Water Potential Gradient Research Articles

Reverse conductivity for water transport and related anatomy in fine roots of six temperate tree species – a potential limitation for hydraulic redistribution

Hydraulic redistribution (HR), the passive reallocation of water along plant structures following a water potential gradient, is an important mechanism for plant survival under drought. For example, trees with deeper roots reallocate water from deeper moist to shallower, drier soil layers sustaining their upper fine root system. The relevance of HR for temperate forest ecosystems is hardly investigated. Both environmental and tree internal factors limiting the capacity for HR, such as low water potential gradients or root anatomy, respectively, are not well understood. Here we investigate fine root anatomy and related capacity for reverse flow of water of six temperate tree species, i.e. Acer pseudoplatanus, Castanea sativa, Fagus sylvatica, Picea abies, Pseudotsuga menziesii and Quercus robur both in forward and reverse flow direction. Additionally, anatomy of primary and secondary roots was analyzed, to test the hypotheses that root anatomy is similar in primary and secondary roots (H1) and conductivity for forward and reverse flow of water in fine roots is identical (H2). In contrast to the two gymnosperm species, most anatomical parameters, e.g. hydraulic conduit diameter and conduit density, were distinctly different between primary and secondary roots in the angiosperms. Therefore, H1 was rejected for angiosperm trees. The reverse flow of water in fine roots was reduced by approx. 40 % compared to the forward flow in angiosperms, while there was no difference in the gymnosperms. Thus, H2 was rejected for angiosperms. This reduction may be caused by vessel structure (e.g. tapering or secondary thickening elements), or perforation plate and pit architecture (e.g. width of aperture opening). Because of the reduced conductivity of reverse water flow, the ability of angiosperm trees to redistribute water along their root system might be lower than expected.

A physiological model for predicting dynamics of tree stem‐wood non‐structural carbohydrates

AbstractNon‐structural carbohydrates (NSC) are considered important in the metabolism of wood plants, and they are highly dynamic. Accurate and detailed modelling of NSC dynamics may increase the understanding of physiological processes.We constructed a physiological model that simulates NSC dynamics from photosynthesis, transpiration and growth carbon demand. With this model, we tested the hypotheses that production, allocation and storage explain tree‐ and forest‐level carbohydrates balance, and especially that cambial activities explain the major portion of carbon consumption.We applied this model at one plot in Harvard Forest, Massachusetts, USA. The predicted results of stem‐wood NSC dynamics of 20 deciduous broad‐leaved trees corresponded well with measurements. NSC (i.e. product from photosynthesis) allocations depended on thermal time and the development stage of each organ. Water movements driven by water potential gradients in whole‐trees also influenced tree growth, which was a necessary process for predicting the daily NSC dynamics.Synthesis. This model combined wood growth at whole‐tree scale and carbohydrates balance at cellular scale with short‐term vegetation hydraulics. The model provides a foundation to further understand the complex interactions between environmental factors and tree carbon allocation.

Arbuscular mycorrhizal influence on oxidative stress in French bean (Phaseolus vulgaris) under drought conditions

We investigated the antioxidant response in French bean (Phaseolus vulgaris) seedlings in a symbiotic interaction with Arbuscular mycorrhiza fungi (AMF) under drought and studied both shooting and rooting tissues to detect the targeted tissue for AMF effects induced under drought. AMF and Non-AMF French bean seedlings were grown under control (well-watered: WW) and drought-stress (DS) conditions to study their tolerance response by assessing various physiological and biochemical parameters. AMF plants appeared to be sheltered from drought as seen by their higher leaf water potential and production of shoot-biomass. The AMF shoots accumulated less proline than those of non-AMF, while the opposite was observed in roots. Also, in DS AMF plants, lipid peroxides were 60 percent lower than in DS non - AMF plants. However, no significant correlation could be established between antioxidant enzyme activity and the low lipid oxidative damage in AMF plants due to which it appears that the symbiont fungus enhances osmotic adjustment first in the root system, which promotes a water potential gradient that favors entry of water into roots from the soil. This enables increased leaf water potential in DS-AMF plants keeping plants sheltered from oxidative damage, and all these collective effects increase the drought hardiness of seedlings.

Leaf mechanisms involved in the response of Cydonia oblonga trees to water stress and recovery

Quince tree (Cydonia oblonga Mill.) is known for bearing fruits that are rich in nutrients and health-promoting compounds while requiring low inputs of agrochemicals, and maintenance, but no information exists on the mechanisms developed at the level of leaf water relations to confront water stress and recovery. For this reason, the purpose of the present study was to identify the strategy (isohydric or anisohydric) by which quince plants cope with water stress and to further elucidate the resistance mechanisms developed in response to water stress and during recovery. In summer 2016, field-grown own rooted 17-years old quince trees (cv. BA-29) were subjected to two irrigation treatments. Control (T0) plants were drip irrigated (105% ETo) to ensure non-limiting soil water conditions, while T1 plants were irrigated at the same level as used in T0, except that irrigation was withheld for 42 days during the linear fruit growth phase, after which irrigation returned to the levels of T0 (recovery period). During the experimental period, T0 and T1 received a total of 374 and 143 mm water, respectively, including rain water. The quince trees exhibited extreme anisohydric behaviour under the experimental conditions. As water stress developed and during the recovery period, the plants exhibited high hydraulic conductivity, probably the result of resistance to cavitation. From the beginning of water stress to the time of maximum water stress, leaf turgor was maintained, possibly due to active osmotic adjustment (stress tolerance mechanism). This leaf turgor maintenance may have contributed to the high leaf conductance, and, therefore, good leaf productivity. The low quince leaf apoplastic water fraction under water stress could be considered as another drought tolerance characteristic because if the accumulation of water in the apoplasm is avoided a steeper gradient in water potential between the leaf and the soil can take place under water stress, thus favouring water absorption.

Potential Reemergence of Seasonal Soil Moisture Anomalies in North America

AbstractSoil moisture anomalies within the root zone (roughly, soil depths down to ~0.4 m) typically persist only a few months. Consequently, land surface–related climate predictability research has often focused on subseasonal to seasonal time scales. However, in this study of multidecadal in situ datasets and land data assimilation products, we find that root zone soil moisture anomalies can recur several or more seasons after they were initiated, indicating potential interannual predictability. Lead–lag correlations show that this recurrence often happens during one fixed season and also seems related to the greater memory of soil moisture anomalies within the layer beneath the root zone, with memory on the order of several months to over a year. That is, in some seasons, notably spring and summer when the vertical soil water potential gradient reverses sign throughout much of North America, deeper soil moisture anomalies appear to return to the surface, thereby restoring an earlier root zone anomaly that had decayed. We call this process “reemergence,” in analogy with a similar seasonally varying process (with different underlying physics) providing winter-to-winter memory to the extratropical ocean surface layer. Pronounced spatial and seasonal dependence of soil moisture reemergence is found that is frequently, but not always, robust across datasets. Also, some of its aspects appear sensitive to spatial and temporal sampling, especially within the shorter available in situ datasets, and to precipitation variability. Like its namesake, soil moisture reemergence may enhance interannual-to-decadal variability, notably of droughts. Its detailed physics and role within the climate system, however, remain to be understood.

Plasma Membrane-Associated Ca2+-Binding Protein PCaP1 is Involved in Root Hydrotropism of Arabidopsis thaliana.

Root hydrotropism is an essential growth response to water potential gradients in plants. To understand the mechanism, fundamental elements such as MIZU-KUSSEI 1 (MIZ1) have been investigated extensively. We investigated the physiological role of a plasma membrane-associated cation-binding protein (PCaP1) and examined the effect of PCaP1 loss-of-function mutations on root hydrotropism. pcap1 knockout mutants showed a defect in root bending as a hydrotropic response, although gravitropism was normal in pcap1 mutants. When pcap1 seedlings were treated with abscisic acid, a negative regulator of gravitropism, the seedlings showed normal gravitropism. The hydrotropism defect in pcap1 mutants was clearly rescued by introducing the genomic sequence of PCaP1 with an endodermis-specific promoter. Analysis of PCaP1-greenfluorescent protein-expressing roots by confocal laser scanning microscopy revealed that PCaP1 was stably associated with the plasma membrane in most cells, but in the cytoplasm of endodermal cells at the bending region. Furthermore, we prepared a transgenic line overexpressing MIZ1 on the pcap1 background and found that the pcap1 hydrotropism defect was rescued. Our results indicate that PCaP1 in the endodermal cells of the root elongation zone is involved in the hydrotropic response. We suggest that PCaP1 contributes to hydrotropism through a MIZ1-independent pathway or as one of the upstream components that transduce water potential signals to MIZ1.

The comparative xylem structure and function of petioles and twigs of mistletoe Loranthus europaeus and its host Quercus pubescence

The xylem structure of the petiole and twig revealed in this study increase our knowledge about mistletoe and host function and their interaction. Although transpiration and photosynthesis relationships between mistletoes and their hosts have been studied extensively, only a few studies have been performed on their comparative anatomical structures. To our knowledge, no studies have focused on comparisons between the petiole and twig xylem structures of mistletoe and its host, although xylem structure is closely connected to tree physiology. In this study, the xylem structure and hydraulic conductivity of petioles and current-year twigs of the mistletoe Loranthus europaeus Jacq. and its host Quercus pubescens Willd. were measured. It was found that the petiole conduit lumen area, xylem area, and xylem-to-cross-sectional area ratio were, respectively, 1.4, 6.2, and 4.4 times higher in Quercus compared to Loranthus. The anatomical differences in xylem between both species were more pronounced in twigs than in petioles. Although the studied petiole conduit traits differed between Quercus and Loranthus, specific hydraulic conductivities were similar in both species. The similar specific hydraulic conductivities for both water competing species indicate that Loranthus and Quercus reach the same transpiration rates under good water supply conditions and under similar leaf water potential. The narrower conduits in Loranthus could make the xylem of this hemiparasite well adapted to withstand a higher water potential gradient without cavitation. Nevertheless, further studies are needed to define the anatomical and morphological traits involved in xylem cavitation in mistletoe, traits which could provide more information about the function and survival of mistletoe under conditions of higher water potential gradient.

Water transport, perception, and response in plants.

Sufficient water availability in the environment is critical for plant survival. Perception of water by plants is necessary to balance water uptake and water loss and to control plant growth. Plant physiology and soil science research have contributed greatly to our understanding of how water moves through soil, is taken up by roots, and moves to leaves where it is lost to the atmosphere by transpiration. Water uptake from the soil is affected by soil texture itself and soil water content. Hydraulic resistances for water flow through soil can be a major limitation for plant water uptake. Changes in water supply and water loss affect water potential gradients inside plants. Likewise, growth creates water potential gradients. It is known that plants respond to changes in these gradients. Water flow and loss are controlled through stomata and regulation of hydraulic conductance via aquaporins. When water availability declines, water loss is limited through stomatal closure and by adjusting hydraulic conductance to maintain cell turgor. Plants also adapt to changes in water supply by growing their roots towards water and through refinements to their root system architecture. Mechanosensitive ion channels, aquaporins, proteins that sense the cell wall and cell membrane environment, and proteins that change conformation in response to osmotic or turgor changes could serve as putative sensors. Future research is required to better understand processes in the rhizosphere during soil drying and how plants respond to spatial differences in water availability. It remains to be investigated how changes in water availability and water loss affect different tissues and cells in plants and how these biophysical signals are translated into chemical signals that feed into signaling pathways like abscisic acid response or organ development.

Water potential gradient between sapwood and heartwood as a driving force in water accumulation in wetwood in conifers

Wetwood, water accumulation in the heartwood of tree trunks, is a defect of forest trees and needs to be improved for wood utilization. To understand the mechanism of wetwood formation, differences in water potential between sapwood and heartwood and their seasonal changes were investigated. The water potential of specimens sampled from tree trunks using an increment corer was measured by psychrometry at four-week intervals for 2 years in two coniferous species. A water potential gradient from sapwood to heartwood was identified that could provide a driving force of the radial water movement, and this gradient was found to fluctuate seasonally. The two species studied differed in seasonal change in water potential. The source of the water potential gradient is thought to be heartwood-accumulated potassium and water-soluble heartwood substance. The hypothesis is proposed that water movement from sapwood to heartwood during wetwood formation involves vaporous rather than liquid water and occurs over a limited season.

C4 savanna grasses fail to maintain assimilation in drying soil under low CO2 compared with C3 trees despite lower leaf water demand

Abstract C4 photosynthesis evolved when grasses migrated out of contracting forests under a declining atmospheric CO2 concentration ([CO2]a) and drying climate around 30 million years ago. C4 grasses are hypothesised to benefit from improved plant–water relations in open habitats such as savannas, giving advantages over C3 plants under low [CO2]a. But experimental evidence in a low CO2 environment is limited, and comparisons with C3 trees are needed to understand savanna vegetation patterns. To test whether stomatal conductance (gS) and CO2 assimilation (A) are maintained in drier soil for C4 grasses than C3 trees, particularly under low [CO2]a, we investigated photosynthesis and plant–water relations of three C3 tree and three C4 grass species grown at 800, 400 or 200 ppm [CO2]a over moderate wetting–drying cycles. C4 grasses had a lower soil‐to‐leaf water potential gradient than C3 trees, especially at 200 ppm [CO2]a, indicating reduced leaf water demand relative to supply. Yet the dependence of gS and A on predawn leaf water potential (a measure of soil water availability) was greater for the C4 grasses than trees, particularly under low [CO2]a. Our findings establish that gS and A are not maintained in drier soil for C4 grasses compared with C3 trees, suggesting that this mechanism was not prevailing in the expansion of C4‐dominated grasslands under low [CO2]a. This inherent susceptibility to sudden decreases in soil water availability justifies why C4 grasses have not evolved a resistant xylem allowing operation under drought, but instead shut down below a water potential threshold and rapidly recover. We point to this capacity to respond to transient water availability as a key overlooked driver of C4 grass success under low [CO2]a. A plain language summary is available for this article.

Disentanglement of the secrets of aluminium in acidophilic tea plant (Camellia sinensis L.) influenced by organic and inorganic amendments

Field experiment was carried out for four years in mature tea (Camellia sinensis L.) growing plot to investigate the impacts of different doses of inorganic and organic fertilizers on aluminium (Al) distribution pattern in soil and different parts of tea plant, leaf pigment concentration, gas exchange parameters, as well as the yield of tea. Results indicated that application of 6 × 103 kg compost ha−1 significantly increased the dry matter yields of tea. Pluckable shoot of tea plant were markedly stimulated in the presence of Al irrespective of treatment imposed. Furthermore, Al induced growth stimulation in tea plant was facilitated by higher photosynthesis rate as well as gas exchange parameters. For the present experiment, Tea Research Association Heavy Metal Contamination Index (TRAHMCI) decreases with increase the fertilizer dose and all the experimental soils were found non-polluted with respect to Al. Localization of Al in the root apex predominantly accumulated in the cortex. The translocation of Al from root to shoot was driven by the gradient in hydrostatic pressure and water potential. In all tea infusions influenced by different treatments, Al concentrations were within the maximum permissible limit of Al in drinking water by Provisional Tolerable Weekly Intake (PTWI) established by the Joint FAO/WHO Expert Committee on Food Additives (JECFA, 2 mg kg−1 bw−1) and the tolerable weekly intake (TWI) established by EFSA (European Food Safety Authority, 1 mg kg−1 bw−1). Application of stepwise multiple regression model indicates that around 75% of the variability in the yield of the crop can be expressed by the selected parameters under study. The Hierarchical cluster analysis reveals that two homogenous groups of treatment can be formed based on all the studied parameters.

Determining in-situ unsaturated soil hydraulic conductivity at a fine depth scale with heat pulse and water potential sensors

Unsaturated hydraulic conductivity (K) of surface soil changes substantially with space and time, and it is of great importance for many ecological, agricultural, and hydrological applications. In general, K is measured in the laboratory, or more commonly, predicted using soil water retention curve and saturated hydraulic conductivity. In the field, K can be determined through infiltration experiments. However, none of these approaches are capable of continuously monitoring K in-situ at fine depth scales. In this study, we propose and investigate an approach to continuously estimate fine depth-scale K dynamics under field conditions. Evaporation rate and change in water storage in a near-surface soil layer are measured with the heat pulse method. Then, water flux density at the lower boundary of the soil layer is estimated from evaporation rate, change in water storage, and rainfall or irrigation rate using a simple water balance approach. Finally, K values at different soil depths are derived using the Buckingham-Darcy equation from water flux densities and measured water potential gradients. A field experiment is performed to evaluate the performance of the proposed approach. K values at 2-, 4-, 7.5-, and 12.5-cm depths are estimated with the new approach. The results show that in-situ K estimates vary with time following changes in soil water content, and the K-water content relationship changes with depth due to the difference in bulk density. In-situ estimated K-matric potential curves agree well with those measured in the laboratory. In-situ K estimates also show good agreement with the Mualem-van Genuchten model predictions, with an average root mean square error in log10 (K, mm h−1) of 0.54 and an average bias of 0.17. The new approach provides reasonable in-situ K estimates and has potential to reveal the influences of natural soil conditions on hydraulic properties as they change with depth and time.

MIZ1 regulates ECA1 to generate a slow, long-distance phloem-transmitted Ca2+ signal essential for root water tracking in Arabidopsis

Ever since Darwin postulated that the tip of the root is sensitive to moisture differences and that it "transmits an influence to the upper adjoining part, which bends towards the source of moisture" [Darwin C, Darwin F (1880) The Power of Movement in Plants, pp 572-574], the signal underlying this tropic response has remained elusive. Using the FRET-based Cameleon Ca2+ sensor in planta, we show that a water potential gradient applied across the root tip generates a slow, long-distance asymmetric cytosolic Ca2+ signal in the phloem, which peaks at the elongation zone, where it is dispersed laterally and asymmetrically to peripheral cells, where cell elongation occurs. In addition, the MIZ1 protein, whose biochemical function is unknown but is required for root curvature toward water, is indispensable for generating the slow, long-distance Ca2+ signal. Furthermore, biochemical and genetic manipulations that elevate cytosolic Ca2+ levels, including mutants of the endoplasmic reticulum (ER) Ca2+-ATPase isoform ECA1, enhance root curvature toward water. Finally, coimmunoprecipitation of plant proteins and functional complementation assays in yeast cells revealed that MIZ1 directly binds to ECA1 and inhibits its activity. We suggest that the inhibition of ECA1 by MIZ1 changes the balance between cytosolic Ca2+ influx and efflux and generates the cytosolic Ca2+ signal required for water tracking.

New insights into the apple fruit dehydration process at the cellular scale by 3D continuum modeling

An accurate understanding of the dehydration kinetics of biological materials is essential to optimize their dehydration processes and to produce high-quality dried products. For a soft, cellular material like fruit, the underlying mass transport and deformation mechanisms at the cellular scale play a key role here. To improve our insight into the cellular scale dehydration kinetics, a 3D model is developed to quantify the impact of the changes in the cellular structure of apple (parenchyma) cells during dehydration on the tissue sorption isotherm and water permeability. As a step beyond the current state-of-the-art models, the model incorporates the changes in the cellular structure over entire dehydration process, starting from a turgid cell down to the occurrence of (free) shrinkage, plasmolysis (detachment of the cell membrane from the cell wall) or lysis (rupture of the cell membrane). Regarding the tissue sorption isotherm, plasmolysis induced a reduction in the equilibrium water content (up to 60%) compared to the free-shrinkage or lysis cases at the same water activity level. On the other hand, the tissue water permeability was found to increase up to five times when lysis occurs, compared to free shrinkage or plasmolysis. A parametric study also quantified the dependency of tissue permeability on the cell wall thickness, the cell membrane permeability, the cell size and the elongation aspect ratio of the cell. We identified that the dehydrated, shrunken cellular tissue reduces the outgoing water flux compared to fresh tissue for the same water potential gradient. As such, dehydrated tissue forms a barrier against further moisture removal from the fresh tissue below.

Hydraulic redistribution affects modeled carbon cycling via soil microbial activity and suppressed fire.

Hydraulic redistribution (HR) of water from moist to drier soils, through plant roots, occurs world-wide in seasonally dry ecosystems. Although the influence of HR on landscape hydrology and plant water use has been amply demonstrated, HR's effects on microbe-controlled processes sensitive to soil moisture, including carbon and nutrient cycling at ecosystem scales, remain difficult to observe in the field and have not been integrated into a predictive framework. We incorporated a representation of HR into the Community Land Model (CLM4.5) and found the new model improved predictions of water, energy, and system-scale carbon fluxes observed by eddy covariance at four seasonally dry yet ecologically diverse temperate and tropical AmeriFlux sites. Modeled plant productivity and microbial activities were differentially stimulated by upward HR, resulting at times in increased plant demand outstripping increased nutrient supply. Modeled plant productivity and microbial activities were diminished by downward HR. Overall, inclusion of HR tended to increase modeled annual ecosystem uptake of CO2 (or reduce annual CO2 release to the atmosphere). Moreover, engagement of CLM4.5's ground-truthed fire module indicated that though HR increased modeled fuel load at all four sites, upward HR also moistened surface soil and hydrated vegetation sufficiently to limit the modeled spread of dry season fire and concomitant very large CO2 emissions to the atmosphere. Historically, fire has been a dominant ecological force in many seasonally dry ecosystems, and intensification of soil drought and altered precipitation regimes are expected for seasonally dry ecosystems in the future. HR may play an increasingly important role mitigating development of extreme soil water potential gradients and associated limitations on plant and soil microbial activities, and may inhibit the spread of fire in seasonally dry ecosystems.

Sap Flow Measurements in a Socotra Dragon’s Blood Tree (Dracaena cinnabari) in its Area of Origin

For the first time, in situ field measurements of sap flow were conducted in adult Dracaena cinnabari plant native to the arid tropical climate of Socotra Island. The heat field deformation (HFD) method was applied using both single and multi-point sensors to study azimuthal and radial sap flow variability in stem, roots and first-order branches over two weeks during a winter monsoon. The main aim of this work was to monitor sap flow in adult D. cinnabari in-situ to better understand its physiological adaptation to extreme arid environments. The second aim was to compare our results with earlier sap flow measurements in adult D. draco uing the same HFD method. The last question we wanted to answer was comparison of sap flow measurements in both, young and adult Dracaena species. We found that sap flow magnitude is low and of a similar range in all observed D. species. High sap flow variability was recorded in different parts of adult D. cinnabari plant which changed throughout the day responding to interplay between intrinsic and extrinsic water potential gradients induced by sunlight. Maximum sap flow levels had variable pattern around stem in response to sun exposure, similarly as it was observed in adult D. draco plant. Sensors installed tangentially in stem xylem showed that water transport in adult D. cinnabari may move in lateral direction. This work also presents several methodological aspects detected from earlier observations of dicots which proved to be more pronounced in adult D. species. These methodologies relate to interpreting negative sap flow rates in conjunction with established axial flow reversal during hydraulic redistribution usually occurring under low evaporative demands and dry soil. Conversely, flow reversal during the day under high evaporative demands and wet soil may designate lateral water movement induced by internal water redistribution.

Hydrotropism: how roots search for water

Fresh water is an increasingly scarce resource for agriculture. Plant roots mediate water uptake from the soil and have developed a number of adaptive traits such as hydrotropism to aid water foraging. Hydrotropism modifies root growth to respond to a water potential gradient in soil and grow towards areas with a higher moisture content. Abscisic acid (ABA) and a small number of genes, including those encoding ABA signal transducers, MIZ2/GNOM, and the hydrotropism-specific MIZ1, are known to be necessary for the response in Arabidopsis thaliana, whereas the role of auxin in hydrotropism appears to vary depending on the plant species. This review will describe recent progress characterizing the hormonal regulation of hydrotropism. Recent advances in identifying the sites of hydrotropic perception and response, together with its interaction with gravitropism, will also be discussed. Finally, I will describe putative mechanisms for perception of the water potential gradient and a potential role for hydrotropism in acclimatizing plants to drought conditions.

Depressed hydraulic redistribution of roots more by stem refilling than by nocturnal transpiration for Populus euphratica Oliv. in situ measurement.

During the night, plant water loss can occur either through the roots, as hydraulic redistribution (HR), or through the leaves via the stoma, as nocturnal transpiration (E n), which was methodologically difficult to separate from stem refilling (R e). While HR and E n have been reported across a range of species, ecosystem, and climate zone, there is little understanding on the interactions between E n and/or R e and HR. As water movement at night occurs via gradients of water potential, it is expected that during periods of high atmospheric vapor pressure deficit (VPD), water loss via E n will override water loss via HR. To test this hypothesis, sap flow in stems and roots of Populus euphratica Oliv. trees, growing in a riparian zone in a hyperarid climate, was measured once in a year. Nocturnal stem sap flow was separated into E n and R e using the “forecasted refilling” method. Substantial nocturnal sap flow (38% of 24‐hr flux on average) was observed and positively correlated with VPD; however, the strength of the correlation was lower (R 2 = .55) than diurnal sap flow (E d) (R 2 = .72), suggesting that nocturnal stem sap flow was attributed to both water loss through the canopy and replenishment of water in stem tissues. Partitioning of nocturnal sap flow shows that R e constituted approximately 80%, and E n ~20%, of nocturnal sap flow. The amount of root sap flow attributed to redistribution was negatively related to E d (R 2 = .69) and the amount of acropetally sap flow in stems, R e (R 2 = .41) and E n (R 2 = .14). It was suggested that the magnitude of HR is more strongly depressed by R e that was recharge to the water loss via E d than by E n. It was consistent with whole‐tree water balance theory, that the nighttime upward sap flow to xylem, stem refilling and transpiration, may depress hydraulic redistribution of roots.

Culm sheaths affect height growth of bamboo shoots in Fargesia yunnanensis

Culm sheath is an important morphological characteristic of bamboos; however, its physiological functions are still unknown. The main goal of this study was to examine the physiological functions of culm sheaths and their effects on the height growth of Fargesia yunnanensis Hsueh et Yi shoots. The elongating internodes were analyzed for determining the soluble sugar and starch contents, as well as the water potential of different sections. The transpiration rates were also determined and compared between culm sheaths and foliage leaf blades. The effects of culm sheaths on the shoot height growth were also analyzed. The results showed that the outer zone had higher starch but lower soluble sugar contents than the inner zone in the developing shoots. The transpiration rate of culm sheaths was far higher than that of leaves. The water potential of the outer zone was lower than that of the inner zone in bamboo culms. The growth of bamboo culms decreased without culm sheaths. When one strip of culm sheaths was peeled off longitudinally from young culms, the bare side of the culms curved rapidly after one night, and the water potential gradient between the inner and outer zones significantly decreased, further demonstrating that sheaths have significant effects on bamboo culm height growth.

Growth is required for perception of water availability to pattern root branches in plants

Water availability is a potent regulator of plant development and induces root branching through a process termed hydropatterning. Hydropatterning enables roots to position lateral branches toward regions of high water availability, such as wet soil or agar media, while preventing their emergence where water is less available, such as in air. The mechanism by which roots perceive the spatial distribution of water during hydropatterning is unknown. Using primary roots of Zea mays (maize) we reveal that developmental competence for hydropatterning is limited to the growth zone of the root tip. Past work has shown that growth generates gradients in water potential across an organ when asymmetries exist in the distribution of available water. Using mathematical modeling, we predict that substantial growth-sustained water potential gradients are also generated in the hydropatterning competent zone and that such biophysical cues inform the patterning of lateral roots. Using diverse chemical and environmental treatments we experimentally demonstrate that growth is necessary for normal hydropatterning of lateral roots. Transcriptomic characterization of the local response of tissues to a moist surface or air revealed extensive regulation of signaling and physiological pathways, some of which we show are growth-dependent. Our work supports a "sense-by-growth" mechanism governing hydropatterning, by which water availability cues are rendered interpretable through growth-sustained water movement.