Abscisic acid and mefluidide in the regulation of cold hardiness development in Solanum tuberosum L. potato

Plants of Solanum tuberosum L. potato do not cold acclimate when exposed to low temperature such as 5°C, day/night. When ABA (45 M) was added to the culture medium, stem-cultured plantlets of S. tuberosum, cv. Red Pontiac, either grown at 20°C/15°C, day/night, or at 5°C, increased in cold hardiness from –2°C (killing temperature) to –4.5°C. The increase in cold hardiness could be inhibited in both temperature regimes if cycloheximide (70 M) was added to the culture medium at the inception of ABA treatment. Cycloheximide did not inhibit cold hardiness development, however, when it was added to the culture medium 3 days after ABA treatment.When pot-grown plants were foliar sprayed with mefluidide (50 M), ABA content increased from 10 nmol to 30 nmol g^–1 dry weight and plants increased in cold hardiness from –2°C to about –3.5°C. The increases in free ABA and cold hardiness occurred only in plants grown at 20°C/15°C; neither ABA nor cold hardiness increased in plants grown at 5°C.The results suggest that an increase in ABA and a subsequent de novo synthesis of proteins are required for the development of cold hardiness in S. tuberosum regardless of temperature regime, and that the inability to synthesize ABA at low temperature, rather than protein synthesis, appears to be the reason why S. tuberosum does not cold acclimate.     

Modulation of carrier-mediated uptake of abscisic acid by methyl jasmonate in Phaseolus coccineus L

The effects of methyl jasmonate and jasmonic acid on uptake of abscisic acid (ABA) by suspension-cultured runner-bean cells and subapical runner-bean root segments have been investigated. Increasing concentrations of methyl jasmonate inhibit ABA uptake by the cultured cells with a K _i of 22±3 M. This is not due to cytoplasmic acidification or to effects on metabolism of ABA, and is not additive with inhibition of radioactive ABA uptake by nonradioactive ABA. Uptake of indol-3-yl acetic acid (IAA) is unaffected by methyl jasmonate. The maximum effect of nonradioactive ABA in inhibiting uptake of radioactive ABA, previously shown to reflect saturation of an ABA carrier, is generally greater than the effect of maximally inhibitory concentrations of methyl jasmonate. Similar results were obtained with root segments, but longer incubation times were necessary to observe inhibitory effects of methyl jasmonate. Demethylation of methyl jasmonate to jasmonic acid does not appear to be required since similar concentrations of jasmonic acid had no observable direct effect on ABA uptake other than that attributable to cytoplasmic acidification. Histidine reagents, a proton ionophore and acidic external pH all affect in parallel the inhibition by methyl jasmonate and nonradioactive ABA of uptake of radioactive ABA by the cultured cells. There is no effect of ABA or nonradioactive methyl jasmonate on uptake of radioactive methyl jasmonate by the cultured cells. It is proposed that methyl jasmonate interacts with the ABA carrier. Various models for this interaction are discussed.Abbreviations ABA abscisic acid - DMO 5,5-dimethyloxazolidine-2,4-dione - IAA indol-3-yl acetic acid     

Effects of NaCl and KNO3 concentrations on the abscisic acid content of Dunaliella sp. (Chlorophyta)

Abscisic acid (ABA) is a hormone which has a number of roles during the life cycle of a plant. We demonstrated the occurrence of ABA in a halotolerant green alga, Dunaliella sp. isolated from a salt pond near Adelaide, South Australia, using thin layer chromatography (TLC), high performance liquid chromatography (HPLC) and gas chromatography-mass spectrometry (GC-MS). The variation of cellular ABA and protein content during the growth of an axenic clonal culture of Dunaliella sp. was investigated under different concentrations of NaCl and KNO₃.Experimental results can be summarized as follow: (1) ABA content was changed with the growth stage of culture: A rapid increase in ABA content was observed in the logarithmic phase. After this, the content rapidly decreased to very low values. (2) ABA content was also affected by the NaCl concentration. The content had a minimum value at the NaCl concentration (15%) where growth rate was maximal, and higher values at higher or lower concentrations of NaCl. (3) The ABA content also increased with decreasing nitrogen concentration of the medium.     

Model systems for the immunolocalisation of cis,trans abscisic acid in plant tissues

To explore the feasibility of immunolocalisation of endogenous abscisic acid (ABA), model systems were developed for testing quantitatively the sensitivity of the second antibody peroxidase/antiperoxidse (PAP) method for immunolocalisation of ABA on plant tissues. Exogenous (±)ABA was fixed to carrot sections on glass slides or to homogenised pea cotyledon material on microtitre plates, either directly by carbodiimide fixation or by glutaraldehyde fixation of ABA-protein conjugates linked through the C1 carboxyl by 1-ethyl-3(3-dimethyl-amino-propyl) carbodiimide hydrochloride (EDC). Backgrounds were decreased by including 0.1% normal goat serum in the incubations, by including 0.1% Triton X-100 as a wetter, by including glycine in the rinses after EDC fixation and by using low-pH rinses after incubation with the primary antibody. Serum antibodies recognising the peptide bond between the protein and abscisic acid were removed by preincubating the serum with acetic acid conjugated to protein. Positives were only accepted when they could be eliminated by adding an excess of ABA-protein conjugate in the primary antiserum. By using a soluble peroxidase reaction product to facilitate quantitation, the limit of reliable exogenous ABA detection was found to be only of the order of 1 pmol. For the histochemical immunolocalisation of endogenous ABA, better antisera and lower backgrounds will be required.The efficiency of fixation of exogenous ABA was determined using [³H] or [¹⁴C]ABA. When aqueous EDC or di-isopropyl carbodiimide (IPC) were used the fixation efficiency was low (up to 5%), but much higher efficiencies (up to 80%) were obtained using IPC vapour with freeze-dried material. Similarly efficient fixation of endogenous ABA in pea cotyledon material, as determined by gas chromatography-mass spectrometry analysis, was obtained using the same technique. The PAP method failed to detect fixed endogenous ABA in pea cotyledons, even though the total tissue amounts present exceeded 1 pmol, evidence that not enough of the ABA was accessible to the antibody.Abbreviations ABA abscisic acid - ACE-ALP acetic acid-alkaline phosphatase - EDC 1-ethyl-3(3-dimethyl-amino-propyl) carbodiimide hydrochloride - GC-MS gas chromatographymass spectrometry - IgG Immunoglobulin G - HSA humanserum albumin - IPC dinsopropyl carbodiimide - LINK goat anti-rabbit IgG - OD optical density - PAP peroxidase/rabbit antiperoxidase complex     

Response ofBrassica napus L. Microspore-derived embryos to exogenous abscisic acid and desiccation

Summary Microspore-derived embryos fromBrassica napus cv. Topas (low erucic acid) and Reston (high erucic acid) were subjected to treatment with abscisic acid (ABA) during late-stage embryo development and then dried under controlled relative humidities to mature dry seed levels of moisture. Exogenously medium-supplied ABA arrested growth and development, reduced moisture content, increased total fatty acids on a dry weight basis, and stimulated systhesis of proteins in microspore-derived embryos. ABA also resulted in a higher proportion of 22∶1 in cv. Reston (high 22∶1) and increased the level of fatty acid unsaturation in cv. Topas (low 22∶1). The accumulation of two proteins that co-migrated with cruciferin and napin on sodium dodecyl sulfate-polyacrylamide gel electrophoresis and two-dimensional gels were also promoted by exposure to ABA, and the degree of accumulation was dependent on the concentration and time of application of ABA. Controlled desiccation of microspore embryos, used to simulate normal maturation and dehydration of zygotic embryos during seed development, did not seem to cause an increase of either storage proteins, total fatty acids, or 22∶1 (in cv. Reston), suggesting that dehydration is not a prerequisite for these processes, at least in culturedBrassica embryos.     

Abscisic acid and ancymidol promote conversion of somatic embryos to plantlets and secondary embryogenesis inAsparagus officinalis L.

Summary The effects of abscisic acid (ABA) (0, 0.09 μM, 0.19 μM, 0.28 μM, and 0.38 μM) or ancymidol (0, 0.98 μM, 1.95 μM, 2.93 μM, 3.90 μM) in embryo germination medium on the conversion of primary embryos to plantlets and secondary embryogenesis were evaluated for asparagus. ABA and ancymidol each significantly enhanced both responses. ABA was more effective than ancymidol in promoting the conversion of primary embryos to plantlets, while the converse was true for the production of secondary embryos. The most effective treatments for embryo conversion were 0.19 and 0.28 μM ABA; 75–77% bipolar and 55–57% globular embryos converted to plantlets. For secondary embryogenesis, the most effective treatments were 1.95 and 2.93 μM ancymidol; 99–101 and 84–86 somatic embryos were produced from 10 globular and 10 bipolar embryos, respectively. Bipolar embryos generally converted to plantlets better than globular embryos, but more secondary embryos were produced from globular embryos than from bipolar embryos in all treatments. ABA and ancymidol also affected the morphology of the plantlets produced. The plantlets from the embryos incubated on the medium with ancymidol had strong and thick shoots and roots, while those on the medium with ABA had long, thin shoots and short thin roots.     

Changes in abscisic acid and its glucose ester in Phaseolus vulgaris L. during chilling and water stress

Levels of free and conjugated abscisic acid (ABA) were determined in leaves and roots of intact bean (Phaseolus vulgaris L., cv. Mondragone) seedlings under chilling (3C) and drought as well as during recovery from stress. Abscisic acid-glucose ester (ABAGE) was the only conjugate releasing free ABA after alkaline hydrolysis of the crude aqueous extracts. During the first 20–30 h chilled plants rapidly dehydrated and wilted without any change in ABA and ABAGE levels. Subsequently, leaf and root ABA levels increased and plants regained turgor. ABAGE concentration showed a slight increase in leaves but not in roots. Upon recovery from chilling a transient, but significant, rise in leaf ABA content was observed, while no appreciable change in ABAGE was found. Drought triggered ABA accumulation in leaves and roots, while a rise in ABAGE content was detected only in leaf tissues. Recovery from stress caused a drop in ABA levels without a correspondent increase in ABAGE concentration. We conclude that ABAGE is not a source of free ABA during either chilling or water stress and that only a small proportion of the ABA produced under stress is metabolised to ABAGE during recovery.Abbreviations ABA = abscisic acid - ABAGE = abscisic acid-glucose ester - DW = dry weight - FW = fresh weight - RIA = radioimmunoassay - RWC = relative water content - w = water potential - o = osmotic potential - p = turgor potential     

Modifications of abscisic acid level in winter oilseed rape leaves during acclimation of plants to freezing temperatures

An almost twofold increase in abscisic acid (ABA) content was observed in the leaves of winter oilseed rape plants (Brassica napus L., var. oleifera L., cv. Jantar) grown in the cold (>0°C). This ABA increase took place during the first three days of cold treatment. After 6 days of plant growth in the cold, the level of ABA started to decline or remained constant, depending on the calculation basis: dry weight or disc area units, respectively. The exposure of cold-acclimated plants to night frost (–5°C for 18 h) induced a further increase (65%) in the ABA level, which begun during the first few hours after thawing. The comparison of time courses of frost resistance increments and ABA content changes showed that modifications of ABA level in the cold-treated leaves preceded those of frost resistance, whereas in the frost-pretreated tissues the ABA increase occurred later than that of frost tolerance. Possible interrelations between ABA content, frost tolerance and tissue water potential modifications in the low temperature-affected tissues are discussed.     

Determination of endogenous abscisic acid levels in immature cereal embryos during in vitro culture

Levels of endogenous abscisic acid (ABA) in immature wheat (Triticum aestivum cv. Timmo) and barley (Hordeum vulgare cv. Golden Promise) embryos have been determined by enzyme-linked immunosorbent assay. Embryos of both cereal species showed an increase in ABA content during development on the parent plant. Immature embryos were excised and cultured in vitro on nutrient media that led to precocious germination or on media containing 9% (w/v) mannitol that maintained their developmental arrest. Barley and wheat embryos responded to these culture conditions in an identical manner with respect to changes in morphology, fresh weight, protein and lectin content. However, in complete contrast, the ABA content of barley embryos increased by an order of magnitude during culture on mannitol, whereas that of wheat embryos showed no significant change. The results are discussed within the context of the role of ABA in the regulation of embryo development.Abbreviations ABA abscisic acid - BGA barley-germ agglutinin - dpa days post anthesis - ELISA enzyme-linked immunosorbent assay - GC-MS gas chromatography-mass spectrometry - WGA wheat-germ agglutinin     

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     

Abscisic acid plays critical role in ozone‐induced taxol production of Taxus chinensis suspension cell cultures

Exposure to ozone induced a rapid increase in the levels of the phytohormone abscisic acid (ABA) and sequentially followed by the enhancement of Taxol production in suspension cell cultures of Taxus chinensis. The observed increases in ABA and Taxol were dependent on the concentration of ozone applied to T. chinensis cell cultures. To examine the role of ABA in ozone‐induced Taxol production, we pretreated the cells with ABA biosynthesis inhibitor fluridone to abolish ozone‐triggered ABA generation and assayed the effect of fluridone on ozone‐induced Taxol production. The results showed that pretreatment of the cells with fluridone not only suppressed the ozone‐triggered ABA generation but also blocked the ozone‐induced Taxol production. Moreover, our data indicate that the effect of ABA on Taxol production of T. chinensis cell cultures is dose‐dependent. Interestingly, the suppression of fluridone on ozone‐induced Taxol production was reversed by exogenous application of low dose of ABA, although treatment of low dose ABA alone had no effect on Taxol production of the cells. Together, the data indicated that ozone was an efficient elicitor for improving Taxol production of plant cell cultures. Furthermore, we demonstrated that ABA played critical roles in ozone‐induced Taxol production of T. chinensis suspension cell cultures. © 2011 American Institute of Chemical Engineers Biotechnol. Prog., 2011     

Stomatal responses to jasmonic acid, linolenic acid and abscisic acid in wild-type and ABA-deficient tomato plants

Wild-type and abscisic acid (ABA) -deficient (sitiens) tomato plants were used to analyse the effects of abscisic acid (ABA), butyric acid (BA), jasmonic acid (JA) and linolenic acid (LA) on assimilation and transpiration rates in detached leaves taking up those substances into the transpiration stream. BA did not affect assimilation and transpiration rates. ABA decreased assimilation and transpiration in both wild-type and ABA-deficient mutants. JA reduced the assimilation rate in both lines but induced a significant reduction of transpiration in the wild type only. The response to LA in both lines was slower than that to JA.     

Role of abscisic acid in cotton fiber development

Fibers of three cotton cultivars varying widely in their final fiber length, i.e., long staple (Gossypium hirsutum H-4), middle staple (G. Hirsutum H-8), and short staple (G. Arboretum G. Cot-15) were analyzed to study the role of ABA in fiber elongation and dry matter accumulation, in vivo and in vitro. The fibers were analyzed for different growth parameters and endogenous ABA content during the entire period of their development using indirect ELISA by raising the antibodies against ABA. From growth analysis, cotton fiber development was divided into four distinct phases, (i) initiation, (ii) elongation, (iii) secondary thickening, and (iv) maturation. An inverse correlation between final fiber length and ABA content was observed in all the cultivars. In long staple cultivar (H-4), rapid ABA accumulation started after fiber had attained peak elongation growth while, in short staple cultivar (G. Cot-15), ABA accumulation was observed even during elongation growth. Significant inhibition in length of short and middle staple cultivars as compared to long staple cultivar was observed in in vitro grown fibers when media were supplemented with ABA (1, 3, and 5 mg/l). The addition of growth promoters like NAA and GA, along with ABA, has reduced the inhibition in fiber elongation in all the cultivars. These results suggest a regulatory role of ABA in cotton fiber elongation along with auxins and gibberellins. Published in Russian in Fiziologiya Rastenii, 2006, Vol. 53, No. 1, pp. 68–74. The text was submitted by the authors in English.     

Effect of abscisic acid application on root isoflavonoid concentration and nodulation of wild-type and nodulation-mutant soybean plants

Isoflavonoids (daidzein, genistein, and coumestrol) are involved in induction of nod genes in Bradyrhizobium japonicum and may be involved in nodule development as well. Abscisic acid (ABA) may also impact nodulation since ABA is reportedly involved in isoflavonoid synthesis. The current study was conducted to evaluate whether ABA plays a role in differential nodulation of a hypernodulated soybean (Glycine max L. Merr.) mutant and the Williams parent. Exogenous ABA application resulted in a decrease in nodule number and weight in both lines. Isoflavonoid concentrations were also markedly decreased in response to ABA application in both inoculated and noninoculated soybean roots. The inoculation treatment itself resulted in a marked increase in isoflavonoid concentrations of NOD1-3, regardless of ABA levels, while only slight increases occurred in Williams. The nodule numbers of both soybean lines across several ABA concentration treatments were highly correlated with the concentration of all three isoflavonoids. However, differences in internal levels of ABA between lines were not detected when grown in the absence of external ABA additions. It is concluded that differential nodule expression between the wild type and the hypernodulated mutant is not likely due to differential ABA synthesis.     

Jasmonic acid and salicylic acid play minor roles in stomatal regulation by CO2, abscisic acid,darkness, vapor pressure deficit and ozone

Jasmonic acid (JA) and salicylic acid (SA) regulate stomatal closure, preventing pathogen invasion into plants. However, to what extent abscisic acid (ABA), SA and JA interact, and what the roles of SA and JA are in stomatal responses to environmental cues, remains unclear. Here, by using intact plant gas-exchange measurements in JA and SA single and double mutants, we show that stomatal responsiveness to CO₂, light intensity, ABA, high vapor pressure deficit and ozone either did not or, for some stimuli only, very slightly depended upon JA and SA biosynthesis and signaling mutants, including dde2, sid2, coi1, jai1, myc2 and npr1 alleles. Although the stomata in the mutants studied clearly responded to ABA, CO₂, light and ozone, ABA-triggered stomatal closure in npr1-1 was slightly accelerated compared with the wild type. Stomatal reopening after ozone pulses was quicker in the coi1-16 mutant than in the wild type. In intact Arabidopsis plants, spraying with methyl-JA led to only a modest reduction in stomatal conductance 80 min after treatment, whereas ABA and CO₂ induced pronounced stomatal closure within minutes. We could not document a reduction of stomatal conductance after spraying with SA. Coronatine-induced stomatal opening was initiated slowly after 1.5–2.0 h, and reached a maximum by 3 h after spraying intact plants. Our results suggest that ABA, CO₂ and light are major regulators of rapid guard cell signaling, whereas JA and SA could play only minor roles in the whole-plant stomatal response to environmental cues in Arabidopsis and Solanum lycopersicum (tomato).     

Abscission-promoting activities of abscisic acid and five abscisic acid analogs in cotton seedlings and explants

The abscission-promoting activities of abscisic acid (ABA) and 5 ABA analogs were examined in cotton (Gossypium hirsutum L. cv LG102) seedlings and cotyledonary node explants. The analogs tested included a series of acetylenic derivatives that differ in the oxidation state of the C-1 atom, a 2,3 dihydro-derivative of ABA and a 2,3 dihydro-derivative of an acetylenic analog with a C-1 carboxyl moiety. ABA and all five analogs were active in stimulating petiole abscission in explants. Following treatment with 100,µM ABA or analog, 50% abscission of explants was observed after 29 h and complete abscission occurred within 40 h. With one exception, none of the treatments resulted in an increase in explant ethylene production. Pretreatment of the explants with the ethylene antagonist silver thiosulfate completely abolished the abscission-promoting activities of ABA and all of the analogs. Daily application of ABA or any of the analogs had no effect on cotyledon abscission in intact seedlings. The implications of the results with respect to the development of a commercial ABA-like regulator as well as to ABA structure-activity studies are discussed.Mention of trademark or proprietary product does not constitute a guarantee or warranty of the product by the U.S. Department of Agriculture and does not imply its approval to the exclusion of other products that may also be suitable.     

Immunolocalization of abscisic acid by monoclonal antibodies in Lavandula stoechas L. leaves

A monoclonal antibody was used to localize abscisic acid in Lavandula stoechas L. plants treated with 1 M abscisic acid. ABA localization was performed using EDC as the only fixative, which preserves antigenicity and improves the specificity of monoclonal antibodies. A relationship was observed between the effect of abscisic acid on chloroplast ultrastructure and ABA accumulation. Gold particles accumulated on swollen chloroplasts. Our findings provide cytochemical evidence that the chloroplast is a trapping compartment, and yield valuable information on the relation between chloroplast ultrastructure and ABA accumulation.Abbreviations ABA abscisic acid - BSA bovine serum albumin - EDC 1-(3-dimethyl-aminopropyl)-3 ethyl carbodiimide - IgG immunoglobulin G - mAB monoclonal antibodies - pAB polyclonal antibodies     

Cryopreservation of immature spring wheat zygotic embryos using an abscisic acid pretreatment

Spring wheat (Triticum aestivum L.) zygotic embryos were successfully cryopreserved, without the addition of exogenous cryoprotectants, using only an abscisic acid (ABA) pretreatment. Optimum survival was obtained when embryos were cultured in vitro for 10 days on semisolid Murashige and Skoog (MS) nutrient medium supplemented with 0.5 mg/L (±) ABA prior to cryopreservation. The embryos resumed growth within three days when returned to MS medium devoid of ABA but containing 2mg/L 2,4-dichlorophenoxyacetic acid. The embryogenic calli produced from these embryos exhibited normal plant regeneration on auxin-free media. Changes in dw/fw ratio, as well as the esterified fatty acid and sucrose concentrations correlated positively with the development of tolerance to cryopreservation.NRCC Publication No. 33519     

Abscisic acid concentrations are correlated with leaf area reductions in two salt-stressed rapid-cycling Brassica species

The greater sensitivity of B. carinata to salinity in comparison to B. napus has been linked to a greater reduction in net assimilation rate. Apparently this is not due to ion toxicity; the cause is unknown. In this report, we test the hypothesis that increases in abscisic acid (ABA) are involved in the reduction of growth by salinity. Salinity (8 dS m^–1) caused an increase of ABA concentrations in the shoot, root and callus of both species. ABA concentrations were lower in the salt-tolerant species, B. napus, than the salt-sensitive species, B. carinata, both in the whole plant and callus. Leaf expansion for both species was equally sensitive to ABA; salt stress did not significantly alter sensitivity to applied ABA. The growth inhibition increased in a hyperbolic manner with an increase in endogenous ABA concentration. These results indicate that ABA in salt-stressed plants may play a role in the inhibition of growth. The photosynthesis of salt-sensitive species, B. carinata, was also decreased by salinity, corresponding to the reduction in growth. The decreased photosynthesis does not appear to be the cause of the growth reduction, because photosynthesis was not inhibited by short-term exposure to salinity and photosynthesis was poorly correlated with endogenous ABA concentrations.     

Abscisic acid biosynthesis in roots

The pathway of water-stress-induced abscisic acid (ABA) biosynthesis in etiolated and light-grown leaves has been elucidated (see A.D. Parry and R. Horgan, 1991, Physiol. Plant. 82, 320–326). Roots also have the ability to synthesise ABA in response to stress and it was therefore of interest to examine root extracts for the presence of carotenoids, including those known to be ABA precursors in leaves. All-trans- and 9-cis-neoxanthin, all-trans- and 9-cis-violaxanthin, antheraxanthin (all potential ABA precursors), lutein and -carotene were identified on the basis of absorbance spectra, reactions with dilute acid, retention times upon high-performance liquid chromatography and by comparison with leaf carotenoids that had been analysed by mass spectrometry. The source of the extracted carotenoids was proved to be root tissue, and not contaminating compost or leaf material. The levels of total carotenoids in roots varied between 0.03–0.07% of the levels in light-grown leaves (Arabidopsis thaliana (L.) Heynh, Nicotiana plumbaginifolia Viv., Phaseolus vulgaris L. and Pisum sativum L.) up to 0.27% (Lycopersicon esculentum Mill.). The relative carotenoid composition was very different from that found in leaves, and varied much more between species. All-trans-neoxanthin and violaxanthin were the major carotenoids present (64–91 % of the total), but while Lycopersicon contained 67–80% all trans-neoxanthin, Phaseolus, Pisum and Zea mays L. contained 61–79% all-trans-violaxanthin. Carotenoid metabolism also varied between species, with most of the carotenoids in older roots of Phaseolus being esterified. Roots and leaves of the ABA-deficient aba mutant of Arabidopsis had reduced epoxy-xanthophyll levels compared to the wild-type.Abbreviations ABA abscisic acid - r.p.HPLC reversed-phase high performance liquid chromatography The authors would like to thank Dr. B.H. Davies for helpful discussions and Mrs. A.F. Rees for her excellent technical assistance. A.D.P. was supported by a grant from the Agricultural and Food Research Council, from whom funds were also obtained to purchase the HPLC-photodiode-array detector.