Conditioning period, CO2 and GR24 influence ethylene biosynthesis and germination of Striga hermonthica

Germination of witchweed ( Striga hermonthica [Del.] Benth), an important root parasite on poaceous crops, requires pretreatment 'conditioning' in a warm moist environment and a subsequent exposure to a stimulant. The roles of conditioning period, CO₂ and a strigol analogue (GR24) in ethylene biosynthesis and germination of the parasite were investigated. Conditioning increased the seeds' capacity to oxidize exogenous 1-aminocyclopropane-1-carboxylic acid (ACC). Exogenous CO₂ increased the seeds capacity to oxidize ACC by 3- to 9-fold. A combination of GR24 and ACC increased ethylene production by more than 3-fold in comparison with the rates obtained using these compounds separately. Aminoethoxyvinylglycine (AVG) completely inhibited ethylene induction by GR24, but not by ACC. A GR24 treatment, made subsequent to conditioning in GR24, did not induce ethylene. However, seeds conditioned in GR24 and then given 1 m M ACC produced 293 nl l⁻¹ ethylene. ACC oxidase (ACCO) activity in crude extracts was increased by conditioning and CO₂. The enzyme displayed an absolute requirement for ascorbate. Absence of exogenous Fe²⁺ reduced enzyme activity only by 14%. GR24 applied during conditioning reduced germination in response to a subsequent GR24 treatment. ACC was, invariably, less effective in inducing S. hermonthica germination than GR24 even at concentrations which induce more ethylene than concurrent GR24 treatments. The results are consistent with a model in which conditioning removes a restriction on the ethylene biosynthetic pathway in S. hermonthica seeds. GR24 modulates the key enzymes in ethylene biosynthesis. The stimulant suppresses ethylene biosynthesis in unconditioned seeds and promotes it in conditioned ones. Germination of S. hermonthica results from the joint action of GR24 and the ethylene it induces.     

Enhancement of ethylene biosynthesis and germination by cytokinins and 1-aminocyclopropane-l-carboxylic acid in Striga asiatica seeds

Germination of witchweed ( Striga asiatica [L.] Kuntze), an important parasitic weed on several poaceous crops, is stimulated by several synthetic and natural compounds. We investigated the role of ethylene biosynthesis and action in cytokinin-induced germination. Conditioned Striga seeds treated with distilled water, 1-aminocyclopro-pane-1-carboxylic acid (ACC) or the cytokinins thidiazuron (TDZ), trans zeatin (TZ), benzyladenine (BA) and kinetin (KIN) produced little ethylene. Treatments with cytokinin-ACC combinations enhanced ethylene production. The relative order of activity of the cytokinins in elicitation of the phytohormone was TDZ > TZ > BA > KIN. Germination in response to distilled water and ACC treatments was negligible. Induction of germination by cytokinins varied from low (0%) to moderate (52%). Seeds treated with cytokinin-ACC combinations displayed high rates of germination. The observed germination was positively correlated (γ= 0. 8 and 0. 9) with ethylene production. Germination was reduced by silver thiosulphate (STS) and CoCl₂, inhibitors of ethylene action and ACC oxidase, respectively. Aminoethoxyvi-nylglycine (AVG), an ACC-synthase inhibitor, reduced TDZ-induced Striga germination. However, the inhibitory effect of AVG was overcome by addition of ACC. The results are consistent with a model in which Striga germination and embryo growth are limited by low capacity of the seeds to oxidize ACC. The cytokinins promote ACC conversion into ethylene and consequent Striga germination by enhancing ACC oxidase activity and/or synthesis.     

Effect of short-chain fatty acids on the ethylene pathway in embryonic axes of Cicer arietinum during germination

The effect of short-chain saturated fatty acids (C₅–C₁₀) on the biosynthesis of ethylene in embryonic axes of chick-pea ( Cicer arietinum L.) seeds was investigated. The emergence of radicle and fresh weight of embryonic axes diminished with increasing number of carbons. The inhibition of germination caused by lower concentrations (1 m M ) of fatty acids (C₅–C₁₀) was partially reversed by exogenous 1-aminocyclopropane-1-carboxylic acid (ACC), whereas exogenous ethylene was able to overcome the inhibitory effect provoked by all concentrations (1–5 m M ) of applied fatty acids (C₅–C₁₀). Ethylene production rates, and enzyme activities of ACC synthase and ACC oxidase decreased concomitantly with the molecular mass and increasing concentration of fatty acids. The inhibitory effect of these acids on ethylene production seems to result not only from a decreased ACC synthesis, but also from an enhancement of 1-malonylamino)cyclopropane-1-carboxylic acid (MACC) synthesis.     

α-Aminoisabutyric acid, propyl gallate and cobalt ion and the mode of inhibition of ethylene production by cotyledonary segments of cocklebur seeds

Using cotyledonary segments of cocklebur ( Xanthium pennsylvanicum Wallr. ) seeds, the inhibitory effect of α-aminoisobutyric acid (AIB) on ethylene production was compared with that of propyl gallate and CoCl₂. Of these inhibitors only AIB was effective in causing the accumulation of endogenous free 1-aminocyclopropane-l-carboxylic acid (ACC) in the tissue. The degree of inhibition of ethylene production by AIB decreased markedly with increasing concentrations of pre-loaded ACC, while the inhibition by propyl gallate and CoCl₂ changed little. Kinetic analysis showed that AIB competitively inhibited the conversion of pre-loaded ACC to ethylene, but propyl gallate and CoC_l2 did not. Short-chain organic acids and analogues of AIB, such as acetic, propionic, butyric and cyclopropanecarboxylic acids, did not inhibit ethylene production by the segments. Thus, additional support for the competitive mode of inhibitory action of AIB on the conversion of free ACC to ethylene was provided.
A conjugated hydrolysable ACC was found to be present in abundance in cotyledons of this seed. However, its content in the tissue was hardly affected by treatment with the three inhibitors and by administration of exogenous ACC, suggesting that the conjugated ACC was not directly involved in ethylene production.     

Metabolism of 1-aminocyclopropane-1-carboxylic acid in ripening apple fruits

In preclimacteric apple fruits ( Malus × domestica Borkh. cv. Golden Delicious) ethylene production is controlled by the rates of 1-aminocyclopropane-1-carboxylic acid (ACC) synthesis, and by its metabolism to ethylene by the ethylene-forming enzyme and to 1-(malonylamino)cyclopropane-1-carboxylic acid (MACC) by malonyl CoA-ACC transferase. The onset of the climacteric in ethylene production is associated with an increase in the activity of the ethylene-forming enzyme in the pulp and with a rise in the activity of ACC synthase. Malonyl transferase activity is very high in the skin of immature fruit, decreases sharply before the onset of the climacteric, and remains nearly constant thereafter. More than 40% of the ACC synthesized in the skin and around 5% in the flesh, are diverted to MACC at early climacteric. At the climacteric peak there are substantial gradients in ethylene production between different portions of the tissue, the inner cortical tissues producing up to twice as much as the external tissues. This increased production is associated with, and apparently due to, increased content of ACC synthase. Less than 1% of the synthesized ACC is diverted to MACC in the flesh of climacteric apples. In contrast, the skin contains high activity of malonyl transferase, and correspondingly high levels [1000 nmol (g dry weight)⁻¹] of MACC.     

Enhancement of ethylene biosynthesis and germination with thidiazuron and some selected auxins in Striga asiatica seeds

Witchweed [ Striga asiatica (L.) Kuntze], an economically important parasitic weed on several poaceous crops, is difficult to control. In nature, germination and subsequent morphogenesis of Striga are cued to specific host-derived chemical signals. Seeds (approximately 2.4 mg) treated with thidiazuron (TDZ) or the auxins 2,4-dichlorophenoxy-acetic acid (2,4-D), 1-naphthalene acetic acid (NAA), or 2-(4-chloro- o -tolyloxy) propionic acid (MCPP) produced little ethylene (66-138 nl l⁻¹). Combinations of TDZ with the auxins increased ethylene production by 4- to 18-fold. Ethylene production was strongly inhibited (86–92%) by aminoethoxyvinylglycine (AVG), inhibitor of 1-aminocyclopropane-1-carboxylic acid (ACC) synthase. Ethylene evolved from seeds treated with TDZ in combination with 2,4-D increased after a lag period and was promoted by a pretreatment in 2,4-D. TDZ or any of the auxins, at the rates tested, effected negligible to low levels of germination (0 to 16%), whereas mixtures of TDZ with the above auxins stimulated 38 to 84% germination. Test solutions containing TDZ and indole-3-acetic acid (IAA) were, however, less effective. TDZ/auxin-induced germination was inhibited by AVG and the ethylene action inhibitor silver thiosulfate (STS). The inhibitory effect of the former was reversed by treatment with ACC. In vitro studies revealed negligible germination (< 1%) on control medium. Seeds germinating on media containing TDZ alone developed into seedlings with distinct shoots and rudimentary roots. Seeds germinating on media containing 2,4-D, irrespective of TDZ concentration, were induced to form calli. The results are consistent with a model in which both germination and subsequent morphogenesis in Striga are associated with exogenous and endogenous phytohormones.     

Synergistic effect of 1-aminocyclopropane-1-carboxylic acid and ethylene during senescence of isolated carnation petals

The effects of ethylene (C₂H₄), (2-chloroethyl)phosphonic acid (ethefon) and 1-aminocyclopropane-1-carboxylic acid (ACC) on senescence of isolated intact petals and of upper petal parts of carnation flowers ( Dianthus caryophyllus L. cv. White Sim) were investigated.
Isolated upper petal parts did not respond to treatment with ethefon or ACC. These tissues did, however, show severe wilting in intact petals that were treated with ethefon or ACC. When isolated upper petal parts were simultaneously treated with ACC and ethefon or ACC and ethylene, a marked synergistic effect on senescence was found. Treatment of isolated petals with radiolabeled ACC led to the accumulation of radiolabeled ACC and N-malonyl-ACC (MACC) in the upper parts. The formation of ethylene and the malonylation of ACC were inhibited by pretreatment of the flower with the inhibitor of ethylene action, silver thiosulphate (STS), which indicates that both were induced by endogenously produced ethylene. Treatment of isolated upper parts with ACC slightly increased their ethylene production. However, when these petal parts were simultaneously treated with ethylene and ACC, the conversion of ACC to ethylene was markedly stimulated.
The results indicate that, in intact petals, ethylene may be translocated from the basal to the upper part where it stimulates the activity of the ethylene-forming enzyme (EFE), thereby making the tissue receptive to ACC.
In addition, it was found that upon incubation of petal portions in radiolabeled ACC, both the petal tissue and the incubation solutions produced radiolabeled carbon dioxide. This was shown to be due to microorganisms that were able to metabolize the carbon atoms in the 2 and 3 position of ACC into carbon dioxide.     

Ethylene-dependent action of gibberellin in seed germination of Amaranthus caudatus

The role of gibberellins in the germination of seeds of Amaranthus caudatus L. was examined. Tetcyclacis (BAS 106), an inhibitor of gibberellin biosynthesis, inhibited germination of the seeds. The inhibition caused by BAS 106 was antagonised by gibberellin A₄₊₇ (GA₄₊₇). Ethephon (2-chloroethylphosphonic acid) and 1-aminocyclopropane-1-carboxylic acid (ACC) could replace GA₄₊₇. Ethephon and ACC counteracted also the side effects of BAS 106 that are not reversible by GA₄₊₇. The rate of seed germination was not increased by gibberellin in the presence of aminoethoxyvinylglycine (AVG). AVG increased the effect of BAS 106. GA₄₊₇ could not reverse the effect of BAS 106 when AVG was simultaneously applied. The results indicate that the biosynthesis of endogenous gibberellins may be required for germination of A. caudatus seeds and that main physiological effects of GA₄₊₇ on seed germination may depend on ethylene biosynthesis.     

Effects of propylene and oxygen on the ethylene-producing system of apples

Hypobaric conditions and treatments with ethylene and the ethylene analogue propylene were used to investigate effects of oxygen and elhylene on 1-aminocyclopropane-1-carboxylic acid (ACC) content, ACC synthase activity and ethylene production of apples ( Malus sylveslris Mill. cv. Golden Delicious). Prcclimacteric apples were stored in air at 6.6 kPa (reduced pressure); 6.6 kPa ventilated with pure O₂; 6.6 kPa ventilated with 2600 μl 1⁻¹ C₂H₄; and in air at 101.3 kPa (atmospheric pressure) for 4 months at 4°C. No ACC synthase activity was detectable in apples stored at 6.6 kPa, whereas ACC synthase activity was induced in apples stored at 6.6 kPa and ventilated with either O₂ or C₂H₄. In a further experiment, preclimacteric apples were stored for 14 days either in air at 20 kPa or at 20 kPa ventilated with pure O₂. Both treatments were supplied with 58 500 μl 1⁻¹ propylene from day 0 to day 9 or from day 9 to day 12. Ethylene production of apples treated with propylene from day 0 to day 9 increased earlier than ethylene production of untreated apples. Propylene treatment from day 9 to day 12 did not stimulate ethylene production. Ethylene and propylene induced and stimulated extractable ACC synthase activity and ACC formation of apples. Oxygen enhanced this effect. The results also suggest inhibition of in vivo ACC synthase activity by propylene.     

3-Hydroxypropyl amide formation from 1-aminocyclopropane-1-carboxylic acid by a cell-free ethylene-forming system from olive leaves

The low ethylene yield in a cell-free ethylene-forming system from olive tree leaves ( Olea europaea L. cv. Picual) was investigated. During the incubation, 1-aminocyclopropane-1-carboxylic acid (ACC) was extensively transformed into 3-hydroxypropyl amide (HPA). Enzyme extract, Mn²⁺ and oxygen are responsible for this reaction. Horseradish peroxidase (EC 1.11.1.7) can substitute for the enzyme extract in this reaction. HPA formation could be one reason for the poor in vitro conversion efficiency of ACC to ethylene.     

Effects of Temperature and dl-Strigol on Seed Conditioning and Germination of Witchweed (Striga asiatica)

Seed conditioning and germination in witchweed (Striga asiatica(L.) Kuntze) were temperature-dependent. With higher conditioningtemperatures, shorter conditioning time was required for germinationwith terminal dl-strigol (strigol) treatment at 30 °C. Maximumgermination (80–100%) was obtained by conditioning inwater at 20, 25, 30 and 35 °C for 14, 7, 5 and 3 d, respectively,and terminally treating with 10^–6 M strigol at 30 °C.Seeds conditioned in 10^–8 M strigol instead of water germinatedmuch less with the same terminal strigol treatment. Generally,conditioning was slower when seeds were conditioned in strigolrather than water. The reduction in germination rate by pretreatmentin strigol or pretreatment at low temperatures could be overcomeby increasing the terminal strigol concentration in the germinationtest. Conditioned seeds did not germinate at 10 and 15 °Cwith a terminal 10^–6 M strigol treatment but yielded closeto maximum germination at 25, 30 and 35 °C with the sameterminal strigol treatment. To obtain maximum germination, boththe minimum conditioning temperature and the minimum germinationtemperature for conditioned seeds were 20 °C. Factors suchas conditioning time, and strigol concentration and temperatureduring conditioning and/or germination determine whether seedsremain in the conditioning phase or shift to a germination phase. dl-Strigol, germination stimulation, parasitic plants, seed conditioning, seed germination, Striga asiatica, temperature, weed control     

Chromium-induced inhibition of ethylene evolution in bean (Phaseolus vulgaris) leaves

The influence of chromium concentration on ethylene production in bean plants ( Phaseolus vulgaris L. cv. Contender) was investigated. A Cr ion-induced inhibition of ethylene synthesis from endogenous 1-aminocyclopropane-1-carboxylic acid (ACC) was observed within both leaf discs floated on 2 m M CrO²⁻₄ or Cr³⁺ and leaf discs from plants cultured in nutrient solutions containing 10, 20 or 40 μ M CrO²⁻₄. However, Cr ions supplied either to plants with the nutrient solution or to discs with the incubation medium rather increased the conversion of exogenous ACC to ethylene. Primary leaves of plants exposed to CrO²⁻₄-containing nutrient solutions showed a statistically insignificant decrease of ACC-synthase activity. In the trifoliolate leaves of plants exposed to 10 μ M CrO²⁻₄, in which a significant decrease of ethylene production from endogenous ACC was observed, a substantial increase of ACC synthase was found. These results indicate that Cr ion-induced inhibition of ethylene production is not due to a breakdown of membrane integrity, which is necessary for ethylene forming enzyme activity, but caused by metabolic alterations leading to decreased ACC availability. Chromium ions may act by inhibiting ACC synthase activity or by diverting a metabolic step prior to the ACC synthase catalyzed reaction.     

Regulation by CO2 of 1-aminocyclopropane-1-carboxylic acid conversion to ethylene in climacteric fruits

Cheverry, J. L., Sy, M. O., Pouliquen, J. and Marcellin, P. 1988. Regulation by CO₂ of 1-aminocyclopropane-1-carboxylic acid conversion to ethylene in climateric fruits. - Physiol. Plant. 72: 535–540.
A high CO₂ concentration (20%) at 20°C rapidly and strongly inhibited the development of the climacteric ethylene burst in apple ( Malus domestica Borkh. cv. Granny Smith) and avocado ( Persea americana Mill. cv. Fuerte) fruits and did not change 1-aminocyclopropane-l-carboxylic acid (ACC) content. Treatment with 20% CO₂ markedly decreased ACC-dependent ethylene biosynthesis at 20°C in climacteric pericarp tissues. It is suggested, therefore, that high CO₂ levels inhibit conversion of ACC to ethylene.
Synthesis of the ethylene forming enzyme (EFE) was enhanced when intact preclimacteric apples or early climacteric avocados were pretreated for 40 h with 10 μ11^-1 ethylene. When CO₂ (20%) and ethylene were both applied, a reduced stimulatory effect of ethylene on EFE synthesis was observed. A high CO₂ concentration enhanced EFE acivity in excised tissues of apples and avocados incubated with ACC (2 m M ) and cycloheximide (1 m M ) or 2–5-norbornadiene (5 ml 1^-1). In the autocatalytic process, 20% CO₂ antagonized the stimulation of EFE synthesis by ethylene, but promoted EFE activity.     

De novo shoot morphogenesis and plant growth of mustard (Brassica juncea) in vitro in relation to ethylene

Role of ethylene in de novo shoot morphogenesis from explants and plant growth of mustard ( Brassica juncea cv. India Mustard) in vitro was investigated, by culturing explants or plants in the presence of the ethylene inhibitors aminoethoxyvinylglycine (AVG) and AgNO₃. The presence of 20 μ M AgNO₃ or 5 μ M AVG in culture medium containing 5 μ M naphthaleneacetic acid and 10 μ M benzyladenine were equally effective in promoting shoot regeneration from leaf disc and petiole explants. However, AgNO₃ greatly enhanced ethylene production which reached a maximum after 14 days, whereas ethylene levels in the presence of AVG remained low during 3 weeks of culture. The promotive effect of AVG on shoot regeneration was overcome by exogenous application of 25 μ M 2-chloroethylphosphonic acid (CEPA), but AgNO_3-induced regeneration was less affected by CEPA. For whole plant culture, AVG did not affect plant growth, although it decreased ethylene production by 80% and both endogenous levels of 1-aminocyclopropane-1-carboxylate (ACC) synthase and ACC by 70–80%. In contrast, AgNO₃ stimulated all 3 parameters of ethylene synthesis. Both AgNO₃ and CEPA were inhibitory to plant growth, with more severe inhibition occuring in AgNO₃. Leaf discs derived from plants grown with AVG or AgNO₃ were highly regenerative on shoot regeneration medium without ethylene inhibitor, but the presence of AgNO₃ in the medium was inhibitory to regeneration of those derived from plants grown with AgNO₃.     

Calcium regulation of exogenous and endogenous 1-aminocylopropane-1-carboxylic acid bioconversion to ethylene

Ethylene production and overall levels of free and conjugated 1-aminocyclopropane-1-carboxylic acid (ACC) were studied in parenchymatous tissues, excised from clmacteric apples ( Malus domestica Borkh. cv. Granny Smith) and infiltrated with an incubation medium containing 0, 1, 10 or 100 m M Ca²⁺, with or without exogenous ACC (2 m M ). Irrespective of whether exogenous ACC was applied or not, ethylene production was inhibited to the same extent (40%) by an apoplastic effect of 100 m M Ca²⁺. In the absence of external ACC, the inhibition was associated with an increase in total endogenous ACC and may be related to a reduction in the rate of the last step of ethylene pathway. This suggests that the ethylene-forming enzyme (EFE) is localized in the plasma membrane. Low Ca²⁺ concentrations (1 m M ) enhanced basal ethylene synthesis due to influx of Ca²⁺ into the cytosol, while overall concentrations of ACC in the tissue decreased. However, 1 m M Ca²⁺ did not stimulate ACC-dependent ethylene formation. Thus, Ca²⁺ influx may stimulate the translocation of endogenous ACC from synthesis or storage compartment (s) to reactive site(s) of the plasma membrane. The concentration of 10 m M Ca²⁺ had no effect on basal ethylene production and appears to represent a balance point between the stimulating and inhibiting effects of 1 and 100 m M Ca²⁺, respectively, Furthermore, the charge-times of exogenous ACC observed with 0, 1 and 10 m M Ca²⁺ suggest that EFE is located on the inner side of the plasma membrane.     

Quantitative relationships between osmotic potential amd epicotyl growth in Vigna radiata as affected by osmotic stress and cotyledon excision

Ethylene biosynthesis in leaf discs of tobacco ( Nicotiana tabacum L. cv. Xanthi), as measured by the conversion of L-[3,4-¹⁴C]-methionine to ¹⁴C₂H₄, was markedly inhibited by exogenous ethylene. This inhibition was accompanied by a decrease in total (free + conjugated) content of 1-aminocyclopropane-1-carboxylic acid (ACC), most of which appeared in its conjugated inactive form. The autoinhibitory effect of ethylene was reversible and could be relieved by Ag⁺. The Ag⁺-treated leaf discs, with or without ethylene, contained only free ACC at an increased level. The results suggest that in tobacco leaves, the autoinhibition of ethylene production resulted from reduction in the availability of free ACC, through both suppression of ACC formation and increased ACC conjugation.     

Stimulation of somatic embryogenesis in carrot by ethylene: Effects of modulators of ethylene biosynthesis and action

Endogenous levels of ethylene appeared to he suhoptimal for somatic embryogenesis in a suspension culture of carrot. Low concentrations of 1-aminocyclopropane-1-carboxylic acid (ACC). 2-chloroethylphosphonic acid (ethephon) and elhylene stimulated embryogenesis whereas higher concentrations were inhibitory. The stimulation by ACC was through its conversion to ethylene. whereas the inhibition by ACC was not. Low concentrations of AgNO₃. an inhibitor of ethylene action, inhibited embryo-genesis but stimulated ethylene production. Aminoethoxyvinylglycine (AVG) and aminooxyacetic acid (AOA). commonly used inhibitors of ACC synthase. inhibited both embryogenesis and ethylene production. However, the inhibition of embryogenesis was not related to the inhibition ote ethylene production. Very low concentrations of AVG stimulated embryo production in a way unrelated to its effect on ethylene production. Salicylic acid and CoCl₂. inhibitors of ACC oxidase in other systems, inhibited embryogenesis but. again, in way(s) unrelated to their inhibition of ethylene production. In fact, low concentrations of salicylic acid stimulated rather than inhibited ethylene production. The results show that in suspension-cultured cells, caution is warranted in the interpretation of results obtained with agents presumed to inhibit ethylene biosynthesis. The stimulation of somatic embryogenesis by ethylene unequivocally shows that the inhibition of embryo development by 2.4-dichlorophenoxyacetic acid (2.4-D) and other auxins cannot be through their stimulatory effect on ethylene production.     

Ethylene synthesis and growth in embryogenic tissue of Norway spruce: Effects of oxygen, 1-aminocyclopropane-1-carboxylic acid, benzyladenine and 2,4-dichlorophenoxyacetic acid

Ethylene production from an embryogenic culture of Norway spruce ( Picea abies L.) was generally low. ca 2.5 nl g⁻¹ h⁻¹, whereas 1-aminocyclopropane-1 -carboxylic acid (ACC) concentration was high, fluctuating between 50 and 500 nmol g⁻¹ during the 11-day incubation period. Hypoxia (2.5 and 5 kPa O₂) rapidly inhibited ethylene production without subsequent accumulation of ACC. Exogenous ACC (1, 10 and 100 μ M ) did not increase ethylene production, but the highest concentrations inhibited tissue growth. Ethylene (7 μl I⁻¹) did not inhibit growth either when supplied as ethephon in the medium or in a continuous flow system. Benzyladenine (BA) had little effect on ethylene production, although it was necessary for sustaining the ACC level. Omission of 2.4-dichloro-phenoxyacetic acid (2.4-D) from the medium caused ethylene production to increase from about 2.5 to 7 nl g⁻¹ h⁻¹ within the 11-day incubation period. Although 2.4-D did not specifically alter the endogenous level of ACC, the lowest ACC level, 33 nmol g⁻¹, was observed in tissue treated with 2.4-D (22.5 μ M ) and no BA for 11 days. Data from this treatment were used to estimate the kinetic constants for ACC oxidase, the apparent K_m was 50 μ M and V_max 2.7 nl g⁻¹ h⁻¹. Growth of the tissue was strongly inhibited by 2.4-D in the absence of BA, but weakly in the presence of BA (4.4 μ M ). The results suggest that ethylene or ACC may be involved in the induction of embryogenic tissue and in the early stages of embryo maturation.     

Regulation of the germination of Rhus coriaria, a post-fire pioneer, by heat, ash, pH, water potential and ethylene

Germination of the post-fire pioneer species Rhus coriaria , in Pinus halepensis forests on Mount Carmel, Israel, is restricted to the ash covered microsites under large burned pine trees, where the germination of other species is strongly inhibited. The aim of this study was to examine the effect of heat, ash cover, pH, water potential (Ψ _Π ) and ethylene on germination of R. coriaria seeds, in order to identify the causes of this unique germination pattern. Pre-heating to 120–140°C for 15 min was essential for the induction of seed germination. Germination percentage was increased by ash cover of 1.2 and 2.4 kg m⁻² (1 and 2 cm, respectively) but inhibited by ash cover of 6.0 kg m⁻² (5 cm). Wet pine ash from a recently burned forest had pH of 10 and Ψ _Π of −0.26 MPa. Under such conditions the germination of R. coriaria was reduced by ca 80%. On the other hand, germination was stimulated by 0.03–0.10 p.p.m. ethylene which was released by wet ash. The post-fire germination of R. coriaria is regulated by the balance between the stimulating effects of fire heat and the ethylene released by the ash, and the inhibition caused by the high pH and the low Ψ _Π caused by the ash. Its mode of dispersal by birds and these ecophysiological attributes direct germination of R. coriaria to preferred microsites under the burned canopies of large pine trees. These microsites are characterized by improved nutrition and low competition.