Abstract: Ethylene regulates many aspects of plant growth and development. During the post-harvest handling of ornamental plants, ethylene presence must be minimized to prevent quality decrease of post-harvest life, such as petal abscission and leaf senescence. To minimize the ethylene effect, several prevention strategies were developed, such as inhibition of ethylene biosynthesis and ethylene perception. This review described the inhibition of ethylene perception through chemical compounds application. Inhibition of ethylene perception is a more effective technique compared to ethylene biosynthesis in preventing ethylene effect. This was due to both endogenous and exogenous ethylene can be blocked. 1-Methylcyclopropene (MCP) is one of the common chemical compound used as ethylene inhibitor. Two formulations of 1-MCP had been developed in the recent years, gas-released powder (volatile 1-MCP) and water-soluble powder (sprayable 1-MCP). Despite having the same active ingredient, the two formulations had different affinity in ethylene effect prevention. The effectiveness of 1-MCP had been widely investigated on several ornamental plants such as Chrysanthemum, Pelargonium, Kalanchoe, Grevillea. Recently, 1-MCP had been used to improve other plants such as and Phalaenopsis, Curcuma and Ginger.
INTRODUCTION
Ethylene is a gaseous plant hormone with a sweet and ether-like odour. The compound is a symmetrical-two carbon compound and has a molecular weight1 of 28.05. Ethylene is easily released from plant tissue and diffused in the gas phase through the intercellular space and sides. The concentration of ethylene can be measured by two methods, gas chromatography with flame or photoionization detector and laser-acoustic technique2. The concentration of ethylene in water is 4.4×10–9 M equal to 1 μL L–1 in the gas phase3 at 25°C.
Ethylene triggered various responses in plant. It releases seed dormancy, hastens shoot and root growth of dormant iris, tulip, freesia and gladiolus, promotes fruit development and ripening, induces lateral cell expansion, promotes the elongation growth of submerged aquatic species, induces roots and root hair formation, promotes the senescence and abscission process of leaf and flower and regulates the ratio of male to female flower in Cucurbitaceae3,4.
Ethylene release causes quality loss in postharvest life of ornamental plants. In most species, ethylene hastens petal wilting. But in some, it exhibits little to no effect. In highly sensitive plant such as Pelargonium5, ethylene immediately causes petal abscission. Plant responses vary depending on the temperature, concentration, stage of plant development, duration of exposure. Generally the ethylene responsiveness increases along with organ aging6.
Senescence is a combination of physiological process in living organism that follows physiological maturity and enhances the programmed cells, tissues and organs death7. In monocarpic plants, senescence lead to the death of entire plants after reproductive development finished. Whereas in polycarpic plants, it does not lead to the death of entire plants but is limited to parts of flower, fruit and old leaf and the plant continues to develop8.
The most visible symptom of flower senescence are wilting or withering9, whereas the visible symptom of leaf senescence is yellowing. Flower wilting or withering is caused by the loss of plant turgor from water stress and liquid logging of the tissue in the entire cut flowering stem. Flower wilting is usually followed by color change, slow dehydration and desiccation5,9. Leaf yellowing due to chlorophyll degradation usually starts from the leaf margin and spreads to the lamina10. Chlorophyll breakdown decreases photosynthesis activity, inhibiting rooting process of the cutting and promotes susceptibility to Botrytis11,12. Reports regarding the improvement of post-harvest shelf life of ornamental plants through ethylene perception techniques were reviewed in this article. Ethylene promotes the senescence and abscission of leaf and flower, which characterized by wilting, premature rooting and chlorophyll loss13. The application of exogenous ethylene 1 μL L–1 caused complete petal abscission of several Pelargonium flower14-16 within 2 h.
ENDOGENOUS AND EXOGENOUS ETHYLENE SOURCES
The ethylene production rate depend on the type of tissue and the stage of plant development. Production of endogenous ethylene in flowers was commonly proceeded in three phase at the stage of flower development: low production in young flowers, strong acceleration during senescence and rapid decrease at the end of senescence. In Ecballium elaterium and digitalis, ethylene production increased during flower and petal abscission17,18, whereas in carnation, it occurs several days or several hours after the full flower opening or pollination19-22. In petunia and carnation, the increase of ethylene production occurred during the first 30 min after pollination22-24. Ethylene production raised during pollination correlated with the increasing capacity of 1-aminocyclopropane-1-carboxylic acid (ACC) oxidase (ACO)21,25. The production of endogenous ethylene was also affected by environmental conditions. Darkness or shady conditions increased ethylene production and caused flower abscission26. The production of endogenous ethylene of Capsicum annuum increased when the plant was placed in 80% shade27.
Ethylene was also found in the transit or storage containers of a post-harvest handling environment. Ethylene from ripening fruits, senescing flowers, dying or decaying plant material, the smoke of the combustion car engines and cigarette smoke were sources of exogenous ethylene. The exposure of illumining gas and tobacco smoke increased the petal abscission of geranium flower and other members, indicating the presence of ethylene in the illuminating gas and tobacco smoke28.
CHEMICAL COMPOUNDS AS AN ETHYLENE INHIBITOR
Ethylene response could be inhibited by inhibition of ethylene biosynthesis and/or action29. Ethylene biosynthesis could be inhibited through the conversion of S-adenosyl L-methionine (AdoMet) to ACC or the conversion of ACC to ethylene. Some chemical compounds and environmental manipulation such as Aminoethoxyvinylglycine (AVG) and amino-oxyacetic acid (AOA) could be applied to inhibit the formation of ACC from AdoMet, whereas, Co2+, alpha-aminoisobutyric acid and low O2 concentration could be used to inhibit ethylene formation from ACC. The second method ethylene action inhibition or prevention of ethylene binding using compounds such as Diazocyclopentadine (DACP), 2,5-norbornadine (2,5-NBD), 1-MCP and silver thiosulfate (STS)29.
Inhibition of ethylene action by inhibiting the binding process of ethylene to the receptor was more effective than preventing the synthesis of exogenous or endogenous ethylene29,30. The inhibition of ethylene action prevented both exogenous and endogenous ethylene effects. While the inhibition of ethylene synthesis only prevented the production of endogenous ethylene and exogenous ethylene will still bind to the receptor and caused response in plants. The AVG application followed by continuous ethylene exposure (1 μL L–1) was less effective than STS to inhibit buds drop of “White Christmas” Schlumbergera truncata (Haw.) and also prevented petal abscission of potted Pelargonium x hortorum30,31. In tropical ornamental plants, the application of AOA reported to delay the onset of flower abscission, decreased ovary growth, delay senescence and block pollination-induced ethylene production in orchid flowers32-34.
The quality loss during post-harvest from ethylene could be reduced or eliminated by pre-treating plants with inhibitors. In the 1970s, silver thiosulfate (STS) was discovered as an effective inhibitor in increasing the post-harvest live on ornamental plants. The STS suppress ethylene action35, it binded to ethylene receptor, leading to the suppression of endogenous and exogenous ethylene effect. STS prevented chlorophyll loss36, decreased petal abscission, increased postharvest quality of Pelargonium hortonum and other ornamental crops16 but also inhibited rooting and decreased rooting quality37. Despite of the STS efficacy, silver was toxic in plant cells and a potential metal pollutant, contaminating ground water38. Therefore, STS application had been restricted in some countries, such as in the Netherlands39.
A series of cyclopropene, such as cyclopropene (CP), 1-MCP, 3-MCP and 3,3-dimethylcyclopropene (3,3-DMCP) were shown as an effective chemical compound to block ethylene binding site. The 3-MCP and 3,3-DMCP were effective in higher concentration than CP and 1-MCP. Unfortunately, CP was unstable in the form of liquid or dilute gas even at -78°C, because the compound seemed to polymerize at room temperature40. Therefore, amongst all the substituted CPs, 1-MCP was one of the most useful and more effective in higher temperatures.
1-Methylcyclopropene (1-MCP) was developed in the early 1990s as an environmental friendly, non-toxic and effective ethylene blocker. 1-MCP quickly became a popular alternative chemical to treat and minimize post-harvest loss in horticultural crops. Effectivity of 1-MCP depends on exposure duration, cultivars, temperature, concentration, stage of development and plant maturity6,41.
1-MCP marketed in a powder form that will form gas (volatile 1-MCP) after water or buffer addition. In order to achieve maximum efficacy, 1-MCP must be applied in closed area to prevent loss. New formulation of 1-MCP was water soluble powder (sprayable 1-MCP) and was intended for widespread application. DeEll42 reported that this formulation was effective in maintaining ‘Barlett’ Pear quality and comparable to 1-MCP. The pre-harvest application 100-150 μL L–1 a water soluble powder of 1-MCP on apples at 7 and 14 days before harvest could reduce advance maturity in the fruit, delay color development and reduced ethylene production43,44. The effect of 1-MCP significantly influenced post-harvest life of several ornamental plants. 1-MCP delayed petal abscission on phlox flowers, P. peltatum, P. zonale and P. x hortonum14,16,45,46, prevents bud, flowers and leaves abscission from Begonia, Rosa ‘Royal’ and ‘Sunset’, Lilium ‘Stargazer’, Kalanchoe blossfeldiana ‘Tropicana’47-51, reduced fresh weight loss of Lupinus havardii ‘Texas Sapphire’52 and increased flower longevity Gravillea and Dianthus53,54.
APPLICATION OF 1-MCP
1-MCP could be applied in several horticultural crops to prevent ethylene negative effect on ornamental cuttings or flowers (Table 1). Mostly, 1-MCP was used as volatile application in an enclosed area. Recently, the new formulation and application of sprayable type with similar capability as volatile 1-MCP were developed. Nevertheless, in several crops such as Pelargonium, sprayable 1-MCP was less effective than volatile 1-MCP in improving the post-harvest quality of cuttings and flower. Sprayable 1-MCP required a much longer time to diffuse into the tissue compared to the volatile type. Moreover, the application of sprayable 1-MCP at low concentration in open space was not effective in preventing the deteriorative effects of ethylene. Seemingly, due to evaporation of the sprayed solution and some molecules of 1-MCP in the solution were lost during the gassing phase, which caused insufficient time for the compound to diffuse into plant cell and inactivate ethylene receptor. Therefore, in open-space application, higher concentration of sprayable 1-MCP was required.
| Table 1: | Application of 1-MCP on ornamental plants |
The application of sprayable 1-MCP with the concentration up to 25 times volatile 1-MCP reported to be ineffective in improving postharvest life of Pelargonium zonale55.
The application of sprayable 1-MCP with1 μL L–1 exposure of 1-MCP for 4-6 h was effective in reducing petal abscission of Pelargonium x hortonum14,69 and in reducing the loss of quality in storage cuttings of Pelargonium such as leaf yellowing but did not improve rooting ability13,37. In cut rosses, 1-MCP and also combination with vase solution were effective in maintaining post-harvest life of roses and chrysanthemum58,70,71. The application of 1-MCP on young flowers was more effective than older flowers in increasing the post-harvest life of Pelargonium15,16. In the case of near-senescence flower, e.g. Pelargonium, 1-MCP did not improve the flower longevity70, suggesting the differences of response in each cultivars. Treatment with 0.1 μL L–1 of 1-MCP for 1 h on P. xhortonum ‘Kim’, ‘Veronica’ and ‘Cotton Candy’, that were less sensitive to ethylene, was enough to reduce petal abscission. However in the case a more sensitive P. xhortonum ‘Fox’, 12-24 h of exposure was required to reduce petal abscission14.
The efficacy and the optimum concentration of sprayable 1-MCP were very cultivar-dependent. The response of 1-MCP did not completely inhibit petal abscission in two ethylene sensitive zonal and regal Pelargonium cultivars, such as Pelargonium x hortonum Bailey and Pelargonium x domesticum Bailey14,69. Application of sprayable 1-MCP in enclosed and open space were effective in reducing number of senescence leaves exhibited by leaf yellowing or browning. Ethylene-induced leaf yellowing was caused by chlorophyll degradation. Matile et al.72, reported that ethylene accelerated chlorophyll degradation by enhancing the activation of chlorophyllase in conversion chlorophyll a and b to chlorophyllide and phytol. Generally, leaf hue and chroma were used for color change quantification from green to the initiation of yellowing11,73. High leaf chroma and low leaf hue indicates leaves yellowing, meanwhile the low chroma indicates that the leaves remain green11. Nevertheless, at the end phase senescence, the chroma decreased, leaving brown to yellow appearance in leaves. Therefore, the low value of leaf chroma may indicated not only green but also brown leaves. In general, consequence of 1-MCP application in cuttings was the increase of endogenous ethylene production. Reported by Kadner and Druege13 showed that endogenous ethylene in Pelargonium cuttings increased as a response to the 1-MCP application. Another report also reported the increase of endogenous ethylene in citrus and coriander after 1-MCP application74,75. In vegetative tissue, 1-MCP application can act as a negative feedback control of ethylene production. The non-activation of the ethylene receptor would inhibit the down regulating action of ethylene and allow for uncontrolled ethylene synthesis13. In contrast, the application of 1-MCP in floret clearly decreases the production of endogenous ethylene. Seglie et al.76 reported that the treatments 1-MCP and DPCA reduced the endogenous ethylene production in carnation76. So far, there was no clear explanation for these phenomenon but it was suggested that several factors such as flower maturity and the ratio of peduncle, sepal and petal may contributed to the production of endogenous ethylene as a response to 1-MCP application. Wue et al.77 reported that the application of 1-MCP decreased endogenous ethylene production in petals but it increased endogenous ethylene production in sepal.
Recently, 1-MCP had been widely used for several tropical ornamental plants. 1-MCP prevents ethylene production during pollination and prevented senescence of the Phalaenopsis ‘Herbert Hager’ flower46, promoted the quality characteristics of the highest water uptake, the best retention of anthocyanin content and the lowest browning appearance in Curcuma aeruqinosa and improved inflorescence longevity of the torch ginger Etlingera elatior78. Since 1-MCP was effective in improving post-harvest life of tropical ornamental plants it could be one of the potential compound to be applied as a post-harvest life quality preservation techniques of tropical ornamental plants.
CONCLUSION AND FUTURE PERSPECTIVE
Inhibition of ethylene biosynthesis and inhibition of ethylene perception are two methods to inhibit ethylene response. Volatile 1-MCP is one of the potential chemical compounds but the application was not simple. New 1-MCP formulation, sprayable1-MCP was more simple because it can be used in the field as a pre-harvest treatment. Therefore, sprayable 1-MCP is recommended as a more reliable compound to be used for ornamental plant especially for ornamental plant industry in the tropics.
SIGNIFICANCE STATEMENT
Ethylene is a plant hormone released to regulate physiological effect on ornamental plants. The presence of ethylene accelerate quality reduction in ornamental plants. Despite being one of the most effective ethylene-inhibiting compounds, 1-MCP has not been used widely for tropical ornamental plants. This review covers the potential application of 1-MCP recorded by researchers as a positive approach to minimize the obstacles that occurs in tropical ornamental plant industry.