Xylem Sap ABA Research Articles

Plant growth regulators and phloem exudates modulate root nodulation of soybean.

A time-course study was conducted on the effect of synthetic ABA and phloem exudate on root nodulation, nitrogenase activity, and dry matter content of pouch-grown soybean [Glycine max (L.) Merr.]. A hypernodulating mutant (NOD1-3) derived from the cultivar Williams was evaluated. The normally-nodulating cultivar Williams 82 served as the control. All phases of nodulation (nodule initiation, development and function) were inhibited by 1-50 μM synthetic ABA treatment of roots of pouch-grown soybean. The ABA effect was more pronounced for the treatment made 24 h prior to inoculation. The magnitude of inhibition of nodulation by ABA and by phloem exudate from inoculated Williams 82 was decreased when application was delayed to 48 h post-inoculation. Synthetic indoleacetic acid and benzyladenine (1 and 5 μM) also inhibited the number of nodules per plant, and the magnitude of inhibition was greater in the normally-nodulating Williams 82 cultivar than in the NOD1-3 hypernodulating mutant. Changes in endogenous free ABA in cotyledons, leaves, and xylem sap of seedlings were monitored at 24-h intervals after inoculation in both lines, and appeared to respond to inoculation. Addition of crude phloem exudate from Williams 82 or hypernodulating NOD1-3 was inhibitory to root nodulation of NOD1-3 - this effect was concentration dependent. Partially-purified (acidic aqueous fraction) phloem exudate from Williams 82 suppressed nodulation in NOD1-3 by 54% (averaged over a 7-d time-course), while partially-purified phloem exudate (acidic aqueous fraction) from NOD1-3 was much less inhibitory (23%). ABA levels in the phloem fractions applied were nearly identical (15.6 vs 16.0 ng mL-1), which indicates that ABA was not directly affecting nodulation control. Partially-purified extract (acidic aqueous fraction) from leaves of nodulated NOD1-3 was much less inhibitory to nodulation of NOD1-3 seedlings than was the comparable fraction from Williams 82. Although free ABA applied exogenously was capable of altering nodulation, it does not appear that endogenous ABA levels directly account for the autoregulatory control of nodulation in soybean. This implies that there may be factor(s) moving in the phloem other than ABA that possibly impact expression of root nodulation. Alternatively, it may be that plant development is responsive to interactions among plant growth regulators mediated by subtle changes in individual hormone concentrations. The possibility remains that normally-nodulating and hypernodulating lines have differential sensitivity to ABA, and that this explains altered nodulation control at similar levels of endogenous ABA.

Correlations of stomatal conductance with hydraulic and chemical factors in several deciduous tree species in a natural habitat.

Recent research in whole-plant stomatal physiology, conducted largely with potted plants in controlled environments, suggests that stomatal conductance (gs ) might be more closely linked to plant chemical variables than to hydraulic variables. To test this in a field situation, seasonal gs was examined in relation to a number of plant and environmental variables in 11 temperate, deciduous forest tree species. Stomatal conductance was generally better correlated with environmental variables (air temperature, vapor pressure deficit, PPFD) than with plant variables, and slightly better correlated with plant hydraulic variables (shoot water and osmotic potentials) than with plant chemical variables (xylem sap ABA concentration, xylem sap pH). We examined a model, developed previously for maize, which describes regulation of gs by xylem sap ABA concentration with leaf water status acting to modify stomatal sensitivity to the ABA signal. This model explained slightly more variation in seasonal gs in the forest trees than did single plant variables but not more variation than most single environmental variables. Response surface models, especially those incorporating environmental variables, were more consistently successful at explaining gs across species.

Root and xylem ABA changes in response to soil drying in two woody plants

Two woody plants,Platycladus orientalise (tolerant to drought) andAcacia auriculiformis (sensitive to drought), have been subjected to rapid and slow soil drying. ABA levels in their roots and xylem sap have been determined using radioimmunoassay (RIA, sensitivity is 0.4 pmol per assay vial) with a monoclonal antibody against ( + )-ABA. ABA contents ofP. orientalise andA. auriculiformis growing in well watered soil are 0.3 and 2.5 nmol · g ow −1 in roots and 1.6 and 0.4 μmol in xylem saps, respectively. A rapid soil drying has been applied to these two plants with soil water content (SWC) being reduced to 0.02 and 0.06 g · g ow −1 respectively. Under such treatment, ABA was increased by 22 times and 2 times in roots and by 7 times and 34 times in xylem saps respectively forP. orientalise andA. auriculiformis. After rewatering for 6 d, ABA in roots and xylem sap of both species returned to control levels. When a slow soil drying was applied, SWC was reduced to 0.1 and 0.13 g · g OW −1 respectively forP. orientalise andA. auriculiformis. ABA was increased by 5 times and 1.6 times in roots and by 6 times and 19 times in xylem saps respectively for these two plants. ABA in roots and xylem saps decreased to near control levels 8 d after watering. Plant leaf water potentials of both plants hardly changed at times when root and xylem ABA showed substantial increase in response to soil drying. It is concluded that ABA levels in the roots and xylem saps ofP. orientalise andA. auriculiformis are more sensitively regulated than leaf water potential in response to soil drying and can act as a chemical signal in root-shoot communications of the drought stress.

Concentration and Delivery of Abscisic Acid in Xylem Sap are Greater at the Shoot Base than at a Target Leaf Nearer to the Shoot Apex

Abstract: Samples of xylem sap from 5‐week‐old Ricinus corn‐munis L. were obtained after severing a lamina, or shoot, from plants pressurized at the roots with air to raise hydrostatic xylem water potentials to atmospheric. In situ sap flow gauges, and mass flow measurements, showed that removing the lamina approximately doubted sap flow rate through the petiole stub that remained attached to the plant. This was a consequence of flow out of the roots being diverted along this low‐resistance pathway and away from leaves higher in the canopy. Leaf and whole shoot excision temporarily released extra solutes in to sap as it discharged from the cut petiole or from the hypo‐cotyl stump. This contamination prevented the use of sap extracted from detached lamina by overpressurizing in a Scholan‐der bomb. To minimise distortions to sap flow and wound‐induced contamination, estimates of in planta concentration and delivery (concentration × sap flow rate) of ABA and osmolality in xylem sap were made using sap flow rates measured before excision and concentrations in flowing sap collected approximately 30 mm after excision. At this time, effects of excision on solute contamination had subsided. The approach revealed that withholding water from upper roots increased ABA delivery from roots into the shoot base 3‐fold. However, approximately half this ABA was lost en route to the youngest fully open leaf. This loss of ABA may explain the slow stomatal response to drying of upper roots shown by R. communis.

Drought tolerance, xylem sap abscisic acid and stomatal conductance during soil drying: a comparison of canopy trees of three temperate deciduous angiosperms.

Patterns of water relations, xylem sap abscisic acid concentration ([ABA]) and stomatal aperture were characterized and compared in drought-sensitive black walnut (Juglans nigra L.), less drought-sensitive sugar maple (Acer saccharum Marsh.) and drought-tolerant white oak (Quercus alba L.) trees co-occurring in a second-growth forest in Missouri, USA. There were strong correlations among reduction in predawn leaf water potential, increased xylem sap [ABA] and stomatal closure in all species. Stomatal conductance was more closely correlated with xylem sap ABA concentration than with ABA flux or xylem sap pH and cation concentrations. In isohydric black walnut, increased concentrations of ABA in the xylem sap appeared to be primarily of root origin, causing stomatal closure in response to soil drying. In anisohydric sugar maple and white oak, however, there were reductions in midday leaf water potential associated with stomatal closure, making it uncertain whether drought-induced xylem sap ABA was of leaf or root origin. The role of root-originated xylem sap ABA in these species as a signal to the shoot of the water status of the roots is, therefore, less certain.

Root restriction affects leaf growth and stomatal response: the role of xylem sap ABA

The effects of a restricted root zone (RRZ) on leaf and root growth, water relations, stomatal conductance ( g s) and xylem sap abscisic acid concentration ([ABA]) were studied in pepper ( Capsicum annuum cv Bellboy) plants. Plants were grown in soil columns of two different volumes (0.5 l (restricted root) and 2.25 l (unrestricted root) control) in a controlled environment growth cabinet. Another group of plants were grown without root restriction but a gradual soil drying treatment was imposed. Root restriction for 19 days reduced leaf and root growth almost to the rate shown by plants exposed to soil drying. Leaf water potential of RRZ plants was similar to that of unrestricted plants, but, g s was reduced in plants undergoing root restriction. The slight increase of xylem sap ABA concentration in RRZ plants did not fully account for the large reductions in leaf growth and g s.

Variability among species of stomatal control under fluctuating soil water status and evaporative demand: modelling isohydric and anisohydric behaviours

Stomatal control of species with contrasting stomatal behaviours have been investigated under natural fluctuations of evaporative demand and soil water status. Sunflower and barley (anisohydric behaviour) have a daytime leaf water potential (ΨI) which markedly decreases with evaporative demand during the day and is lower in droughted than in watered plants. In contrast, maize and poplar (isohydric behaviour) maintain a nearly constant ΨI during the day at a value which does not depend on soil water status until plants are close to death. Plants were also subjected to a range of soil water potentials under contrasting air vapour pressure deficits (VPD, from 0.5 to 3 kPa) in the field, in the greenhouse or in a growth chamber. Finally, plants or detached leaves were fed with varying concentrations of artificial ABA. Stomatal conductance of well-watered plants had no response to VPD when plants were grown in natural soils, suggesting that the opposite result observed in many laboratory experiments might be linked to the low unsaturated hydraulic conductivity of usual potting substrates. The response of stomatal conductance of all studied species to the concentration of ABA in pressurized xylem sap ([ABA]xyl) was the same whether ABA had an endogenous origin (droughted plants) or was artificially fed. However stomatal response of maize and poplar to [ABA]xyl markedly changed with varying evaporative demand or ΨI, whereas this was not the case in sunflower or barley. This suggests that isohydric behaviour is linked to an interaction between hydraulic and chemical information, while anisohydric behaviour is linked to an absence of interaction. In all cases, [ABA]xyl was related to soil water status with common relationships for different experimental conditions, but with markedly different responses among species. Diurnal variations of [ABA]xyl with evaporative demand were small in all studied species. Results are synthesized in a model which accounts for observed behaviours of gs, ΨI and [ABA]xyl in fluctuating conditions and for several species. The validity of this model, in particular the physiological meaning of [ABA]xyl, is discussed.

Relative importance of osmotic-stress and ion-specific effects on ABA-mediated inhibition of leaf expansion growth in Phaseolus vulgaris

Abbreviations: The osmotic and ion-specific components of salt-induced inhibition of leaf expansion growth were investigated in beans grown from 12 h to several days in either NaCl-containing solution cultures, an isosmotic concentrated macronutrient solution, or a vermiculite–compost mixture with low Na+ but high Cl– availability. Inhibition of leaf expansion and leaf ABA increase was more intense in the NaCl than in the isosmotic macronutrient treatment. Root Na+ was highly correlated to inhibition of leaf expansion and leaf or xylem sap ABA. When Na+ was sequestered in soil, salinized plants showed no reduction in leaf expansion or ABA increase, regardless of the presence of high leaf Cl– concentrations. Stomatal conductance exhibited an exponential relationship with the reciprocal value of xylem sap ABA. Our results indicate that an ion-specific effect caused by Na+ in roots may account for an ABA-mediated reponse of both stomatal closure and leaf expansion inhibition.

Whole‐plant ABA flux and the regulation of water loss in Cedrella odorata

ABSTRACTThree‐month‐old Cedrella odorata seedlings 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 with a 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).

Collection of Xylem Sap at Flow Rate Similar to in vivo Transpiration Flux

We have explored a method to collect xylem sap using a Scholander pressure chamber for potted plants. Intact root system in pots which fitted the pressure chamber was pressurised at a pneumatic pressure numerically equal to the absolute value of shoot water potential. The rate of xylem flow obtained from the stem stump under such pressure was found similar to the rate of transpiration before detopping. The rate of pressurised flow from detopped roots was linearly related to the pressure applied in both well-watered and soil-dried plants. The osmotic concentration of the xylem sap was negatively related to the rate of volume flow, suggesting the necessity to collect xylem sap at in vivo flow rate if original solute concentration is to be evaluated. The concentration of ABA in the xylem sap, however, did not show such a relationship with water flux. Both well-watered and soil-dried plants showed the concentration of ABA in xylem sap largely stable with a range of volume flow rate, indicating a linear relationship between the rate of ABA delivery through xylem and that of volume flow. We also compared the concentrations of ABA in xylem sap sequentially collected from pressurised roots with that from detached shoots of the same plants. The concentration of ABA in the initial saps from shoots showed to be similar to that from roots. However, a decrease in the concentration of ABA in the xylem sap collected from detached leaf or twig was observed when more volume of sap was collected, which might also be dependent on the plant species and the volume of xylem vessels concerned.

How Do Roots Control Xylem Sap ABA Concentration in Response to Soil Drying?

Maize (Zea mays L.) and Leucaena leucocephala (a tropical tree) were subjected to different degrees of soil drying and changes in their root and xylem ABA concentrations were measured simultaneously. Additional treatments such as defoliation, light sheltering and stem girdling were applied so that phloem transport into roots was manipulated. When soil was dried, ABA concentration in xylem sap increased almost linearly with that of roots in both species. Such an increase was relatively plateaued at high root ABA concentrations, suggesting that severe soil drying might have hindered some parts of roots from delivering ABA into xylem flow. When 3 H-ABA was load

ABA xylem concentrations determine maximum daily leaf conductance of field‐grown Vitis vinifera L. plants

ABSTRACTDifferences in maximum leaf conductance in grapevine plants growing in soils with contrasting water availabilities during mid‐summer in Portugal could be accounted for by differences in the concentration of ABA in xylem sap. This conclusion is reinforced by the observation that the relationship between leaf conductance and endogenous ABA concentration can be mimicked by the application of exogenous ABA to leaves detached from irrigated plants. During the day, leaf conductance decreased after a morning peak, even when the leaves remained in a constant environment at a moderate temperature and leaf‐to‐air vapour pressure difference. This decline in leaf conductance was not a consequence of an increase in the xylem ABA concentration or the rate of delivery of this compound by the transpiratory stream. The afternoon depression in leaf conductance was associated with an apparent limitation in stomatal opening potential, which persisted even when detached leaves were fed with water and rehydrated. The reason for this inhibition has still to be identified.

Root-shoot signalling in sunflower plants with confined root systems

Sunflower plants were grown hydroponically under controlled conditions with the root systems confined in small containers. Root confinement inhibited the growth of sunflower plants as indicated by reduction in both leaf and cotyledon area and root and shoot fresh weight. This effect was more pronounced in shoots. Root confinement favored the accumulation of potassium in the roots and shoots, and the exudation of potassium and water in excised roots. Xylem sap from root confined plants inhibited cotyledon expansion as revealed by bioassay with decapited sunflower seedlings. In addition decapited control plants incubated in ABA solution also showed cotyledon growth reduction. Xylem sap ABA analysis indicated a 7-times higher concentration in root confined than control plants. Our results suggest the synthesis of a chemical signal in the roots of plants subjected to mechanical stress which can be responsible for the inhibition of plant growth.

Characterization of a wilty sunflower (Helianthus annuusL.) mutant II. Water relations, stomatal conductance, abscisic acid content in leaves and xylem sap of plants subjected to water deficiency

The response of w-1, a wilty sunflower (Helianthus annuus L.) mutant, to water stress is described in comparison with the control line (W-1). Detached leaves of w-1 strongly dehydrated during the first 30 min without significant changes in leaf conductance, whereas W-1 responded rapidly to water loss by reducing stomatal aperture. After 2 h stress ABA increased slightly in w-1, while W-1 leaves showed a 20-fold increase. When water stress was imposed to potted plants by water withholding, w-1 quickly dehydrated, and lost turgor, while W-1 maintained positive turgor values for a longer period. Wild-type plants responded to small changes in leaf water potential by accumulating ABA and by closing stomata, whereas in the mutant significant changes in ABA content and in stomatal conductance were found only at very low water potentials. In another experiment in which water was withheld under high relative humidity, when soil water content started to decrease W-1 rapidly closed stomata in the absence of any change in leaf water status and the reduction in conductance was paralleled by a rise in xylem sap ABA concentration. By contrast the mutant started to accumulate ABA in the xylem sap and to close stomata when soil water content and leaf water potential were dramatically reduced. The low endogenous ABA levels and the inability to synthesize the hormone rapidly either in the leaves or in the roots seem to be responsible for the high sensitivity of w-1 to water stress.

Does Xylem Sap ABA Control the Stomatal Behaviour of Water-Stressed Sycamore (Acer pseudoplatanusL.) Seedlings?

The present experiment was designed to test the hypothesis that ABA is produced by the roots when they encounter drying soil and is transported to the shoot through the xylem sap to inhibit stomatal conductance quite independently of any change in the leaf water status. Sycamore seedlings were grown with their root systems divided equally between two containers. Water was withheld from one container while the other container was kept well-watered. Abscisic acid concentration in roots, xylem sap and leaves were evaluated. The role of xylem sap ABA as a sensitive indicator of the effect of soil drying is discussed

ABA AS A ROOT SIGNAL IN GRAPEVINES UNDER WATER STRESS

ABA implication in root signals of water stress has been suggested by several authors. To verify this hypothesis in grapevines, this experiment has been carried out. One-year-old own rooted cuttings of grapevine cultivar Cabernet Sauvignon were exposed to water stress. After three months of growth, water was completely withdrawn for nine days, till the plants reached the wilting point. The plants were then rewatered. During the whole period, root hydraulic conductivity was measured with a pressure bomb; xylem sap samples were collected, as well as leaf and root samples. ABA concentration in these samples was measured using Radio Immuno Assay with DBPA1, a monoclonal antibody for ABA. The concentration of xylem sap ABA was 68.2 mg m-3 at the start of the experiment. After eight days of stress it was 1863.6 mg m-3, 27 × higher. On the ninth day the plants were rewatered, and the xylem sap ABA decreased at 100.2 mg m-3, keeping this level for eight more days. Leaf ABA showed high levels of this inhibitor, with a peak in correspondence with the maximum stress. A similar behaviour was attained by roots. In grapevine, ABA seems to be involved in a water stress root signal directed to the canopy.

Changes in the concentration of ABA in xylem sap as a function of changing soil water status can account for changes in leaf conductance and growth

Abstract. Maize seedlings (Zea mays L. John Innes F1 hybrid) were grown in a greenhouse in l‐m‐long tubes of soil. When the plants were well established, water was withheld from half of the tubes. Control plants were watered every day during the 20‐d experimental period. The soil drying treatment resulted in a substantial restriction of stomatal conductance and a limitation in shoot growth, even though there was no detectable difference in the water relations of watered and unwatered plants. From day 7 of the soil drying treatment, xylem ABA concentrations (measured using the sap exuded from detopped plants) were substantially increased in unwatered plants compared to values recorded with sap from plants watered every day. Measurements of water potential through the profile of unwatered soil suggest that xylem ABA concentrations reflects the extent of soil drying. Leaf ABA content was a much less sensitive indicator of the effect of soil drying and during the whole of experimental period there was no significant difference between ABA concentration in leaves of well watered and unwatered plants. In a second set of experiments, ABA was fed to part of the roots of potted maize plants to manipulate xylem ABA concentration. These manipulations suggested that the increases in ABA concentration in xylem sap, which resulted from soil drying, were adequate to explain the observed variation in stomatal conductance and might also explain the restriction in leaf growth rate. These results are discussed in the light of recent work which suggests that stomatal responses to soil drying are partly attributable to an as‐yet unidentified inhibitor of stomatal opening.

Sequential response of whole plant water relations to prolonged soil drying and the involvement of xylem sap ABA in the regulation of stomatal behaviour of sunflower plants

SUMMARYSunflower plants (Helianihus animus cv. Tall Single Yellow} were grown in the greenhouse in drain pipes (100 mm inside diameter and 1 m long) rilled with John Innes No. 2 compost. When the fifth leaf had emerged, half of the plants were left unwatered for 6 days, rewatered for 2 days and then not watered for another 12 days. Measurements of water relations and abaxial stomatal conductance were made at each leaf position at regular intervals during the experimental period. Estimates were also made of soil water potentials along the soil profile and of ABA concentrations in xylem sap and leaves.Soil drying led to some reduction in stomatal conductance alter only 3 days but leaf turgors were not reduced until day 13 (6 days after rewatering). When the water relations of leaves did change, older leases became substantially dehydrated while high turgors were recorded in younger leaves. Leaf ABA content measured on the third youngest leaf hardly changed over the first 13 days of the experiment, despite substantial soil drying, while xylem ABA concentrations changed very significantly and dynamically as soil water status varied, even when there was no effect of soil drying on leaf water relations. We argue that the highest ABA concentrations in the xylem, found as a result of substantial soil drying, arise from synthesis in both the roots and the older leaves, and act to delay the development of water deficit in younger leases.In other experiments ABA solutions were watered on to the root systems of sunflower plants to increase ABA concentrations in xylem sap. The stomatal response to applied ABA was quantitatively very similar to that to ABA generated as a result of soil drying. There was a log‐linear relationship between the reduction of leaf conductance and the increase of ABA concentration m xylem sap.