Synthesis and metabolism of abscisic acid in detached leaves of Phaseolus vulgaris L. after loss and recovery of turgor

Metabolism of abscisic acid (ABA) was studied after wilting and upon recovery from water stress in individual, detached leaves of Phaseolus vulgaris L. (red kidney bean). Loss of turgor was correlated with accumulation of ABA and its metabolites, resulting in a 10-fold increase in the level of phaseic acid (PA) and a doubling of the level of conjugated ABA. The level of conjugated ABA in turgid leaves was no higher than that of the free acid. These results indicate that accumulation of ABA in wilted leaves resulted from a stimulation of ABA synthesis, rather than from a release from a conjugated form or from inhibition of the metabolism of ABA. The rate of synthesis of ABA was at its maximum between 2.5 and 5 h after turgor was lost, and slackened there-after. In wilted leaves, the rate of conversion of ABA to PA climbed steadly until it matched the rate of synthesis, after about 7.5 h. Upon rehydration of sections from wilted leaves, the rate of synthesis of ABA dropped close to zero within about 3 h, while the rate of conversion to PA accelerated. Formation of PA was two to four times faster than in sections maintained in the wilted condition; it reached a rate sufficient to convert almost one-half of the ABA present in the tissue to PA within 1 h. In contrast, the alternate route of metabolism of ABA, synthesis of conjugated ABA, was not stimulated by rehydration. The role of turgor in the stimulation of the conversion of ABA to PA was investigated. When leaves that had been wilted for 5 h were rehydrated to different degrees, the amount of ABA which disappeared, or that of PA which accumulated during the next 3 h, did not depend linearly on the water potential of the rehydrated leaf. Rather, re-establishment of the slightest positive turgor was sufficient to result in maximum stimulation of conversion of ABA to PA.Abbreviations ABA abscisic acid - DPA dihydrophaseic acid - PA phaseic acid - _leaf leaf water potential - osmotic pressure     

Abscisic acid metabolism —vacuolar/extravacuolar distribution of metabolites

The biotransformation of abscisic acid (ABA) was studied in cell suspension cultures of Lycopersicon esculentum. The ABA was converted by the cells to phaseic acid, nigellic acid, dihydrophaseic acid, abscisic acid--D-glucopyranosyl ester (ABA-Glc) and other ABA and phaseic acid conjugates. Investigation of their cellular distribution showed that the conjugated forms were located only in the vacuoles whereas ABA and its acidic metabolites were found mainly in the extravacuolar fractions. Our results, together with a number of studies on the increase of ABA-Glc as a response to stress, allow us to propose that ABA-Glc is irreversibly compartmented in the vacuoles of plant cells.Abbreviations ABA abscisic acid - ABA-Glc -D-glucopyranosyl ester of ABA - DPA 4-dihydrophaseic acid; nigellic acid=3-methyl-5-(1-hydroxy-2-hydroxymethyl-6-dimethyl-4-oxo-cyclohex-2-enyl)-penta-2Z, 4E-dienoic acid - PA phaseic acid     

Metabolism of R,S-[2-14C]abscisic acid by non-stressed and water-stressed detached leaves of wheat (Triticum aestivum L.)

Metabolism of R,S-[2-¹⁴C]abscisic acid (ABA) was studied in detached leaves of six wheat (Triticum aestivum) cultivars, using non-stressed leaves or leaves water stressed by desiccation to 90% of their original fresh weight. The rate constant of ABA metabolism was similar in nonstressed leaves of all cultivars. Water stress resulted in significantly lower rate constants in two cultivars which accumulated high levels of ABA when stressed, the constants decreasing by a factor of about 1.5. Rate constants for the remainder of the cultivars were not significantly different from those for the non-stressed controls. It was calculated that if decreased metabolism was the mechanism for the accumulation of ABA following water stress the rate constants of metabolism would have to be reduced by a factor of between 25 and 70. The results therefore support the hypothesis that enhanced synthesis rather than reduced degradation is the main process by which ABA levels are elevated following experimentally induced water stress. There were differences between the six cultivars in the products of ABA metabolism. Over the time period studied, oxidation to phaseic acid and dihydrophaseic acid as well as to other unidentified metabolites appeared to be the predominant pathway of ABA metabolism, rather than conjugation to ABA glucose ester and other more polar compounds.Abbreviations ABA abscisic acid - ABAGE abscisic-acid glucose ester - DPA dihydrophaseic acid - PA phaseic acid     

Abscisic acid levels and metabolism in the leaf epidermal tissue of Tulipa gesneriana L. and Commelina communis L.

We have shown the presence of abscisic acid (ABA) in abaxial epidermal strips taken from Tulipa gesneriana and Commelina communis and that the ABA level rises in the epidermis when leaves are water stressed. ABA levels had risen 50% in the abaxial epidermis of C. communis 30 min after the leaves lost 10% of their fresh weight. Epidermis from both T. gesneriana and C. communis metabolize [¹⁴C]ABA to several products probably including phaseic acid (PA) and dihydrophaseic acid (DPA).Abbreviations ABA abscisic acid - RIA radioimmunoassay - PA phaseic acid - DPA dihydrophaseic acid - TLC thin-layer chromatography - GC gas chromatography     

The relationship between stomatal resistance and abscisic-acid levels in leaves of water-stressed bean plants

Leaf water potentials of Phaseolus vulgaris L. plants exposed to a -3.0 bar root medium were reduced to between -7 and -9 bars within 25 min and remained constant for the next several hours. This treatment led to considerable variation between leaves in both abscisic-acid (ABA) content and R_s, although the two were well correlated after a 5-h treatment. There was an apparent 7-fold increase in leaf ABA levels necessary to initiate stomatal closure when plants were exposed to a -3.0 bar treatment, but when plants were exposed to a -5.0 bar stress R_s values increased prior to any detectable rise in ABA levels. To explain these seemingly contradictory results, we suggest that the rate of ABA synthesis in the leaf, rather than the total ABA content, determines the status of the stomatal aperture.Abbreviations ABA abscisic acid - PEG polyethylene glycol - R_s stomatal diffusion resistance of lower leaf surface - leaf water potential     

Growth response of barley and tomato to nitrogen stress and its control by abscisic acid,water relations and photosynthesis

Barley (Hordeum vulgare L.) and tomato Lycopersicon esculentum Mill.) were grown hydroponically and examined 2, 5, and 10 d after being deprived of nitrogen (N) supply. Leaf elongation rate declined in both species in response to N stress before there was any reduction in rate of dryweight accumulation. Changes in water transport to the shoot could not explain reduced leaf elongation in tomato because leaf water content and water potential were unaffected by N stress at the time leaf elongation began to decline. Tomato maintained its shoot water status in N-stressed plants, despite reduced water absorption per gram root, because the decline in root hydraulic conductance with N stress was matched by a decline in stomatal conductance. In barley the decline in leaf elongation coincided with a small (8%) decline in water content per unit area of young leaves; this decline occurred because root hydraulic conductance was reduced more strongly by N stress than was stomatal conductance. Nitrogen stress caused a rapid decline in tissue NO ₃ ^- pools and in NO ₃ ^- flux to the xylem, particularly in tomato which had smaller tissue NO ₃ ^- reserves. Even in barley, tissue NO ₃ ^- reserves were too small and were mobilized too slowly (60% in 2 d) to support maximal growth for more than a few hours. Organic N mobilized from old leaves provided an additional N source to support continued growth of N-stressed plants. Abscisic acid (ABA) levels increased in leaves of both species within 2 d in response to N stress. Addition of ABA to roots caused an increase in volume of xylem exudate but had no effect upon NO ₃ ^- flux to the xylem. After leaf-elongation rate had been reduced by N stress, photosynthesis declined in both barley and tomato. This decline was associated with increased leaf ABA content, reduced stomatal conductance and a decrease in organic N content. We suggest that N stress reduces growth by several mechanisms operating on different time scales: (1) increased leaf ABA content causing reduced cell-wall extensibility and leaf elongation and (2) a more gradual decline in photosynthesis caused by ABA-induced stomatal closure and by a decrease in leaf organic N.Abbreviation and symbols ABA abscisic acid - c_i leaf internal CO₂ concentration - L_p root hydraulic conductance     

Light stimulation of proline synthesis in water-stressed barley leaves

The effect of light on [¹⁴C]glutamate conversion to free proline during water stress was studied in attached barley (Hordeum vulgare L.) leaves which had been trimmed to 10 cm in length. Plants at the three-leaf stage were stressed by flooding the rooting medium with polyethylene glycol 6000 (osmotic potential-19 bars) for up to 3 d. During this time the free proline content of 10-cm second leaves rose from about 0.02 to 2 mol/leaf while free glutamate content remained steady at about 0.6 mol/leaf. In stressed leaves, the amount of [¹⁴C]glutamate converted to proline in a 3-h period of light or darkness was taken to reflect the in-vivo rate of proline biosynthesis because the following conditions were met: (a) free-glutamate levels were not significantly different in light and darkness; (b) both tracer [¹⁴C]-glutamate and [¹⁴C]proline were rapidly absorbed; (c) rates of [¹⁴C]proline oxidation and incorporation into protein were very slow. As leaf water potential fell, more [¹⁴C]glutamate was converted to proline in both light and darkness, but at any given water potential in the range-12 to-20 bars, illuminated leaves converted twice as much [¹⁴C]glutamate to proline.     

Movement and compartmentation of abscisic acid in guard cells of Valerianella locusta: Effects of osmotic stress,external H+-concentration and fusicoccin

Epidermal peels of Valerianella locusta were acid-treated for 1 h at pH 3.9 to kill all cells other than guard cells. These guard-cell preparations were used to explore the steady-state one-way fluxes and the cytoplasmic and vacuolar contents of abscisic acid (ABA). The method of compartmental analysis has been applied. The intracellular ABA concentrations were surprisingly high. At an external pH of 5.8 the cytoplasm contained 1.28 mmol·dm^-3 of ABA, twice of the amount which accumulated in the vacuoles (0.57 mmol·dm^-3). The fluxes of ABA at the plasmalemma (_oc=_oc=0.43 fmol · cell ^–1 · h ^–1) were higher than those at the tonoplast (_cv=_vc=0.12 fmol · cell ^–1 · h ^–1). Moderate stress (0.1 and 0.3 mol·dm^-3 sorbitol in the medium) caused a change in the kinetics of ABA movement. The rate constants of the fluxes from the cytoplasm into the vacuole (_cv) and into the apoplast (_co) were increased while the rate constant of the flux from the vacuoles into the cytoplasm (_vc) was decreased. As a consequence the amount of ABA sequestered in the vacuole remained unchanged; the cytoplasmic ABA content, however, was reduced to only 20% of that found in the control treatments (no sorbitol in the medium). Under moderate stress, one Valerianella guard cell released rapidly about 0.36 fmol·cell^-1 to its direct cell-wall space. This surprising result is discussed in regard to rapid stomatal closure under reduced water supply.Abbreviations ABA abscisic acid - FC fusicoccin     

Compartmental distribution and redistribution of abscisic acid in intact leaves

Using a computer model written for whole leaves (Slovik et al. 1992, Planta 187, 14–25) we present in this paper calculations of abscisic acid (ABA) redistribution among different leaf tissues and their compartments in relation to stomatal regulation under drought stress. The model calculations are based on experimental data and biophysical laws. They yield the following results and postulates: (i) Under stress, compartmental pH-shifts come about as a consequence of the inhibition of the pH component of proton-motive forces at the plasmalemma. There is a decrease of net proton fluxes by about 8.6 nmol · s^–1 · m^–2. (ii) Using stress-induced pH-shifts we demonstrate how stress intensities can be quantified on a molecular basis. (iii) As the weak acid ABA is the only phytohormone which behaves in vivo and in vitro ideally according to the Henderson-Hasselbalch equation, pH-shifts induce a complicated redistribution amongst compartments in the model leaf. (iv) The final accumulation of ABA in guard-cell walls is intensive: up to 16.1-fold compared with only up to 3.4-fold in the guard-cell cytosol. We propose that the binding site of the guard-cell ABA receptor faces the apoplasm. (v) A twoto three-fold ABA accumulation in guard-cell walls is sufficient to induce closure of stomata. (vi) The minimum time lag until stomata start to close is 1–5 min; it depends on the stress intensity and on the guard-cell sensitivity to ABA: the more moderate the stress is, the later stomata start to close or they do not close at all. (vii) In the short term, there is almost no influence of the velocity of pH-shifts on the velocity of the ABA redistribution, (viii) Six hours after the termination of stress there is still an ABA concentration 1.4-fold the initial level in the guard-cell cytosol (delay of ABA relaxation, aftereffect), (ix) The observed induction of net ABA synthesis after onset of stress may be explained by a decrease in cytosolic ABA degradation. About 1 h after onset of stress the model leaf would start to synthesise ABA (and its conjugates) automatically, (x) This ABA net synthesis serves to inform roots via an increased ABA concentration in the phloem sap. The stress-induced ABA redistribution is per se not sufficient to feed the ploem sap with ABA. (xi) The primary target membrane of stress is the plasmalemma, not thylakoids. (xii) The effective stress sensor, which induces the proposed signal chain finally leading to stomatal closure, is located in epidermal cells. Mesophyll cells are not capable of creating a significant ABA signal to guard cells if the epidermal plasmalemma conductance to undissociated molecular species of ABA (HABA) is indeed higher than the plasmalemma conductance of the mesophyll (plasmodesmata open), (xiii) All model conclusions which can be compared with independent experimental data quantitatively fit to them. We conclude that the basic experimental data of the model are consistent. A stress-induced ABA redistribution in the leaf lamina elicits stomatal closure.Abbreviations ABA abscisic acid - CON vacuolar ABA conjugates We are grateful to Prof. U. Heber (Lehrstuhl Botanik I, University of Würzburg, FRG) for stimulating discussions. This work has been performed within the research program of the Sonderforschungsbereich 251 (TP 3 and 4) of the University of Würzburg. It has been also supported by the Fonds der Chemischen Industrie.     

Abscisic-acid contents and concentrations in protoplasts from guard cells and mesophyll cells ofVicia faba L.

The abscisic-acid (ABA) contents of isolated guard-cell protoplasts and mesophyll-cell protoplasts fromVicia faba were determined by high-pressure liquid chromatography followed by gas chromatography. The amounts of ABA found immediately after preparation of the protoplasts varied from 90 to 570 amol per guard-cell protoplast, and from 75 to 100 amol per mesophyll-cell protoplast. These contents correspond to concentrations between 36 and 230 mol per liter in guard-cell protoplasts and between 2.7 and 3.3 mol per liter in mesophyll-cell protoplasts. During exposure of protoplasts to betaine concentrations of 0.3, 0.5, and 0.8 mol·l^-1 at 0° and 20°C for 30 min, ABA contents as well as the fractions of ABA that leaked into the medium remained constant for both protoplast types. There was no evidence for net production of ABA in isolated protoplasts subjected to osmotic stress.Abbreviation ABA abscisic acid     

Abscisic acid and water transport in sunflowers

The role of abscisic acid (ABA) in the transport of water and ions from the root to the shoot of sunflower plants (Helianthus annuus) was investigated by application of ABA either to the root medium or to the apical bud. The exudation at the hypocotyl stump of decapitated seedlings was measured with and without hydrostatic pressure (0–0.3 MPa) applied to the root. All ABA concentrations tested (10^-10–10^-4 mol·l^-1) promoted exudation. Maximal amounts of exudate (200% of control) were obtained with ABA at 10^-6·mol·l^-1 and an externally applied pressure of 0.1 MPa. The effect was rapid and long-lasting, and involved promotion of ion release to the xylem (during the first hours) as well as an increase in hydraulic conductivity. Abscisic acid applied to the apical bud had effects similar to those of the rootapplied hormone. Increased rates of exudation were also obtained after osmotic stress was applied to the root; this treatment increased the endogenous level of ABA in the root as well as in the shoot. Water potentials of the hypocotyls of intact plants increased when the roots were treated with ABA at 5°C, whereas stomatal resistances were lowered. The results are consistent with the view that ABA controls the water status of the plant not only by regulating stomatal transpiration, but also by regulating the hydraulic conductivity of the root.Abbreviations and symbols ABA abscisic acid - Tv volume flow - Lp hydraulic conductivity - PEG polyethyleneglycol - water potential - osmotic potential - osmotic value - P hydrostatic pressure     

The role of cis-carotenoids in abscisic acid biosynthesis

Evidence has been obtained which is consistent with 9-cis-neoxanthin being a major precursor of abscisic acid (ABA) in higher plants. A mild, rapid procedure was developed for the extraction and analysis of carotenoids from a range of tissues. Once purified the carotenoids were identified from their light-absorbance properties, reactions with dilute acid, high-performance liquid chromatography R_ts, mass spectra and the quasiequilibria resulting from iodine-catalysed or chlorophyllsensitised photoisomerisation. Two possible ABA precursors, 9-cis-neoxanthin and 9-cis-violaxanthin, were identified in extracts of light-grown and etiolated leaves (of Lycopersicon esculentum, Phaseolus vulgaris, Vicia faba, Pisum sativum, Cicer arietinum, Zea mays, Nicotiana plumbaginifolia, Plantago lanceolata and Digitalis purpurea), and roots of light-grown and etiolated plants (Lycopersicon, Phaseolus and Zea). The 9,9-di-cisisomer of violaxanthin was synthesised but its presence was not detected in any extracts. Levels of 9-cis-neoxanthin and all-trans-violaxanthin were between 20- to 100-fold greater than those of ABA in light-grown leaves. The levels of 9-cis-violaxanthin were similar to those of ABA but unaffected by water stress. Etiolated Phaseolus leaves contained reduced amounts of carotenoids (15–20% compared with light-grown leaves) but retained the ability to synthesise large amounts of ABA. The amounts of ABA synthesised, measured as increases in ABA and its metabolites phaseic acid and dihydrophaseic acid, were closely matched by decreases in the levels of 9-cis-neoxanthin and all-trans-violaxanthin. In etiolated seedlings grown on 50% D₂O, deuterium incorporation into ABA was similar to that into the xanthophylls. Relative levels of carotenoids in roots and light-grown and etiolated leaves of the ABA-deficient mutants, notabilis, flacca and sitiens were the same as those found in wild-type tomato tissues.Abbreviations ABA abscisic acid - DPA dihydrophaseic acid - GC-MS gas chromatography-mass spectrometry - HPLC high-performance liquid chromatography - PA phaseic acid - t trans - Xan xanthoxin - flc flacca - not notabilis - sit sitiens The authors would like to thank the following for their help and advice: G. Britton (Department of Biochemistry, University of Liverpool, UK), B.H. Davies (Department of Biochemistry, University of Wales, Aberystwyth), P. Molnar, J. Szabolcs, D.C. Walton (Department of Biology, Suny, Syracuse, N.Y., USA), and Mr. J.K. Heald for his expert operation of the mass spectrometer. A.D.P. was supported initially by a Science and Engineering Research Council CASE award with Shell Biosciences, Sittingbourne, Kent, UK, and later by a Agricultural and Food Research Council (AFRC) grant. M.J.B. received a NATO fellowship. The mass spectrometer and HPLC-photodiode-array detector were purchased with funds provided by the AFRC.     

Variations in abscisic acid, indole-3-acetic acid and zeatin riboside concentrations in two Mediterranean shrubs subjected to water stress

Water stress induced an increase in endogenous concentrations of ABA in Lavandula stoechas L. plants to 13100 pmol ABA g^–1 FW, which may contribute to the maintenance of water relations between the second and the third day of water stress treatment. After the third day, a sharp decrease in ABA levels was observed to 2630 pmol ABA g^–1 FW, together with a decrease in water content and water potential and a loss of plant response to water stress. Water deficit did not induce an increase in endogenous ABA concentration, which remained at 514 pmol ABA g^–1 FW in Rosmarinus officinalis L., which is more sclerophyllous than L. stoechas. Nevertheless, the relative water content of Rosmarinus officinalis L. after seven days of water stress decreased more than 40% and reached values of –3.2 MPa. R. officinalis showed lower levels of ABA, but significantly higher levels of IAA and ZR than L. stoechas (4 times and 6 times respectively in well watered-plants). The increase in ABA levels is not a common mechanism in these two Mediterranean shrubs which survive under water stress conditions.Abbreviations ABA abscisic acid - d days of water stress treatment - DW dry weight - FW fresh weight - IAA indole-3-acetic acid - RP Reversed Phase - RWC relative water content - TW turgid weight - WC water content - ZR zeatin riboside - water potential     

Isolation and characterization of a barley mutant with abscisic-acid-insensitive stomata

A barley (Hordeum vulgare L.) mutant (cool) with leaf transpiration unaffected by the application of 1 mM abscisic acid (ABA) was isolated from the population of M2 seedlings using thermography (electronic visualization, and quantitation of the temperature profiles on the surface of the leaves). Stomata of the mutant plants were insensitive to exogenously applied ABA, darkness, and such desiccation treatments as leaf excision and drought stress. The evaporative cooling of the leaves of the cool barley was always higher than that of the wild-type barley, even without ABA application, indicating that the diffusive resistance of the mutant leaves to water loss was always lower. Guard-cell morphology and stomatal density as well as ABA level and metabolism were seemingly unaltered in the mutant plants. In addition, gibberellin-induced -amylase secretion and precocious embryo germination in the mutant barley was inhibited by ABA to the same extent as in the wild-type barley.Abbreviations ABA (±) cis-trans abscisic acid - GA gibberellin     

Translocation and metabolism of glycine betaine by barley plants in relation to water stress

The glycine betaine which accumulated in shoots of young barley plants (Hordeum vulgare L.) during an episode of water stress did not undergo net destruction upon relief of stress, but its distribution among plant organs changed. During stress, betaine accumulated primarily in mature leaves, whereas it was found mainly in young leaves after rewatering. Well-watered, stressed, and stressed-rewatered plants were supplied with [methyl-¹⁴C]betaine (8.5 nmol) via an abraded spot on the second leaf blade, and incubated for 3 d. In all three treatments the added ¹⁴C migrated more or less extensively from the second leaf blade, but was recovered quantitatively from various plant organs in the form of betaine; no labeled degradation products were found in any organ. When 0.5 mol of [methyl-¹⁴C]betaine was applied via an abraded spot to the second leaf blades of well-watered, mildly-stressed, and stressed-rewatered plants, ¹⁴C was translocated out of the blades at velocities of about 0.2–0.3 cm/min which were similar to velocities found for applied [¹⁴C]sucrose. Heat-girdling of the sheath prevented export of [¹⁴C]betaine from the blade. When 0.5 mol [³H]sucrose and 0.5 mol [¹⁴C]betaine were suppled simultaneously to second leaf blades, the ³H/¹⁴C ratio in the sheath tissue was the same as that of the supplied mixture. After supplying tracer [¹⁴C]betaine aldehyde (the immediate precursor of betaine) to the second leaf blade, the ¹⁴C which was translocated into the sheath was in the form of betaine. Thus, betaine synthesized by mature leaves during stress behaves as an inert end product and upon rewatering is translocated to the expanding leaves, most probably via the phloem. Accordingly, it is suggested that the level of betaine in a barley plant might serve as a useful cumulative index of the water stress experienced during growth.     

Effects of water stress on cellular ultrastructure and on concentrations of endogenous abscisic acid and indole-3-acetic acid in Fatsia japonica leaves

Ultrastructural alterations in mesophyll cells as well as variations in bulk leaf endogenous ABA and IAA concentrations were studied in water-stressed field-grown plants of Fatsia japonica. Under water deficit cellular membranes were modified and an increase in vesicles was observed. The main damage to the chloroplasts included thylakoid swelling and disruption of the chloroplast envelope. Concomitant variations in abscisic acid and indole-3-acetic acid were observed. Despite the expected increased in endogenous ABA concentration in relation to water stress, after the highest concentration of ABA, observed at predawn in severely stressed plants (29-1), there was a sharp decline from 2768 pmol g fw^–1 to 145 pmol g fw^–1; thus in severely stressed plants ABA levels were not related to changes in bulk leaf ABA contents. Water stress did not influence the concentrations of indole-3-acetic acid, although the increase in the endogenous abscisic acid concentration could be related with the ultrastructural changes.Abbreviations ABA abscisic acid - IAA indole-3-acetic acid - leaf water potential     

Correlation between loss of turgor and accumulation of abscisic acid in detached leaves

Mature leaves of Phaseolus vulgaris L. (red kidney bean), Xanthium strumarium L. (cocklebur), and Gossypium hirsutum L. (cotton) were used to study accumulation of abscisic acid (ABA) during water stress. The water status of individual, detached leaves was monitored while the leaves slowly wilted, and samples were cut from the leaves as they lost water. The leaf sections were incubated at their respecitive water contents to allow ABA to build up or not. At least 8 h were required for a new steady-state level of ABA to be established. The samples from any one leaf covered a range of known water potentials (), osmotic pressures (), and turgor pressures (p). The and p values were calculated from pressure-volume curves, using a pressure bomb to measure the water potentials. Decreasing water potential had little effect on ABA levels in leaves at high turgor. Sensitivity of the production of ABA to changes in progressively increased as turgor approached zero. At p=1 bar, ABA content averaged 4 times the level found in fully turgid samples. Below p=1 bar, ABA content increased sharply to as much as 40 times the level found in unstressed samples. ABA levels rose steeply at different water potentials for different leaves, according to the at which turgor became zero. These differences were caused by the different osmotic pressures of the leaves that were used; must cqual - for turgor to be zero. Leaves vary in , not only among species, but also between plants of one and the same species depending on the growing conditions. A difference of 6 bars (calculated at =0) was found between the osmotic pressures of leaves from two groups of G. hirsutum plants; one group had previously experienced periodic water stress, and the other group had never been stressed. When individual leaves were subsequently wilted, the leaves from stress-conditioned plants required a lower water potential in order to accumulate ABA than did leaves from previously unstressed plants. On the basis of these results we suggest that turgor is the critical parameter of plant water relations which controls ABA production in water-stressed leaves.Abbreviations ABA abscisic acid - me-ABA abscisic-acid methyl ester - leaf water potential - osmotic pressure - p volumeaveraged turgor - volumetric modulus of elasticity     

Determination of carbon-reduction-cycle intermediates in leaves of Arbutus unedo L. suffering depressions in photosynthesis after application of abscisic acid or exposure to dry air

Gas exchange and contents of photosynthetic intermediates of leaves of Arbutus unedo L. were determined with the aim of recognizing the mechanisms of inhibition that were responsible for the midday depression of photosynthesis following exposure to dry air, and the decline in photosynthetic capacity following application of abscisic acid (ABA). Rapidly killed (<0.1 s) leaf samples were taken when gas analysis showed reduced CO₂ assimilation. Determination of the contents of 3-phosphoglyceric acid (PGA), ribulose 1,5-bisphosphate (RuBP), triose phosphates, fructose 1,6-bisphosphate and hexose phosphates in the samples showed that significant variation occurred only in the level of PGA. As a result, the ratio PGA/RuBP decreased with increasing inhibition of photosynthesis, particularly when application of ABA had been the cause. A comparison of metabolite patterns did not bring out qualitative differences that would have indicated that effects of ABA and of dry air had been caused by separate mechanisms. Depression of photosynthesis occurred in the presence of sufficient RuBP which indicated that the carboxylation reaction of the carbon-reduction-cycle was inhibited after application of ABA or exposure to dry air.Abbreviations and symbols ABA abscisic acid - C _a partial pressure of CO₂ in the ambient air - C _i partial pressure of CO₂ in the intercellular spaces - I quantum flux - PGA 3-phosphoglyceric acid - RuBP ribulose 1,5-bisphosphate - I _L leaf temperature - w water-vapor pressure difference between leaf and air     

Transport and metabolism of [214-C]abscisic acid in maize root

The tips of intact maize (cv. LG 11) roots, maintained vertically, were pretreated with a droplet of buffer solution or a bead of anion exchange resin, both containing [2¹⁴-C]abscisic acid (ABA). A significant basipetal ABA movement was observed and two metabolites of ABA (possibly phaseic acid and dihydrophaseic acid) were found. ABA pretreatment enhanced the gravireaction of 10 mm apical root segments kept both in the dark and in the light. The possibility that ABA could be one of the endogenous growth inhibitors produced or released by the cap cells is discussed.Abbreviations ABA abscisic acid - 3,3-DGA 3,3-dimethyl-glutaric acid - DPA dihydrophaseic acid - PA phaseic acid - GCMS gas chromatography-mass spectrometry