A comparative study of the effects of abscisic acid and methyl jasmonate on seedling growth of rice

The effects of abscisic acid (ABA) and methyl jasmonate (MJ) on growth of rice seedlings were compared. The lowest tested concentration of ABA and MJ that inhibited seedling growth was found to be 4.5 and 0.9 µM, respectively. Growth inhibition by ABA is reversible, whereas that by MJ is irreversible. GA₃ was found to be more effective in reversing inhibition of shoot growth by ABA than by MJ. KCl partially relieved MJ-inhibited, but not ABA-inhibited, growth of rice seedlings. The beneficial effect of K⁺ on growth of rice seedlings in MJ medium could not be replaced by Li⁺, Na⁺ or Cs⁺. MJ treatment caused a marked release of K⁺ into the medium. In order to understand whether cell wall-bound peroxidase activity was inversely related to rice seedling growth, effects of ABA and MJ on cell wall-bound peroxidase activity were also examined. Results indicated that both ABA and MJ increased cell wall-bound peroxidase activity in roots and shoots of rice seedlings. Although MJ (4.5 µM) was less effective in inhibiting root growth than ABA (9 µM), MJ was found to increase more cell wall-bound peroxidase activity in roots than ABA.     

Further Studies on the Relationship between Growth Regulators and Tuberization of Dahlias

Abscisic acid (ABA) and gibberellin (GA) treatments, applied to budless leaf cuttings of Dahlia under long day conditions, did not increase the total weight of the cuttings but did affect the region where photosynthates accumulate. GA treatment inhibited the growth of tubers and roots, and enhanced thickening at the petiole base. ABA did not affect the weight of roots or petiole, but promoted the growth of tuberous roots. Anatomical examination of the tuberization revealed that it is caused by inter-fascicular cambial activity producing parenchyma cells inward. No anatomical difference could be observed between the GA-induced thickening at the petiole base and the natural or ABA-induced thickening of the roots. In single node cuttings, taken from various sites along the branch, tuberization ability increased with proximity to the base of the branch, where axillary buds are more inhibited. Short-day conditions which promote tuberization, increased the endogenous level of ABA-like inhibitors in intact plants. External conditions and growth substances which inhibit growth also inhibited tuberization. Benzyladenine treatments did not affect tuberization. It is suggested that in addition to their respective indirect effects via the enhancement or suppression of top growth, GA and ABA directly control tuberization. ABA levels which increased under short-day conditions seem to produce a sink in the roots. GA probably inhibits tuberization by interfering with the trans-location of photosynthates to the root area.     

黄瓜茎端ABA/GA_4比值与花芽性别分化的关系(英文)

在同一环境条件下,单雌性株茎端ABA含量显著高于雌雄同株的,而GA_4水平却正好相反。 较高的ABA/GA_4比值有利于黄瓜雌性分化,反之则有利于雄性分化。促进黄瓜雄性分化的生长物质GA_3和AgNO_3会引起黄瓜茎端ABA/GA_4比值的下降,而促进其雌性分化的生长物质乙烯利(Ethrel)则会引起ABA/GA_4比值的上升。     

The Role of Gibberellin, Abscisic Acid, and Sucrose in the Regulation of Potato Tuber Formation in Vitro

The effects of plant hormones and sucrose (Suc) on potato (Solanum tuberosum L.) tuberization were studied using in vitro cultured single-node cuttings. Tuber-inducing (high Suc) and -noninducing (low Suc or high Suc plus gibberellin [GA]) media were tested. Tuberization frequencies, tuber widths, and stolon lengths were measured during successive stages of development. Endogenous GAs and abscisic acid (ABA) were identified and quantified by high-performance liquid chromatography and gas chromatography-mass spectrometry. Exogenous GA_4/7 promoted stolon elongation and inhibited tuber formation, whereas exogenous ABA stimulated tuberization and reduced stolon length. Indoleacetic acid-containing media severely inhibited elongation of stolons and smaller sessile tubers were formed. Exogenous cytokinins did not affect stolon elongation and tuber formation. Endogenous GA₁ level was high during stolon elongation and decreased when stolon tips started to swell under inducing conditions, whereas it remained high under noninducing conditions. GA₁ levels were negatively correlated with Suc concentration in the medium. We conclude that GA₁ is likely to be the active GA during tuber formation. Endogenous ABA levels decreased during stolon and tuber development, and ABA levels were similar under inducing and noninducing conditions. Our results indicate that GA is a dominant regulator in tuber formation: ABA stimulates tuberization by counteracting GA, and Suc regulates tuber formation by influencing GA levels.     

GIBBERELLIN RELATIONSHIPS IN THE ‘ALASKA’ PEA (PISUM SATIVUM)

'Alaska’ peas (Pisum sativum L.) grown under a 16-hr photoperiod at 20 ± 1 C and an 8-hr dark period at 16 ± 1 C in their ontogeny exhibit two periods of sensitivity to applied gibberellin (GA), namely, prior to and subsequent to but not during the linear phase of stem elongation. This paper describes experiments conducted primarily with seedlings. Growth-saturating doses of GA, applied to dry seeds before planting (10⁻³ m) and to the shoot tips of 3-day-old seedlings (10 μg), evoked growth rates equal to the growth rate of etiolated seedlings. Sensitivity of seedlings to applied GA decreased with age through the first 2 to 3 weeks of development; by the time seedlings were about 14 days of age and had four elongating internodes they no longer responded to applied GA. As endogenous growth rate diminished late in ontogeny, the plants again became sensitive to applied GA. Growth response was used as a criterion for determining apparent translocation of applied GA. ‘Alaska’ pea seedlings appeared to transport GA, both acropetally from the cotyledons and basipetally from the shoot tip, to all internodes with remaining extension potential. Excision of both cotyledons at any time during the first 9 days of development caused a significant reduction of growth rate, and applied GA did not restore normal growth rate. No evidence was found that the cotyledons supply endogenous GA to the shoot axis in normal seedling development. It is suggested that the normal growth rate of light-grown ‘Alaska’ peas is correlated with the rate of synthesis of GA and that GA is rate-limiting for stem elongation during early seedling development and during the period of decreasing growth rate and onset of apex senescence.     

Metabolic changes during rooting in stem cuttings of Casuarina equisetifolia L.: effects of auxin, the sex and the type of cutting on rooting

Rooting in terminal shoot and lateral shoot cuttings from 10-year-old elite trees of Casuarina equisetifolia L. in different sex groups was achieved after 20 days when the basal ends of the cuttings were dipped for 3 h in 20 ppm indole-3-butyric acid (IBA). Shoots derived from male plants rooted better than their female and monoecious counterparts, and the lateral shoots were more responsive to rooting than the terminal shoots. During rooting, the metabolic activities varied in both lateral shoot and terminal shoot cuttings derived from plants under different sex groups. Peroxidase and polyphenoloxidase activities were high during root initiation and showed a sharp decline thereafter. The polyphenoloxidase activity was higher in the lateral shoot than the terminal shoot cuttings. The rooted plantlets survived and established well in the field.Abbreviations IAA indole-3-acetic acid - IBA indole-3-butyric acid - NAA 1-naphthaleneacetic acid - PVP polyvinylpyrrolidone     

Endogenous Levels of Growth Regulators and Their Relationship to the Rooting of Dahlia Cuttings

The relationship between the level of endogenous growth regulators and the rooting ability of dahlia cuttings was investigated. Easy-to-root vegetative cuttings were compared with difficult-to-root, flower bud bearing cuttings and the easy-to-root cv. Choot Hashani was compared with the difficnlt-to-root cv. Orpheo. No differences in the level of extracted or diffusable auxin, nor in the level of rooting cofactors, were found between easy-to-root and difficult-to-root cuttings. No activity could be found at the R_f zone corresponding to Cofactor 4 which is considered the main rooting cofactor in other plants. Higher inhibitor activity levels (assayed by the wheat coleoptile test and the mung bean rooting test) were found in difficult-to-root than in easily rooted cuttings. Furthermore, the inhibitor activity level in exadates from decapitated plants, was higher in esodates from plants which bore difficult-to-root cuttings than from plants bearing easy-to-root ones. Grafting of an easy-to-root plant onto a difficult-to-root stock caused inhibition of rooting in the cuttings taken from the scion. Results suggest that inhibitors formed in the root system, move upwards and accumulate in the plant, there causing the inhibition of rooting. Evidence from chromatographic comparisons and exogenous treatments with ABA indicate that the main inhibitor involved in the inhibition of rooting in dahlia cuttings is not ABA.     

Abscisic acid-induced growth inhibition of sweet potato (Ipomoea batatas L.) in vitro

The inhibitory effects of abscisic acid (ABA) on in vitro growth and development of axillary buds from nodal segments of sweet potato (Ipomoea batatas L.) was investigated. ABA at concentrations of 0.01, 0.1, 1.0 or 10.0 mg 1^-1 inhibited axillary bud and root development and subsequent plantlet growth. ABA at 10 mg 1^-1 completely inhibited axillary shoot development but did not affect the viability of cv. Jewel explants over a culture period of 365 days. Transfer of nodal segments cultured for 90, 180 or 365 days from basal medium containing 10 mg 1^-1 ABA to growth regulator-free media resulted in rapid and normal plantlet development. Gibberellic acid at 0.1, 1.0 or 10.0 mg 1^-1 in the presence of ABA at 0.1, 1.0 or 10.0 mg 1^-1 did not counteract the ABA-induced growth inhibition. Although ABA totally inhibited the growth of 6 sweet potato plant introductions at a concentration of 10.0 mg 1^-1, the efficacy of ABA as a suppressant of shoot growth varied with genotype.Abbreviations ABA abscisic acid - GA gibberellic acid - cDNA complementary DNA - PI plant introduction - SE standard error     

In vitro multiplication of Nothapodites foetida (Wight.) Sleumer through seedling explant cultures

In vitro multiplication of Nothapodites foetida (Wight.) Sleumer was achieved using axenic seedling explant cultures. Isolated nodes (1.0-1.2 cm) and shoot tips (1.0-1.5 cm) cultured in Murashige and Skoog's agar medium containing varying concentrations of TDZ, BA and combinations of 2iP and GA3. Single shoot (0.8-1.2 cm) was regenerated in each culture after 6 weeks. Axillary shoots were then excised and recultured for 8 weeks in medium containing TDZ (0.05 mgL-1) which formed shoots (about 4 in no.; 2 cm) from the basal node. Axillary branches (2) which formed on 60% of these shoots after 10-12 weeks of culture were separated and recultured in the same medium for 8 weeks. Three shoots (0.8-1.0 cm) per culture were regenerated. Shoots of 0.8-1.8 cm length were subcultured on a low cytokinin (0.01 mgL-1 TDZ) regime to induce shoot elongation (2.0-3.5 cm) in 4 weeks. Shoot cuttings were rooted (60%) in the medium containing IBA (1.5 mgL-1). Rooted plantlets established in pots (90%) after hardening resumed normal growth in 3 months.     

THE EFFECT OF MALEIC HYDRAZIDE ON THE GROWTH RESPONSE OF PLANTS TO GIBBERELLIC ACID

Maleic hydrazide (MH) and gibberellic acid (GA) were applied alone and in combination at various doses to dwarf and tall varieties of garden pea, and their effect on stem extension measured. Combinations of MH and 3-indolylacetic acid (IAA) were also studied. Stern extension of dwarf peas was accelerated by GA and inhibited by MH. Their effects were not additive, since MH reduced the response to GA at all concentrations of each tested. IAA did not affect stem extension, whether applied alone or in combination with MH. Stem extensions of tall peas was not affected by GA or IAA alone. MH severely inhibited growth and this inhibition was not reduced either by GA or by IAA. At low doses MH broke apical dominance and side branches developed; extension of these was stimulated by GA and IAA and extension of the main axis correspondingly still further reduced. The results show that MH prevents the response to GA of GA-sensitive plants. It is suggested that the rapid growth of tall peas, as compared with that of dwarfs, and their lack of response to GA, are due to a greater capacity to synthesize a 'GA-like hormone'. Growth of tall peas is much more drastically inhibited by MH than that of dwarf peas and the suggestion is made that the inhibition of shoot growth induced by MH is due primarily to blocking the activity of the postulated 'GA-like hormone'.     

Effect of indoleacetic acid,abscisic acid,root tips and coleoptile tips on growth and curvature of maize roots

The effect of externally applied indoleacetic acid (IAA) and abscisic acid (ABA) on the growth of roots of Zea mays L. was measured. Donor blocks of agar with IAA or ABA were placed laterally on the roots and root curvature was measured. When IAA was applied to vertical roots, a curvature directed toward the donor block was observed. This curvature corresponded to a growth inhibition at the side of the root where the donor was applied. When IAA was applied to horizontal roots from the upper side, normal geotropic downward bending was delayed or totally inhibited. The extent of retardation and the inhibition of curvature were found to depend on the concentration of IAA in the donor block. ABA neither induced curvature in vertical roots nor inhibited geotropic curvature in horizontal roots; thus the growth of roots was not inhibited by ABA. However, when, instead of donor blocks, root tips or coleoptile tips were placed onto vertical roots, a curvature of the roots was observed.Abbreviations ABA abscisic acid - IAA 3-indoleacetic acid     

Roots of willow (Salix viminalis L.) show marked tolerance to oxygen shortage in flooded soils and in solution culture

Responses to soil flooding and oxygen shortage were studied in field, glasshouse and controlled environment conditions. Established stools ofSalix viminalis L., were compared at five field sites in close proximity but with contrasting water table levels and flooding intensities during the preceding winter. There was no marked effect of site on shoot extension rate, time to half maximum length or final length attained. When rooted cuttings were waterlogged for 4 weeks in a glasshouse, soil redox potentials quickly decreased to below zero. Shoot extension was slowed after a delay of 20 d, while, in the upper 100 mm of soil, formation and outgrowth of unbranched adventitious roots with enhanced aerenchyma development was promoted after 7 d. At depths of 100–200 mm and 200–300 mm, extension by existing root axes was halted by soil flooding, while adventitious roots from above failed to penetrate these deeper zones. After 4 weeks waterlogging, all arrested root tips recommenced elongation when the soil was drained; their extension rates exceeding those of roots that were well-drained throughout. Growth in fresh mass was also stimulated. The additional aerenchyma found in adventitious roots in the upper 100 mm of soil may have been ethylene regulated since gas space development was inhibited by silver nitrate, an ethylene action inhibitor. The effectiveness of aerenchyma was tested by blocking the entry of atmospheric oxygen into plants with lanolin applied to lenticels of woody shoots of plants grown in solution culture. Root extension was halved, while shoot growth remained unaffected. H Lambers Section editor     

Changes in endogenous growth regulators in nasturtium leaves during senescence

Summary By use of lettuce-hypocotyl and wheat-coleoptile bioassay, the presence of both gibberellin (GA)-like and abscisic-acid(ABA)-like components in acidic ethyl-acetate extracts of fully expanded nasturtium (Tropaeolum majus) leaves has been shown. During senescence of detached leaves there was a progressive decline in GA-like components and an increase in ABA-like components. Pretreatment of detached leaves with GA₃ or kinetin prevented changes in the levels of endogenous growth regulators and delayed senescence. The observations provide experimental verification for the concept that senescence is associated with changes in endogenous growth regulators.     

Increased endogenous abscisic Acid maintains primary root growth and inhibits shoot growth of maize seedlings at low water potentials

Roots of maize (Zea mays L.) seedlings continue to grow at low water potentials that cause complete inhibition of shoot growth. In this study, we have investigated the role of abscisic acid (ABA) in this differential growth sensitivity by manipulating endogenous ABA levels as an alternative to external applications of the hormone. An inhibitor of carotenoid biosynthesis (fluridone) and a mutant deficient in carotenoid biosynthesis (vp 5) were used to reduce the endogenous ABA content in the growing zones of the primary root and shoot at low water potentials. Experiments were performed on 30 to 60 hour old seedlings that were transplanted into vermiculite which had been preadjusted to water potentials of approximately −1.6 megapascals (roots) or −0.3 megapascals (shoots). Growth occurred in the dark at near-saturation humidity. Results of experiments using the inhibitor and mutant approaches were very similar. Reduced ABA content by either method was associated with inhibition of root elongation and promotion of shoot elongation at low water potentials, compared to untreated and wild-type seedlings at the same water potential. Elongation rates and ABA contents at high water potential were little affected. The inhibition of shoot elongation at low water potential was completely prevented in fluridone-treated seedlings during the first five hours after transplanting. The results indicate that ABA accumulation plays direct roles in both the maintenance of primary root elongation and the inhibition of shoot elongation at low water potentials.     

Thermoinductive Regulation of Gibberellin Metabolism in Thlaspi arvense L. (II. Cold Induction of Enzymes in Gibberellin Biosynthesis)

Vernalization of Thlaspi arvense L. results in the alteration of gibberellin (GA) metabolism such that the metabolism and turnover of the GA precursor ent-kaur-16-en-19-oic acid (kaurenoic acid) is dramatically increased. This cold-induced change in GA metabolism is restricted to the shoot tip, the site of perception of cold in this species (J.P. Hazebroek, J.D. Metzger [1990] Plant Physiol 94: 157-165). In the present report additional biochemical information about the nature of this low-temperature-regulated process is provided. The endogenous levels of kaurenoic acid in leaves and shoot tips of plants were estimated by combined gas chromatography-chemical ionization mass spectrometry at various times after 4 weeks of vernalization at 6[deg]C. The endogenous levels in shoot tips declined 10-fold by 2 d after the plants were returned to 21[deg]C; this decline continued such that there was nearly 50-fold less kaurenoic acid by 10 d after the end of vernalization. No effect of vernalization on the endogenous levels of kaurenoic acid in leaves was observed. An in vitro enzyme assay was developed to monitor changes in the ability of tissues to convert kaurenoic acid to ent-7[alpha]-hydroxykaur-16-en-19-oic acid (7-OH kaurenoic acid). The activity of this enzyme rapidly increased in microsomal extracts from shoot tips following the end of vernalization. No thermoinduced increase in activity was observed in leaves. The enzymic oxidation of ent-kaurene to ent-kaurenol was also induced in shoot tips by vernalization. However, this reaction does not appear to be rate limiting for GA biosynthesis, because substantial amounts of kaurenoic acid accumulated in noninduced shoot tips. These results corroborate our hypothesis that the conversion of kaurenoic acid to 7-OH kaurenoic acid is the primary step in GA metabolism regulated by vernalization in Thlaspi shoot tips.     

Auxin-induced ethylene triggers abscisic acid biosynthesis and growth inhibition

The growth-inhibiting effects of indole-3-acetic acid (IAA) at high concentration and the synthetic auxins 7-chloro-3-methyl-8-quinolinecarboxylic acid (quinmerac), 2-methoxy-3,6-dichlorobenzoic acid (dicamba), 4-amino-3,6, 6-trichloropicolinic acid (picloram), and naphthalene acetic acid, were investigated in cleavers (Galium aparine). When plants were root treated with 0.5 mM IAA, shoot epinasty and inhibition of root and shoot growth developed during 24 h. Concomitantly, 1-aminocyclopropane-1-carboxylic acid (ACC) synthase activity, and ACC and ethylene production were transiently stimulated in the shoot tissue within 2 h, followed by increases in immunoreactive (+)-abscisic acid (ABA) and its precursor xanthoxal (xanthoxin) after 5 h. After 24 h of treatment, levels of xanthoxal and ABA were elevated up to 2- and 24-fold, relative to control, respectively. In plants treated with IAA, 7-chloro-3-methyl-8-quinolinecarboxylic acid, naphthalene acetic acid, 2-methoxy-3,6-dichlorobenzoic acid, and 4-amino-3,6,6-trichloropicolinic acid, levels of ethylene, ACC, and ABA increased in close correlation with inhibition of shoot growth. Aminoethoxyvinyl-glycine and cobalt ions, which inhibit ethylene synthesis, decreased ABA accumulation and growth inhibition, whereas the ethylene-releasing ethephon promoted ABA levels and growth inhibition. In accordance, tomato mutants defective in ethylene perception (never ripe) did not produce the xanthoxal and ABA increases and growth inhibition induced by auxins in wild-type plants. This suggests that auxin-stimulated ethylene triggers ABA accumulation and the consequent growth inhibition. Reduced catabolism most probably did not contribute to ABA increase, as indicated by immunoanalyses of ABA degradation and conjugation products in shoot tissue and by pulse experiments with [(3)H]-ABA in cell suspensions of G. aparine. In contrast, studies using inhibitors of ABA biosynthesis (fluridone, naproxen, and tungstate), ABA-deficient tomato mutants (notabilis, flacca, and sitiens), and quantification of xanthophylls indicate that ABA biosynthesis is influenced, probably through stimulated cleavage of xanthophylls to xanthoxal in shoot tissue.     

Epibrassinolide-inhibition of development of excised, adventitious and intact roots of tomato (Lycopersicon esculentum): comparison with the effects of steroidal estrogens

The brassinosteroid, 24-epibrassinolide, caused observable inhibition of the growth of aseptically-cultured excised tomato ( Lycopersicon esculentum Mill. cv. Best of All) roots at a concentration of 0.01 μ M . Root formation and outgrowth in tomato shoot cuttings and root biomass in intact tomato seedlings were also depressed by epibrassinolide, but only at a higher concentration (0.1μ M ). Concentrations of the steroidal estrogens, estrone and estradiol (as sulphate derivatives), in excess of 1.0 μ M , reduced cultured root growth as well as root number and total (but not mean) root length in shoot cuttings. Seedling root growth was only slightly inhibited by estradiol but not by estrone. Both epibrassinolide and estrogens caused morphological abnormalities, such as epinasty and leaf-inrolling in cuttings and seedlings. Epibrassinolide stimulated extension growth of the hypocotyl and epicotyl in cuttings and of the hypocotyl in seedlings, but estrogens exerted no effects on shoot growth.     

Antibodies to phase related proteins in juvenile and mature Prunus avium

In the search for biochemical and molecular markers of juvenility in trees proteins have been identified which are preferentially or differentially expressed in either the juvenile or the mature phases of Prunus avium cv. Stella. These fall into two classes: those which are phase-related and those which may be affected by root-shoot distance. N-terminal amino acid sequence data from some of these proteins were used to produce polyclonal antibodies to corresponding synthetic peptides in order to determine if they could be used as markers of phase state in woody plants. Western blot analysis was performed on proteins extracted from three sources; juvenile trees, mature trees and rooted cuttings from mature trees. The results showed that the antibodies recognised differentially-expressed proteins. In particular, one antibody to a juvenile specific protein cross-reacted with a polypeptide of approx. 28 kDa which was present in greater amounts in shoot tips of juvenile P. avium cv. Stella seedlings compared with rooted cuttings of mature plants placed in the same growth environment.     

Ascorbic acid and reactive oxygen species are involved in the inhibition of seed germination by abscisic acid in rice seeds

The antagonism between abscisic acid (ABA) and gibberellin (GA) plays a key role in controlling seed germination, but the mechanism of antagonism during this process is not known. The possible links among ABA, reactive oxygen species (ROS), ascorbic acid (ASC), and GA during rice seed germination were investigated. Unlike in non-seed tissues where ROS production is increased by ABA, ABA reduced ROS production in imbibed rice seeds, especially in the embryo region. Such reduced ROS also led to an inhibition of ASC production. GA accumulation was also suppressed by a reduced ROS and ASC level, which was indicated by the inhibited expression of GA biosynthesis genes, amylase genes, and enzyme activity. Application of exogenous ASC can partially rescue seed germination from ABA treatment. Production of ASC, which acts as a substrate in GA biosynthesis, was significantly inhibited by lycorine which thus suppressed the accumulation of GA. Consequently, expression of GA biosynthesis genes was suppressed by the low levels of ROS and ASC in ABA-treated seeds. It can be concluded that ABA regulates seed germination in multiple dimensions. ROS and ASC are involved in its inhibition of GA biosynthesis.     

On the role of abscisic Acid and gibberellin in the regulation of growth in rice

Submergence induces rapid elongation of rice coleoptiles (Oryza sativa L.) and of deepwater rice internodes. This adaptive feature helps rice to grow out of the water and to survive flooding. Earlier, we found that the growth response of submerged deepwater rice plants is mediated by ethylene and gibberellin (GA). Ethylene promotes growth, at least in part, by increasing the responsiveness of the internodal tissue to GA. In the present work, we examined the possibility that increased responsiveness to GA was based on a reduction in endogenous abscisic acid (ABA) levels. Submergence and treatment with ethylene led, within 3 hours, to a 75% reduction in the level of ABA in the intercalary meristem and the growing zone of deepwater rice internodes. The level of GA₁ increased fourfold during the same time period. An interaction between GA and ABA could also be shown by application of the hormones. ABA inhibited growth of submerged internodes, and GA counteracted this inhibition. Our results indicate that the growth rate of deepwater rice internodes is determined by the ratio of an endogenous growth promoter (GA) and a growth inhibitor (ABA). We also investigated whether ABA is involved in regulating the growth of rice coleoptiles. Rice seedlings were grown on solutions containing fluridone, an inhibitor of carotenoid and, indirectly, of ABA biosynthesis. Treatment with fluridone reduced the level of ABA in coleoptiles and first leaves by more than 75% and promoted coleoptile growth by more than 60%. Little or no enhancement of growth by fluridone was observed in barley, oat, or wheat. The involvement of ABA in determining the growth rate of rice coleoptiles and deepwater rice internodes may be related to the semiaquatic growth habit of this plant.