A novel, non-invasive, online-monitoring, versatile and easy plant-based probe for measuring leaf water status

Water for agriculture: more crop per drop

Global crop production in agriculture depends on water availability. Future scenarios predict increasing occurrence of flash floods and rapidly developing droughts accompanied by heatwaves in humid regions that rely on rain-fed agriculture. It is challenging to maintain high crop yields, even in arid and drought-prone regions that depend on irrigation. The average water demand of crops varies significantly, depending on plant species, development stage, and climate. Most crops, such as maize and wheat, require relatively more water during the vegetative phase compared to the ripening phase. In this review, we explain WUE and options to improve water use and thus crop yield. Nutrient management might represent another possibility to manipulate water uptake and use by plants. An emerging topic involves agroforest co-cultivation, where trees in the system facilitate water transfer through hydraulic lift, benefiting neighbouring crops. Other options to enhance crop yield per water use are discussed.

Water exchange between the Chlorenchyma and the Hydrenchyma and its physiological role in leaves with Crassulacean acid metabolism

Direct and non-destructive measurements of plant-water relations of plants exhibiting the Crassulacean acid metabolism (CAM) photosynthetic pathway are seldom addressed, with most findings inferred from gas exchange measurements. The main focus of this paper was to study how the water exchange between the chlorenchyma and the hydrenchyma depends on and follows the CAM photosynthetic diel pattern using non-invasive and continuous methods. Gas exchange and leaf patch clamp pressure probe (LPCP) measurements were performed on Aloe vera (L.) Burm f., a CAM species, and compared to measurements on banana (Musa acuminata Colla), a C3 species. The LPCP output pressure, Pp , of Aloe vera plants follows its diel CAM photosynthetic cycle, reversed to that observed in banana and other C3 species. The four phases of CAM photosynthesis can also be identified in the diel LPCP output pressure, Pp , cycle. The Pp values in Aloe vera are determined by the hydrenchyma turgor pressure, with both parameters being reversely related. A non-invasive and continuous assessment of the water exchange between the chlorenchyma and the hydrenchyma in CAM plants, namely, by following the changes in the hydrenchyma turgor pressure, is presented. However, showing once more how the LPCP output pressure, Pp , depends on the leaf structure, such an approach can be used to study plant-water relations in other CAM species with a leaf structure similar to Aloe vera, with the hydrenchyma composing most of the leaf volume.

Dual effect of the presence of fruits on leaf gas exchange and water relations of olive trees

The presence of fruits provokes significant modifications in plant water relations and leaf gas exchange. The underlying processes driving these modifications are still uncertain and likely depend on the water deficit level. Our objective was to explain and track the modification of leaf-water relations by the presence of fruits and water deficit. With this aim, net photosynthesis rate (AN), stomatal conductance (gs), leaf osmotic potential (Ψπ), leaf soluble sugars and daily changes in a variable related to leaf turgor (leaf patch pressure) were measured in olive trees with and without fruits at the same time, under well-watered (WW) and water stress (WS) conditions. Leaf gas exchange was increased by the presence of fruits, this effect being observed mainly in WW trees, likely because under severe water stress, the dominant process is the response of the plant to the water stress and the presence of fruits has less impact on the leaf gas exchange. Ψπ was also higher for WW trees with fruits than for WW trees without fruits. Moreover, leaves from trees without fruits presented higher concentrations of soluble sugars and starch than leaves from trees with fruits for both WW and WS, these differences matching those found in Ψπ. Thus, the sugar accumulation would have had a dual effect because on one hand, it decreased Ψπ, and on the other hand, it would have downregulated AN, and finally gs in WW trees. Interestingly, the modification of Ψπ by the presence of fruits affected turgor in WW trees, the change in which can be identified with leaf turgor sensors. We conclude that plant water relationships and leaf gas exchange are modified by the presence of fruits through their effect on the export of sugars from leaves to fruits. The possibility of automatically identifying the onset of sugar demand by the fruit through the use of sensors, in addition to the water stress produced by soil water deficit and atmosphere drought, could be of great help for fruit orchard management in the future.

Physiological and Structural Responses to Prolonged Water Deficit in Young Trees of Two Olive Cultivars

This study aimed to characterize the physiological and structural responses of potted one-year-old olive trees belonging to two olive cultivars—‘Nocellara del Belice’ and ‘Cerasuola’—exposed to prolonged drought under greenhouse conditions. Two irrigation treatments based on evapotranspiration (ET) were imposed for 69 days, i.e., well-watered (WW, 100% ET) and drought-stressed (DS, 10−30% ET). Leaf stomatal conductance (gs), stem water potential (Ψstem), transpiration (E), photosynthetic capacity (Amax), water use efficiency (WUE), stem (Kstem) and root (Kroot) hydraulic conductance, trunk diameter variations (TDV), and leaf patch attenuated pressure fluctuations (pp, a proxy of the inverse of leaf turgor pressure) were measured in WW and DS trees at different stages of the experiment. Leaf gs did not significantly differ between cultivars under DS, whereas differences in Ψstem only became significant at the end of prolonged drought, when ‘Nocellara del Belice’ experienced Ψstem < −4 MPa. ‘Cerasuola’ trees expressed the best WUE under drought, although they were more susceptible to photoinhibition under optimal plant water status. Both cultivars tended to increase their Kstem at the end of the drought period. A marked reduction in Kroot occurred in ‘Cerasuola’ plants after prolonged drought; however, a similar mechanism was not observed in ‘Nocellara del Belice’. The ratio between Kstem and Kroot exponentially increased towards the end of the prolonged drought period in both cultivars, but more markedly in ‘Cerasuola’. TDV and pp trends suggested that ‘Cerasuola’ plants keep better plant water status under severe drought compared to ‘Nocellara del Belice’ by maintaining high leaf turgor and reduced trunk diameter fluctuations. These responses may be related to reduced cell wall elasticity and xylem vessel size and/or wall thickness—drought avoidance mechanisms. The Kstem/Kroot ratio can serve as an indicator of drought stress avoidance mechanisms to compare genotype-specific responses to drought stress.

Non-invasive assessment of the physiological role of leaf aerenchyma in Hippeastrum Herb. and its relation to plant water status

The leaf patch clamp pressure probe combined with gas exchange measurements provides a non-invasive approach for measuring leaf aerenchyma pressure and study its physiological role in plants. The non-invasive leaf patch clamp pressure probe (LPCP) measures the output pressure, P_p, in response to the pressure applied by two magnets clamped to a leaf. In many plant species, it has been observed that the diel pattern of P_p follows the changes in the leaf turgor pressure reversely. The genus Hippeastrum comprises 143 species and many hybrids and cultivars of high economic value within Amaryllidaceae. Their leaves are characterized by the presence of aerenchyma composed of lacunae, running throughout the leaf and composing most of the mesophyll volume. In Hippeastrum, the diel changes of the LPCP output pressure are the reverse of that observed on the air pressure in the leaf aerenchyma, P_a, which depends on the changes in the leaf vapor pressure occurring during photosynthesis. A theoretical model is proposed and confirmed experimentally by LPCP and gas exchange measurements. The output pressure, P_p, in Hippeastrum can be related to the plant water status through the gas exchange processes that occur during photosynthesis. Considering the natural habitats of Hippeastrum species, these results agree with the physiological role of leaf aerenchyma in facilitating gas transport and light scattering in leaves, thus contributing to the photosynthetic efficiency of these plants under adverse environments. A second, but supplemental, interpretation of the LPCP output pressure, P_p, when applied on species in which the aerenchyma constitutes most of the mesophyll volume is presented.

High-throughput phenotyping reveals differential transpiration behaviour within the banana wild relatives highlighting diversity in drought tolerance

Crop wild relatives, the closely related species of crops, may harbour potentially important sources of new allelic diversity for (a)biotic tolerance or resistance. However, to date, wild diversity is only poorly characterized and evaluated. Banana has a large wild diversity but only a narrow proportion is currently used in breeding programmes. The main objective of this study was to evaluate genotype-dependent transpiration responses in relation to the environment. By applying continuous high-throughput phenotyping, we were able to construct genotype-specific transpiration response models in relation to light, VPD and soil water potential. We characterized and evaluated six (sub)species and discerned four phenotypic clusters. Significant differences were observed in leaf area, cumulative transpiration and transpiration efficiency. We confirmed a general stomatal-driven 'isohydric' drought avoidance behaviour, but discovered genotypic differences in the onset and intensity of stomatal closure. We pinpointed crucial genotype-specific soil water potentials when drought avoidance mechanisms were initiated and when stress kicked in. Differences between (sub)species were dependent on environmental conditions, illustrating the need for high-throughput dynamic phenotyping, modelling and validation. We conclude that the banana wild relatives contain useful drought tolerance traits, emphasising the importance of their conservation and potential for use in breeding programmes.

On-line monitoring of plant water status: Validation of a novel sensor based on photon attenuation of radiation through the leaf

Non-destructive real-time monitoring of leaf water status is important for precision irrigation practice to increase water productivity and reduce its use. To this end, we tested and validated a novel leaf sensor (Leaf Water Meter, LWM), based on the photon attenuation during the passage of the light through the leaf, to monitor plant water status. Four woody species were subjected to multiple cycles of dehydration and re-hydration, and the signals recorded by the LWM were compared with classical measurements of plant water relations (relative water content and water potential). A good agreement between the signals recorded by LWM sensor and the destructive measurements, throughout the repeated water stress and rewatering cycles, was found across all species. These results demonstrate that LWM sensor is a sensitive, non-destructive and easy-to-handle device to reliably monitor in continuous fashion leaf water status. In conclusion, this sensor may be considered a promising tool for smart irrigation scheduling in precision agriculture context to decrease water wastage in light of global change and increasing conflicts over water demand.

Disentangling the link between leaf photosynthesis and turgor in fruit growth

Despite the importance of understanding plant growth, the mechanisms underlying how plant and fruit growth declines during drought remain poorly understood. Specifically, it remains unresolved whether carbon or water factors are responsible for limiting growth as drought progresses. We examine questions regarding the relative importance of water and carbon to fruit growth depending on the water deficit level and the fruit growth stage by measuring fruit diameter, leaf photosynthesis, and a proxy of cell turgor in olive (Olea europaea). Flow cytometry was also applied to determine the fruit cell division stage. We found that photosynthesis and turgor were related to fruit growth; specifically, the relative importance of photosynthesis was higher during periods of more intense cell division, while turgor had higher relative importance in periods where cell division comes close to ceasing and fruit growth is dependent mainly on cell expansion. This pattern was found regardless of the water deficit level, although turgor and growth ceased at more similar values of leaf water potential than photosynthesis. Cell division occurred even when fruit growth seemed to stop under water deficit conditions, which likely helped fruits to grow disproportionately when trees were hydrated again, compensating for periods with low turgor. As a result, the final fruit size was not severely penalized. We conclude that carbon and water processes are able to explain fruit growth, with importance placed on the combination of cell division and expansion. However, the major limitation to growth is turgor, which adds evidence to the sink limitation hypothesis.

Equipment and protocol for measurement of extracellular electrical signals, gas exchange and turgor pressure in plants

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Following the step by step described in this protocol is it is possible to measure long-distance electrical signaling induced by several stimuli, including wounding, current application, irrigation, and others.

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Extracellular electrical signal measurements were associated to gas exchange measurements using an infra-red gas analyzer (IRGA, model Li-6400, Li-Cor) and turgor pressure measurements using a patch clamp pressure probe (ZIM-probe, YARA ZIM-plant Technology).

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The technique is interesting to study electrical signaling in association with other physiological parameters in plants.

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We present results that validate the protocol and discuss the troubleshooting that may compromise measurements and data collection.

We present a detailed protocol for measuring extracellular electrical signals in plants using the electrode insertion technique. Using this approach, it is possible to measure long-distance electrical signaling induced by several stimuli, including wounding, current application, irrigation, burning, and others. Additionally, we describe how to associate gas exchange measurements using an infra-red gas analyzer (IRGA, Model Li-6400, Li-Cor) and turgor pressure measurements using a patch clamp pressure probe (ZIM-probe, YARA ZIM-plant Technology) to measure extracellular electrical signals.

• The method requires a complete electrical circuit that includes a measuring device (amplifier and voltmeter) and electrodes that provide a contact between the biological material and the equipment.

• The infra-red gas analyzer (IRGA), needs to be grounded because it is an important source of noise for electrophysiological measurements.

• The ZIM-probe did not cause any interference in electrical signal measure.

• Our approach is useful for plant physiologists wishing to implement the technique of measuring electrical signals in plants, in association with other parameters of plant physiology. In addition, our text was written for agricultural and biological scientists who are not electronics specialists.

Chloride nutrition improves drought resistance by enhancing water deficit avoidance and tolerance mechanisms

Chloride (Cl-), traditionally considered harmful for agriculture, has recently been defined as a beneficial macronutrient with specific roles that result in more efficient use of water (WUE), nitrogen (NUE), and CO2 in well-watered plants. When supplied in a beneficial range of 1-5 mM, Cl- increases leaf cell size, improves leaf osmoregulation, and reduces water consumption without impairing photosynthetic efficiency, resulting in overall higher WUE. Thus, adequate management of Cl- nutrition arises as a potential strategy to increase the ability of plants to withstand water deficit. To study the relationship between Cl- nutrition and drought resistance, tobacco plants treated with 0.5-5 mM Cl- salts were subjected to sustained water deficit (WD; 60% field capacity) and water deprivation/rehydration treatments, in comparison with plants treated with equivalent concentrations of nitrate, sulfate, and phosphate salts. The results showed that Cl- application reduced stress symptoms and improved plant growth during water deficit. Drought resistance promoted by Cl- nutrition resulted from the simultaneous occurrence of water deficit avoidance and tolerance mechanisms, which improved leaf turgor, water balance, photosynthesis performance, and WUE. Thus, it is proposed that beneficial Cl- levels increase the ability of crops to withstand drought, promoting a more sustainable and resilient agriculture.

Detecting Mild Water Stress in Olive with Multiple Plant-Based Continuous Sensors

A comprehensive characterization of water stress is needed for the development of automated irrigation protocols aiming to increase olive orchard environmental and economical sustainability. The main aim of this study is to determine whether a combination of continuous leaf turgor, fruit growth, and sap flow responses improves the detection of mild water stress in two olive cultivars characterized by different responses to water stress. The sensitivity of the tested indicators to mild stress depended on the main mechanisms that each cultivar uses to cope with water deficit. One cultivar showed pronounced day to day changes in leaf turgor and fruit relative growth rate in response to water withholding. The other cultivar reduced daily sap flows and showed a pronounced tendency to reach very low values of leaf turgor. Based on these responses, the sensitivity of the selected indicators is discussed in relation to drought response mechanisms, such as stomatal closure, osmotic adjustment, and tissue elasticity. The analysis of the daily dynamics of the monitored parameters highlights the limitation of using non-continuous measurements in drought stress studies, suggesting that the time of the day when data is collected has a great influence on the results and consequent interpretations, particularly when different genotypes are compared. Overall, the results highlight the need to tailor plant-based water management protocols on genotype-specific physiological responses to water deficit and encourage the use of combinations of plant-based continuously monitoring sensors to establish a solid base for irrigation management.

Surface Density of the Spongy and Palisade Parenchyma Layers of Leaves Extracted From Wideband Ultrasonic Resonance Spectra

The wide band and air-coupled ultrasonic resonant spectroscopy together with a modified Simulated Annealing metaheuristic algorithm and a 1D layered acoustic-model are used to resolve the structure of plant leaves. In particular, this paper focuses on the extraction of the surface density of the different layers of tissue in leaves having a relatively simple structure. There are three main reasons to select the surface density as the focus of this study: (i) it is a parameter directly extracted by the proposed technique and it requires no further processing, (ii) it is relevant in order to study the dynamic of the water within the different tissues of the leaves and also to study the differential development of the different tissues, and (iii) unlike other parameters provided by this technique (like resonant frequency, impedance, ultrasonic elastic modulus, or ultrasonic damping), this parameter can be easier to understand as it is a direct measure of mass per unit surface. The selection of leaves with a simple structure is justified by the convenience of avoiding an unnecessary complication of the data extraction step. In this work, the technique was applied to determine the surface density of the palisade and spongy parenchyma layers of tissue of Ligustrum lucidum, Vitis vinifera, and Viburnum tinus leaves. The first species was used to study the variation of the surface density at full turgor with the thickness of the leaf, while the two other species were used to study the variation of the surface densities with the variation in the leaf relative water content. Consistency of the results with other conventional measurements (like overall surface density, and cross-section optical and cryo-SEM images) is discussed. The results obtained reveal the potential of this technique; moreover, the technique presents the additional advantage that can be applied in-vivo as it is completely non-invasive, non-destructive, fast, and equipment required is portable.

A Cultivar-Sensitive Approach for the Continuous Monitoring of Olive (Olea europaea L.) Tree Water Status by Fruit and Leaf Sensing

Sustainable irrigation is crucial to reduce water use and management costs in modern orchard systems. Continuous plant-based sensing is an innovative approach for the continuous monitoring of plant water status. Olive (Olea europaea L.) genotypes can respond to drought using different leaf and fruit physiological and morphological mechanisms. This study aimed to identify whether fruit and leaf water dynamics of two different olive cultivars were differently affected by water deficit and their response to changes of midday stem water potential (Ψ_stem), the most common indicator of plant water status. Plant water status indicators such as leaf stomatal conductance (g_s) and Ψ_stem were measured in the Sicilian olive cultivars Nocellara del Belice (NB) and Olivo di Mandanici (MN), in stage II and III of fruit development. Fruit gauges and leaf patch clamp pressure probes were mounted on trees and their raw data were converted in relative rates of fruit diameter change (RR_fruit) and leaf pressure change (RR_leaf), sensitive indicators of tissue water exchanges. The analysis of diel, diurnal and nocturnal fluctuations of RR_fruit and RR_leaf highlighted differences, often opposite, between the two cultivars under water deficit. A combination of statistical parameters extrapolated from RR_fruit and RR_leaf diurnal and nocturnal curves were successfully used to obtain significant multiple linear models for the estimation of midday Ψ_stem. Fruit and leaf water exchanges suggest that olive cultivar can either privilege fruit or leaf water status, with MN likely preserving leaf water status and NB increasing fruit tissue elasticity under severe water deficit. The results highlight the advantages of the integration of fruit and leaf water dynamics to estimate plant water status and the need for genotype-specific models in olive.

Determining the scale at which variation in a single gene changes population yields

Plant trait diversity is known to influence population yield, but the scale at which this happens remains unknown: divergent individuals might change yields of immediate neighbors (neighbor scale) or of plants across a population (population scale). We use Nicotiana attenuata plants silenced in mitogen-activated protein kinase 4 (irMPK4) - with low water-use efficiency (WUE) - to study the scale at which water-use traits alter intraspecific population yields. In the field and glasshouse, we observed overyielding in populations with low percentages of irMPK4 plants, unrelated to water-use phenotypes. Paired-plant experiments excluded the occurrence of overyielding effects at the neighbor scale. Experimentally altering field arbuscular mycorrhizal fungal associations by silencing the Sym-pathway gene NaCCaMK did not affect reproductive overyielding, implicating an effect independent of belowground AMF interactions. Additionally, micro-grafting experiments revealed dependence on shoot-expressed MPK4 for N. attenuata to vary its yield per neighbor presence. We find that variation in a single gene, MPK4, is responsible for population overyielding through a mechanism, independent of irMPK4's WUE phenotype, at the aboveground, population scale.

Instantaneous and non-destructive relative water content estimation from deep learning applied to resonant ultrasonic spectra of plant leaves

BACKGROUND

Non-contact resonant ultrasound spectroscopy (NC-RUS) has been proven as a reliable technique for the dynamic determination of leaf water status. It has been already tested in more than 50 plant species. In parallel, relative water content (RWC) is highly used in the ecophysiological field to describe the degree of water saturation in plant leaves. Obtaining RWC implies a cumbersome and destructive process that can introduce artefacts and cannot be determined instantaneously.

RESULTS

Here, we present a method for the estimation of RWC in plant leaves from non-contact resonant ultrasound spectroscopy (NC-RUS) data. This technique enables to collect transmission coefficient in a [0.15-1.6] MHz frequency range from plant leaves in a non-invasive, non-destructive and rapid way. Two different approaches for the proposed method are evaluated: convolutional neural networks (CNN) and random forest (RF). While CNN takes the entire ultrasonic spectra acquired from the leaves, RF only uses four relevant parameters resulted from the transmission coefficient data. Both methods were tested successfully in Viburnum tinus leaf samples with Pearson's correlations between 0.92 and 0.84.

CONCLUSIONS

This study showed that the combination of NC-RUS technique with deep learning algorithms is a robust tool for the instantaneous, accurate and non-destructive determination of RWC in plant leaves.

Transcription profile analysis identifies marker genes to distinguish salt shock and salt stress after stepwise acclimation in Arabidopsis thaliana and Zea mays

Many physiological and molecular responses to salt stress have been investigated after a salt shock. However, salt shock rarely happens in agricultural practice. In the field, salts accumulate gradually due to poor agricultural management. Thus in salinity research, it is more reasonable to investigate plant reaction after stepwise acclimation to salt stress. Previous studies demonstrate that salt shock induces Phase 0, a short-term effect that shows transient water loss and rapid turgor decrease; salt stress after stepwise acclimation avoids Phase 0 effects and induces Phase 1. During Phase 1, plants show maintenance of turgor. In this study, salt shock and stepwise acclimation to salt stress were separated at physiological and transcriptional levels. Four major experiments were conducted: 1) leaf turgor changes were monitored in real time after salt application to separate Phase 0 and Phase 1 effects at the physiological level, 2) RNA-sequence analysis was conducted in Arabidopsis thaliana L. to identify potential marker genes that are involved in plant water relations to distinguish Phase 0 and Phase 1 at transcript level, 3) these selected marker gene candidates were identified in Arabidopsis at different Phase 0 and Phase 1 time points via qRT-PCR, 4) these candidates were further evaluated in Zea mays L. (a model plant for applied research in plant physiology and an important crop plant) via qRT-PCR. In future salinity research, marker genes that are both applicable in Arabidopsis and maize have the potential to differentiate salt shock and stepwise acclimation to salt stress.

Signal coordination before, during and after stomatal closure in response to drought stress

Signal coordination in response to changes in water availability remains unclear, as does the role of embolism events in signaling drought stress. Sunflowers were exposed to two drought treatments of varying intensity while simultaneously monitoring changes in stomatal conductance, acoustic emissions (AE), turgor pressure, surface-level electrical potential, organ-level water potential and leaf abscisic acid (ABA) concentration. Leaf, stem and root xylem vulnerability to embolism were measured with the single vessel injection technique. In both drought treatments, it was found that AE events and turgor changes preceded the onset of stomatal closure, whereas electrical surface potentials shifted concurrently with stomatal closure. Leaf-level ABA concentration did not change until after stomata were closed. Roots and petioles were equally vulnerable to drought stress based on the single vessel injection technique. However, anatomical analysis of the xylem indicated that the increased AE events were not a result of xylem embolism formation. Additionally, roots and stems never reached a xylem pressure threshold that would initiate runaway embolism throughout the entire experiment. It is concluded that stomatal closure was not embolism-driven, but, rather, that onset of stomatal closure was most closely correlated with the hydraulic signal from changes in leaf turgor.

Chloride as a macronutrient increases water-use efficiency by anatomically driven reduced stomatal conductance and increased mesophyll diffusion to CO2

Chloride (Cl- ) has been recently described as a beneficial macronutrient, playing specific roles in promoting plant growth and water-use efficiency (WUE). However, it is still unclear how Cl- could be beneficial, especially in comparison with nitrate (NO3- ), an essential source of nitrogen that shares with Cl- similar physical and osmotic properties, as well as common transport mechanisms. In tobacco plants, macronutrient levels of Cl- specifically reduce stomatal conductance (gs ) without a concomitant reduction in the net photosynthesis rate (AN ). As stomata-mediated water loss through transpiration is inherent in the need of C3 plants to capture CO2 , simultaneous increase in photosynthesis and WUE is of great relevance to achieve a sustainable increase in C3 crop productivity. Our results showed that Cl- -mediated stimulation of larger leaf cells leads to a reduction in stomatal density, which in turn reduces gs and water consumption. Conversely, Cl- improves mesophyll diffusion conductance to CO2 (gm ) and photosynthetic performance due to a higher surface area of chloroplasts exposed to the intercellular airspace of mesophyll cells, possibly as a consequence of the stimulation of chloroplast biogenesis. A key finding of this study is the simultaneous improvement of AN and WUE due to macronutrient Cl- nutrition. This work identifies relevant and specific functions in which Cl- participates as a beneficial macronutrient for higher plants, uncovering a sustainable approach to improve crop yield.

Disentangling the net: concomitant xylem and over-bark size measurements reveal the phloem-generated turgor signal behind daytime stem swelling in the mangrove Avicennia marina

Daytime stem shrinking is a well recorded phenomenon: trees dip into their internal water stores to meet imbalances between water supply and demand. Uncertainty surrounds the mechanisms behind the unusual pattern of daytime stem swelling, presented by species like the mangrove Avicennia marina (Forssk.) Vierh., and which is thought to originate in the osmotic adjustment of storage tissues. We performed on-xylem radius change (XRC) and whole-stem radius change (SRC) measurements with point dendrometers to ascertain if the swelling of the stem is due to an increase in the thickness of the phloem and inner-bark (BRC). We measured leaf water potential (ψleaf), sap flow, leaf turgor and microclimate to understand the coupling between BRC and transpiration-driven changes in XRC. Our results present direct evidence of the type of mechanism responsible for daytime stem swelling. Inner-bark thickness increase concurrent with XRC decrease, sap-flow increase and leaf turgor loss, reveals a phloem-generated turgor signal behind daytime stem swelling of tree stems. On-xylem measurements were highly heterogeneous due to the variability in the three dimensional fish-net wood structure of the stem of A. marina. As daytime decreases in XRC were compensated by BRC and correlated with conditions of high water demand, we suggest a potential role of daytime stem swelling in the hydraulic safety of A. marina.

Water relations determine short time leaf growth patterns in the mangrove Avicennia marina (Forssk.) Vierh

High-resolution leaf growth is rarely studied despite its importance as a metric for plant performance and resource use efficiency. This is in part due to methodological challenges. Here, we present a method for in situ leaf growth measurements in a natural environment. We measured instantaneous leaf growth on a mature Avicennia marina subsp. australasica tree over several weeks. We measured leaf expansion by taking time-lapse images and analysing them using marker tracking software. A custom-made instrument was designed to enable long-term field studies. We detected a distinct diel growth pattern with leaf area shrinkage in the morning and leaf expansion in the afternoon and at night. On average, the observed daily shrinkage was 37% of the net growth. Most of the net growth occurred at night. Diel leaf area shrinkage and recovery continued after growth cessation. The amount of daily growth was negatively correlated with shrinkage, and instantaneous leaf growth and shrinkage were correlated with changes in leaf turgor. We conclude that, at least in this tree species, instantaneous leaf growth patterns are very strongly linked to, and most likely driven by, leaf water relations, suggesting decoupling of short-term growth patterns from carbon assimilation.

Irrigation of Greenhouse Crops

Precision agricultural greenhouse systems indicate considerable scope for improvement of irrigation management practices, since growers typically irrigate crops based on their personal experience. Soil-based greenhouse crop irrigation management requires estimation on a daily basis, whereas soilless systems must be estimated on an hourly or even shorter interval schedule. Historically, irrigation scheduling methods have been based on soil or substrate monitoring, dependent on climate or time with each having both strengths and weaknesses. Recently, plant-based monitoring or plant reflectance-derived indices have been developed, yet their potential is limited for estimating the irrigation rate in order to apply proper irrigation scheduling. Optimization of irrigation practices imposes different irrigation approaches, based on prevailing greenhouse environments, considering plant-water-soil relationships. This article presents a comprehensive review of the literature, which deals with irrigation scheduling approaches applied for soil and soilless greenhouse production systems. Irrigation decisions are categorized according to whether or not an automatic irrigation control has the ability to support a feedback irrigation decision system. The need for further development of neural networks systems is required.

Fruit and Leaf Sensing for Continuous Detection of Nectarine Water Status

Continuous assessment of plant water status indicators provides the most precise information for irrigation management and automation, as plants represent an interface between soil and atmosphere. This study investigated the relationship of plant water status to continuous fruit diameter (FD) and inverse leaf turgor pressure rates (p _p) in nectarine trees [Prunus persica (L.) Batsch] throughout fruit development. The influence of deficit irrigation treatments on stem (Ψ _stem) and leaf water potential, leaf relative water content, leaf stomatal conductance, and fruit growth was studied across the stages of double-sigmoidal fruit development in 'September Bright' nectarines. Fruit relative growth rate (RGR) and leaf relative pressure change rate (RPCR) were derived from FD and p _p to represent rates of water in- and outflows in the organs, respectively. Continuous RGR and RPCR dynamics were independently and jointly related to plant water status and environmental variables. The independent use of RGR and RPCR yielded significant associations with midday Ψ _stem, the most representative index of tree water status in anisohydric species. However, a combination of nocturnal fruit and leaf parameters unveiled an even more significant relationship with Ψ _stem, suggesting a changing behavior of fruit and leaf water flows in response to pronounced water deficit. In conclusion, we highlight the suitability of a dual-organ sensing approach for improved prediction of tree water status.

Improved Wheat Growth and Yield by Delayed Leaf Senescence Using Developmentally Regulated Expression of a Cytokinin Biosynthesis Gene

Delaying leaf senescence in plants, especially under water stress conditions, can help to maintain the remobilization of stored nutrients in source-sink relationships, thus leading to improved crop yields. Leaf senescence can be delayed by plant hormones such as cytokinin. Here, the Isopentenyl transferase (IPT) gene, encoding a cytokinin biosynthesis enzyme, driven by a modified AtMYB32xs promoter was transformed into wheat. Transgenic wheat plants exhibited delayed leaf senescence, retaining chlorophyll for longer under controlled environment conditions. Selected independent transgenic events and their corresponding nulls were grown under field conditions for two consecutive years under well-watered and water stress treatments using automated rainout shelters. Three independent transgenic events had improved canopy green cover, lower canopy temperatures, and higher leaf water potential than their respective non-transgenic nulls, with no abnormality in morphology and phenology. Increased grain yield was observed in transgenic events under both water treatments, with the yield increase more pronounced under water stress (26-42%). These results have shown that delayed leaf senescence using the chimeric transgene AtMYB32xs-p::IPT can be a useful strategy to achieve grain yield gains in wheat and potentially other crops for sustainable food production.

The Pressure Is On - Epiphyte Water-Relations Altered Under Elevated CO2

Vascular epiphytes are a major biomass component of forests across the globe and they contribute to 9% of global vascular plant diversity. To improve our understanding of the whole-plant response of epiphytes to future climate change, we investigated for the first time both individual and combined effects of elevated CO2 (560 ppm) and light on the physiology and growth of two epiphyte species [Tillandsia brachycaulos (CAM) and Phlebodium aureum (C3)] grown for 272 days under controlled conditions. We found that under elevated CO2 the difference in water loss between the light (650 μmol m-2s-1) and shade (130 μmol m-2s-1) treatment was strongly reduced. Stomatal conductance (g s) decreased under elevated CO2, resulting in an approximate 40-45% reduction in water loss over a 24 h day/night period under high light and high CO2 conditions. Under lower light conditions water loss was reduced by approximately 20% for the CAM bromeliad under elevated CO2 and increased by approximately 126% for the C3 fern. Diurnal changes in leaf turgor and water loss rates correlated strong positively under ambient CO2 (400 ppm) and high light conditions. Future predicted increases in atmospheric CO2 are likely to alter plant water-relations in epiphytes, thus reducing the canopy cooling potential of epiphytes to future increases in temperature.

Daytime stem swelling and seasonal reversal in the peristaltic depletion of stored water along the stem of Avicennia marina (Forssk.) Vierh

Diurnal courses of stem radial water dynamics represent the sum of all internal and external conditions affecting tree water relations. Changes in stem radius due to early morning water depletion and night time refilling of storage tissues is generally well documented. This study seeks to understand the unusual daytime refilling of stem elastic storage tissues present in the mangrove species Avicennia marina (Forssk.) Vierh, which deviates from our traditional understanding of hydraulics in terrestrial trees. We explored the relationship of this pattern to other water-related physiological processes and environmental variables, and investigated the seasonal changes in the timing and time lags of peak swelling at different stem heights, in order to understand the 'peristaltic' depletion of internally stored water. Our findings show that daytime stem swelling occurs year-round, even on days when leaf water potentials dropped to values lower than -4 MPa. The amplitude of stem swelling was strongly positively correlated to daily light sums more often than to measures of water availability in air and soil, especially in winter. There was also a clear seasonal reversal in the timing and direction of the 'peristaltic' depletion of water along the stem, with an earlier onset of shrinking in the upper (median = 10:00 h) than in the lower stem (median = 12:00 h) in winter, but an earlier onset of shrinking in the lower (median = 08:00 h) than in the upper stem (median = 11:00 h) in summer. This time lag was closely correlated to daily temperature, with a clear switch in the direction of peristaltic stem shrinking at the start of the growing season. We propose that sugar loading/unloading and changes in source-sink activity play a role in the endogenous osmotic adjustment responsible for daytime stem swelling and the seasonal switch in the direction of peristaltic water storage depletion in A. marina.

Water ascent in trees and lianas: the cohesion-tension theory revisited in the wake of Otto Renner

The cohesion-tension theory of water ascent (C-T) has been challenged over the past decades by a large body of experimental evidence obtained by means of several minimum or non-invasive techniques. The evidence strongly suggests that land plants acquire water through interplay of several mechanisms covered by the multi-force theory of (U. Zimmermann et al. New Phytologist 162: 575-615, 2004). The diversity of mechanisms includes, for instance, water acquisition by inverse transpiration and thermodynamically uphill transmembrane water secretion by cation-chloride cotransporters (L.H. Wegner, Progress in Botany 76:109-141, 2014). This whole plant perspective was opened by Otto Renner at the beginning of the last century who supported experimentally the strictly xylem-bound C-T mechanism, yet anticipated that the water ascent involves both the xylem conduit and parenchyma tissues. The survey also illustrates the known paradigm that new techniques generate new insights, as well as a paradigm experienced by Max Planck that a new scientific idea is not welcomed by the community instantly.

Ultrasonic Sensing of Plant Water Needs for Agriculture

Fresh water is a key natural resource for food production, sanitation and industrial uses and has a high environmental value. The largest water use worldwide (~70%) corresponds to irrigation in agriculture, where use of water is becoming essential to maintain productivity. Efficient irrigation control largely depends on having access to reliable information about the actual plant water needs. Therefore, fast, portable and non-invasive sensing techniques able to measure water requirements directly on the plant are essential to face the huge challenge posed by the extensive water use in agriculture, the increasing water shortage and the impact of climate change. Non-contact resonant ultrasonic spectroscopy (NC-RUS) in the frequency range 0.1-1.2 MHz has revealed as an efficient and powerful non-destructive, non-invasive and in vivo sensing technique for leaves of different plant species. In particular, NC-RUS allows determining surface mass, thickness and elastic modulus of the leaves. Hence, valuable information can be obtained about water content and turgor pressure. This work analyzes and reviews the main requirements for sensors, electronics, signal processing and data analysis in order to develop a fast, portable, robust and non-invasive NC-RUS system to monitor variations in leaves water content or turgor pressure. A sensing prototype is proposed, described and, as application example, used to study two different species: Vitis vinifera and Coffea arabica, whose leaves present thickness resonances in two different frequency bands (400-900 kHz and 200-400 kHz, respectively), These species are representative of two different climates and are related to two high-added value agricultural products where efficient irrigation management can be critical. Moreover, the technique can also be applied to other species and similar results can be obtained.

Simultaneous recording of diurnal changes in leaf turgor pressure and stem water status of bread wheat reveal variation in hydraulic mechanisms in response to drought

Information about water relations within crop canopies is needed to improve our understanding of canopy resource distribution and crop productivity. In this study, we examined the dehydration/rehydration kinetics of different organs of wheat plants using ZIM-probes that continuously monitor water status non-destructively. ZIM-probes were clamped to the flag leaf and penultimate leaf of the same stem to monitor changes in turgor pressure, and a novel stem probe was clamped to the peduncle (just below the spike of the same stem) to monitor changes in stem water status. All organs behaved similarly under well-watered conditions, dehydrating and recovering at the same times of day. When water was withheld, the behaviour diverged, with the leaves showing gradual dehydration and incomplete recovery in leaf turgor pressure during the night, but the stem was affected to a lesser extent. Penultimate leaves were the most severely affected, reaching turgor loss point before the flag leaf. Upon rewatering, turgor pressure recovered but the output patch-pressure of the probes (Pp) oscillated at ~30min periods in all organs of most plants (n=4). Oscillations in Pp were attributed to oscillations in stomatal opening and appear to only occur above a threshold light intensity. The mechanisms identified in this study will be beneficial for crop productivity because the flag leaf is the source of most photoassimilates in developing grains, so the plant's ability to maintain flag leaf hydration at the expense of older leaves should moderate the impact of drought on yield. Stomatal oscillations could increase water use efficiency as the plant attempts to rehydrate after drought.

Neural network implementation for a reversal procedure for water and dry matter estimation on plant leaves using selected LED wavelengths

An inversion method based on a neural network was used to estimate water and dry matter contents on plant leaves, from transmittance and reflectance measurements, using light emitting diodes (LEDs) at specific wavelengths in NIR and FIR. The preliminary results for the predicted water content by the neural network method showed a RMSE value of 0.0027 g/cm(2) and |σ| value of approximately 3.53%, computed on 127 plant leaf samples over 51 species. Dry matter estimation also was performed, which showed potential implementation after future improvements. We believe this inversion method could be implemented in a portable system based on any silicon platform with the capability to perform in situ measurements on plant tissue.

Development of a diurnal dehydration index for spring barley phenotyping

Spectral and thermal assessments may enable the precise, high-throughput and low-cost characterisation of traits linked to drought tolerance. However, spectral and thermal measurements of the canopy water status are influenced by the crops' soil coverage, the size of the biomass and other properties such as the leaf angle distribution. The aim of this study was to develop a referenced spectral method that would be minimally influenced by potentially perturbing factors for retrieving the water status of differing cultivars. Sixteen spring barley cultivars were grown in field trials under imposed drought stress, natural drought stress and irrigated conditions. The relative leaf water content of barley plants declines diurnally from pre-dawn until the afternoon, and other plant traits such as the biomass change little throughout the day. As an indicator of the current drought stress, pre-dawn and afternoon values of the relative leaf water content were assessed spectrally. Diurnal changes in reflectance are only slightly influenced by other perturbing factors. A new spectral index (diurnal dehydration index) was developed by using the wavelengths 730 and 457nm collected from an active spectrometer. This index allowed the differentiation of the drought tolerance of barley plants. The diurnal dehydration index was significantly related to final biomass, grain yield and harvest index and significantly different between cultivars. Compared with other indices, the diurnal dehydration index offered a higher stability in retrieving the water status of barley plants. Due to its diurnal assessment, the index was barely influenced by the differences in cultivars biomass at the time of measurement. It may represent a valuable tool for assessing the water status or drought tolerance in breeding nurseries.

Spatio-temporal water dynamics in mature Banksia menziesii trees during drought

Southwest Australian Banksia woodlands are highly diverse plant communities that are threatened by drought- or temperature-induced mortality due to the region's changing climate. We examined water relations in dominant Banksia menziesii R. Br. trees using magnetic leaf patch clamp pressure (ZIM-) probes that allow continuous, real-time monitoring of leaf water status. Multiple ZIM-probes across the crown were complemented by traditional ecophysiological measurements. During summer, early stomatal downregulation of transpiration prevented midday balancing pressures from exceeding 2.5 MPa. Diurnal patterns of ZIM-probe and pressure chamber readings agreed reasonably well, however, ZIM-probes recorded short-term dynamics, which are impossible to capture using a pressure chamber. Simultaneous recordings of three ZIM-probes evenly spaced along leaf laminas revealed intrafoliar turgor gradients, which, however, did not develop in a strictly basi- or acropetal fashion and varied with cardinal direction. Drought stress manifested as increasing daily signal amplitude (low leaf water status) and occasionally as rising baseline at night (delayed rehydration). These symptoms occurred more often locally than across the entire crown. Microclimate effects on leaf water status were strongest in crown regions experiencing peak morning radiation (East and North). Extreme spring temperatures preceded the sudden death of B. menziesii trees, suggesting a temperature- or humidity-related tipping point causing rapid hydraulic failure as evidenced by collapsing ZIM-probe readings from an affected tree. In a warmer and drier future, increased frequency of B. menziesii mortality will result in significantly altered community structure and ecosystem function.

Compartment specific response of antioxidants to drought stress in Arabidopsis

Compartment specific changes in ascorbate and glutathione contents were studied during drought stress in Arabidopsis thaliana Col-0 and in ascorbate and glutathione deficient mutants vtc2-1 and pad2-1, respectively, over a time period of 10 days. The results of this study revealed a strong decrease of glutathione contents in both mutants (up to 52% in mitochondria of pad2-1 and 40% in nuclei of vtc2-1) at early time points when drought stress was not yet measurable in leaves even though the soil showed a drop in relative water contents. These results indicate that glutathione is used at early time points to signal drought stress from roots to leaves. Such roles could not be confirmed for ascorbate which remained unchanged in most cell compartments until very late stages of drought. During advanced drought stress the strong depletion of ascorbate and glutathione in chloroplasts (up to 50% in Col-0 and vtc2-1) and peroxisomes (up to 56% in Col-0) could be correlated with a strong accumulation of H2O2. The strong increase of H2O2 and ascorbate in vacuoles (up to 111%) in wildtype plants indicates that ascorbate plays an important role for the detoxification of ROS in vacuoles during drought stress.

Gate control: guard cell regulation by microbial stress

Terrestrial plants rely on stomata, small pores in the leaf surface, for photosynthetic gas exchange and transpiration of water. The stomata, formed by a pair of guard cells, dynamically increase and decrease their volume to control the pore size in response to environmental cues. Stresses can trigger similar or opposing movements: for example, drought induces closure of stomata, whereas many pathogens exploit stomata and cause them to open to facilitate entry into plant tissues. The latter is an active process as stomatal closure is part of the plant's immune response. Stomatal research has contributed much to clarify the signalling pathways of abiotic stress, but guard cell signalling in response to microbes is a relatively new area of research. In this article, we discuss present knowledge of stomatal regulation in response to microbes and highlight common points of convergence, and differences, compared to stomatal regulation by abiotic stresses. We also expand on the mechanisms by which pathogens manipulate these processes to promote disease, for example by delivering effectors to inhibit closure or trigger opening of stomata. The study of pathogen effectors in stomatal manipulation will aid our understanding of guard cell signalling.

Monitoring plant response to environmental stimuli by ultrasonic sensing of the leaves

Described here is the application of a technique based on the excitation, sensing and spectral analysis of thickness resonances of plant leaves using air-coupled and wide-band ultrasound pulses (150-900 kHz) to monitor variations in leaf properties caused by plant responses to different environmental stimuli, such as a sudden variation in light intensity (from 2000 to 150 μmol m(-2) s(-1)), sudden watering after a drought period, and along the diurnal cycle (3-5 days, with continuous variation in light intensity from 150 to 2000 μmol m(-2) s(-1) and change in temperature of about 5°C). Four different widely available species, both monocots and dicots and evergreen and deciduous, with different leaf features (shape, size, thickness, flatness, vascular structure), were selected to test the technique. After a sudden decrease in light intensity, and depending on the species, there was a relative increase in the thickness resonant frequency from 8% to 12% over a 25- to 50-min period. After sudden watering, the relative increase in the resonant frequency varied from 5% to 30% and the period from 10 to 400 min. Finally, along the diurnal cycle, the measured relative variation is between 4% and 10%. The technique revealed differences in both the amplitude of the frequency oscillations and the kinetics of the leaf response for different species and also within the same species, but for specimens grown under different conditions that present different cell structures at the tissue level. The technique can be equally applied to the leaves of any species that present thickness resonances.

Salt adaptation requires efficient fine-tuning of jasmonate signalling

Understanding the mechanism by which plants sense, signal and respond to salinity stress is of great interest to plant biologists. In stress signalling, often the same molecules are involved in both damage-related and adaptive events. To dissect this complexity, we compared the salinity responses of two grapevine cell lines differing in their salinity tolerance. We followed rapid changes in the cellular content of sodium and calcium, apoplastic alkalinisation and slower responses in the levels of jasmonic acid, its active isoleucine conjugate and abscisic acid, as well as of stilbenes. Differences in timing and sensitivity to either the lanthanoid Gd or exogenous calcium provide evidence for an adaptive role of early sodium uptake through non-selective cation channels acting upstream of Ca(2+) and H(+) fluxes. We find a correlation of salt sensitivity with unconstrained jasmonate (JA) signalling, whereas salt adaptation correlates with tight control of jasmonic acid and its isoleucine conjugate, accompanied by accumulation of abscisic acid and suppression of stilbenes that trigger defence-related cell death. The data are discussed by a model where efficient fine-tuning of JA signalling determines whether cells will progress towards adaptation or programme cell death.

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.

Investigating water transport through the xylem network in vascular plants

Our understanding of physical and physiological mechanisms depends on the development of advanced technologies and tools to prove or re-evaluate established theories, and test new hypotheses. Water flow in land plants is a fascinating phenomenon, a vital component of the water cycle, and essential for life on Earth. The cohesion-tension theory (CTT), formulated more than a century ago and based on the physical properties of water, laid the foundation for our understanding of water transport in vascular plants. Numerous experimental tools have since been developed to evaluate various aspects of the CTT, such as the existence of negative hydrostatic pressure. This review focuses on the evolution of the experimental methods used to study water transport in plants, and summarizes the different ways to investigate the diversity of the xylem network structure and sap flow dynamics in various species. As water transport is documented at different scales, from the level of single conduits to entire plants, it is critical that new results be subjected to systematic cross-validation and that findings based on different organs be integrated at the whole-plant level. We also discuss the functional trade-offs between optimizing hydraulic efficiency and maintaining the safety of the entire transport system. Furthermore, we evaluate future directions in sap flow research and highlight the importance of integrating the combined effects of various levels of hydraulic regulation.

The reflectivity in the S-band and the broadband ultrasonic spectroscopy as new tools for the study of water relations in Vitis vinifera L

The large water requirements of Vitis vinifera L. together with an increase in temperature and drought events imply the need for irrigation in the driest areas of its distribution range. Generous watering may reduce grape quality so irrigation should be precisely regulated through the development of new methods of accurate irrigation scheduling based on plant 'stress sensing'. Two new methods, the reflectivity in the S-band and the broadband ultrasonic spectroscopy, can be used as non-invasive and reproducible techniques for the study of plant water relations in V. vinifera. On one hand, the measurement of reflectance at frequencies around 2.4 GHz gives an excellent accuracy when the changes in the existing area (S) between two reflectance curves are correlated with the relative water content (RWC). On the other hand, an improvement of the broadband ultrasonic spectroscopy based on the enlargement of the analysis frequency window provides, apart from the determination of the turgor loss point (TLP), additional information about the leaves without additional computational cost or additional leaf information requirements. Before TLP, the frequency associated with the maximum transmittance (f/f(o)), the macroscopic elastic constant of the leaf in the Z direction (c(33)) and, specially, the variation of the attenuation coefficient with the frequency (n), were highly correlated with changes in RWC. Once turgor is lost, a shift in the parameters directly related to the attenuation of the signal was also observed. The use of both techniques allows for a more convincing knowledge of the water status in V. vinifera.

Ultrasonic spectroscopy allows a rapid determination of the relative water content at the turgor loss point: a comparison with pressure-volume curves in 13 woody species

The turgor loss point (TLP), which is considered a threshold for many physiological processes, may be useful in plant-breeding programs or for the selection of reforestation species. Obtaining TLP through the standard pressure-volume (p-v) curve method in a large set of species is highly time-consuming and somewhat subjective. To solve this problem, we present an objective and a less time-consuming technique based on the leaf resonance able to calculate the relative water content (RWC) at TLP (RWCTLP). This method uses air-coupled broadband ultrasonic spectroscopy to obtain the sigmoidal relation between RWC and the standardized resonant frequency (f/fo). For the 13 species measured, the inflexion point of the RWC-f/fo relationship ( ) was not statistically different from the value of RWC at the TLP obtained with the p-v curves (RWCTLP p-v).

Non-destructive estimation of root pressure using sap flow, stem diameter measurements and mechanistic modelling

BACKGROUND

Upward water movement in plants via the xylem is generally attributed to the cohesion-tension theory, as a response to transpiration. Under certain environmental conditions, root pressure can also contribute to upward xylem water flow. Although the occurrence of root pressure is widely recognized, ambiguity exists about the exact mechanism behind root pressure, the main influencing factors and the consequences of root pressure. In horticultural crops, such as tomato (Solanum lycopersicum), root pressure is thought to cause cells to burst, and to have an important impact on the marketable yield. Despite the challenges of root pressure research, progress in this area is limited, probably because of difficulties with direct measurement of root pressure, prompting the need for indirect and non-destructive measurement techniques.

METHODS

A new approach to allow non-destructive and non-invasive estimation of root pressure is presented, using continuous measurements of sap flow and stem diameter variation in tomato combined with a mechanistic flow and storage model, based on cohesion-tension principles.

KEY RESULTS

Transpiration-driven sap flow rates are typically inversely related to stem diameter changes; however, this inverse relationship was no longer valid under conditions of low transpiration. This decoupling between sap flow rates and stem diameter variations was mathematically related to root pressure.

CONCLUSIONS

Root pressure can be estimated in a non-destructive, repeatable manner, using only external plant sensors and a mechanistic model.

Evaluation of diel patterns of relative changes in cell turgor of tomato plants using leaf patch clamp pressure probes

Relative changes in cell turgor of leaves of well-watered tomato plants were evaluated using the leaf patch clamp pressure probe (LPCP) under dynamic greenhouse climate conditions. LPCP changes, a measure for relative changes in cell turgor, were monitored at three different heights of transpiring and non-transpiring leaves of tomato plants on sunny and cloudy days simultaneously with whole plant water uptake. Clear diel patterns were observed for relative changes of cell turgor of both transpiring and non-transpiring leaves, which were stronger on sunny days than on cloudy days. A clear effect of canopy height was also observed. Non-transpiring leaves showed relative changes in cell turgor that closely followed plant water uptake throughout the day. However, in the afternoon the relative changes of cell turgor of the transpiring leaves displayed a delayed response in comparison to plant water uptake. Subsequent recovery of cell turgor loss of transpiring leaves during the following night appeared insufficient, as the pre-dawn turgescent state similar to the previous night was not attained.

Leaf patch clamp pressure probe measurements on olive leaves in a nearly turgorless state

The non-invasive leaf patch clamp pressure (LPCP) probe measures the attenuated pressure of a leaf patch, P(p) , in response to an externally applied magnetic force. P(p) is inversely coupled with leaf turgor pressure, P(c) , i.e. at high P(c) values the P(p) values are small and at low P(c) values the P(p) values are high. This relationship between P(c) and P(p) could also be verified for 2-m tall olive trees under laboratory conditions using the cell turgor pressure probe. When the laboratory plants were subjected to severe water stress (P(c) dropped below ca. 50 kPa), P(p) curves show reverse diurnal changes, i.e. during the light regime (high transpiration) a minimum P(p) value, and during darkness a peak P(p) value is recorded. This reversal of the P(p) curves was completely reversible. Upon watering, the original diurnal P(p) changes were re-established within 2-3 days. Olive trees in the field showed a similar turnover of the shape of the P(p) curves upon drought, despite pronounced fluctuations in microclimate. The reversal of the P(p) curves is most likely due to accumulation of air in the leaves. This assumption was supported with cross-sections through leaves subjected to prolonged drought. In contrast to well-watered leaves, microscopic inspection of leaves exhibiting inverse diurnal P(p) curves revealed large air-filled areas in parenchyma tissue. Significantly larger amounts of air could also be extracted from water-stressed leaves than from well-watered leaves using the cell turgor pressure probe. Furthermore, theoretical analysis of the experimental P(p) curves shows that the propagation of pressure through the nearly turgorless leaf must be exclusively dictated by air. Equations are derived that provide valuable information about the water status of olive leaves close to zero P(c) .

Concomitant dendrometer and leaf patch pressure probe measurements reveal the effect of microclimate and soil moisture on diurnal stem water and leaf turgor variations in young oak trees

Tree water relations and their dependence on microclimate and soil moisture were studied over several months in young oaks (Quercus robur L.) subjected in large lysimeter-based open top chambers to environments with a controlled soil water supply. Automated single point dendrometers and the recently developed leaf patch clamp pressure (LPCP) probe were used for monitoring water-related stem radius variations (ΔW) and turgor-dependent leaf patch pressures (Pp). Both parameters showed distinct diurnal patterns with sharp negative and positive peaking of ΔW and Pp, respectively, after solar noon and recovery to initial levels in the evening. During the day, varying solar radiation was responsible for short time fluctuations of Pp in the range of minutes to hours reflecting feedback regulation of leaf turgor by sunlight driven stomatal movements. At longer timescales, i.e. days to months, atmospheric vapour pressure deficit (VPD) and soil water content (SWC) were the main determinants of ΔW and Pp. Daily minimum and maximum values of ΔW and Pp decreased and increased, respectively, with increasing VPD or decreasing SWC and recovery of ΔW and Pp in the evening was impeded by low SWC. In well-watered oaks, daily positive peaking of Pp preceded daily negative peaking of ΔW; these time lags gradually increased with increasing soil drought, suggesting hydraulic uncoupling of stem and leaves.

Ion exchangers NHX1 and NHX2 mediate active potassium uptake into vacuoles to regulate cell turgor and stomatal function in Arabidopsis

Intracellular NHX proteins are Na(+),K(+)/H(+) antiporters involved in K(+) homeostasis, endosomal pH regulation, and salt tolerance. Proteins NHX1 and NHX2 are the two major tonoplast-localized NHX isoforms. Here, we show that NHX1 and NHX2 have similar expression patterns and identical biochemical activity, and together they account for a significant amount of the Na(+),K(+)/H(+) antiport activity in tonoplast vesicles. Reverse genetics showed functional redundancy of NHX1 and NHX2 genes. Growth of the double mutant nhx1 nhx2 was severely impaired, and plants were extremely sensitive to external K(+). By contrast, nhx1 nhx2 mutants showed similar sensitivity to salinity stress and even greater rates of Na(+) sequestration than the wild type. Double mutants had reduced ability to create the vacuolar K(+) pool, which in turn provoked greater K(+) retention in the cytosol, impaired osmoregulation, and compromised turgor generation for cell expansion. Genes NHX1 and NHX2 were highly expressed in guard cells, and stomatal function was defective in mutant plants, further compromising their ability to regulate water relations. Together, these results show that tonoplast-localized NHX proteins are essential for active K(+) uptake at the tonoplast, for turgor regulation, and for stomatal function.

Air-coupled ultrasonic resonant spectroscopy for the study of the relationship between plant leaves' elasticity and their water content

Air-coupled wideband ultrasonic piezoelectric transducers are used in the frequency range 0.3 to 1.3 MHz to excite and sense first-order thickness resonances in the leaves of four different tree species at different levels of hydration. The phase and magnitude spectra of these resonances are measured, and the inverse problem solved; that is, leaf thickness and density, ultrasound velocity, and the attenuation coefficient are obtained. The elastic constant in the thickness direction (c33) is then determined from density and velocity data. The paper focuses on the study of c33, which provides a unique, fast, and noninvasive ultrasonic method to determine leaf elasticity and leaf water content.

Relationship between ultrasonic properties and structural changes in the mesophyll during leaf dehydration

The broad-band ultrasonic spectroscopy technique allows the determination of changes in the relative water content (RWC) of leaves with contrasting structural features. Specifically, the standardized frequency associated with the maximum transmittance (f/f(o)) is strongly related to the RWC. This relationship is characterized by the existence of two phases separated by an inflexion point (associated with the turgor loss point). To obtain a better understanding of the strong relationship found between RWC and f/f(o), this work has studied the structural changes experienced by Quercus muehlenbergii leaves during dehydration in terms of ultrasounds measurements, cell wall elasticity, leaf thickness, leaf density, and leaf structure. The results suggest that the decrease found in f/f(o) before the turgor loss point can be attributed to the occurrence of changes in the estimation of the macroscopic effective elastic constant of the leaf (c(33)), mainly associated with changes in the bulk modulus of elasticity of the cell wall (ε). These changes are overriding or compensating for the thickness decreases recorded during this phase. On the other hand, the high degree of cell shrinkage and stretching found in the mesophyll cells during the second phase seem to explain the changes in the acoustic properties of the leaf beyond the turgor loss point. The formation of large intercellular spaces, which increased the irregularity in the acoustic pathway, may explain the increase of the attenuation coefficient of ultrasounds once the turgor loss point threshold is exceeded. The direct measurement of c(33) from ultrasonic measurements would allow a better knowledge of the overall biomechanical properties of the leaf further than those derived from the P-V analysis.

Calcium delivery and storage in plant leaves: exploring the link with water flow

Calcium (Ca) is a unique macronutrient with diverse but fundamental physiological roles in plant structure and signalling. In the majority of crops the largest proportion of long-distance calcium ion (Ca(2+)) transport through plant tissues has been demonstrated to follow apoplastic pathways, although this paradigm is being increasingly challenged. Similarly, under certain conditions, apoplastic pathways can dominate the proportion of water flow through plants. Therefore, tissue Ca supply is often found to be tightly linked to transpiration. Once Ca is deposited in vacuoles it is rarely redistributed, which results in highly transpiring organs amassing large concentrations of Ca ([Ca]). Meanwhile, the nutritional flow of Ca(2+) must be regulated so it does not interfere with signalling events. However, water flow through plants is itself regulated by Ca(2+), both in the apoplast via effects on cell wall structure and stomatal aperture, and within the symplast via Ca(2+)-mediated gating of aquaporins which regulates flow across membranes. In this review, an integrated model of water and Ca(2+) movement through plants is developed and how this affects [Ca] distribution and water flow within tissues is discussed, with particular emphasis on the role of aquaporins.