Xylem ABA Concentration Research Articles

Alternation of wet and dry sides during partial rootzone drying irrigation enhances leaf ethylene evolution

Soil drying increases endogenous ABA and ACC concentrations in planta, but how these compounds interact to regulate stomatal responses to soil drying and re-watering is still unclear. To determine the temporal dynamics and physiological significance of root, xylem and leaf ABA and ACC concentrations in response to deficit irrigation (DI) or partial rootzone drying (PRD-F) and re-watering, these variables were measured in plants exposed to similar whole pot soil water contents. Both DI and PRD-F plants received only a fraction of the irrigation supplied to well-watered (WW) plants, either to all (DI) or part (PRD-F) of the rootzone of plants grown in split-pots. Both DI and PRD-F induced partial stomatal closure, increased root ABA and ACC accumulation consistent with local soil water content, but did not affect xylem or leaf concentrations of these compounds compared to WW plants. Two hours after re-watering all (DI-RW) or part of the rootzone (PRD-A) to the same soil water content, stomatal conductance returned to WW values or further decreased respectively. Re-watering the whole rootzone had no effect on xylem and leaf ABA and ACC concentrations, while re-watering the dry side of the pot in PRD plants had no effect on xylem and leaf ABA concentrations but increased xylem and leaf ACC concentrations and leaf ethylene evolution. Leaf water potential was similar between all irrigation treatments, with stomatal conductance declining as xylem ABA concentrations and leaf ACC concentrations increased. Prior to re-watering PRD plants, accounting for the spatial differences in soil water uptake best explained variation in xylem ACC concentration suggesting root-to-shoot ACC signalling, but this model did not account for variation in xylem ACC concentration after re-watering the dry side of PRD plants. Thus local (foliar) and long-distance (root-to-shoot) variation in ACC status both seem important in regulating the temporal dynamics of foliar ethylene evolution in plants exposed to PRD.

Stem girdling uncouples soybean stomatal conductance from leaf water potential by enhancing leaf xylem ABA concentration

To understand the impact of shoot-to-root ABA transport on water potential of, and xylem ABA concentration in, different plant tissues during soil drying, soybean (Glycine max cv. Siverka) plants were subjected to drought and girdling in a factorial experiment. Girdling was achieved by surgically excising the phloem tissue from just above the cotyledonary node. After girdling and withholding water, ABA concentrations were determined in xylem saps extracted from individual leaves, detached shoots and de-topped roots, after measuring stomatal conductance (gs), tissue water potentials, and root ABA concentrations. Soil drying decreased water potential throughout the plant and approximately doubled xylem ABA concentrations, coinciding with stomatal closure. Girdling slightly enhanced water potential, especially in droughted plants. Girdling diminished the soil-drying induced increase in xylem sap ABA concentration, and completely prevented root tissue ABA accumulation. Furthermore, girdling decreased root ABA concentration and increased leaf xylem ABA concentration of well-watered (WW) plants. Stomatal conductance declined linearly with leaf water potential only in intact plants, while gs declined as leaf xylem ABA concentration increased, independently of girdling. Thus shoot to root ABA transport not only determines (soil-drying induced) root ABA accumulation, but also limits ABA accumulation in the shoot to maintain stomatal opening of WW plants.

Physiological response of post-veraison deficit irrigation strategies and growth patterns of table grapes (cv. Crimson Seedless)

To determine whether partial root-zone drying (PRD) offers physiological advantages compared with regulated deficit irrigation (RDI), a 3 year long-experiment was conducted on a commercial vineyard of ‘Crimson Seedless’ table grapes (Vitis vinifera L.). Four different drip irrigation treatments were imposed: (i) a Control treatment irrigated at 110% of seasonal crop evapotranspiration (ETc), (ii), a regulated deficit irrigation (RDI) treatment irrigated similar to Control before veraison and at 50% of the Control treatment post-veraison, (iii) a partial root-zone drying (PRD) irrigated similar to RDI but alternating (every 10–14 days) the dry and wet side of the root-zone, and (iv) a null irrigation treatment (NI) which only received the natural precipitation and occasional supplementary irrigation when midday stem water potential (Ψs) dropped below −1.2 MPa. Post-veraison, PRD vines accumulated greater localized soil and plant water deficit at midday than RDI vines, but maintained similar pre-dawn water potential (Ψpd) values. Stomatal conductance (gs) of PRD vines remained high, likely because there was sufficient root water uptake from irrigated soil. Xylem ABA concentration ([ABA]xylem) did not change yet intrinsic WUE (WUEi) decreased compared to RDI vines, probably because PRD induced greater root density and root development at depth, allowing greater water uptake from roots in the wet part of the soil profile. Vegetative growth was only decreased by severe deficit irrigation (NI) although total leaf area index (LAI) was also affected by PRD in the 1st and 3rd year.. PRD can be considered a useful strategy in semiarid areas with limited water resources because sustained water use maintained assimilation rates despite greater stress than conventional RDI strategy, which may be related to root and morphological adjustment.

Different irrigation regimes affect xylem ABA concentrations and the physical berry quality of table grapes at harvest and during postharvest life

Different irrigation regimes affect xylem ABA concentrations and the physical berry quality of table grapes at harvest and during postharvest life

Enhanced root growth of the brb (bald root barley) mutant in drying soil allows similar shoot physiological responses to soil water deficit as wild-type plants.

The genetics, molecular biology and nutrient uptake of plant root hair mutants have been studied in detail, but their physiological responses to soil drying have not. Thus, the root hairless brb (bald root barley) barley (Hordeum vulgare L.) mutant and its wild type (WT) were grown in drying soil. Well-watered, pre-tillering plants showed no genotypic differences in daily transpiration and leaf elongation rate, and the ratio of day to night leaf elongation (D/N, a sensitive indicator of water stress). After withholding water for 25 days, root hydraulic conductivity and xylem ABA concentration were similar between genotypes, but WT plants had more tillers and D/N was more than halved in brb. To avoid possible developmental and nutritional differences confounding responses to water deficit, pre-tillering plants were allowed to dry soils of high and low phosphorus (P) status. Although leaf area, leaf water potential and shoot fresh weight (FW) were similar in the two genotypes, root FW of brb was greater by 44 and 18% in a high and low P soil respectively. This adaptive response allowed brb to maintain similar shoot growth and transpiration as WT plants, despite decreased effective root surface area in the absence of root hairs.

Sap fluxes from different parts of the rootzone modulate xylem ABA concentration during partial rootzone drying and re-wetting.

Previous studies with partial rootzone drying (PRD) irrigation demonstrated that alternating the wet and dry parts of the rootzone (PRD-Alternated) increased leaf xylem ABA concentration ([X-ABA]leaf) compared with maintaining the same wet and dry parts of the rootzone (PRD-Fixed). To determine the relative contributions of different parts of the rootzone to this ABA signal, [X-ABA]leaf of potted, split-root tomato (Solanum lycopersicum) plants was modelled by quantifying the proportional water uptake from different soil compartments, and [X-ABA]leaf responses to the entire pot soil-water content (θpot). Continuously measuring soil-moisture depletion by, or sap fluxes from, different parts of the root system revealed that water uptake rapidly declined (within hours) after withholding water from part of the rootzone, but was rapidly restored (within minutes) upon re-watering. Two hours after re-watering part of the rootzone, [X-ABA]leaf was equally well predicted according to θpot alone and by accounting for the proportional water uptake from different parts of the rootzone. Six hours after re-watering part of the rootzone, water uptake by roots in drying soil was minimal and, instead, occurred mainly from the newly irrigated part of the rootzone, thus [X-ABA]leaf was best predicted by accounting for the proportional water uptake from different parts of the rootzone. Contrary to previous results, alternating the wet and dry parts of the rootzone did not enhance [X-ABA]leaf compared with PRD-Fixed irrigation. Further work is required to establish whether altered root-to-shoot ABA signalling contributes to the improved yields of crops grown with alternate, rather than fixed, PRD.

Physiological and gene expression responses of sunflower (Helianthus annuus L.) plants differ according to irrigation placement

To investigate effects of soil moisture heterogeneity on plant physiology and gene expression in roots and leaves, three treatments were implemented in sunflower plants growing with roots split between two compartments: a control (C) treatment supplying 100% of plant evapotranspiration, and two treatments receiving 50% of plant evapotranspiration, either evenly distributed to both compartments (deficit irrigation – DI) or unevenly distributed to ensure distinct wet and dry compartments (partial rootzone drying – PRD). Plants receiving the same amount of water responded differently under the two irrigation systems. After 3 days, evapotranspiration was similar in C and DI, but 20% less in PRD, concomitant with decreased leaf water potential (Ψleaf) and increased leaf xylem ABA concentration. Six water-stress responsive genes were highly induced in roots growing in the drying soil compartment of PRD plants, and their expression was best correlated with local soil water content. On the other hand, foliar gene expression differed significantly from that of the root and correlated better with xylem ABA concentration and Ψleaf. While the PRD irrigation strategy triggered stronger physiological and molecular responses, suggesting a more intense and systemic stress reaction due to local dehydration of the dry compartment of PRD plants, the DI strategy resulted in similar water savings without strongly inducing these responses. Correlating physiological and molecular responses in PRD/DI plants may provide insights into the severity and location of water deficits and may enable a better understanding of long-distance signalling mechanisms.

Two potato (Solanum tuberosum) varieties differ in drought tolerance due to differences in root growth at depth.

To test the hypothesis that root growth at depth is a key trait explaining some genotypic differences in drought tolerance in potato (Solanum tuberosum L.), two varieties (Horizon and Maris Piper) differing in drought tolerance were subjected to different irrigation regimes in pots in a glasshouse and in the field under a polytunnel. In the glasshouse, both cultivars showed similar gas exchange, leaf water potential, leaf xylem ABA concentration and shoot biomass independently of whether plants were grown under well watered or water deficit conditions. Under well watered conditions, root growth was three-fold higher in Horizon compared with Maris Piper, 3 weeks after emergence. Water deficit reduced this difference. In the polytunnel, applying 60% or less irrigation volume compared with full irrigation significantly decreased tuber yield in Maris Piper but not in Horizon. This was coincident with the higher root density of Horizon in deep soil layers (>40cm), where water content was stable. The results suggest that early vigorous root proliferation may be a useful selection trait for maintaining yield of potato under restricted irrigation or rainfall, because it rapidly secures access to water stored in deep soil layers. Although selecting for vigorous root growth may assist phenotyping screening for drought tolerance, these varieties may require particular environmental or cultural conditions to express root vigour, such as sufficiently deep soils or sufficient water shortly after emergence.

Xylem sap collection and extraction methodologies to determine in vivo concentrations of ABA and its bound forms by gas chromatography-mass spectrometry (GC-MS)

BackgroundAccurate quantification of xylem sap ABA concentrations is important to underpin models of root-to-shoot ABA signalling to predict the physiological effects of soil drying. Growing tomato plants in a whole plant pressure chamber allowed sequential xylem sap collection from a detached leaf, the petiole stub of an otherwise intact plant and finally the de-topped root system of the same plant, to determine the impact of xylem sap sampling methodology on xylem ABA concentration. Since xylem sap can contain bound forms of ABA, a novel gas chromatography-mass spectrometry (GC-MS) procedure was developed to chemically separate free ABA from two in planta bound ABA forms known as Adducts I and II and ABA-glucose-ester (ABA-GE).ResultsXylem sap ABA concentrations were highly dependent on the sampling methodology used: the highest concentrations were detected in sap collected by applying an overpressure to detached leaves following the measurement of leaf water potential. Irrespective of xylem sap source, the wild-type cultivars Ailsa Craig and Rheinlands Ruhm had higher free ABA concentrations than a range of ABA-deficient mutants (notabilis, flacca and sitiens). However, in the mutants, concentrations of bound forms of ABA were similar to wild-type plants, and similar to free ABA concentrations.ConclusionsAlthough xylem concentrations of these bound ABA forms and ABA-GE suggest they have a limited physiological impact on ABA homeostasis in tomato, the methods developed here will allow a more complete understanding of ABA biochemistry and root-to-shoot signalling in species known to have higher concentrations of these compounds.

Partial rootzone drying increases water-use efficiency of lemon Fino 49 trees independently of root-to-shoot ABA signalling.

To determine whether irrigation strategy altered the sensitivity of Citrus leaf gas exchange to soil, plant and atmospheric variables, mature (16-year-old) Fino 49 lemon trees (Citrus limon (L.) Burm. fil. grafted on Citrus macrophylla Wester) were exposed to three irrigation treatments: control (irrigated with 100% of crop potential evapotranspiration, ETc), deficit irrigation (DI) and partial rootzone drying (PRD) treatments,which received 75% ETc during the period of highest evaporative demand and 50% ETc otherwise. Furthermore, to assess the physiological significance of root-to-shoot ABA signalling, the seasonal dynamics of leaf xylem ABA concentration ([X-ABA]leaf) were evaluated over two soil wetting-drying cycles during a 2-week period in summer. Although stomatal conductance (gs) declined with increased leaf-to-air vapour pressure deficit (LAVPD), lower leaf water potential and soil water availability, [X-ABA]leaf was only related to stomatal closure in well irrigated trees under moderate (<2.5kPa) atmospheric vapour pressure deficit (VPD). Differences in [X-ABA]leaf were not detected between treatments either before or immediately after (<12h) rewatering the dry side of PRD trees. Leaf water potential was higher in control trees, but decreased similarly in all irrigation treatments as daily LAVPD increased. In contrast, DI and PRD trees showed lower stomatal sensitivity to LAVPD than control trees. Although DI and PRD decreased stomatal conductance and photosynthesis, these treatments did not significantly decrease yield, but PRD increased crop water use efficiency (WUE) by 83% compared with control trees. Thus PRD-induced enhancement of crop WUE in a semiarid environment seems to involve physiological mechanisms other than increased [X-ABA]leaf.

Root water potential integrates discrete soil physical properties to influence ABA signalling during partial rootzone drying

To investigate the influence of different growing substrates (two mineral, two organic) on root xylem ABA concentration ([ABA](root)) and the contribution of the drying root system to total sap flow during partial rootzone drying (PRD), sunflower (Helianthus annuus L.) shoots were grafted onto the root systems of two plants grown in separate pots. Sap flow through each hypocotyl was measured below the graft union when one pot ('wet') was watered and other ('dry') was not. Each substrate gave unique relationships between dry pot matric potential (Psi(soil)), volumetric water content ((v)) or penetrometer resistance (Q) and either the fraction of photoperiod sap flow from roots in drying soil or [ABA](root). However, decreased relative sap flow, and increased [ABA](root), from roots in drying soil varied with root water potential (Psi(root)) more similarly across a range of substrates. The gradient between Psi(soil) and Psi(root) was greater in substrates with high sand or peat proportions, which may have contributed to a more sensitive response of [ABA](root) to Psi(soil) in these substrates. Whole plant transpiration was most closely correlated with the mean Psi(soil) of both pots, and then with detached leaf xylem ABA concentration. Although Psi(root) best predicted decreased relative sap flow, and increased [ABA](root), from roots in drying soil across a range of substrates, the inaccessibility of this variable in field studies requires a better understanding of how measurable soil variables (Psi(soil), (v), Q) affect Psi(root).

Genetic analysis of physiological components of salt tolerance conferred by Solanum rootstocks. What is the rootstock doing for the scion?

Grafting desirable crop varieties on stress-tolerant rootstocks provides an opportunity to increase crop salt tolerance. Here, a commercial hybrid tomato variety was grafted on two populations of recombinant inbred lines developed from a salt-sensitive genotype of Solanum lycopersicum var. cerasiforme, as female parent, and two salt-tolerant lines, as male parents, from S. pimpinellifolium, the P population, and S. cheesmaniae, the C population, to identify an easy screening method for identifying rootstocks conferring salt tolerance in terms of fruit yield. Potential physiological components of salt tolerance were assessed in the scion: leaf biomass, [Na(+)], nutrition, water relations and xylem ABA concentration. A significant correlation between scion fruit yield and scion leaf fresh weight, water potential or the ABA concentration was found in the C population under salinity, but the only detected QTL did not support this relationship. The rootstocks of the P population clearly affected seven traits related to the sodium, phosphorous and copper concentrations and water content of the scion leaf, showing heritability estimates around 0.4 or higher. According to heritability estimates in the P population, up to five QTLs were detected per trait. QTLs contributing over 15% to the total variance were found for P and Cu concentrations and water content of the scion leaf, and the proportion of fresh root weight. Correlation and QTL analysis suggests that rootstock-mediated improvement of fruit yield in the P population under salinity is mainly explained by the rootstock's ability to minimise perturbations in scion water status.

MEASURING SAP FLOW IN "TWO ROOT-ONE SHOOT" GRAFTED PLANTS TO MODEL SHOOT XYLEM ABA CONCENTRATION DURING PARTIAL ROOTZONE DRYING

Quantifying sap fluxes and ABA concentrations of xylem saps collected from different parts of the root system may assist in modelling leaf xylem ABA concentration ([X-ABA](leaf)) of plants grown with heterogeneous soil moisture distribution. To investigate the contribution of different parts of the root system to total sap flow and [X-ABA](leaf), individual shoots were grafted onto the root systems of two plants grown in two separate compartments and sap flow sensors placed on each hypocotyl below the graft union. Two different irrigation regimes were imposed: conventional deficit irrigation (DI) where both pots received the same volumes of water, and partial rootzone drying (PRD) where only one pot (designated wet) was watered and other (designated dry) dried the substrate. In plants grown in 3 L pots that were vertically divided into two watertight compartments, soil water content (theta) and sap flow were continuously monitored. In plants grown in 2 appressed 0.43 L pots, xylem sap was collected independently from each root system (after severing the shoot below the graft union) to measure root xylem ABA concentration. During PRD, sap flow from roots in drying soil decreased: this relationship was described by a threshold theta(dry) (at which sap flow from roots in drying soil started to decrease) and sensitivity (the rate of decline in sap flow). The threshold theta(dry) varied with both theta(wet) and soil volume. As theta(wet) increased, the threshold theta(dry) increased but the sensitivity of sap flow to theta(dry) was independent of theta(wet). At a similar theta(wet), as soil volume increased, the threshold theta(dry) decreased. Root xylem ABA concentration increased as theta declined in both DI and PRD plants. A simple model that weighted the ABA contributions of wet and dry root systems to [X-ABA](leaf) according to the sap flow from each, showed that with moderate soil drying, simulated [X-ABA](leaf) was higher under PRD than DI.

Partial phenotypic reversion of ABA-deficient flacca tomato (Solanum lycopersicum) scions by a wild-type rootstock: normalizing shoot ethylene relations promotes leaf area but does not diminish whole plant transpiration rate.

To evaluate the role of root-synthesized ABA in regulating growth and stomatal behaviour under well-watered conditions, isogenic wild-type (WT) and ABA-deficient flacca (flc) tomato (Solanum lycopersicum) were reciprocally and self-grafted just below the cotyledonary node. Since flc scions had lower leaf water potentials due to higher transpiration rates, a subset of all graft combinations was grown under a shoot misting treatment to minimize differences in shoot water status. Misting did not alter the relative effects of the different graft combinations on leaf area. WT scions had the greatest leaf area and lowest whole plant transpiration rate irrespective of the rootstock, implying that shoot ABA biosynthesis was sufficient to account for a WT shoot phenotype. In WT scions, the rootstock had no effect on detached leaf ethylene evolution or xylem concentrations of ABA or the ethylene precursor 1-aminocyclopropane-1-carboxylic acid (ACC). In flc scions, although the WT rootstock suppressed stomatal conductance of individual leaves, there was no detectable effect on whole plant transpiration rate. However, leaf area of flc/WT (scion/rootstock) plants increased 1.6-fold compared to flc self-grafts. WT rootstocks increased xylem ABA concentration in flc scions (relative to flc self-grafts) up to 3-fold, and resulted in xylem ACC concentrations and detached leaf ethylene evolution similar to WT scions. Since the WT rootstock normalized shoot ethylene relations but only partially restored the leaf area of flc scions (relative to that of WT scions), shoot ABA biosynthesis can directly promote leaf area via an unknown, ethylene-independent, mechanism.

The rhizosphere bacterium Variovorax paradoxus 5C-2 containing ACC deaminase does not increase systemic ABA signaling in maize (Zea mays L.)

Soil inoculation with the ACC deaminase-containing rhizobacterium Variovorax paradoxus 5C-2 increased pea (Pisum sativum) growth and yield in both well watered and drying soil, with an attenuation of systemic ACC signaling likely key in the latter case.1 However, inoculated plants also had increased xylem ABA concentrations (which may also promote growth) in drying soil. Possible mediation of ABA levels by V. paradoxus 5C-2 was investigated in two experiments in which maize (Zea mays) growth was promoted. Xylem ABA concentration of both inoculated and uninoculated plants increased similarly as leaf water potential decreased. Furthermore, hormone flow modeling showed a decreased phloem flow of ABA back to the root. Thus Variovorax paradoxus 5C-2 does not intensify ABA signaling in planta.

A quantitative approach to developing more mechanistic gas exchange models for field grown potato: A new insight into chemical and hydraulic signalling

In this study we introduce new gas exchange models that are developed under natural conditions of field grown potato. The new models could explain about 85% of the stomatal conductance variations, which was much higher than the well-known gas exchange models such as the Ball-Berry model [Ball, Woodrow, Berry, 1987. In: Nijhoff, M. (Eds.), Progress in Photosynthesis Research, vol. 4. Dordrecht, The Netherlands, pp. 5.221–5.224]. To overcome the limitations of previous models in simulating stomatal conductance when plants are exposed to drought stress, we proposed a down-regulating factor of chemical and hydraulic signalling on stomatal conductance as exp(− β[ABA])exp(− δ| ψ|) in which [ABA] and | ψ| are xylem ABA concentration and absolute value of leaf or stem water potential. In this study we found that stem water potential could be a very reliable indicator of how plant water status affects the stomatal conductance regulation. While previous models considered the same weighting for relative humidity and photosynthesis rate, we found that relative humidity has a more pronounced regulating effect on stomatal conductance than photosynthesis rate and the weightings for relative humidity and photosynthesis rate, respectively, were significantly higher and lower than unity.

Comparative effects of partial rootzone drying and deficit irrigation on growth and physiology of tomato plants

The effects of partial rootzone drying (PRD), deficit irrigation (DI), and full irrigation (FI) on tomato physiology were investigated. In PRD and DI plants, leaf water potential values and stomatal conductance were significantly lower, while xylem ABA concentration was greater compared to FI plants. Photosynthesis was similar for all treatments. Water use efficiency was improved by PRD and DI, which reduced fruit dry weight, but had no effect on dry weight of leaves and stems.

Accounting for sap flow from different parts of the root system improves the prediction of xylem ABA concentration in plants grown with heterogeneous soil moisture.

When soil moisture is heterogeneous, sap flow from, and ABA status of, different parts of the root system impact on leaf xylem ABA concentration ([X-ABA]leaf). The robustness of a model for predicting [X-ABA]leaf was assessed. ‘Two root-one shoot’ grafted sunflower (Helianthus annuus L.) plants received either deficit irrigation (DI, each root system received the same irrigation volumes) or partial rootzone drying (PRD, only one root system was watered and the other dried the soil). Irrespective of whether relative sap flow was assessed using sap flow sensors in vivo or by pressurization of de-topped roots, each root system contributed similarly to total sap flow during DI, while sap flow from roots in drying soil declined linearly with soil water potential (Ψsoil) during PRD. Although Ψsoil of the irrigated pot determined the threshold Ψsoil at which sap flow from roots in drying soil decreased, the slope of this decrease was independent of the wet pot Ψsoil. Irrespective of whether sap was collected from the wet or dry root system of PRD plants, or a DI plant, root xylem ABA concentration increased as Ψsoil declined. The model, which weighted ABA contributions of each root system according to the sap flow from each, almost perfectly explained [X-ABA] immediately above the graft union. That the model overestimated measured [X-ABA]leaf may result from changes in [X-ABA] along the transport pathway or an artefact of collecting xylem sap from detached leaves. The implications of declining sap flow through partially dry roots during PRD for the control of stomatal behaviour and irrigation scheduling are discussed.

Modelling the effects of soil water limitations on transpiration and stomatal regulation of cauliflower

Quantitative knowledge of the drought adaptation processes of crops is an important prerequisite for efficient irrigation management. To study the adaptation of stomatal resistance and transpiration of cauliflower to three different drought situations an experiment using containers with 220 l of soil volume was conducted with three different water supply regimes to obtain a wide variation of plant reactions in time and intensity. One model of transpiration and three models of stomatal conductance based on either soil water potential, leaf water potential, or root signals were developed and parameterised with the experimental data. Specific transpiration, i.e. transpiration per unit leaf area, could be well described with a model based on soil water potential, but the threshold water potential below which specific transpiration declined was dependent on the atmospheric evaporative demand, characterised by potential transpiration. Stomatal resistance of unstressed plants was dependent on the radiation environment, and the threshold soil water potential also increased with increasing atmospheric transpiration requirement. Models using leaf water potential or simulated stomatal closure based on xylem ABA concentration gave also satisfactory descriptions of the experimental data but had other shortcomings like high correlations between model parameters and difficult input requirements that limit their usefulness for the prediction of effects of water limitations.

Soil moisture heterogeneity during deficit irrigation alters root-to-shoot signalling of abscisic acid.

The effects of different irrigation techniques on leaf xylem ABA concentration ([X-ABA]leaf) were compared in tomato (Lycopersicon esculentum Mill.). During partial rootzone drying (PRD), water was distributed unevenly to the root system such that part was irrigated while the remainder was allowed to dry the soil. During conventional deficit irrigation (DI), plants received the same volume of water as PRD plants, but water was distributed evenly to the entire root system. When the plant root system was allowed to explore two separate soil compartments, DI plants had a higher [X-ABA]leaf than PRD plants with moderate soil drying, but PRD plants had a higher [X-ABA]leaf than DI plants as the soil dried further. The difference in [X-ABA]leaf between the two sets of plants was not because of differences in either whole pot soil water content (θpot) or leaf water potential (Ψleaf). To investigate the contribution of different parts of the root system to [X-ABA]leaf, individual shoots were grafted onto the root systems of two plants grown in two separate pots, so that the graft union had the appearance of an inverted 'Y'. After sap collection from detached leaves, removal of the shoot below the graft union allowed sap collection from each root system. Again, DI plants had a higher [X-ABA]leaf than PRD plants when the soil was relatively wet, but the opposite occurred as the soil dried. Root xylem ABA concentration ([X-ABA]root) increased exponentially as soil water content (θ) declined. In DI plants, [X-ABA]root from either pot (or the arithmetic mean of [X-ABA]root) accounted for a similar amount of the variation in [X-ABA]leaf. In PRD plants, [X-ABA]root from the watered side underestimated [X-ABA]leaf, whereas [X-ABA]root from the dry side overestimated [X-ABA]leaf. The arithmetic mean of [X-ABA]root best explained the variation in [X-ABA]leaf, implying continued sap flow from the dry part of the root system (Jdry) at soil water potentials (Ψsoil) at which Jdry had ceased in previous studies of PRD plants (Yao et al. 2001). Evaluating the relationship between Jdry and Ψsoil may assist in maintaining export of ABA (and other growth regulators) from the drying part of the root system, to achieve desirable horticultural outcomes during PRD.