Exogenous ethylene inhibits sprout growth in onion bulbs

Background and Aims

Exogenous ethylene has recently gained commercial interest as a sprouting inhibitor of onion bulbs. The role of ethylene in dormancy and sprouting of onions, however, is not known.

Methods

A cultivar (Allium cepa ‘Copra’) with a true period of dormancy was used. Dormant and sprouting states of onion bulbs were treated with supposedly saturating doses of ethylene or with the ethylene-action inhibitor 1-methylcyclopropene (1-MCP). Initial sprouting was determined during storage at 18 °C by monitoring leaf blade elongation in a specific size class of leaf sheaths. Changes in ATP content and sucrose synthase activity in the sprout leaves, indicators of the sprouting state, were determined. CO₂ and ethylene production of onion bulbs during storage were recorded.

Key results

Exogenous ethylene suppressed sprout growth of both dormant and already sprouting onion bulbs by inhibiting leaf blade elongation. In contrast to this growth-inhibiting effect, ethylene stimulated CO₂ production by the bulbs about 2-fold. The duration of dormancy was not significantly affected by exogenous ethylene. However, treatment of dormant bulbs with 1-MCP caused premature sprouting.

Conclusions

Exogenous ethylene proved to be a powerful inhibitor of sprout growth in onion bulbs. The dormancy breaking effect of 1-MCP indicates a regulatory role of endogenous ethylene in onion bulb dormancy.Key words: Bulb dormancy, Allium cepa, onion, sprout growth, ethylene, CO₂ production, respiration, 1-methylcyclopropene     

温度、GA3和乙烯对中国水仙休眠的解除

自然条件下,4月至7月下旬期间,中国水仙鳞茎的呼吸速率逐渐下降,7月底至8月底开始上升之后又下降。30℃高温下的鳞茎发芽延缓,发芽率下降;15℃低温有利于打破休眠,7月下旬前的鳞茎,其休眠解除所需的低温处理时间长,GA3不能破除,还延缓其休眠时间,萌发率也下降;而7月下旬后的鳞茎,短时间低温或施用GA3均能破除其休眠,提高鳞茎的发芽率和发芽整齐度;乙烯在任何阶段都有助于水仙鳞茎休眠的解除。自然条件下7月下旬前水仙鳞茎可能是内生休眠,其休眠较难打破,需要长时间的低温或施用适当浓度的乙烯,7月下旬后可能是环境休眠,短时间的低温、GA3或乙烯都能破除其休眠。     

The Role of Growth Substances in the Regulation of Onion Bulb Dormancy

The correlation between sprouting and changes in endogenousgrowth substances was investigated in stored onion bulbs (Alliumcepa c.v. Rijnsburger and Lancastrian). In the main experimentsbulbs were removed from store at approximately fortnightly intervals,samples were assessed for percentage sprouting and non-sproutingbulbs were either extracted for hormone assay or treated withgrowth substances in an attempt to induce sprouting. In otherexperiments the hormone content of bulbs at different stagesof sprouting was assessed. Growth-inhibitor and gibberellinactivity decreased before sprouting, but there was an increasein gibberellin and auxin activity as sprouting commenced. Gibberellinactivity was highest in bulbs with well-developed sprouts whereasauxin activity occurred mainly in bulbs in which early sproutdevelopment was visible only on their being cut open. Therewas no conclusive evidence that bulb dormancy could be brokenby application of the gibberellins GA₃ and GA_4/7, or the auxin1-naphthylacetic acid (NAA). Maleic hydrazide (MH) completelyinhibited root and sprout development but the growth retardant(2-chloroethyl)trimethylammonium chloride (CCC) was mainly effectivein reducing root development and sprouting was only slightlyinhibited.     

The effect of bulb size on the storage-life of onions

In 2 years, 1972 and 1973, onion bulbs were harvested on various dates, graded into a range of sizes and stored at ambient temperatures. Periodic assessments showed that the number of bulbs that both rotted and sprouted increased with increasing bulb size. Although rotting can be controlled with benomyl seed dressing the increased tendency of large bulbs to sprout could have commercial implications. The date of commencement of sprouting was not affected by bulb size, but the percentage increase of sprouting was faster in the larger bulbs. Delayed harvesting also led to increased sprouting in store.     

The influence of temperature on weight loss from stored onion bulbs due to desiccation, respiration and sprouting

When onion bulbs were stored for 9 months at 2, 7.5, 15 or 25 °C and 70% r.h., the losses due to desiccation increased with temperature but less than 20 % was due to respiration at any of the storage temperatures. Respiration rates of onion bulbs transferred from 2 to 25 °C were higher from February onwards than those of bulbs stored continuously at 25 °C. Conversely, bulbs transferred from 25 to 2 °C respired less from February onwards than those kept at 2 °C. Sprouting, at the final assessment in June, was highest at 15 and 7.5 °C and lowest at 2 °C. Total weight loss was above 45 % in all the storage treatments except at 2 °C (12%). Storage at 7.5 °C is suitable until March but long-term storage until June requires low temperatures.     

The effect of low temperature and GA3 treatments on dormancy breaking and activity of antioxidant enzymes in Fritillaria meleagris bulblets cultured in vitro

We investigated the effect of low temperature and gibberellic acid (GA₃) treatment on dormancy in Fritillaria meleagris L. bulbs. Also, we studied the effect of dormancy breaking on the antioxidant enzymes activity. To overcome dormancy, bulbs require a period (4–8 weeks) of exposure to low temperature. Bulbs regenerated in vitro were grown in the dark on medium without growth regulators at the standard (24 °C) or at low temperatures (4 and 15 °C) for 4, 6, 8 and 10 weeks. Bulbs were collected after 3, 4 and 5 weeks of cooling at 4 °C. To investigate the influence of GA₃ on dormancy, bulbs were treated for 24 h with GA₃ solutions with 1, 2 and 3 mg l^?1 concentrations. During the period of growth of bulbs at 4 °C, regeneration of bulbs was very weak, while at 15 °C the number of regenerated bulbs increased significantly. Improved bulb sprouting was achieved by a short treatment with gibberellin. Low temperature also represents a kind of oxidative stress for the plant. The activity of superoxide dismutase, catalase (CAT) and peroxidase (POX) in bulbs of F. meleagris L. grown in vitro and ex vitro increased with decreasing temperature in contrast to glutathione reductase. POX showed generally lower activity than CAT which indicates that major role in the breaking dormancy and preparing bulbs for sprouting have catalases.     

Nature of enhanced respiration during sprouting of aged potato seed-tubers

Respiration of 18-month-old Solarium tuberosum L. tubers was about 53% greater than that of 6-month-old tubers during sprouting at 23°C; yet, a significant loss of sprout vigor in the older tubers was apparent. Involvement of alternative oxidase (AO) in the age-induced difference in tuber respiration was assessed. AO was only detected in immunoblots if tissue disks from tubers were pre-incubated for 24 h prior to isolation of submitochondrial membrane particles (SMPs). No AO¹ was detected in SMPs from nonincubated tuber tissue of either age, indicating that it was not contributing to tuber respiration during sprouting as previously thought. Respiratory control and ADP/O ratios indicated that oxidative phosphorylation was fully coupled to electron transport in mitochondria isolated from 6- and 18-month-old tubers. Cytochrome c oxidase (EC 1.9.3.1) activities of intact mitochondria were also not affected by tuber age. The difference in respiration during sprouting was unique to whole tubers, as oxygen consumption by mitochondria from young and oid tubers was equal on a milligram protein basis. Sprouting 18-month-old tubers had 15% more mitochondrial protein per gram fresh weight than did 6-month-old tubers. Older tubers also produced more ATP than younger tubers prior to and during sprouting, through a fully coupled, Cyt-mediated respiratory pathway, reduced sprout vigor notwithstanding. From 5 to 10 days of sprouting, coinciding with development of the age-induced difference in whole-tuber respiration, ATP concentration in 18-month-old tubers increased to become 52% higher than that in 6-month-old tubers. ATP synthase (EC 3.6.1.34), assessed by SDS-PAGE and immunoblots of β- and oligomycin-sensitivity conferring protein-subunits, also increased as a proportion of SMP protein in older tubers during this period. Relative to 6-month-old tubers, the increased respiration and associated oxidative phosphorylation of 18-rnonth-old tubers during sprouting were probably in response to a lower adenylate energy charge (AEC) prior to sprouting (from 0 fo 5 days). From 5 to 10 days of sprouting, AEC of 18-rnonth-old tubers increased to equal that of 6-month-old tubers and the two tuber ages maintained the same AEC for the remainder of the 20-day sprouting interval. Higher respiration and lower AEC of older tubers in storage at 4°C, along with the fact that older tubers respired at a higher rate to achieve the same AEC as younger tubers during sprouting, indicate greater utilization of ATP by older tubers.     

Rhythmic Growth in Excised Sprout-leaves of Onion Bulbs

Sprout leaf preparations, consisted of the inner 6 or 7 sprout leaves attached to the base plate of bulbs of Allium cepa cv. Downy Yellow Globe. The preparations excised from dormant bulbs were capable of sprouting. The elongation of the sprout leaves in the light was inhibited by a field pretreatment of 2000 mg/1 M.H.-30 (30% maleic hydrazide) and stimulated by incubation in the dark. Rapid cooling of the bulbs prior to excising of the preparations had no effect on sprouting. The ability of the sprout leaf preparations to elongate followed a cyclical time course, displaying maxima 8 to 12 weeks apart. Pretreatment with M.H.-30 inhibited this periodicity.     

Sprouting of adult Purkinje cell axons in lesioned mouse cerebellum: “Non-permissive” versus “permissive” environment

Adult rat Purkinje cells are extremely resistant to axotomy and, although they lack spontaneous regeneration, are able to sprout. Axon sprouting is a late process that occurs mainly 6 to 18 months after the lesion and results from an interplay between Purkinje cell intrinsic properties and chemical remodeling of the glial scar. To better appraise the role of the local environment in the late sprouting, we performed new axotomy experiments in mice. In this species, unlike the rat, there is no cavitation because the post-lesional necrotic tissue is invaded by astrocytes and incorporated into the glial scar. In this scarring tissue, chondroitin sulfate proteoglycans (CS-PGs) and PSA-NCAM are present one week after the lesion, but the time courses of their expression differ: the former are transiently expressed and rapidly disappears (by one month), thus preventing early sprouting and providing a negative spatiotemporal correlation with the late sprouting. PSA-NCAM expression, which is maintained up till 12 months, is by itself not sufficient to attract the sprouts, since the core of the glial scar—which exhibits high level of PSA-NCAM—is always devoid of them. Finally, by using a double experimental approach (lesion and graft) aimed at providing a permissive environment to the terminal bulbs of axotomized Purkinje cells, we show that the presence of grafted cerebellum at the lesion site neither changes the time course of the sprouting nor enhances the Purkinje cell axonal regeneration. Nevertheless, these experiments have revealed a new type of altered Purkinje cells, the “irritated” Purkinje cells with a high potentiality for axon sprouting.     

Effect of octanoic acid on ethylene-mediated flower induction in Dutch iris

Treatment of Dutch iris (Iris × hollandica Hoog. cv. Sapphire Beauty) bulbs with ethylene prior to precooling stimulated flowering in bulbs of various sizes. In large sized bulbs exposure to ethylene followed by precooling resulted in 100% flowering over a five months period after planting. Flowering in control bulbs which were not treated with ethylene prior to precooling was limited to 67% during the same five months period. In medium sized bulbs flowering in the ethylene treatment was 90% compared to 75% in the control. However, the biggest stimulation of flowering by ethylene was found in small sized bulbs (from 16 to 56%). Application of octanoic acid for a short time period prior to exposure to ethylene stimulated flowering in all bulb sizes. After five months the final percentage flowering in large and medium sized bulbs of the octanoic acid plus ethylene treatment did not differ from that of the ethylene only treatment. However, the initial rate of flowering was higher in the former treatment. In small bulbs the percentage flowering was much higher in the octanoic acid plus ethylene treatment than in the ethylene only treatment. The results of this study indicate that, just as in certain flowers, fruit and seeds, treatment with octanoic acid stimulates ethylene sensitivity in Dutch iris bulbs. The sensitivity of untreated bulbs to ethylene was highest in large bulbs and lowest in small bulbs. This correlated well with the endogenous octanoic acid content of the bulbs. Octanoic acid levels were highest in large bulbs and lowest in small Bulbs. It appears that the endogenous levels of octanoic acid in the bulbs is determined prior to the onset of dormancy.     

Regulation of summer dormancy by water deficit and ABA in Poa bulbosa ecotypes

BACKGROUND AND AIMS: Survival of many herbaceous species in Mediterranean habitats during the dry, hot summer depends on the induction of summer dormancy by changes in environmental conditions during the transition between the winter (growth) season to the summer (resting) season, i.e. longer days, increasing temperature and drought. In Poa bulbosa, a perennial geophytic grass, summer dormancy is induced by long days, and the induction is enhanced by high temperature. Here the induction of summer dormancy in a Mediterranean perennial grass by water deficit under non-inductive photoperiodic conditions is reported for the first time. METHODS: Plants grown under 22/16 degrees C and non-inductive short-day (9 h, SD) were subjected to water deficit (WD), applied as cycles of reduced irrigation, or sprayed with ABA solutions. They were compared with plants in which dormancy was induced by transfer from SD to inductive long-day (16 h, LD). Responses of two contrasting ecotypes, from arid and mesic habitats were compared. Dormancy relaxation in bulbs from these ecotypes and treatments was studied by comparing sprouting capacity in a wet substrate at 10 degrees C of freshly harvested bulbs to that of dry-stored bulbs at 40 degrees C. Endogenous ABA in the bulbs was determined by monoclonal immunoassay analysis. KEY RESULTS: Dormancy was induced by WD and by ABA application in plants growing under non-inductive SD. Dormancy induction by WD was associated with increased levels of ABA. Bulbs were initially deeply dormant and their sprouting capacity was very low, as in plants in which dormancy was induced by LD. Dormancy was released after 2 months dry storage at 40 degrees C in all treatments. ABA levels were not affected by dormancy relaxation. CONCLUSIONS: Summer dormancy in P. bulbosa can be induced by two alternative and probably additive pathways: (1) photoperiodic induction by long-days, and (2) water deficit. Increased levels of endogenous ABA are involved in both pathways.     

Inhibition of flowering in vivo by existing fruits and applied growth regulators in Citrus unshiu

In the Satsuma mandarin ( Citrus unshiu Marc.) the presence of the fruit results in a gradual inhibition of flowering and of bud sprouting. This inhibitory effect starts several months before the onset of the winter rest period and lasts until the end of the accumulation of carotenoids in the fruit peel, more than one month after the completion of fruit growth. During all this time and until natural bud sprouting, flowering and bud sprouting are inhibited by exogenous gibberellic acid. Peak responses to this growth regulator coincide with periods of maximal rates of flowering inhibition by the fruit. Kinetin and abscisic acid, applied at the time of peak response to gibberellic acid, inhibited flowering and reduced the number of shoots developed through the reduction of the number of shoots formed per sprouted node, but failed to reduce the number of nodes which sprouted. The same pattern of sprouting was obtained in trees treated with gibberellic acid during the winter rest period or several months earlier. It is concluded that some step leading to flowering and which determines the differences in sensitivity of the buds to this growth regulator has taken place already at this early date.     

Respiration of mitochondria isolated from tulip bulbs stored at 5 and 17°C

For the production of good quality flowers, tulip ( Tulipus gesneriana L.) bulbs need a period of low temperature. In cultivar Apeldoom a treatment of 12 weeks at 5 C can be used. In the bulb scales the respiratory metabolism has to adjust to this low temperature. Mitochondria isolated from the bulb scales are able to use succinate. NADH and pyruvate as respiratory substrates. Respiratory characteristics of these mitochondria changed after transfer of the bulbs to 5°C and the adaptation was complete within 2 weeks. Both state 3 respiration and respiratory control increased. Alternative pathway capacity was constitutively present at both 5°C and 17°C: it was low and substrate dependent at both temperatures. During the following weeks of the treatment no significant changes took place. However, when bulbs were transferred to 17°C after storage at 5°C. various responses could be demonstrated. In bulbs only cooled for a short period no readjustment to this higher temperature occurred. In bulbs stored for longer periods the change depended on the duration of the 5°C treatment. The nature of the re-adaptation is discussed     

Age of potato seed-tubers influences protein synthesis during sprouting

The effect of seed-tuber age on the ability of tuber tissue to synthesize protein during sprouting was examined. As seed-tuber age advanced from 4 to 32 months (at 4°C, 95% relative humidity), soluble protein concentration of tubers decreased linearly, with a concomitant increase in free amino acid concentration. The age-induced loss of tuber protein may thus be due to increased proteolysis, decreased protein synthesis, or both. Five- and 17-month-old seed-tubers were compared for their ability to incorporate radiolabeled amino acids into soluble protein at equivalent stages of sprout development. Tuber respiration was profiled through each sprouting stage to characterize the physiological status of the seed-tubers prior to incorporation studies. Five-month-old seed-tubers maintained a constant rate of respiration during sprouting. In contrast, respiration of 17-month-old tubers increased as sprout dry matter increased, resulting in a 2- to 3-fold greater respiratory rate from the older tubers, relative to the younger tubers, at similar stages of sprout development. Prior to sprouting, the rate of incorporation of amino acids into trichloroacetic acid-precipitable protein of tissue from 5-month-old tubers was 2. 9-fold higher than that from 17-month-old tubers. More importantly, protein-synthetic capacity of tissue from younger tubers increased about 1. 7-fold during sprout development. Despite the higher respiratory activity and faster total sprout dry matter accumulation from older seed-tubers, protein synthesis remained at a low and constant level through all stages of sprouting. Protein-synthetic capacity thus declines with advancing tuber age, and this may contribute to reduced growth potential during the latter stages of establishment by affecting the ability of seed-tubers to synthesize enzymes involved in mobilization and translocation of tuber reserves to developing plants.     

Direct and acclimatory responses of dark respiration and translocation to temperature

BACKGROUND AND AIMS: Accounting for the acclimation of respiration of plants to temperature remains a major problem in analysis of carbon balances of plants and ecosystems. Translocation of carbohydrates out of leaves in the dark requires energy from respiration. In this study relationships between the responses of leaf respiration and translocation to temperature are examined. METHODS: Direct and acclimatory responses to temperature of respiration and translocation in the dark were investigated in mature leaves of soybean and amaranth. In some cases translocation from leaves was prevented by heat-girdling the phloem in the leaf petiole, or photosynthesis during the previous day was altered. KEY RESULTS: In both species short-term increases in temperature early in the dark period led to exponential increases in rates of respiration. However, respiration rates decreased toward the end of the dark period at higher temperatures. Stopping translocation largely prevented this decrease in respiration, suggesting that the decrease in respiration was due to low availability of substrates. In soybean, translocation also increased with temperature, and both respiration and translocation fully acclimated to temperature. In amaranth, translocation in the dark was independent of temperature, and respiration did not acclimate to temperature. Respiration and translocation rates both decreased with lower photosynthesis during the previous day in the two species. CONCLUSIONS: Substrate supply limited total night-time respiration in both species at high temperatures and following days with low photosynthesis. This resulted in an apparent acclimation of respiration to high temperatures within one night in both species. However, after long-term exposure to different temperatures there was no evidence that lack of substrates limited respiration in either species. In amaranth, respiration did not limit translocation rates over the temperature range of 20-35 degrees C.     

The effect of defoliation on storage potential of bulbs of the onion (Allium cepa)

Preharvest defoliation of onion bulbs did not increase rotting in store but did increase sprouting. The level of ethyl acetate-soluble inhibitory substances present in bulbs after harvest was not correlated with bulb dormancy.     

Effects of anaerobiosis on ethylene production, respiration and flowering in iris bulbs

Ethylene production of iris bulbs (Iris hollandica cv. Ideal) was very low. When stored at 30°C, production was 12–20 pmol C₂H₄ (kg fresh weight)^?1 h^?1. Higher temperatures (35°C, 40°C) enhanced the ethylene production; a treatment with 40°C for ca 7 days caused a 3 times higher ethylene production than at 30°. During anaerobic storage (in 100% N₂) ethylene production was equal to that of control bulbs. When after a 7 day period of anaerobiosis the N₂ was replaced by air, a burstlike ethylene production was observed. Twenty-four h after the replacement, ethylene production was equal to control values again. The effects of this production of ethylene on mitochondrial respiration and flowering were investigated. When mitochondria were isolated immediately after the anaerobic treatment (before the enhanced ethylene production) alternative pathway capacity was not detectable, a situation also occurring in control bulbs. When mitochondria were isolated 24 h after the end of the anaerobiosis (after the ethylene burst) uninhibited respiration did not change significantly, but a capacity of the alternative pathway was observed. The increase in alternative pathway capacity after anaerobiosis was partly inhibited by 2,5-norbornadiene (NBD), an ethylene antagonist. Fermentation occurred during anaerobiosis: ethanol concentrations increased during the treatment and decreased when air was supplied. When bulbs were exposed to ethanol vapour the alternative pathway was induced but only when very high ethanol levels in the bulbs were reached. The amount of ethanol accumulated in the bulbs during a 7 day anaerobic treatment was far too low to explain the observed induction of alternative pathway capacity. Flowering percentages were enhanced after a 24 h treatment with ethylene and after a 7 day anaerobic treatment. NBD significantly inhibited the effect of exogenous ethylene and of anaerobiosis on flowering. Ethanol was not able to induce flowering. The burst-like production of ethylene after anaerobiosis probably is responsible for the effects on respiration and flowering.     

Effects of Methyl Jasmonate on Anthocyanin Accumulation, Ethylene Production, and CO2 Evolution in Uncooled and Cooled Tulip Bulbs

Effects of methyl jasmonate (JA-Me) on anthocyanin accumulation, ethylene production, and CO₂ evolution in uncooled and cooled tulips (Tulipa gesneriana L. cvs. Apeldoorn and Gudoshnik) were studied. JA-Me stimulated anthocyanin accumulation in stems and leaves from uncooled and cooled bulbs of both cultivars. The highest level of anthocyanin accumulation was observed in leaves from cooled bulbs treated with 200 μL/liter JA-Me. In sprouting bulbs treated with 100 μL/liter and higher concentrations of JA-Me, the ethylene production began to increase at 3 days after treatment, being extremely greater in uncooled bulbs than in cooled ones. JA-Me also stimulated CO₂ evolution in both cultivars, depending on its concentrations. CO₂ evolution in sprouting bulbs was not affected by cooling treatment. These results suggest that anthocyanin accumulation by JA-Me in tulip leaves is not related to ethylene production stimulated by JA-Me. Received October 10, 1997; accepted November 17, 1997