hydraulic properties. Although it is not yet clear to what extent this scenario alters the current proposal for the hydraulic circuit models, it introduces new insights when studying plants that are able to easily overcome water restrictions. In this context, our aim was to explore water adjustments in a halotolerant model (Beta vulgaris) by studying the coordination between the root in terms of root hydraulic conductivity (Lpr) and the shoot as reflected in the stomatal conductance (gs). The root water pathways were also analysed in terms of root suberization (apoplastic barrier) and aquaporin transcript levels (cell-to-cell pathway). Beta vulgaris showed the ability to rapidly lose (4 h) and gain (24 h) turgor when submitted to salt stress (200 mM). The reduction profile observed in Lpr and gs was consistent with a coupled process. The tuning of the root water flow involved small variations in the studied aquaporin's transcripts before anatomical modifications occurred. Exploring Lpr enhancement after halting the stress contributed to show not only a different profile in restoring Lpr but also the capacity to uncouple Lpr from gs. Beta vulgaris root plays a key role and can anticipate water loss before the aerial water status is affected.

Molecular mechanisms of foliar water uptake in a desert tree

Water deficits severely affect growth, particularly for the plants in arid and semiarid regions of the world. In addition to precipitation, other subsidiary water, such as dew, fog, clouds and small rain showers, may also be absorbed by leaves in a process known as foliar water uptake. With the severe scarcity of water in desert regions, this process is increasingly becoming a necessity. Studies have reported on physical and physiological processes of foliar water uptake. However, the molecular mechanisms remain less understood. As major channels for water regulation and transport, aquaporins (AQPs) are involved in this process. However, due to the regulatory complexity and functional diversity of AQPs, their molecular mechanism for foliar water uptake remains unclear. In this study, Tamarix ramosissima, a tree species widely distributed in desert regions, was investigated for gene expression patterns of AQPs and for sap flow velocity. Our results suggest that the foliar water uptake of T. ramosissima occurs in natural fields at night when the humidity is over a threshold of 85 %. The diurnal gene expression pattern of AQPs suggests that most AQP gene expressions display a circadian rhythm, and this could affect both photosynthesis and transpiration. At night, the PIP2-1 gene is also upregulated with increased relative air humidity. This gene expression pattern may allow desert plants to regulate foliar water uptake to adapt to extreme drought. This study suggests a molecular basis of foliar water uptake in desert plants.

PIP1 aquaporins: Intrinsic water channels or PIP2 aquaporin modulators?

The highly conserved plant aquaporins, known as Plasma membrane Intrinsic Proteins (PIPs), are the main gateways for cell membrane water exchange. Years of research have described in detail the properties of the PIP2 subfamily. However, characterizing the PIP1 subfamily has been difficult due to the failure to localize to the plasma membrane. In addition, the discovery of the PIP1-PIP2 interaction suggested that PIP1 aquaporins could be regulated by a complex posttranslational mechanism that involves trafficking, heteromerization and fine-tuning of channel activity. This review not only considers the evidence and findings but also discusses the complexity of PIP aquaporins. To establish a new benchmark in PIP regulation, we propose to consider PIP1-PIP2 pairs as functional units for the purpose of future research into their physiological roles.

Multiscale model of a freeze-thaw process for tree sap exudation

Sap transport in trees has long fascinated scientists, and a vast literature exists on experimental and modelling studies of trees during the growing season when large negative stem pressures are generated by transpiration from leaves. Much less attention has been paid to winter months when trees are largely dormant but nonetheless continue to exhibit interesting flow behaviour. A prime example is sap exudation, which refers to the peculiar ability of sugar maple (Acer saccharum) and related species to generate positive stem pressure while in a leafless state. Experiments demonstrate that ambient temperatures must oscillate about the freezing point before significantly heightened stem pressures are observed, but the precise causes of exudation remain unresolved. The prevailing hypothesis attributes exudation to a physical process combining freeze-thaw and osmosis, which has some support from experimental studies but remains a subject of active debate. We address this knowledge gap by developing the first mathematical model for exudation, while also introducing several essential modifications to this hypothesis. We derive a multiscale model consisting of a nonlinear system of differential equations governing phase change and transport within wood cells, coupled to a suitably homogenized equation for temperature on the macroscale. Numerical simulations yield stem pressures that are consistent with experiments and provide convincing evidence that a purely physical mechanism is capable of capturing exudation.

Aquaporins in Plants

Aquaporins are membrane channels that facilitate the transport of water and small neutral molecules across biological membranes of most living organisms. In plants, aquaporins occur as multiple isoforms reflecting a high diversity of cellular localizations, transport selectivity, and regulation properties. Plant aquaporins are localized in the plasma membrane, endoplasmic reticulum, vacuoles, plastids and, in some species, in membrane compartments interacting with symbiotic organisms. Plant aquaporins can transport various physiological substrates in addition to water. Of particular relevance for plants is the transport of dissolved gases such as carbon dioxide and ammonia or metalloids such as boron and silicon. Structure-function studies are developed to address the molecular and cellular mechanisms of plant aquaporin gating and subcellular trafficking. Phosphorylation plays a central role in these two processes. These mechanisms allow aquaporin regulation in response to signaling intermediates such as cytosolic pH and calcium, and reactive oxygen species. Combined genetic and physiological approaches are now integrating this knowledge, showing that aquaporins play key roles in hydraulic regulation in roots and leaves, during drought but also in response to stimuli as diverse as flooding, nutrient availability, temperature, or light. A general hydraulic control of plant tissue expansion by aquaporins is emerging, and their role in key developmental processes (seed germination, emergence of lateral roots) has been established. Plants with genetically altered aquaporin functions are now tested for their ability to improve plant tolerance to stresses. In conclusion, research on aquaporins delineates ever expanding fields in plant integrative biology thereby establishing their crucial role in plants.

Aquaporins in Coffea arabica L.: Identification, expression, and impacts on plant water relations and hydraulics

Plant aquaporins (AQPs) are involved in the transport of water and other small solutes across cell membranes, and thus play major roles in the regulation of plant water balance, as well as in growth regulation and response to abiotic stress factors. Limited information is currently available about the presence and role of AQPs in Coffea arabica L., despite the economic importance of the species and its vulnerability to drought stress. We identified candidate AQP genes by screening a proprietary C. arabica transcriptome database, resulting in the identification of nine putative aquaporins. A phylogenetic analysis based on previously characterized AQPs from Arabidopsis thaliana and Solanum tuberosum allowed to assign the putative coffee AQP sequences to the Tonoplast (TIP) and Plasma membrane (PIP) subfamilies. The possible functional role of coffee AQPs was explored by measuring hydraulic conductance and aquaporin gene expression on leaf and root tissues of two-year-old plants (C. arabica cv. Pacamara) subjected to different experimental conditions. In a first experiment, we tested plants for root and leaf hydraulic conductance both before dawn and at mid-day, to check the eventual impact of light on AQP activity and plant hydraulics. In a second experiment, we measured plant hydraulic responses to different water stress levels as eventually affected by changes in AQPs expression levels. Our results shed light on the possible roles of AQPs in the regulation of C. arabica hydraulics and water balance, opening promising research lines to improve the sustainability of coffee cultivation under global climate change scenarios.

Influence of arbuscular mycorrhizal fungi and treated wastewater on water relations and leaf structure alterations of Viburnum tinus L. plants during both saline and recovery periods

Nowadays, irrigation with low quality water is becoming an alternative to satisfy the needs of crops. However, some plant species have to deal with high salinity of reclaimed water, by adapting their physiological behaviour during both saline and recovery periods and developing morphological changes in their leaves. The application of arbuscular mycorrhizal fungi (AMF) could also be a suitable option to mitigate the negative effects of this kind of water, although the effectiveness of plant-AMF association is influenced by many factors. In this work, during forty weeks, the combined effect of Glomus iranicum var. tenuihypharum and two types of water: control, C, EC<0.9 dS m(-1) and reclaimed water, RW (with EC: 4 dS m(-1) during a first saline period and EC: 6 dS m(-1) during a second saline period) was evaluated for laurustinus plants (Viburnum tinus L.) transplanted in soil. This was followed by a recovery period of eight weeks, when all the plants were irrigated in the control irrigation conditions. Seasonal and daily changes in stem water potential (Ψstem), stomatal conductance (gs), photosynthesis (Pn) and leaf internal CO2 concentration (Ci) of laurustinus plants were evaluated. Leaf structure alterations, nutrient imbalance, height and leaf hydraulic conductivity (Kleaf) were also determined. Due to the high difficulty of absorbing water from the soil, RW plants showed a high volumetric water content (θv) in soil. The stem water potential and the stomatal conductance (gs) values were reduced in RW plants throughout the second saline period. These decreases were also found during the day. Leaf Ca(2+)/Na(+) and K(+)/Na(+) ratios diminished in RW plants respect to the C plants due to the Na(+) accumulation, although height and chlorophyll content values did not show statistical differences. Leaves from RW plants showed a significantly thicker mesophyll than Control leaves as a consequence of high EC. The area of palisade parenchyma (PP) increased while the area of spongy parenchyma (SP) decreased in RW leaves with respect to the C leaves. These structural changes could be considered as a strategy to maximize photosynthesis potential in saline conditions. Mycorrhizal inoculation improved the water status of both C and RW plants by increasing their Ψstem and gs values. As regards leaf structure, AMF showed an opposite effect to salinity for PP and SP. At the end of the recovery period, hardly any statistical differences of physiological parameters were found between treatments, although a tendency to improve them was observed in inoculated plants. In any case, the leaf structural changes and the great reduction in Kleaf observed at Ψleaf below -1.5 MPa would constitute an important mechanism for laurustinus plants to reduce the water loses produced by salinity.

Flowers under pressure: ins and outs of turgor regulation in development

BACKGROUND

Turgor pressure is an essential feature of plants; however, whereas its physiological importance is unequivocally recognized, its relevance to development is often reduced to a role in cell elongation.

SCOPE

This review surveys the roles of turgor in development, the molecular mechanisms of turgor regulation and the methods used to measure turgor and related quantities, while also covering the basic concepts associated with water potential and water flow in plants. Three key processes in flower development are then considered more specifically: flower opening, anther dehiscence and pollen tube growth.

CONCLUSIONS

Many molecular determinants of turgor and its regulation have been characterized, while a number of methods are now available to quantify water potential, turgor and hydraulic conductivity. Data on flower opening, anther dehiscence and lateral root emergence suggest that turgor needs to be finely tuned during development, both spatially and temporally. It is anticipated that a combination of biological experiments and physical measurements will reinforce the existing data and reveal unexpected roles of turgor in development.

Regulation of photosynthesis and stomatal and mesophyll conductance under water stress and recovery in olive trees: correlation with gene expression of carbonic anhydrase and aquaporins

The hypothesis that aquaporins and carbonic anhydrase (CA) are involved in the regulation of stomatal (g s) and mesophyll (g m) conductance to CO2 was tested in a short-term water-stress and recovery experiment in 5-year-old olive plants (Olea europaea) growing outdoors. The evolution of leaf gas exchange, chlorophyll fluorescence, and plant water status, and a quantitative analysis of photosynthesis limitations, were followed during water stress and recovery. These variables were correlated with gene expression of the aquaporins OePIP1.1 and OePIP2.1, and stromal CA. At mild stress and at the beginning of the recovery period, stomatal limitations prevailed, while the decline in g m accounted for up to 60% of photosynthesis limitations under severe water stress. However, g m was restored to control values shortly after rewatering, facilitating the recovery of the photosynthetic rate. CA was downregulated during water stress and upregulated after recovery. The use of structural equation modelling allowed us to conclude that both OePIP1.1 and OePIP2.1 expression could explain most of the variations observed for g s and g m. CA expression also had a small but significant effect on g m in olive under water-stress conditions.

Aquaporins: highly regulated channels controlling plant water relations

Plant growth and development are dependent on tight regulation of water movement. Water diffusion across cell membranes is facilitated by aquaporins that provide plants with the means to rapidly and reversibly modify water permeability. This is done by changing aquaporin density and activity in the membrane, including posttranslational modifications and protein interaction that act on their trafficking and gating. At the whole organ level aquaporins modify water conductance and gradients at key "gatekeeper" cell layers that impact on whole plant water flow and plant water potential. In this way they may act in concert with stomatal regulation to determine the degree of isohydry/anisohydry. Molecular, physiological, and biophysical approaches have demonstrated that variations in root and leaf hydraulic conductivity can be accounted for by aquaporins but this must be integrated with anatomical considerations. This Update integrates these data and emphasizes the central role played by aquaporins in regulating plant water relations.

Mild salt stress conditions induce different responses in root hydraulic conductivity of phaseolus vulgaris over-time

Plants respond to salinity by altering their physiological parameters in order to maintain their water balance. The reduction in root hydraulic conductivity is one of the first responses of plants to the presence of salt in order to minimize water stress. Although its regulation has been commonly attributed to aquaporins activity, osmotic adjustment and the toxic effect of Na+ and Cl- have also a main role in the whole process. We studied the effects of 30 mM NaCl on Phaseolus vulgaris plants after 9 days and found different responses in root hydraulic conductivity over-time. An initial and final reduction of root hydraulic conductivity, stomatal conductance, and leaf water potential in response to NaCl was attributed to an initial osmotic shock after 1 day of treatment, and to the initial symptoms of salt accumulation within the plant tissues after 9 days of treatment. After 6 days of NaCl treatment, the increase in root hydraulic conductivity to the levels of control plants was accompanied by an increase in root fructose content, and with the intracellular localization of root plasma membrane aquaporins (PIP) to cortex cells close to the epidermis and to cells surrounding xylem vessels. Thus, the different responses of bean plants to mild salt stress over time may be connected with root fructose accumulation, and intracellular localization of PIP aquaporins.

Expression of Fragaria vesca PIP aquaporins in response to drought stress: PIP down-regulation correlates with the decline in substrate moisture content

PIP aquaporin responses to drought stress can vary considerably depending on the isoform, tissue, species or level of stress; however, a general down-regulation of these genes is thought to help reduce water loss and prevent backflow of water to the drying soil. It has been suggested therefore, that it may be necessary for the plant to limit aquaporin production during drought stress, but it is unknown whether aquaporin down-regulation is gradual or triggered by a particular intensity of the stress. In this study, ten Fragaria PIP genes were identified from the woodland strawberry (Fragaria vesca L.) genome sequence and characterised at the sequence level. The water relations of F. vesca were investigated and the effect of different intensities of drought stress on the expression of four PIP genes, as well as how drought stress influences their diurnal transcription was determined. PIP down-regulation in the root corresponded to the level of drought stress. Moreover, transcript abundance of two genes highly expressed in the root (FvPIP1;1 and FvPIP2;1) was strongly correlated to the decline in substrate moisture content. The amplitude of diurnal aquaporin expression in the leaves was down-regulated by drought without altering the pattern, but showing an intensity-dependent effect. The results show that transcription of PIP aquaporins can be fine-tuned with the environment in response to declining water availability.

The root cortex cell hydraulic conductivity is enhanced with increasing chromosome ploidy in wheat

Wheat (Triticum spp.) root water uptake is enhanced with increasing chromosome ploidy, but the underlying mechanism is unclear. The leaf transpiration rate (E), individual root (Lp(r)) and cortical cell (Lp(c)) hydraulic conductivity, cortical cell volume (V(c)) and transcription levels of two putative plasma intrinsic aquaporin genes (PIPs) were investigated in wheat seedlings with different chromosome ploidy (Triticum monococcum (2X, AA); Triticum dicoccum (4X, BB); Triticum aestivum (6X, AABBDD)). The E, Lp(r) and Lp(c) of wheat increased with increasing ploidy, but the Vc was reduced. Osmotic stress significantly reduced the E, Lp(c), Lp(r), and the relative mRNA content of TaPIP1;2 and TaPIP2;5 in wheat. Under both well-watered and osmotic stress conditions, the Lp(r) was significantly and positively correlated with the E and Lp(c), and the relative mRNA content of TaPIP1;2 and TaPIP2;5 was significantly and positively correlated with Lp(c) and Lp(r), respectively. For well-watered or osmotically stressed wheat plants, the Lp(c) was reduced, but the L(p)(c)/Lp(r) increased with increasing V(c), suggesting that Vc affects root radical water transport. Thus, the increased Lp(c) and transcription levels of TaPIP1;2 and TaPIP2;5 in wheat roots provides insight into the mechanisms underlying enhanced root water uptake with increasing chromosome ploidy during wheat evolution.

Transpirational demand affects aquaporin expression in poplar roots

Isohydric plants tend to maintain a water potential homeostasis primarily by controlling water loss via stomatal conductance. However, there is accumulating evidence that plants can also modulate water uptake in a dynamic manner. The dynamics of water uptake are influenced by aquaporin-mediated changes in root hydraulics. Most studies in this area have been conducted on herbaceous plants, and less is known about responses of woody plants. Here a study was conducted to determine how roots of hybrid poplar plants (Populus trichocarpa×deltoides) respond to a step change in transpirational demand. The main objective was to measure the expression of selected aquaporin genes and to assess how transcriptional responses correspond to changes in root water flow (Q R) and other parameters of water relations. A subset of plants was grown in shade and was subsequently exposed to a 5-fold increase in light level. Another group of plants was grown at ~95% relative humidity (RH) and was then subjected to lower RH while the light level remained unchanged. Both plant groups experienced a transient drop in stem water potentials. At 28h after the increase in transpirational demand, water potentials recovered. This recovery was associated with changes in the expression of PIP1 and PIP2 subfamily genes and an increase in Q R. Stomata of plants growing at high RH were larger and showed incomplete closure after application of abscisic acid. Since stomatal conductance remained high and unchanged in these plants, it is suggested that the recovery in water potential in these plants was largely driven by the increase in Q R.

Beyond Arabidopsis: the circadian clock in non-model plant species

Circadian clocks allow plants to temporally coordinate many aspects of their biology with the diurnal cycle derived from the rotation of Earth on its axis. Although there is a rich history of the study of clocks in many plant species, in recent years much progress in elucidating the architecture and function of the plant clock has emerged from studies of the model plant, Arabidopsis thaliana. There is considerable interest in extending this knowledge of the circadian clock into diverse plant species in order to address its role in topics as varied as agricultural productivity and the responses of individual species and plant communities to global climate change and environmental degradation. The analysis of circadian clocks in the green lineage provides insight into evolutionary processes in plants and throughout the eukaryotes.

Ion-mediated enhancement of xylem hydraulic conductivity in four Acer species: relationships with ecological and anatomical features

The 'ionic effect', i.e., changes in xylem hydraulic conductivity (k(xyl)) due to variation of the ionic sap composition in vessels, was studied in four Acer species growing in contrasting environments differing in water availability. Hydraulic measurements of the ionic effect were performed together with measurements on the sap electrical conductivity, leaf water potential and vessel anatomy. The low ionic effect recorded in Acer pseudoplatanus L. and Acer campestre L. (15.8 and 14.7%, respectively), which represented two species from shady and humid habitats, was associated with a low vessel grouping index, high sap electrical conductivity and least negative leaf water potential. Opposite traits were found for Acer monspessulanum L. and Acer platanoides L., which showed an ionic effect of 23.6 and 23.1%, respectively, and represent species adapted to higher irradiance and/or lower water availability. These findings from closely related species provide additional support that the ionic effect could function as a compensation mechanism for embolism-induced loss of k(xyl), either as a result of high evaporative demand or increased risk of hydraulic failure.

Coming of leaf age: control of growth by hydraulics and metabolics during leaf ontogeny

Leaf growth is the central process facilitating energy capture and plant performance. This is also one of the most sensitive processes to a wide range of abiotic stresses. Because hydraulics and metabolics are two major determinants of expansive growth (volumetric increase) and structural growth (dry matter increase), we review the interaction nodes between water and carbon. We detail the crosstalks between water and carbon transports, including the dual role of stomata and aquaporins in regulating water and carbon fluxes, the coupling between phloem and xylem, the interactions between leaf water relations and photosynthetic capacity, the links between Lockhart's hydromechanical model and carbon metabolism, and the central regulatory role of abscisic acid. Then, we argue that during leaf ontogeny, these interactions change dramatically because of uncoupled modifications between several anatomical and physiological features of the leaf. We conclude that the control of leaf growth switches from a metabolic to a hydromechanical limitation during the course of leaf ontogeny. Finally, we illustrate how taking leaf ontogeny into account provides insights into the mechanisms underlying leaf growth responses to abiotic stresses that affect water and carbon relations, such as elevated CO2, low light, high temperature and drought.

Lunisolar tidal force and the growth of plant roots, and some other of its effects on plant movements

BACKGROUND

Correlative evidence has often suggested that the lunisolar tidal force, to which the Sun contributes 30 % and the Moon 60 % of the combined gravitational acceleration, regulates a number of features of plant growth upon Earth. The time scales of the effects studied have ranged from the lunar day, with a period of approx. 24.8 h, to longer, monthly or seasonal variations.

SCOPE

We review evidence for a lunar involvement with plant growth. In particular, we describe experimental observations which indicate a putative lunar-based relationship with the rate of elongation of roots of Arabidopsis thaliana maintained in constant light. The evidence suggests that there may be continuous modulation of root elongation growth by the lunisolar tidal force. In order to provide further supportive evidence for a more general hypothesis of a lunisolar regulation of growth, we highlight similarly suggestive evidence from the time courses of (a) bean leaf movements obtained from kymographic observations; (b) dilatation cycles of tree stems obtained from dendrograms; and (c) the diurnal changes of wood-water relationships in a living tree obtained by reflectometry.

CONCLUSIONS

At present, the evidence for a lunar or a lunisolar influence on root growth or, indeed, on any other plant system, is correlative, and therefore circumstantial. Although it is not possible to alter the lunisolar gravitational force experienced by living organisms on Earth, it is possible to predict how this putative lunisolar influence will vary at times in the near future. This may offer ways of testing predictions about possible Moon-plant relationships. As for a hypothesis about how the three-body system of Earth-Sun-Moon could interact with biological systems to produce a specific growth response, this remains a challenge for the future. Plant growth responses are mainly brought about by differential movement of water across protoplasmic membranes in conjunction with water movement in the super-symplasm. It may be in this realm of water movements, or even in the physical forms which water adopts within cells, that the lunisolar tidal force has an impact upon living growth systems.

Combined impacts of irradiance and dehydration on leaf hydraulic conductance: insights into vulnerability and stomatal control

The leaf is a hydraulic bottleneck, accounting for a large part of plant resistance. Thus, the leaf hydraulic conductance (K(leaf) ) is of key importance in determining stomatal conductance (g(s) ) and rates of gas exchange. Previous studies showed that K(leaf) is dynamic with leaf water status and irradiance. For four species, we tested the combined impacts of these factors on K(leaf) and on g(s) . We determined responses of K(leaf) and g(s) to declining leaf water potential (Ψ(leaf) ) under low and high irradiance (<6 and >900 µmol photons m(-2) s(-1) photosynthetically active radiation, respectively). We hypothesized greater K(leaf) vulnerability under high irradiance. We also hypothesized that K(leaf) and g(s) would be similar in their responses to either light or dehydration: similar light-responses of K(leaf) and g(s) would stabilize Ψ(leaf) across irradiances for leaves transpiring at a given vapour pressure deficit, and similar dehydration responses would arise from the control of stomata by Ψ(leaf) or a correlated signal. For all four species, the K(leaf) light response declined from full hydration to turgor loss point. The K(leaf) and g(s) differed strongly in their light- and dehydration responses, supporting optimization of hydraulic transport across irradiances, and semi-independent, flexible regulation of liquid and vapour phase water transport with leaf water status.

The magnitude of hydraulic redistribution by plant roots: a review and synthesis of empirical and modeling studies

Hydraulic redistribution (HR) - the movement of water from moist to dry soil through plant roots - occurs worldwide within a range of different ecosystems and plant species. The proposed ecological and hydrologic impacts of HR include increasing dry-season transpiration and photosynthetic rates, prolonging the life span of fine roots and maintaining root-soil contact in dry soils, and moving rainwater down into deeper soil layers where it does not evaporate. In this review, we compile estimates of the magnitude of HR from ecosystems around the world, using representative empirical and modeling studies from which we could extract amounts of water redistributed by plant root systems. The reported average magnitude of HR varies by nearly two orders of magnitude across ecosystems, from 0.04 to 1.3 mm H(2)O d(-1) in the empirical literature, and from 0.1 to 3.23 mm H(2)O d(-1) in the modeling literature. Using these synthesized data, along with other published studies, we examine this variation in the magnitude of upward and downward HR, considering effects of plant, soil and ecosystem characteristics, as well as effects of methodological details (in both empirical and modeling studies) on estimates of HR. We take both ecological and hydrologic perspectives.

Water release through plant roots: new insights into its consequences at the plant and ecosystem level

Hydraulic redistribution (HR) is the passive movement of water between different soil parts via plant root systems, driven by water potential gradients in the soil-plant interface. New data suggest that HR is a heterogeneous and patchy process. In this review we examine the main biophysical and environmental factors controlling HR and its main implications at the plant, community and ecosystem levels. Experimental evidence and the use of novel modelling approaches suggest that HR may have important implications at the community scale, affecting net primary productivity as well as water and vegetation dynamics. Globally, HR may influence hydrological and biogeochemical cycles and, ultimately, climate.