Modification of growth anisotropy and cortical microtubule dynamics in Arabidopsis hypocotyls grown under microgravity conditions in space

We carried out a space experiment, denoted as Aniso Tubule, to examine the effects of microgravity on the growth anisotropy and cortical microtubule dynamics in Arabidopsis hypocotyls, using lines in which microtubules are visualized by labeling tubulin or microtubule‐associated proteins (MAPs) with green fluorescent protein (GFP). In all lines, GFP‐tubulin6 (TUB6)‐, basic proline‐rich protein1 (BPP1)‐GFP‐ and spira1‐like3 (SP1L3)‐GFP‐expressing using a constitutive promoter, and spiral2 (SPR2)‐GFP‐ and GFP‐65 kDa MAP‐1 (MAP65‐1)‐expressing using a native promoter, the length of hypocotyls grown under microgravity conditions in space was longer than that grown at 1 g conditions on the ground. In contrast, the diameter of hypocotyls grown under microgravity conditions was smaller than that of the hypocotyls grown at 1 g. The percentage of cells with transverse microtubules was increased under microgravity conditions, irrespective of the lines. Also, the average angle of the microtubules with respect to the transverse cell axis was decreased in hypocotyls grown under microgravity conditions. When GFP fluorescence was quantified in hypocotyls of GFP‐MAP65‐1 and SPR2‐GFP lines, microgravity increased the levels of MAP65‐1, which appears to be involved in the maintenance of transverse microtubule orientation. However, the levels of SPR2 under microgravity conditions were comparable to those at 1 g. These results suggest that the microgravity‐induced increase in the levels of MAP65‐1 is involved in increase in the transverse microtubules, which may lead to modification of growth anisotropy, thereby developing longer and thinner hypocotyls under microgravity conditions in space.     

Effects of hypergravity on the elongation growth in radish and cucumber hypocotyls

The elongation growth of the hypocotyls of radish and cucumber seedlings was examined under hypergravity in a newly developed centrifuge (Kasaharaet al. 1995). The effects of hypergravity on elongation growth differed between the two species. The rate of elongation of radish hypocotyls was reduced under basipetal hypergravity (H+20g) but not under acropetal hypergravity (H-13g), as compared to growth under the control conditions (C+1g and C-1g). In cucumber hypocotyls, elongation growth was inhibited not only by basipetal but also by acropetal hypergravity. Under these conditions, the reduction in the elongation growth of both radish and cucumber hypocotyls was accompanied by an increase in their thickness. Although no distinct differences in relative composition of neutral sugars were found, the amounts of cell-wall components (pectic substances, hemicelluloses and cellulose) per unit length of hypocotyls were increased by exposure to hypergravity.     

Germination responses of Discopodium penninervium Hochst. to temperature and light

The germination responses of Discopodium penninervium were tested at different constant and alternating temperature regimes as well as under various light conditions both in the laboratory and glasshouse. Seeds incubated at 10, 15, 20, 25 and 30 °C failed to germinate. When the seeds were incubated at alternating temperatures of 20/12 °C and 30/12 °C under continuous light, germination was 89 and 61%, indicating that the species requires alternating temperatures as a cue for germination. However, germination declined as the amplitude of alternating temperatures increased from 8 °C and was completely inhibited at an amplitude of 23 °C, suggesting that the optimum amplitude is around 8 °C. Germination was less than 10% in light and nil in darkness at 20 °C in the laboratory. In contrast, seeds incubated at 20/12 °C germinated to 96 and 86% in light and darkness, respectively. Seeds incubated under leaf shade in the glasshouse failed to germinate whereas those incubated under direct daylight and darkness germinated to 44 and 50%, respectively, 30 days after sowing. When seeds incubated under leaf shade and in darkness were exposed afterwards to light, final percent germination was 83% from seeds incubated initially under direct daylight, 79% from those incubated under leaf shade and 86% from those incubated in darkness. The requirement for alternating temperatures and light rich in red:far red ratio to break the dormancy of seeds of D. penninervium could restrict germination to gaps in the vegetation. The results conform with the ecology of the species.     

毛脉酸模的组织培养及植株再生

以毛脉酸模下胚轴和幼苗根茎为外植体,以MS为基本培养基,与不同浓度比例的6-BA和2,4-D等植物生长调节剂进行毛脉酸模组织培养的研究。结果表明,毛脉酸模幼苗根茎是诱导丛生芽的最佳材料,诱导丛生芽产生的最佳培养基为MS+6-BA 3.0 mg·L^-1+2,4-D 0.1 mg·L^-1,诱导率为81.2%;生根培养基为无激素的MS培养基,生根率可达92%,在此条件下生根迅速,可获得完整植株;组培苗经过炼苗移栽后成活率可达80%。     

Effects of exopolysaccharide production on liquid vegetative growth,stress survival,and stationary phase recovery in <Emphasis Type="Italic">Myxococcus xanthus</Emphasis>

Exopolysaccharide (EPS) of Myxococcus xanthus is a well-regulated cell surface component. In addition to its known functions for social motility and fruiting body formation on solid surfaces, EPS has also been proposed to play a role in multi-cellular clumping in liquid medium, though this phenomenon has not been well studied. In this report, we confirmed that M. xanthus clumps formed in liquid were correlated with EPS levels and demonstrated that the EPS encased cell clumps exhibited biofilm-like structures. The clumps protected the cells at physiologically relevant EPS concentrations, while cells lacking EPS exhibited significant reduction in long-term viability and resistance to stressful conditions. However, excess EPS production was counterproductive to vegetative growth and viable cell recovery declined in extended late stationary phase as cells became trapped in the matrix of clumps. Therefore, optimal EPS production by M. xanthus is important for normal physiological functions in liquid.     

Morphogenesis of Rice and Arabidopsis Seedlings in Space

Oryza sativa L.) and Arabidopsis (A. thaliana L.) were cultivated for 68.5 hr in the RICE experiment on board during Space Shuttle STS-95 mission, and changes in their growth and morphology were analyzed. Microgravity in space stimulated elongation growth of both rice coleoptiles and Arabidopsis hypocotyls by making their cell walls extensible. In space, rice coleoptiles showed an inclination toward the caryopsis in the basal region and also a spontaneous curvature in the same direction in the elongating region. These inclinations and curvatures were more prominent in the Koshihikari cultivar compared to a dwarf cultivar, Tan-ginbozu. Rice roots elongated in various directions including into the air on orbit, but two thirds of the roots formed a constant angle with the axis of the caryopsis. In space, Arabidopsis hypocotyls also elongated in a variety of directions and about 10% of the hypocotyls grew into the agar medium. No clear curvatures were observed in the elongating region of Arabidopsis hypocotyls. Such a morphology of both types of seedlings was fundamentally similar to that observed on a 3-D clinostat. Thus, it was confirmed by the RICE experiment that rice and Arabidopsis seedlings perform an automorphogenesis under not only simulated but also true microgravity conditions. Received 13 September 1999/ Accepted in revised form 12 October 1999     

Development, survivorship, and reproduction of Helicoverpa armigera (Lepidoptera: Noctuidae) under constant and alternating temperatures

Laboratory studies were conducted to assess the effect of temperature on the survival, development, fecundity, and longevity of Helicoverpa armigera (Hübner) at 11 constant temperatures ranging from 12.5 to 40 degrees C, as well as at five alternating temperature regimes (25-10, 30-15, 32.5-17.5, 35-20, and 35-27.5 degrees C) and under a photoperiod of 16:8 (L:D) h. H. armigera reared at constant temperatures did not develop from egg to adult (emergence) outside the temperature range of 17.5-32.5 degrees C. The alternating conditions expanded this range from 10 to 35 degrees C. The lowest developmental thresholds of the immature stages were estimated by a linear model and ranged from 10.17 (pupal stage) to 11.95 degrees C (egg stage) at constant temperature regimes and from 1.1 to 5.5 degrees C, respectively at alternating temperatures. The values of developmental thresholds estimated using the nonlinear (Lactin-2) model were lower than those estimated by the linear model for constant and alternating temperature regimes except for larval and pupal stages at constant temperatures. Mean adult longevity fluctuated from 34.4 d at 15 degrees C to 7.6 d at 35 degrees C. Females reared under all alternating temperature regimes laid more eggs than females reared at any, except the 25 degrees C, constant temperature treatment. The intrinsic rate of increase was highest at 27.5 degrees C, at both the constant and the corresponding alternating temperature regimes (0.147 and 0.139, respectively). Extreme temperatures had a negative effect on life table parameters.     

Effects of temperature, light, nitrate and pre chilling on seed germination of grassland plants

The effects of factorial combinations of alternating (20/10 ^oC) and constant temperature (20 ^oC), of light (intermittent) and dark, and of distilled water and nitrate on the germination of a range of species of indigenous grassland were investigated in 1979, and in 1980 when the effects of pre-chilling were also studied. Species differed greatly in their response to the eight sets of environmental conditions in 1979. With the exception of Lolium perenne ssp. perenne and Cynosurus cristatus, only a small percentage of seeds of most species was able to germinate in constant temperature in the dark. However, when light was supplied there was moderate germination of Anthoxanthum odoratum, Cerastium fontanum ssp. glabrescens, Festuca rubra, Holcus lanatus and Poa trivialis. Alternating temperature greatly increased the germination of most species and nitrate further increased germination of Agrostis capillaris, Deschampsia caespitosa, H. lanatus, Poa annua and P. trivialis. In alternating temperatures, light increased germination of these species even more than did nitrate. A. capillaris, D. caespitosa and P. annua required all three factors for maximum germination, and another 7% of seeds of A. capillaris also required gibberellic acid. Germination responses following sequential application of factors often differed from those resulting from simultaneous treatment: in particular, the germination of Ranunculus species was greatly enhanced. Although seeds of the species tested in 1980 were more germinable than those of the same species in 1979, they responded similarly to the different factors. Light increased the germination of both Taraxacum officinale and Plantago lanceolata in constant but not in alternating temperature, while nitrate was much more stimulatory to the latter species. About half the seeds of P. lanceolata were dormant. Pre-chilling at 4^oC for 7 days increased subsequent germination of all species when followed by constant temperature, except of A. capillaris in the dark and C. cristatus (already maximal) in the light. When followed by alternating temperature in the dark, pre-chilling greatly decreased germination of A. capillaris. In the light, where germination of most species was maximal, there was little effect of pre-chilling. Longer durations (21 and 42 days) of chilling of older seed gave similar results to the 7 day pre-chilling.     

The effect of clumped planting patterns on epidemics of damping-off disease in cress seedlings

Summary Environmentally controlled experiments on damping-off disease of garden cress (Lepidium sativum) caused by Pythium irregulare were used to study the effect of clumped planting patterns upon epidemic rates. In clumped stands of seedlings the rate of multiplication of disease was, in general, slower than in unclumped, evenly spaced stands of the same overall density. When the number of clumps per unit area was varied, but the proportion of the total area occupied by clumps kept constant, there was no significant change in the rate of linear advance of the disease between different treatments, but the multiplication rate in randomly inoculated plots was higher at 100 clumps/m² than 200 clumps/m² due to an unexpected interaction between the multiplication rate and the distribution of primary disease foci. When the area occupied by clumps was varied, but the number of clumps per unit area and the overall plant density were kept constant, both the multiplication rate and the rate of linear advance of the disease were significantly reduced in the stands with the most condensed clumps.     

Effects of flood-influenced factors on seed germination of Ambrosia tenuifolia

Strong fluctuations are exhibited by populations of the perennial herb Ambrosia tenuifolia in the grasslands of the Salado basin (Province of Buenos Aires, Argentina), an area frequently enduring prolonged floods. Flooding causes the death of most dicotyledon plants of the community, A. tenuifolia among them, opening numerous gaps of various sizes. After the recession of the flood the density of A. tenuifolia seedlings was higher in flooded than in non-flooded plots and it was larger in wider gaps. Canopy removal in non-flooded plots increased field seedling emergence of A. tenuifolia up to the levels found in flooded plots. Responses of the seeds in the soil to gap-associated environmental factors such as light quality and temperature regime were studied both in the field and under controlled were studied both in the field and under controlled conditions. Seedling emergence was significantly enhanced when the red:far-red ratio of natural light reaching the soil surface under the canopy of nonflooded plots was increased by means of copper sulfate filters. The influence of light quality and temperature on germination of the soil seed population was also tested using grassland soil monoliths or mesocosms, transported from the field to the laboratory, in which the canopy was clipped and the soil exposed to either red or far-red light and kept at constant or fluctuating temperatures. Significant seedling emergence was observed only when the soil samples were exposed to red light and incubated at alternating temperatures. No emergence was recorded in samples exposed to far-red light or incubated at a constant 25°C. Seeds stored dry in the laboratory were also stimulated to germinate by red light and alternating temperatures but only after dormancy was sufficiently decreased by low temperature stratification or by low temperature under immersion. The results are consistent with the hypothesis that primary dormancy of A. tenuifolia seeds is decreased by low temperatures in winter even if the seeds are submerged as happens when floods occur. The decrease in dormancy makes the seeds prone to be stimulated to germinate by the Pfr form of phytochrome in combination with alternating temperatures. These conditions are likely to be met in the gaps opened by the flood-caused death of dicotyledon plants.     

Phenotypic plasticity of body pigmentation in Drosophila: influence of a developmental thermoperiodic regime in two sibling species

Abstract Variation of body pigmentation according to growth temperature was analysed in two European populations (Bordeaux and Cordoba) of the two sibling species Drosophila melanogaster (Meigen) and D. simulans (Sturtevant). Results obtained at constant temperatures were compared with those observed under alternating thermal regimes of various amplitudes and with a daily periodicity of 12–12 h. Abdominal pigmentation in segments 5, 6 and 7, and of their sum, was investigated in both species in females only. Thoracic trident pigmentation was studied in males and females but in Drosophila melanogaster only.
Thoracic pigmentation exhibited curvilinear convex reaction norms under constant conditions, with a darker pigmentation in the population (Bordeaux) living in a colder climate. No specific effects of alternating regimes were seen. For abdomen pigmentation, slightly different reaction norms were observed under constant conditions between Bordeaux and Cordoba strains. There were also slight differences between the two species. Alternating temperatures produced a significant decrease in average pigmentation, roughly proportional to the thermal amplitude.
Alternating stressing conditions induced also an increase in the phenotypic variability of abdominal pigmentation, especially in segments 6 and 7. Finally, because an increase of growth temperature results in a monotonically decreasing reaction norm of both abdomen pigmentation in females and a size-related trait, the wing/thorax ratio, we found a linear relationship between the two variables, with a very high overall correlation at constant temperatures ( r  = 0.99). The correlation was slightly less under alternating conditions, due to a broader dispersal of the experimental values around the regression lines.     

Seed germination traits of Trapella sinensis (Trapellaceae), an endangered aquatic plant in Japan: Conservation implications

We investigated the effects of cold stratification, temperature, light, and oxygen conditions on seed germination of Trapella sinensis Oliver, an endangered aquatic plant in Japan. Seeds had physiological dormancy, and final germination rate increased with an increasing period of cold stratification. Seeds of T. sinensis had an almost absolute requirement for aerobic conditions to germinate. Also, alternating temperatures significantly promoted germination regardless of light conditions, although final germination percentage was twice to four times higher in light under constant temperature conditions. Suitable sites for germination of T. sinensis appear to be shallow water with alternating temperatures and sufficient oxygen and sediment anoxia, caused by eutrophication of water bodies, may have prevented recruitment of T. sinensis from seed.     

Effect of continuous darkness on circadian morphological rhythms in pinealocytes of the Chinese hamster,Cricetulus griseus

Summary Effects of a short-term exposure to continuous darkness on 24-h morphological variations in pinealocytes in the superficial pineal of the Chinese hamster (Cricetulus griseus) were examined. Pinealocytes contained type-1, -2 and -3 synaptic ribbons (SR), which had a central dense structure showing rod-like, various and ring-like profiles, respectively, and the quantity of each type of SR was expressed by SR index. 24-h changes in the type-1 and type-3 SR indices persisted in darkness and thus may be endogenous in nature. As under alternating light and dark (LD) conditions, the type-2 SR indices were almost constant over a 24-h period under continuous darkness, but the indices were larger in animals under darkness than in those under LD conditions. The 24-h variations in the nuclear and cytoplasmic volumes were abolished after exposing animals to darkness for 7 days, suggesting that these rhythms may be regulated exogenously. The amount of condensed chromatin exhibited a circadian change; this rhythm persisted under darkness. The results suggest that 24-h variations in the nuclear and cytoplasmic volumes in pinealocytes of the Chinese hamster are regulated by mechanisms different from those controlling the rhythms in SR and chromatin, and that the changes in the nuclear and cytoplasmic volumes and chromatin are related to the change in synthetic activity of pinealocytes.     

An endogenous growth inhibitor, 3-hydroxy-β-ionone. II. Changes in its levels associated with the light-induced growth inhibition of hypocotyls of Phaseolus vulgaris

Dark-grown, 4-day-old seedlings of dwarf bean ( Phuseolus vulgaris L. cv. Morocco) and tall bean (cv. Kentucky Wonder) were transferred to conditions of continuous light at various fluence rates, and the resulting changes in growth rates and concentration of 3-hydroxy-β-ionone, a novel endogenous growth inhibitor, were monitored. The light-induced inhibition of growth and the levels of the inhibitor concentration were dependent on the fluence rate of light applied. The growth rate of hypocotyls of both cultivars decreased rapidly and reached a plateau 18 h after the onset of light. However, the more noticeable changes were the marked inhibition of growth of the hypocotyls of the dwarf cultivar. The concentration of 3-hydroxy-β-ionone in the hypocotyls of both cultivars increased after a 20-min lag and reached plateau values after 12 h. The concentration of the inhibitor in the hypocotyls of the dwarf cultivar increased to about 4 to 8 times the level in the nonirradiated controls, while that in the hypocotyls of the tall cultivar increased only about 2- to 3-fold under the same conditions. The high concentration of 3-hydroxy-β-ionone in the dwarf cultivar under steady-state conditions in continuous light, as compared with that in the tall cultivar, may determine the growth habit of these cultivars.     

Interaction of red to far red light ratio and ethylene in regulating stem elongation of <Emphasis Type="Italic">Helianthus annuus</Emphasis>

Sunflower (Helianthus annuus L.) stems showed increased elongation under two types of vegetative shade: canopy shade (low red to far red [R/FR] ratio) and neighbouring proximity shade (FR enrichment). Hypocotyls also elongated more under narrow-band FR light than under narrow-band R light. Ethylene levels were determined in actively elongating 7-day-old hypocotyls and 17-day-old internodes under three R/FR ratios. Ethylene levels were lower in both sunflower hypocotyls and internodes when the R/FR ratio was reduced. Both FR enrichment of normal R/FR ratio and narrow-band FR light with very low light irradiance resulted in reduction in ethylene levels in 7-day-old hypocotyls. Further, in application experiments, sunflower stems grown under low R/FR ratio were more sensitive to ethephon and less sensitive to aminoethoxyvinylglycine (AVG) than stems grown under high R/FR ratio. Low R/FR ratio appears to initiate reduction in ethylene levels in sunflower seedlings, allowing maximum stem elongation. These results, and findings of other authors, suggest that various plant species may have developed different ways of regulating stem elongation and ethylene levels in response to low R/FR ratio.     

Persistence of plant hormone levels in rice shoots grown under microgravity conditions in space: its relationship to maintenance of shoot growth

We investigated the effects of microgravity environment on growth and plant hormone levels in dark‐grown rice shoots cultivated in artificial 1 g and microgravity conditions on the International Space Station (ISS). Growth of microgravity‐grown shoots was comparable to that of 1 g‐grown shoots. Endogenous levels of indole‐3‐acetic acid (IAA) in shoots remained constant, while those of abscisic acid (ABA), jasmonic acid (JA), cytokinins (CKs) and gibberellins (GAs) decreased during the cultivation period under both conditions. The levels of auxin, ABA, JA, CKs and GAs in rice shoots grown under microgravity conditions were comparable to those under 1 g conditions. These results suggest microgravity environment in space had minimal impact on levels of these plant hormones in rice shoots, which may be the cause of the persistence of normal growth of shoots under microgravity conditions. Concerning ethylene, the expression level of a gene for 1‐aminocyclopropane‐1‐carboxylic acid (ACC) synthase, the key enzyme in ethylene biosynthesis, was reduced under microgravity conditions, suggesting that microgravity may affect the ethylene production. Therefore, ethylene production may be responsive to alterations of the gravitational force.     

Potential differences in seed dispersals of low‐height vegetation between single element and windbreak‐like clumps

Wind speed is one of the most important factors for seed wind dispersal. A wind speed reduction region, which could be influenced by vegetation arrangement, will form in the lee of vegetation and therefore affects the seed dispersal. Here, by taking shrub as an example, quantitative differences in seed dispersals of low vegetation between single element and windbreak‐like clumps are numerically investigated. The local variation of stream‐wise wind speed is focused. Empirically parameterized functions of leeward wind distributions are employed. It reveals that the accumulative probability of dispersed seeds from a point source with considering leeward wind reduction could be well fitted by a logistic function. For a fixed release height or vegetation porosity, accumulative probabilities for single element and those for windbreak‐like clumps would intersect at a leeward location. This intersection location decreases linearly with release height but exponentially with porosity. The fitting parameter r₀ (the center of logistic function) for single element increases as the same manner for windbreak‐like clumps, with regard to the increase of release height, porosity, and height. But, the increasing rates for single element are higher than those for windbreak‐like clumps. The fitting parameter p (the power index of logistic function) for single element is generally larger than that for windbreak‐like clumps. With the increase of release height, p decreases at first but increases then for single element, while it shows opposite trend for windbreak‐like clumps. p decreases with porosity for both single element and windbreak‐like clumps. But, the decreasing rate for single element is lower than that for windbreak‐like clumps. p increases exponentially with height for windbreak‐like clumps, while it almost keeps constant for single element. These results suggest the potential importance of vegetation arrangement on seed dispersal and therefore possibly provide additional reason for the disagreement among observed dispersal kernels.     

The effect of dissolved oxygen on the growth of young-of-the-year winter flounder,Pseudopleuronectes americanus

Synopsis The effects of constant and diurnally fluctuating levels of dissolved oxygen on the growth of young-of-the-year winter flounder,Pseudopleuronectes americanus, were examined under controlled laboratory conditions. Fish were exposed for either 10 or 11 weeks to constant levels of 6.7 (high) and 2.2 (low) mg l^–1, and a diurnal fluctuation, ranging from 2.5 to 6.4 mg 0₂l^–1. Growth rates, calculated for both standard length and weight, for fish exposed to low and diurnally fluctuating levels were significantly reduced (p < 0.001) as compared to those for fish exposed to the high level. Growth rates of fish exposed to the high level were over twice those of fish held under low oxygen conditions. Under fluctuating conditions, fish grew at intermediate rates. Following these exposures, all fish were subsequently held at 7.2 mg Oz l^–1 for five weeks. Growth rates increased over two and a half times for fish previously exposed to the low oxygen level and were significantly (p < 0.001) higher than for the other two groups.     

Photoperiod,stress and the distribution of leukocytes in the peripheral blood ofNotophthalmus viridescens

Summary Adult newts,Notophthalmus viridescens, were maintained for 8 days at a constant temperature of 11.0±0.5 °C. In one series, the control animals were kept in constant darkness (DD), while the experimental newts were exposed to alternating 12-hour periods of light and dark (LD). In a second series, controlNotophthalmus lived in DD, and experimental animals lived in constant light (LL). In both series, the newts were sacrificed on the ninth day when blood smears were prepared. Differential counts of the leukocytes of animals that lived under the LD regimen were the same as those of controlNotophthalmus (Table 1). However, in newts that were maintained in LL, the neutrophils increased and the lymphocytes decreased relative to those types of cells in the controls (Table 1). Those changes indicate that continuous light constitutes stress for this species.Supported in part by a grant from the Committee on Research, Travel and Sabbatical Leaves, Colby College     

Somatic embryogenesis induced by the simple application of abscisic acid to carrot (Daucus carota L.) seedlings in culture

Seedlings of carrot (Daucus carota L. cv. Red Cored Chantenay) formed somatic embryos when cultured on medium containing abscisic acid (ABA) as the sole source of growth regulator. The number of embryos per number of seedlings changed depending on the concentration of ABA added to the medium, with a maximum embryo number at 1 × 10⁻⁴M ABA. Seedling age was critical for response to exogenous ABA; no seedling with a hypocotyl longer than 3.0 cm was able to form an embryo. Removal of shoot apices from seedlings completely inhibited the embryogenesis induced by application of exogenous ABA, suggesting that the action of ABA requires some substance(s) that is translocated basipetally from shoot apices through hypocotyls. Histologically, somatic embryos shared common epidermal cells and differentiated not through the formation of embryogenic cell clumps, but directly from epidermal cells. These morphological traits are distinct from those of embryogenesis via formation of embryogenic cell clumps, which has been found in embryogenic carrot cultures established using 2,4-dichlorophenoxyacetic acid or other auxins. These results suggest that ABA acts as a signal substance in stress-induced carrot seedling somatic embryogenesis. Received: 22 April 2000 / Accepted: 8 June 2000