Growth and Development of Roots of Grapevine (Vitis vinifera L.) in Relation to Water Uptake from Soil

Developmental patterns of lateral roots and their vascular differentiationwere investigated for Vitis vinifera L. cv. Shiraz to assessthe likely contribution of lateral roots to total water uptakeof plants subjected to different irrigation regimes. Correlationanalyses showed a significant positive correlation between mainroot diameter and the diameter of first order lateral rootsof well-watered plants, but in water-stressed plants the twowere not significantly correlated. The correlations betweendiameters of first order lateral roots and the diameters ofmain roots were greater than correlations between the lengthsof first order laterals and the diameters of main roots. Thesuberised surface area of well-watered main roots increasedfrom 4% of total surface area at 0·25 cm to 100% at 10cm from the tip, whereas that of stressed plants increased from15% at 0·25 cm to 100% at 5 cm from the tip. In all treatmentsthe highest linear density of first order laterals was about7 laterals cm^-1 of main root. More than 50% of first order lateralshad diameters less than 0·05 cm, and more than 90% ofthem had lengths less than 5 cm. Calculations of axial resistancesbased on xylem diameter measurements suggest that the axialresistances of root segments may not be uniform along rootsas is often assumed in models of water uptake. Water flow intothe main roots via the lateral root pathway is likely to bemuch smaller than that via the direct radial flow pathway asonly about 1% of surface area of main roots is directly occupiedby lateral roots, leaving the other 99% of main root surfacearea available for the direct radial flow pathway.Copyright1994, 1999 Academic Press Axial resistance, grapevine (Vitis vinifera L. cv. Shiraz) roots, root diameter, root length, xylem vessels     

The Contribution of an Apoplastic Pathway to Sodium Uptake by Rice Roots in Saline Conditions

An apoplastic pathway, the so-called bypass-flow, across riceroots to the xylem has been investigated and approximately quantifiedusing the apoplastic tracer dye 8-hydroxy-l,3,6-pyrenetrisulphonicacid (PTS); former nomenclature 3-hydroxy-5,8,10-pyrenetrisulphonicacid. It was confirmed that PTS was confined to a compartmentno greater than the apparent free space in living rice roots.Experimental handling did not contribute to bypass-flow. Riceroots recovered rapidly from severe damage: following root pruning,sodium and calcium uptake returned to steady values in about6 h. Apoplastic flow declined after damage as a first-orderkinetic with a half time of 75 min. Analysis of the pattern of elution of PTS from preloaded roots(intact, excised and heat-killed), and from cellulose, was followedto compare PTS movement in the extracellular compartment withthat of water and small hydrated ions. Consideration is givento the factor by which the bypass-flow estimated with the dyewould need to be corrected to reflect the proportion of thetranspiration stream flowing in the apoplastic pathway. Thedata suggest that this factor would be at least 10 for transpiringrice plants. There was large individual variation in the transport both ofsodium and of the apoplastic tracer PTS to the shoot. Plantswith high sodium transport also had high PTS transport and itis concluded that some proportion of the sodium reaching thexylem in rice does so by a pathway which is also available toPTS, presumably direct apoplastic contact across the endodermis.A median value for the bypass-flow of water (corrected fromPTS) would be 0.5 to 1.0 percent of the transpirational volumeflow, but individuals with the highest sodium transport wouldhave bypass-flow values of several percent. Evidence is discussedwhich suggests that apoplastic transport may increase in stressconditions, and it is argued that bypass-flow is a major contributionto sodium uptake in rice in saline conditions. Key words: Oryza saliva, salinity, roots, radial ion transport, apoplast, bypass-flow     

Net sodium fluxes change significantly at anatomically distinct root zones of rice (Oryza sativa L.) seedlings

Casparian bands of endodermis and exodermis play crucial roles in blocking apoplastic movement of ions and water into the stele of roots through the cortex. These apoplastic barriers differ considerably in structure and function along the developing root. The present study assessed net Na⁺ fluxes in anatomically distinct root zones of rice seedlings and analyzed parts of individual roots showing different Na⁺ uptake. The results indicated that anatomically distinct root zones contributed differently to the overall uptake of Na⁺. The average Na⁺ uptake in root zones in which Casparian bands of the endo- and exo-dermis were interrupted by initiating lateral root primordia (root zone III) was significantly greater than that at the root apex, where Casparian bands were not yet formed (root zone I), or in the region where endo- and exo-dermis with Casparian bands were well developed (root zone II). The measurement of net Na⁺ fluxes using a non-invasive scanning ion-selective electrode technique (SIET) demonstrated that net Na⁺ flux varied significantly in different positions along developing rice roots, and a net Na⁺ influx was obvious at the base of young lateral root primordia. Since sodium fluxes changed significantly along developing roots of rice seedlings, we suggest that the significantly distinct net Na⁺ flux profile may be attributed to different apoplastic permeability due to lateral root primordia development for non-selective apoplastic bypass of ions along the apoplast.     

Abscisic acid‐mediated modifications of radial apoplastic transport pathway play a key role in cadmium uptake in hyperaccumulator Sedum alfredii

Abscisic acid (ABA) is a key phytohormone underlying plant resistance to toxic metals. However, regulatory effects of ABA on apoplastic transport in roots and consequences for uptake of metal ions are poorly understood. Here, we demonstrate how ABA regulates development of apoplastic barriers in roots of two ecotypes of Sedum alfredii and assess effects on cadmium (Cd) uptake. Under Cd treatment, increased endogenous ABA level was detected in roots of nonhyperaccumulating ecotype (NHE) due to up‐regulated expressions of ABA biosynthesis genes (SaABA2, SaNCED), but no change was observed in hyperaccumulating ecotype (HE). Simultaneously, endodermal Casparian strips (CSs) and suberin lamellae (SL) were deposited closer to root tips of NHE compared with HE. Interestingly, the vessel‐to‐CSs overlap was identified as an ABA‐driven anatomical trait. Results of correlation analyses and exogenous applications of ABA/Abamine indicate that ABA regulates development of both types of apoplastic barriers through promoting activities of phenylalanine ammonialyase, peroxidase, and expressions of suberin‐related genes (SaCYP86A1, SaGPAT5, and SaKCS20). Using scanning ion‐selected electrode technique and PTS tracer confirmed that ABA‐promoted deposition of CSs and SL significantly reduced Cd entrance into root stele. Therefore, maintenance of low ABA levels in HE minimized deposition of apoplastic barriers and allowed maximization of Cd uptake via apoplastic pathway.     

Arbuscular mycorrhizal symbiosis increases relative apoplastic water flow in roots of the host plant under both well-watered and drought stress conditions

Background and Aims

The movement of water through mycorrhizal fungal tissues and between the fungus and roots is little understood. It has been demonstrated that arbuscular mycorrhizal (AM) symbiosis regulates root hydraulic properties, including root hydraulic conductivity. However, it is not clear whether this effect is due to a regulation of root aquaporins (cell-to-cell pathway) or to enhanced apoplastic water flow. Here we measured the relative contributions of the apoplastic versus the cell-to-cell pathway for water movement in roots of AM and non-AM plants.

Methods

We used a combination of two experiments using the apoplastic tracer dye light green SF yellowish and sodium azide as an inhibitor of aquaporin activity. Plant water and physiological status, root hydraulic conductivity and apoplastic water flow were measured.

Key Results

Roots of AM plants enhanced significantly relative apoplastic water flow as compared with non-AM plants and this increase was evident under both well-watered and drought stress conditions. The presence of the AM fungus in the roots of the host plants was able to modulate the switching between apoplastic and cell-to-cell water transport pathways.

Conclusions

The ability of AM plants to switch between water transport pathways could allow a higher flexibility in the response of these plants to water shortage according to the demand from the shoot.     

Review article. How does water get through roots?

On the basis of recent results with young primary maize roots, a model is proposed for the movement of water across roots. It is shown how the complex, 'composite anatomical structure' of roots results in a 'composite transport' of both water and solutes. Parallel apoplastic, symplastic and transcellular pathways play an important role during the passage of water across the different tissues. These are arranged in series within the root cylinder (epidermis, exodermis, central cortex, endodermis, pericycle stelar parenchyma, and tracheary elements). The contribution of these structures to the root's overall radial hydraulic resistance is examined. It is shown that as soon as early metaxylem vessels mature, the axial (longitudinal) hydraulic resistance within the xylem is usually not rate-limiting. According to the model, there is a rapid exchange of water between parallel radial pathways because, in contrast to solutes such as nutrient ions, water permeates cell membranes readily. The roles of apoplastic barriers (Casparian bands and suberin lamellae) in the root's endo- and exodermis are discussed. The model allows for special characteristics of roots such as a high hydraulic conductivity (water permeability) in the presence of a low permeability of nutrient ions once taken up into the stele by active processes. Low root reflection coefficients indicate some apoplastic by-passes for water within the root cylinder. For a given root, the model explains the large variability in the hydraulic resistance in terms of a dependence of hydraulic conductivity on the nature and intensity of the driving forces involved to move water. By switching the apoplastic path on or off, the model allows for a regulation of water uptake according to the demands from the shoot. At high rates of transpiration, the apoplastic path will be partially used and the hydraulic resistance of the root will be low, allowing for a rapid uptake of water. On the contrary, at low rates of transpiration such as during the night or during stress conditions (drought, high salinity, nutrient deprivation), the apoplastic path will be less used and the hydraulic resistance will be high. The role of water channels (aquaporins) in the transcellular path is in the fine adjustment of water flow or in the regulation of uptake in older, suberized parts of plant roots lacking a substantial apoplastic component. The composite transport model explains how plants are designed to optimize water uptake according to demands from the shoot and how external factors may influence water passage across roots.     

Ultrastructure of the haustorial interface and apoplastic continuum between host and the root hemiparasiteOlax phyllanthi (Labill.) R. Br. (Olacaceae)

Summary Structural features of haustorial interface parenchyma of the root hemiparasiteOlax phyllanthi are described. Walls contacting host xylem are thickened non-uniformly with polysaccharides, not lignin, and show only a thin protective wall layer when abutting pits in walls of host xylem vessels or tracheids. Lateral walls of interface parenchyma exhibit an expanded middle layer of open fibrillar appearance, sometimes with, but mostly lacking adjoining layers of dense wall material. Free ribosomes and rough endoplasmic reticulum are prominent and occasional wall ingrowths present. Experiments involving transpirational feeding of the apoplast tracers lanthanum nitrate or uranyl acetate to host roots cut below haustorial connections, indicate effective apoplastic transfer from host to parasite root via the haustorium. Deposits of the tracers suggest a major pathway for water flow through host xylem pits, across the thin protective wall layer, and thence into the haustorium via the electronopaque regions of the terminal and lateral walls of the contact parenchyma. Graniferous tracheary elements and walls of parenchyma cells of the body of the haustorium appear to participate in tracer flow as do walls of cortical cells, stele parenchyma and xylem conducting elements of the parasite root, suggesting that both vascular and non-vascular routes are involved in extracytoplasmic transfer of xylem sap from host to parasite. The Casparian strip of the endodermis and the suberin lamella of the exodermis of theOlax root act as barriers to flow within the system.     

The effect of phosphorus nutrition on water flow through the apoplastic bypass in cotton roots

A previous study comparing hydraulic conductivity of intactroots and cortical cells of cotton (Gossypium hirsutum L.) seedlingssuggested that substantial water flow may bypass cell membranesentirely, following a completely apoplastic pathway, and alsosuggested that phosphorus deficiency might increase bypass flow.We used fluorescent apoplastic tracers to quantify flow throughthe apoplastic bypass and to assess the effect of phosphorusdeficiency on bypass flow. Tracer concentration in shoot xylemsap was less than 0.25% of the concentration in the culturalsolution for five different tracers. Phosphorus deficiency reducedthis already low concentration even further, possibly by reducingthe number of newly emerged lateral roots which can providean alternative pathway for water entrance into the stele. Norelationship existed between hydraulic conductivity and bypassflow. We concluded that the apoplastic bypass constituted onlya small fraction of water movement into the stele. The contributionof the apoplastic bypass was not sufficient to explain differencesin hydraulic conductivity between intact roots and corticalcells, or between plants receiving high or low phosphorus treatments. Key words: Apoplastic bypass, phosphorus nutrition, cotton, hydraulic conductivity, lateral roots     

Root Browning in Pinus banksiana Lamb. and Eucalyptus pilularis Sm. 1. Anatomy and Permeability of the White and Tannin Zones

Growing tree roots are characteristically brown with white tips. The browning process, which occurs as the white region matures, has often been attributed to the deposition of suberin in various tissues. However, in pouch-grown tree seedlings of jack pine (Pinus banksiana Lamb.) and eucalyptus (Eucalyptus pilularis Sm.), browning was not linked to suberization but was caused by the deposition of condensed tannins in the walls of all cells external to the stele. Therefore, we propose using the term “tannin zone” to refer to this region of the root. Vitality tests indicated that the cells of the epidermis and cortex were alive in white regions but were dead in brown regions. Following sequential treatment with berberine hemisulfate and potassium thiocyanate, the cortical walls external to the endodermal Casparian band were full of berberine thiocyanate crystals, indicating that they were permeable to berberine. These walls should also be permeable to water and ions, which have smaller molecular dimensions than the tracer dye. Based on the anatomy and permeability of the tannin zone, we predict that its capacity for ion uptake would be reduced compared to the white zone because of a reduced absorptive plasmalemma surface area. In jack pine, some uptake could be effected by the passage cells of the endodermis. The tannin zone should be even less absorptive in eucalyptus because the exodermis remains an apoplastic barrier and the endodermis lacks passage cells. It is difficult to predict the difference between the tannin and white zones with respect to water uptake. Death of the cells external to the endodermis would reduce the resistance of the root to water movement, but deposition of tannins would increase it. The deposition of suberin lamellae in increasing numbers of endodermal cells may also retard water flow. The anatomy and physiological properties of the tannin zone are unique from those of the distal, white zone and the proximal, cork-clad zone.     

The growth,activity and distribution of the fruit tree root system

Summary The paper reviews information, much of it obtained from studies using the East Malling root observation laboratories, on the growth and development of the fruit tree root system. The production of new white root varies from year-to-year, generally being highest in the early years. As trees age, woody roots constitute an increasing fraction of total root length although the contribution made by new root growth to the total root length of established trees is also affected by soil management, being higher for trees under grass than under herbicide. Soil management also affects the balance of short (lateral) to long (extension) roots; under grass there are more lateral roots.Calculation of the rate of water uptake per unit root length needed at various times in the year to meet transpirational demand, suggests that woody roots, which recent experimental work has shown to be capable of absorbing water, must be responsible for much of total water supply.Measurements of VA mycorrhizal infection in field-grown trees indicated, for part of the season, higher per cent infection in trees grown under irrigated grass than under herbicide management. It is suggested that this, which is associated with raised leaf phosphorus levels, may be due at least partly to higher numbers of lateral roots, the root type which becomes infected. The growth and functioning of the root system under field conditions depend upon the production and integration of a range of root types.     

Silicon deposition in the root reduces sodium uptake in rice (Oryza sativa L.) seedlings by reducing bypass flow

Sodium chloride reduces the growth of rice seedlings, which accumulate excessive concentrations of sodium and chloride ions in their leaves. In this paper, we describe how silicon decreases transpirational bypass flow and ion concentrations in the xylem sap in rice (Oryza sativa L.) seedlings growing under NaCl stress. Salt (50 mM NaCl) reduced the growth of shoots and roots: adding silicate (3 mM) to the saline culture solution improved the growth of the shoots, but not roots. The improvement of shoot growth in the presence of silicate was correlated with reduced sodium concentration in the shoot. The net transport rate of Na from the root to shoot (expressed per unit of root mass) was also decreased by added silicate. There was, however, no effect of silicate on the net transport of potassium. Furthermore, in salt-stressed plants, silicate did not decrease the transpiration, and even increased it in seedlings pre-treated with silicate for 7 d prior to salt treatment, indicating that the reduction of sodium uptake by silicate was not simply through a reduction in volume flow from root to shoot. Experiments using trisodium-8-hydroxy-1,3,6-pyrenetrisulphonic acid (PTS), an apoplastic tracer, showed that silicate dramatically decreased transpirational bypass flow in rice (from about 4.2 to 0.8%), while the apparent sodium concentration in the xylem, which was estimated indirectly from the flux data, decreased from 6.2 to 2.8 mM. Direct measurements of the concentration of sodium in xylem sap sampled using Philaenus spumarius confirmed that the apparent reduction was not a consequence of sodium recycling. X-ray microanalysis showed that silicon was deposited in the outer part of the root and in the endodermis, being more obvious in the latter than in the former. The results suggest that silicon deposition in the exodermis and endodermis reduced sodium uptake in rice (Oryza sativa L.) seedlings under NaCl stress through a reduction in apoplastic transport across the root.     

压力室测定根系导水率方法探讨

用压力室连续测定了玉米根系长压和降压过程的导水率,结果表明,降压过程湍 得的根系导水率显著大于用升压过程的,并且前者的相关系数大于后者,这种差异是由于这两个过程中质外体途径细胞壁空间充水量不同造成的,开始升压时,由于细胞壁空间含水量低,质外体途径阻力大,导致非结构阻力;随着压力的升高,细胞壁空间含水量增大,质外体途径导度增大,减小甚至可以消除非结构阻力,降压法可以使根系快速复水,消除传统方法因长时间复水所致根结构的改变。建议用降压法测定根系导水率。     

植物钙素吸收和运转

近年来,钙素在植物体内的吸收和运输研究主要集中在细胞和分子水平,但整株水平上的研究也同样重要.整株水平上的钙吸收和运输包括根细胞的钙吸收、钙离子横向穿过根系并进入木质部、在木质部运输、从木质部移出并进入叶片或果实及在叶片或果实中运转分配等环节,既经过质外体也穿越共质体.钙离子通道、Ca2 -ATP酶和Ca2 /H 反向转运器等参与根细胞的钙吸收.在钙离子横向穿根进入木质部的过程中,需要穿越内皮层和木质部薄壁细胞组织.根系内皮层凯氏带阻挡了Ca2 沿质外体途径由内皮层外侧向内侧的移动,部分Ca2 由此通过离子通道流进内皮层细胞而转入共质体并到达木质部薄壁细胞组织,而由木质部薄壁细胞组织进入中柱质外体可能需要Ca2 -ATP酶驱动;还有一些Ca2 由内皮层细胞运出,沿内皮层内侧的质外体途径进入木质部导管,并通过导管运向枝干.钙离子以螯合态的形式在枝干导管运输;水流速率是影响钙离子沿导管运输的关键因子.钙离子在果实和叶片中的运输和分配不仅通过质外体途径也通过共质体途径.     

Lateral ABA transport in maize roots (Zea mays): visualization by immunolocalization

The intensity of an ABA (abscisic acid) signal as a root-to-shoot signal, as well as its action on root hydraulic conductivity, strongly depends on the distribution of ABA during its radial transport across roots. Therefore ABA was visualized by immunolocalization with monoclonal ABA antibodies under conditions of lateral water flow induced by the application of a pressure gradient to the cut surface of the mesocotyl of maize seedlings. From the labelling of rhizodermis, hypodermis, cortical cells, and endodermis of roots of hydroponically (no exodermis) and aeroponically (with exodermis) grown seedlings it is concluded that the exodermis acts as a barrier to apoplastic transport that controls ABA uptake and efflux, but that the endodermis can easily be overcome via an apoplastic bypass. In longitudinal sections the strongest ABA signals originated from the root cap and the meristematic root tip, which is in agreement with the non-vacuolated cells of these tissues being an effective anion trap for ABA.     

Relationships between structural development and the absorption of ions by the root system of Cucurbita pepo

Summary In both the seminal axis and lateral roots of Cucurbita pepo L. the formation of large central xylem elements and the commencement of secondary cambial activity occur 10–20 cm from the root tip. Concomitant with or slightly preceding these developments there are changes in the structure of the walls of endodermal cells where the lignified casparian band spreads along the radial wall and substances staining with Sudan IV are deposited in both radial and tangential walls. At distances more than 30 cm from the tip of primary roots the radius of the stele increases considerably causing splits in the cortex. The endodermis is stretched and the suberin becomes organized in a lamellar form.Against this background of anatomical change certain of the transport capabilities of the root are retained while others are lost. Using an apparatus for measuring the uptake of tracers by segments of intact roots it was found that neither the uptake nor translocation of potassium seem to be affected by the suberization of the endodermis or by secondary thickening, while the translocation of calcium is virtually eliminated when these processes begin. As the root ages its ability to absorb phosphate declines although the translocation of the phosphate absorbed is much less affected by structural development than that of calcium.The observed rates of potassium uptake by complete root systems could be predicted quite accurately from the average of segment uptake data suggesting that the method used gives reliable results.     

Hydraulic conductivity of rice roots

A pressure chamber and a root pressure probe technique have been used to measure hydraulic conductivities of rice roots (root Lp(r) per m(2) of root surface area). Young plants of two rice (Oryza sativa L.) varieties (an upland variety, cv. Azucena and a lowland variety, cv. IR64) were grown for 31-40 d in 12 h days with 500 micromol m(-2) s(-1) PAR and day/night temperatures of 27 degrees C and 22 degrees C. Root Lp(r) was measured under conditions of steady-state and transient water flow. Different growth conditions (hydroponic and aeroponic culture) did not cause visible differences in root anatomy in either variety. Values of root Lp(r) obtained from hydraulic (hydrostatic) and osmotic water flow were of the order of 10(-8) m s(-1) MPa(-1) and were similar when using the different techniques. In comparison with other herbaceous species, rice roots tended to have a higher hydraulic resistance of the roots per unit root surface area. The data suggest that the low overall hydraulic conductivity of rice roots is caused by the existence of apoplastic barriers in the outer root parts (exodermis and sclerenchymatous (fibre) tissue) and by a strongly developed endodermis rather than by the existence of aerenchyma. According to the composite transport model of the root, the ability to adapt to higher transpirational demands from the shoot should be limited for rice because there were minimal changes in root Lp(r) depending on whether hydrostatic or osmotic forces were acting. It is concluded that this may be one of the reasons why rice suffers from water shortage in the shoot even in flooded fields.     

THE ROOT APEX OF MALVA SYLVESTRIS. III. LATERAL ROOT DEVELOPMENT AND THE QUIESCENT CENTER

Both histological and autoradiographic procedures were used to follow lateral root initiation and development. Lateral roots of M. sylvestris were initiated in the pericycle, and although the endodermis became multiseriate, endodermal derivatives were not incorporated into the lateral root primordium. Apical organization of pre-emergent roots, characterized by two tiers of cortical initials, did not change with growth. During pre-emergent development there was no evidence of cortical lysogeny or quiescent center formation. Quiescent centers were present in both secondary and tertiary roots longer than 0.5 cm.     

Water uptake by roots: effects of water deficit

The variable hydraulic conductivity of roots (Lp(r)) is explained in terms of a composite transport model. It is shown how the complex, composite anatomical structure of roots results in a composite transport of both water and solutes. In the model, the parallel apoplastic and cell-to-cell (symplastic and transcellular) pathways play an important role as well as the different tissues and structures arranged in series within the root cylinder (epidermis, exodermis, cortex, endodermis, stelar parenchyma). The roles of Casparian bands and suberin lamellae in the root's endo- and exodermis are discussed. Depending on the developmental state of these apoplastic barriers, the overall hydraulic resistance of roots is either more evenly distributed across the root cylinder (young unstressed roots) or is concentrated in certain layers (exo- and endodermis in older stressed roots). The reason for the variability of root Lp(r), is that hydraulic forces cause a dominating apoplastic flow of water around protoplasts, even in the endodermis and exodermis. In the absence of transpiration, water flow is osmotic in nature which causes a high resistance as water passes across many membranes on its passage across the root cylinder. The model allows for a high capability of roots to take up water in the presence of high rates of transpiration (high demands for water from the shoot). By contrast, the hydraulic conductance is low, when transpiration is switched off. Overall, this results in a non-linear relationship between water flow and forces (gradients of hydrostatic and osmotic pressure) which is otherwise hard to explain. The model allows for special root characteristics such as a high hydraulic conductivity (water permeability) in the presence of a low permeability of nutrient ions once taken up into the stele by active processes. Low root reflection coefficients are in line with the idea of some apoplastic bypasses for water within the root cylinder. According to the composite transport model, the switch from the hydraulic to the osmotic mode is purely physical. In the presence of heavily suberized roots, the apoplastic component of water flow may be too small. Under these conditions, a regulation of radial water flow by water channels dominates. Since water channels are under metabolic control, this component represents an 'active' element of regulation. Composite transport allows for an optimization of the water balance of the shoot in addition to the well-known phenomena involved in the regulation of water flow (gas exchange) across stomata. The model is employed to explain the responses of plants to water deficit and other stresses. During water deficit, the cohesion-tension mechanism of the ascent of sap in the xylem plays an important role. Results are summarized which prove the validity of the coehesion/tension theory. Effects of the stress hormone abscisic acid (ABA) are presented. They show that there is an apoplastic component of the flow of ABA in the root which contributes to the ABA signal in the xylem. On the other hand, (+)-cis-trans-ABA specifically affects both the cell level (water channel activity) and water flow driven by gradients in osmotic pressure at the root level which is in agreement with the composite transport model. Hydraulic water flow in the presence of gradients in hydrostatic pressure remains unchanged. The results agree with the composite transport model and resemble earlier findings of high salinity obtained for the cell (Lp) and root (Lp(r)) level. They are in line with known effects of nutrient deprivation on root Lp(r )and the diurnal rhythm of root Lp(r )recently found in roots of LOTUS.     

Aquaporin-facilitated water uptake in barley (Hordeum vulgare L.) roots

It is not known to what degree aquaporin-facilitated water uptake differs between root developmental regions and types of root. The aim of this study was to measure aquaporin-dependent water flow in the main types of root and root developmental regions of 14- to 17-d-old barley plants and to identify candidate aquaporins which mediate this flow. Water flow at root level was related to flow at cell and plant level. Plants were grown hydroponically. Hydraulic conductivity of cells and roots was determined with a pressure probe and through exudation, respectively, and whole-plant water flow (transpiration) determined gravimetrically in response to the commonly used aquaporin inhibitor HgCl(2). Expression of aquaporins was analysed by real-time PCR and in situ hybridization. Hydraulic conductivity of cortical cells in seminal roots was largest in lateral roots; it was smallest in the fully mature zone and intermediate in the not fully mature 'transition' zone along the main root axis. Adventitious roots displayed an even higher (3- to 4-fold) cortical cell hydraulic conductivity in the transition zone. This coincided with 3- to 4-fold higher expression of three aquaporins (HvPIP2;2, HvPIP2;5, HvTIP1:1). These were expressed (also) in cortical tissue. The largest inhibition of water flow (83-95%) in response to HgCl(2) was observed in cortical cells. Water flow through roots and plants was reduced less (40-74%). It is concluded that aquaporins contribute substantially to root water uptake in 14- to 17-d-old barley plants. Most water uptake occurs through lateral roots. HvPIP2;5, HvPIP2;2, and HvTIP1;1 are prime candidates to mediate water flow in cortical tissue.     

Water uptake and structural plasticity along roots of a desert succulent during prolonged drought

Desert succulents resume substantial water uptake within 1–2 d of the cessation of drought, but the changes in root structure and hydraulic conductivity underlying such recovery are largely unknown. In the monocotyledonous leaf succulent Agave deserti Engelm. substantial root mortality occurred only for lateral roots near the soil surface; nearly all main roots were alive at 180 d of drought. New main roots were initiated and grew up to 320 mm at soil water potentials lower than – 5·0 MPa, utilizing water from the shoot. The hydraulic conductivity of distal root regions decreased 62% by 45 d of drought and 70% thereafter. After 7 d of rewetting, root hydraulic conductivity was restored following 45 d of drought but not after 90 and 180 d. The production of new lateral roots and the renewed apical elongation of main roots occurred 7–11 d after rewetting following 180 d of drought. Hydraulic conductivity was higher in the distal region than at midroot and often increased again near the root base, where many endodermal cells lacked suberin lamellae. Suberization and xylem maturation were influenced by the availability of moisture, suggesting that developmental plasticity along a root allows A. deserti to capitalize on intermittent or heterogeneous supplies of water.