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The phytohormone abscisic acid (ABA) plays a major role in regulating root growth. Most work to date has investigated the influence of root-sourced ABA on root growth during water stress. Here, we tested whether foliage-derived ABA could be transported to the roots, and whether this foliage-derived ABA had an influence on root growth under well-watered conditions. Using both application studies of deuterium-labelled ABA and reciprocal grafting between wild-type and ABA-biosynthetic mutant plants, we show that both ABA levels in the roots and root growth in representative angiosperms are controlled by ABA synthesized in the leaves rather than sourced from the roots. Foliage-derived ABA was found to promote root growth relative to shoot growth but to inhibit the development of lateral roots. Increased root auxin (IAA) levels in plants with ABA-deficient scions suggest that foliage-derived ABA inhibits root growth through the root growth-inhibitor IAA. These results highlight the physiological and morphological importance, beyond the control of stomata, of foliage-derived ABA. The use of foliar ABA as a signal for root growth has important implications for regulating root to shoot growth under normal conditions and suggests that leaf rather than root hydration is the main signal for regulating plant responses to moisture. [ABSTRACT FROM AUTHOR]

Abscisic acid ( ABA) has been demonstrated to be involved in iron ( Fe) homeostasis, but the underlying mechanism is largely unknown. Here, we found that Fe deficiency induced ABA accumulation rapidly (within 6 h) in the roots of Arabidopsis. Exogenous ABA at 0.5 μM decreased the amount of root apoplastic Fe bound to pectin and hemicellulose, and increased the shoot Fe content significantly, thus alleviating Fe deficiency-induced chlorosis. Exogenous ABA promoted the secretion of phenolics to release apoplastic Fe and up-regulated the expression of AtNRAMP3 to enhance reutilization of Fe stored in the vacuoles, leading to a higher level of soluble Fe and lower ferric-chelate reductase ( FCR) activity in roots. Treatment with ABA also led to increased Fe concentrations in the xylem sap, partially because of the up-regulation of AtFRD3, AtYSL2 and AtNAS1, genes related to long-distance transport of Fe. Exogenous ABA could not alleviate the chlorosis of abi5 mutant resulting from the significantly low expression of AtYSL2 and low transport of Fe from root to shoot. Taken together, our data support the conclusion that ABA is involved in the reutilization and transport of Fe from root to shoot under Fe deficiency conditions in Arabidopsis. [ABSTRACT FROM AUTHOR]

The role of jasmonic acid in the induction of stomatal closure is well known. However, its role in regulating root hydraulic conductivity ( L) has not yet been explored. The objectives of the present research were to evaluate how JA regulates L and how calcium and abscisic acid ( ABA) could be involved in such regulation. We found that exogenous methyl jasmonate ( MeJA) increased L of Phaseolus vulgaris, Solanum lycopersicum and Arabidopsis thaliana roots. Tomato plants defective in JA biosynthesis had lower values of L than wild-type plants, and that L was restored by addition of MeJA. The increase of L by MeJA was accompanied by an increase of the phosphorylation state of the aquaporin PIP2. We observed that MeJA addition increased the concentration of cytosolic calcium and that calcium channel blockers inhibited the rise of L caused by MeJA. Treatment with fluoridone, an inhibitor of ABA biosynthesis, partially inhibited the increase of L caused by MeJA, and tomato plants defective in ABA biosynthesis increased their L after application of MeJA. It is concluded that JA enhances L and that this enhancement is linked to calcium and ABA dependent and independent signalling pathways. [ABSTRACT FROM AUTHOR]

ABSTRACT Whole-genome duplication, or polyploidy, is common in many plant species and often leads to better adaptation to adverse environmental condition. However, little is known about the physiological and molecular determinants underlying adaptation. We examined the drought tolerance in diploid (2x) and autotetraploid (4x) clones of Rangpur lime ( Citrus limonia) rootstocks grafted with 2x Valencia Delta sweet orange ( Citrus sinensis) scions, named V/2xRL and V/4xRL, respectively. Physiological experiments to study root-shoot communication associated with gene expression studies in roots and leaves were performed. V/4xRL was much more tolerant to water deficit than V/2xRL. Gene expression analysis in leaves and roots showed that more genes related to the response to water stress were differentially expressed in V/2xRL than in V/4xRL. Prior to the stress, when comparing V/4xRL to V/2xRL, V/4xRL leaves had lower stomatal conductance and greater abscisic acid (ABA) content. In roots, ABA content was higher in V/4xRL and was associated to a greater expression of drought responsive genes, including CsNCED1, a pivotal regulatory gene of ABA biosynthesis. We conclude that tetraploidy modifies the expression of genes in Rangpur lime citrus roots to regulate long-distance ABA signalling and adaptation to stress. [ABSTRACT FROM AUTHOR]

ABSTRACT MicroRNAs (miRNAs) are part of a novel mechanism of gene regulation that is active in plants under abiotic stress conditions. In the present study, 12 miRNAs were analysed to identify miRNAs differentially expressed in sugarcane subjected to cold stress (4 °C). The expression of miRNAs assayed by stem-loop RT-PCR showed that miR319 is up-regulated in sugarcane plantlets exposed to 4 °C for 24 h. The induction of miR319 expression during cold stress was observed in both roots and shoots. Sugarcane miR319 was also regulated by treatment with abscisic acid. Putative targets of this miRNA were identified and their expression levels were decreased in sugarcane plantlets exposed to cold. The cleavage sites of two targets were mapped using a 5′ RACE PCR assay confirming the regulation of these genes by miR319. When sugarcane cultivars contrasting in cold tolerance were subjected to 4 °C, we observed up-regulation of miR319 and down-regulation of the targets in both varieties; however, the changes in expression were delayed in the cold-tolerant cultivar. These results suggest that differences in timing and levels of the expression of miR319 and its targets could be tested as markers for selection of cold-tolerant sugarcane cultivars. [ABSTRACT FROM AUTHOR]

To determine whether root-to-shoot signalling of soil moisture heterogeneity depended on root distribution, wild-type (WT) and abscisic acid (ABA)-deficient ( Az34) barley ( Hordeum vulgare) plants were grown in split pots into which different numbers of seminal roots were inserted. After establishment, all plants received the same irrigation volumes, with one pot watered (w) and the other allowed to dry the soil (d), imposing three treatments (1 d : 3 w, 2 d : 2 w, 3 d : 1 w) that differed in the number of seminal roots exposed to drying soil. Root distribution did not affect leaf water relations and had no sustained effect on plant evapotranspiration (ET). In both genotypes, leaf elongation was less and leaf ABA concentrations were higher in plants with more roots in drying soil, with leaf ABA concentrations and water potentials 30% and 0.2 MPa higher, respectively, in WT plants. Whole-pot soil drying increased xylem ABA concentrations, but maximum values obtained when leaf growth had virtually ceased (100 n in Az34, 330 n in WT) had minimal effects (<40% leaf growth inhibition) when xylem supplied to detached shoots. Although ABA may not regulate leaf growth in vivo, genetic variation in foliar ABA concentration in the field may indicate different root distributions between upper (drier) and lower (wetter) soil layers. [ABSTRACT FROM AUTHOR]

Deposition of ammonium (NH) from the atmosphere is a substantial environmental problem. While toxicity resulting from root exposure to NH is well studied, little is known about how shoot-supplied ammonium (SSA) affects root growth. In this study, we show that SSA significantly affects lateral root (LR) development. We show that SSA inhibits lateral root primordium (LRP) emergence, but not LRP initiation, resulting in significantly impaired LR number. We show that the inhibition is independent of abscisic acid (ABA) signalling and sucrose uptake in shoots but relates to the auxin response in roots. Expression analyses of an auxin-responsive reporter, DR5:GUS, and direct assays of auxin transport demonstrated that SSA inhibits root acropetal (rootward) auxin transport while not affecting basipetal (shootward) transport or auxin sensitivity of root cells. Mutant analyses indicated that the auxin influx carrier AUX1, but not the auxin efflux carriers PIN-FORMED (PIN)1 or PIN2, is required for this inhibition of LRP emergence and the observed auxin response. We found that AUX1 expression was modulated by SSA in vascular tissues rather than LR cap cells in roots. Taken together, our results suggest that SSA inhibits LRP emergence in Arabidopsis by interfering with AUX1-dependent auxin transport from shoot to root. [ABSTRACT FROM AUTHOR]

To investigate the contribution of different parts of the root system to total sap flow and leaf xylem abscisic acid (ABA) concentration ([X-ABA]leaf), individual sunflower ( Helianthus annuus L.) shoots were grafted onto the root systems of two plants grown in separate pots and sap flow through each hypocotyl measured below the graft union. During deficit irrigation (DI), both pots received the same irrigation volumes, while during partial root zone drying (PRD) one pot (‘wet’) was watered and another (‘dry’) was not. During PRD, once soil water content ( θ) decreased below a threshold, the fraction of sap flow from drying roots declined. As θ declined, root xylem ABA concentration increased in both irrigation treatments, and [X-ABA]leaf increased in DI plants, but [X-ABA]leaf of PRD plants actually decreased within a certain θ range. A simple model that weighted ABA contributions of wet and dry root systems to [X-ABA]leaf according to the sap flow from each, better predicted [X-ABA]leaf of PRD plants than either [X-ABA]dry, [X-ABA]wet or their mean. Model simulations revealed that [X-ABA]leaf during PRD exceeded that of DI with moderate soil drying, but continued soil drying (such that sap flow from roots in drying soil ceased) resulted in the opposite effect. [ABSTRACT FROM AUTHOR]

Plants produce compounds in roots that are transported to shoots via the xylem sap. Some of these compounds are vital for signalling and adaptation to environmental stress such as drought. In this study, we screened the xylem sap using mass spectrometry to quantify the changes in new and previously identified sap constituents under extended drought. We detected and quantified the changes in the concentration of 31 compounds present in the xylem sap under progressively increasing drought stress. We found changes in the hormones abscisic acid (ABA) and cytokinin, and the presence of high concentrations of the aromatic cytokinin 6-benzylaminopurine (BAP). Several phenylpropanoid compounds (coumaric, caffeic and ferulic acids) were found in xylem sap. The concentrations of some of these phenylpropanoid compounds changed under drought. In parallel, an analysis of the xylem sap proteome was conducted. We found a higher abundance of cationic peroxidases, which with the increase in phenylpropanoids may lead to a reduction in lignin biosynthesis in the xylem vessels and could induce cell wall stiffening. The application of new methodologies provides insights into the range of compounds in sap and how alterations in composition may lead to changes in development and signalling during adaptation to drought. [ABSTRACT FROM AUTHOR]

The impact of water deficit progression on cytokinin (CK), auxin and abscisic acid (ABA) levels was followed in upper, middle and lower leaves and roots of Nicotiana tabacum L. cv. Wisconsin 38 plants [wild type (WT)]. ABA content was strongly increased during drought stress, especially in upper leaves. In plants with a uniformly elevated total CK content, expressing constitutively the trans-zeatin O-glucosyltransferase gene ( 35S::ZOG1), a delay in the increase of ABA was observed; later on, ABA levels were comparable with those of WT. As drought progressed, the bioactive CK content in leaves gradually decreased, being maintained longer in the upper leaves of all tested genotypes. Under severe stress (11 d dehydration), a large stimulation of cytokinin oxidase/dehydrogenase (CKX) activity was monitored in lower leaves, which correlated well with the decrease in bioactive CK levels. This suggests that a gradient of bioactive CKs in favour of upper leaves is established during drought stress, which might be beneficial for the preferential protection of these leaves. During drought, significant accumulation of CKs occurred in roots, partially because of decreased CKX activity. Simultaneously, auxin increased in roots and lower leaves. This indicates that both CKs and auxin play a role in root response to severe drought, which involves the stimulation of primary root growth and branching inhibition. [ABSTRACT FROM AUTHOR]

Directed growth of roots in relation to a moisture gradient is called hydrotropism. The no hydrotropic response ( nhr1) mutant of Arabidopsis lacks a hydrotropic response, and shows a stronger gravitropic response than that of wild type (wt) in a medium with an osmotic gradient. Local application of abscisic acid (ABA) to seeds or root tips of nhr1 increased root downward growth, indicating the critical role of ABA in tropisms. Wt roots germinated and treated with ABA in this system were strongly gravitropic, even though they had almost no starch amyloplasts in the root-cap columella cells. Hydrotropically stimulated nhr1 roots, with or without ABA, maintained starch in the amyloplasts, as opposed to those of wt. Hence, the near-absence (wt) or abundant presence ( nhr1) of starch granules does not influence the extent of downward gravitropism of the roots in an osmotic gradient medium. Starch degradation in the wt might help the root sustain osmotic stress and carry out hydrotropism, instead of reducing gravity responsiveness. nhr1 roots might be hydrotropically inactive because they maintain this starch reserve in the columella cells, sustaining both their turgor and growth, and in effect minimizing the need for hydrotropism and at least partially disabling its mechanism. We conclude that ABA and water stress are critical regulators of root tropic responses. [ABSTRACT FROM AUTHOR]

The inhibition of the hydraulic conductivity of individual cortical cells ( Lp) of young roots of cucumber and figleaf gourd by low root temperature (8 °C, LRT) was measured using a cell pressure probe. When LRT was imposed, the Lp of the two species responded differently. Water permeability of cortical cells of chilling-sensitive cucumber decreased by a factor of 10, but there was only a small effect in the chilling-resistant figleaf gourd. Mechanical stresses (pulses of cell turgor pressure) resulted in a similar inhibition for both species by a factor of 6.5. When applied at LRT, abscisic acid (ABA) partially or even completely reversed the effects of chilling and mechanical stresses of both species. At the control temperature of 22 °C, 50 µ m of the aquaporin (AQP) inhibitor HgCl2 acted on root cells of both species, although the effect on root cells of figleaf gourd was small. There was no effect of HgCl2, when AQPs were already closed either by LRT or by mechanical stress. The effect of mechanical stress (pressure pulses) was substantially bigger than that of HgCl2. When AQPs were closed by big pulses in the presence of 50 µ m HgCl2, they could be partially re-opened in the presence of the inhibitor by applying small pulses, suggesting that there are at least two different types of channels present, which respond differently to mechanical stress or to the heavy metal. The presence of 1 µ m ABA in the root medium prevented the effects of LRT and mechanical stress, namely an increase in the half-times of water exchange ( Tw1/2 ∝ 1/ Lp). In the absence of stresses at short Tw1/2, there was no effect of ABA. It is concluded that the responsiveness of AQPs of the two species differs in the presence of LRT but not under conditions of mechanical stress. In both cases, however, ABA has an ameliorative effect. The results suggest that the presence of ABA reduces the activation energy of changes of the conformation of AQPs, when switching between open and closed states. Mechanisms of the gating of AQP activity by LRT and mechanical stresses and the possible role of the stress hormone ABA are discussed. [ABSTRACT FROM AUTHOR]

Gibberellin (GA) promoted rice (Oryza sativaL.) root elongation in a concentration-dependent manner with roots grown in 0.1 µmGA3 being 13.8% longer than controls. On the other hand, the roots of Tan-ginbozu, a semi-dwarf cultivar were 69.8% shorter in comparison with Nipponbare, a normal cultivar. Treatments with 10 µmuniconazole-P and 10 µmabscisic acid (ABA) caused decreases in root length in Tan-ginbozu by 44.6 and 79.2%, respectively. To investigate how GA influences rice root growth, proteome analysis techniques were applied. Extracted proteins were separated by two-dimensional polyacrylamide gel electrophoresis and analysed using an automated protein sequencer and mass spectrometer. Sixteen proteins show differences in accumulation levels as a result of treatment with GA3, uniconazole-P and ABA treatment and/or the difference between the semi-dwarf cultivar, Tan-ginbozu, and normal cultivars. Among these proteins, fructose-bisphosphate aldolase (EC 4.1.2.13) increased in roots treated with GA3, occurred in low levels in Tan-ginbozu roots, and decreased in roots treated with uniconazole-P or ABA. Moreover, roots from seedlings grown in 100 µmglucose were 9.1% longer than controls. These results indicate that increases in fructose-bisphosphate aldolase activity stimulate the glycolytic pathway and may play an important role in the GA-induced growth of roots. [ABSTRACT FROM AUTHOR]

The excision of four out of five primary roots of wheat ( Triticum durum Desf.) seedlings often leads to an enhanced rate of transpiration. Surprisingly this enhancement could be maintained for several hours after root excision and was particularly likely to occur at low irradiances or high atmospheric humidity. This long-term enhancement could not be explained in terms of conventional hydropassive stomatal effects. Elevated rates of transpiration were associated with and possibly caused by increased cytokinin concentrations in shoots of plants with partially excised roots. The single root remaining after excision was able to maintain an adequate water uptake for the continued enhanced transpiration, after only a short transient reduction in leaf water content. The enhanced capacity for water uptake by the remaining root was confirmed by measuring the water flow from detached roots at negative hydrostatic pressure. Even without additional suction, flow from the reduced root system increased about 1.5 h after the start of treatment, suggesting an increase in membrane permeability for water. Although abscisic acid (ABA) concentrations in the roots increased after the root excision treatment, there was no evidence for any enhanced concentration in the xylem sap. The possible role that this accumulation of ABA in roots may have in the apparent increase in hydraulic conductivity after root excision is discussed. [ABSTRACT FROM AUTHOR]

Abstract Shoot and root growth are differentially sensitive to water stress. Interest in the involvement of hormones in regulating these responses has focused on abscisic acid (ABA) because it accumulates in shoot and root tissues under water-limited conditions, and because it usually inhibits growth when applied to well-watered plants. However, the effects of ABA can differ in stressed and non-stressed plants, and it is therefore advantageous to manipulate endogenous ABA levels under water-stressed conditions. Studies utilizing ABA-deficient mutants and inhibitors of ABA synthesis to decrease endogenous ABA levels, and experimental strategies to circumvent variation in plant water status with ABA deficiency, are changing the view of the role of ABA from the traditional idea that the hormone is generally involved in growth inhibition. In particular, studies of several species indicate that an important role of endogenous ABA is to limit ethylene production, and that as a result of this interaction ABA may often function to maintain rather than inhibit shoot and root growth. Despite early speculation that interaction between these hormones may influence many of the effects of water deficit, this topic has received little attention until recently. [ABSTRACT FROM AUTHOR]

ABSTRACTPolypeptide synthesis and accumulation were examined in the roots of tomato seedlings exposed to a polyethylene glycol-imposed water deficit stress. In these roots, the synthesis of a number of polypeptides was induced, while that of several others was enhanced or repressed. To examine the role played by abscisic acid (ABA) in co-ordinating the accumulation of these proteins, water-deficit-stress-responsive polypeptide synthesis was investigated in the roots of the ABA-deficient mutant flacca. In the roots of this mutant, the ability to accumulate a complete set of water-deficit-stress-responsive polypeptides was impaired, indicating that ABA is required for their synthesis. The role of ABA was further examined by exposing the roots of both genotypes to exogenous ABA, which, with one exception, elicited the accumulation of all water-deficit-stress-responsive proteins. Polyethylene glycol-induced polypeptide accumulation was accompanied by a 1·6-fold increase in the level of endogenous ABA in the roots of wild-type plants and a 5-fold increase in the roots of flc. Thus, although the absolute level was lower than that of the wild-type, flc has the capacity to accumulate ABA in its roots. When fluridone was used to prevent the biosynthesis of ABA, the accumulation of several water-deficit-stress-responsive polypeptides was reduced further. The synthesis of polypeptides was also examined in the roots of salt-treated seedlings. Salt altered the accumulation of several polypeptides, all of which were previously observed in water-deficit-stressed roots, indicating that their synthesis was the result of the osmotic component of the salt stress. However, the accumulation of these polypeptides was not impaired in flc roots, indicating that the role played by ABA in regulating their accumulation in salt-and polyethylene glycol-treated roots differs. As such, salt- and water-deficit-stress-induced changes in gene expression may be effected by different mechanisms, at least at the level of polypeptide accumulation. [ABSTRACT FROM AUTHOR]

Three-month-old Cedrella odorata were exposed to a soil-drying treatment. During this period, xylem sap was periodically collected from the plant by applying pneumatic pressure to the roots. This also allowed whole-plant water status to be measured by recording the balancing pressure applied. The concentration of ABA in xylem sap (C) was related to the whole-plant transpiration rate (V) which was measured witha sap flow gauge. The analysis of these paired measurements centred on how the reciprocal of C (R) varied with respect to V. This revealed that (1) the observed increases in C could not be explained by the reductions in V alone, (2) initially, decreases in V were associated with proportional increases in the whole-plant ABA flux (M), and (3) this relationship broke down at low values of V since zero flow was associated with a finite value for C estimated to be 41 pmol ABA mmol-1 H2O. A simple static model is developed from the observations that is able to explain the data well, and the results are discussed in terms of the effects of ABA on stomatal conductance (gsw). [ABSTRACT FROM AUTHOR]

Cytosolic Ca2+ ([Ca2+]i) and elongation growth were measured in the roots of Arabidopsis thaliano. Exposure of plant tissues to high NaCI and abscisic acid (ABA) concentrations results in a reduction in the rate of growth but the mechanism by which growth is inhibited is not understood. Both NaCI and ABA treatments are known to influence [Ca2+, and in this study we measured the effects of salinity and ABA on [Ca2+]i in cells from the meristematic region of Arabidopsis roots. The Ca2+-sensitive dye Fura-2 and ratiometric techniques were used to measure in cells of the root meristem region. Resting [Ca2+]i was found to he between 100 and 200 μmol m-3 in roots of untreated plants. Resting [Ca2+] ichanged in response to changes in the [Ca2+] surrounding growing roots. An increase of external [Ca2+ increased [Ca2+] conversely, a decrease of external [Ca2+] decreased [Ca2+. Exposure of roots to NaCl caused a rapid reduction of [Ca2+]i, a response that was proportional to the external NaCI concentration. Thus, as the NaCl concentration was increased, [Ca2+]i in root meristematic cells decreased. Root elongation was also inhibited in proportion to the external NaCI concentration, with maximal inhibition occurring at 120 mol m-3 NaCI. The [Ca2+]i of root meristem cells also changed in response to ABA. and the magnitude of the effect of ABA was dependent upon ABA concentration. Treatment with 0.2 mmol m-3 ABA caused a momentary increase in [Ca2+]i followed by a decrease after 15 min, but 10 mmol m-3 ABA caused an immediate decline in [Ca2+]i. There was a strong positive correlation between [Ca2+]i and root elongation rates. Experiments with the ABA-deficient Arabidopsis mutant aba-3 indicated that the reduction in [Ca2+]i brought about by NaCI was unlikely to he mediated via changes in endogenous ABA. Experiments with solutes such as sorbitol, KCI and NaNO3 indicated that the effects of NaCl could be mimicked by other solutes and was not specific for NaCl. [ABSTRACT FROM AUTHOR]

The inhibitory effects of PEG on whole-plant growth can exceed the effects of other osmolytes such as NaCl, and this has been ascribed to toxic contaminants, or to reduced oxygen availability in PEG solutions. We investigated another possibility, namely that PEG has an additional inhibitory effect on root water transport which in turn affects leaf development. The effects on first-leaf growth of applications of PEG 6000 or iso-osmotic NaCl to the roots were determined using hydroponically grown maize (Zea mays L.) seedlings. Leaf growth rates were inhibited within minutes of PEG application to the roots and remained inhibited for days. The inhibitory effects on growth of NaCl, and also of KCl and mannitol, were much smaller. The comparative effects of NaCl and PEG on root water transport were determined by assaying pressurized flow through excised roots. PEG induced a 7-fold greater inhibition of flow through live roots than NaCl. Killing of the roots by heat treatment, to reduce cell membrane resistances to solute penetration, nearly doubled the flow rate for roots in NaCl, but not for roots in PEG. We suggest that the greater viscosity of PEG solutions, as compared with NaCl, may be a primary factor contributing to the additional inhibition of water flow through live and killed roots. PEG did not have additional effects on leaf turgor but had a 3 times greater inhibitory effect than NaCl on the irreversible extensibility of the leaves and induced 16 times more leaf accumulation of the growth inhibitory stress hormone abscisic acid (ABA). We conclude that greater inhibition of root water transport by PEG 6000, as compared with NaCl, leads to additional reductions in extensibility, additional ABA accumulation, and a greater inhibition of leaf growth. [ABSTRACT FROM AUTHOR]

Root exudates were sampled from detopped root systems of castor bean (Ricinus communis). Different volume flux rates were imposed by changing the pneumatic pressure around the root system using a Passioura-type pressure chamber. The concentrations of cations, anions, amino acids, organic acids and abscisic acid decreased hyperbolically when flux rates increased from pure root exudation up to values typical for transpiring plants. Concentrations at low and high fluxes differed by up to 40 times (phosphate) and the ratio of substances changed by factors of up to 10. During the subsequent reduction of flux produced by lowering the pneumatic pressure in the root pressure chamber, the concentrations and ratios of substances deviated (at a given flux rate) from those found when flux was increased. The flux dependence of exudate composition cannot therefore be explained by a simple dilution mechanism. Xylem sap samples from intact, transpiring plants were collected using a Passioura-type root pressure chamber. The concentrations of the xylem sap changed diurnally. Substances could be separated into three groups: (1) calcium, magnesium and amino acid concentrations correlated well with the values expected from their concentration-flux relationships, whereas (2) the concentrations of sulphate and phosphate deviated from the expected relationships during the light phase, and (3) nitrate and potassium concentrations in intact plants varied in completely the opposite manner from those in isolated root systems. Abscisic acid concentrations in the root exudate were dependent on the extent of water use and showed strong diurnal variations in the xylem sap of intact plants even in drought-stressed plants. Calculations using root exudates overestimated export from the root system in intact plants, with the largest deviation found for proton flux (a factor of 10). We conclude that root exudate studies cannot be used as the sole basis for estimating fluxes of substances in the xylem of intact plants. Consequences for studying and modelling xylem transport in whole plants are discussed. [ABSTRACT FROM AUTHOR]

Stomatal conductance, leaf water potential, soil water potential and concentration of abscisic acid (ABA) in the xylem sap were measured on maize plants growing in the field, in two treatments with contrasting soil structures. Soil compaction affected the stomatal conductance, but this effect was no longer observed if the soil water potential was increased by irrigation. Differences in leaf water potential did not account for the differences in conductance between treatments. Conversely, the relationship between stomatal conductance and concentration of ABA in the xylem sap was consistent during the experiment. The proposed interpretation is that stomatal conductance was controlled by the root water potential via an ABA message. Control of the stomatal conductance by the leaf water potential or by an effect of mechanical stress on the roots in unlikely. [ABSTRACT FROM AUTHOR]

It is now clear that drying of the soil does not always result in an early change in shoot water status. This may be because stomata close and leaf growth slows to reduce water loss. When this is the case, it is necessary to ask how the change in soil water status has been 'sensed' by the shoot. The current view is that soil drying results in some type of chemical signalling between roots and shoots. The sensitivity of the response and experiments involving the manipulation of small parts of root systems suggest that the signalling involves more than a simple change in root activity in response to soil drying. In this paper, we consider the evidence for chemical signalling between roots and shoots and discuss the possible candidates for such signals. In some plants, root-sourced ABA can apparently influence shoot physiology and growth in the absence of any perturbation of shoot water relations. The ABA produced is quantitatively sufficient to account for the responses observed. Applied ABA can mimic many of the effects of soil drying on plants, including effects at the plasma membrane and on gene expression. Perhaps uniquely, ABA seems to be involved in signalling between different plant organs, and in signalling at the transmembrane and genome levels. We review the effects of ABA on leaf cells with a view to gaining some understanding of how soil drying may influence plant development. [ABSTRACT FROM AUTHOR]

Maize plants were grown in l-m-long tubes of John Innes No. 2 potting compost. From the start of the experimental period, half of the plants were unwatered. Stomatal conductance of these plants was restricted 6d after last watering and continued to decline thereafter. This was despite the fact that as a result of solute accumulation, unwatered plants showed consistently higher leaf turgors than wellwatered plants. Leaf water potentials of unwatered plants were not significantly lower than those of plants that were watered well. Main seminal and nodal roots showed solute regulation in drying soil and continued to grow even in the driest soil, and plants growing in drying soil showed consistently higher root dry weights than did well-watered plants. Water potentials and turgors of the tips of fine roots in the upper part of the column decreased as the soil dried. Soil drying below a water content of around 0.25 g cm[SUP-3] (a bulk soil water potential of between -0.2 and -0.3MPa) resulted in a substantial increase in the ABA content of roots. As soil columns dried progressively from the top, ABA content increased in roots deeper and deeper in the soil. These responses suggest that ABA produced by dehydrating roots and which was subsequently transported to the shoots provided a sensitive indication of the degree of soil drying. [ABSTRACT FROM AUTHOR]

Experiments with isolated roots of wheat plants suggested that when water uptake rates are low, low concentrations of abscisic acid (ABA) may increase the flux of water into roots. This increase was recorded despite an ABA-stimulated reduction in the hydraulic conductance of the whole root system. Hydraulic conductances were measured under steady state conditions. A system is described where the stomatal behaviour and water movement through roots of a single intact plant may be concurrently monitored. Experiments with intact plants confirmed that application of ABA could increase the rate of water movement into roots when uptake rates were low. No such increase was observed at high flux rates. Application of ABA to roots caused partial stomatal closure and caused conductance to oscillate around a reduced mean value. An ABA-stimulated increase in the turgor sensitivity of stomata is postulated and the significance of this effect is discussed. [ABSTRACT FROM AUTHOR]