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Fruit tree growers are often dealt major setbacks when warm temperatures arrive early in the spring and crops blossom early, leaving them susceptible to frost events. For example, in , 40 per cent of the total Ontario apple crop was lost due to a frost in late-May, with some areas severely hit. Climate change is resulting in more erratic weather patterns and is causing significant temperature swings. Production insurance is obviously an option for growers but many want to do their best to prevent crop injury when they can. As the impending frost moved in during spring , some Ontario grape and tree fruit producers used wind machines, and Carter notes that many other growers bought these machines afterwards to have them on hand. They are now available in tractor PTO- or engine-driven models and can be moved between orchards.
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The sustainable fruit production in temperate and boreal regions is often imperiled by spring frosts. The risk of frost damage and the resulting economic losses have been increasing in the recent years as a result of the global climate change. Among the many approaches in mitigating frost damages, an ethylene-based compound, ethephon has proven to be effective in delaying bloom time in many fruit species and, thereby, avoid frost damage.
However, effective concentrations of ethephon are often associated with harmful effects on fruit trees, which largely limit its use. Relatively, limited research attention has been given to understand the mechanisms underlying this ethylene-mediated bloom delay, thus hindering the progress in exploring its potential in frost protection.
Recent advances in omics and bioinformatics have facilitated the identification of critical molecular and biochemical pathways that govern the progression of bud dormancy in deciduous woody perennials. In this review, we summarized our current understanding of the function of ethylene and its interaction with other networks in modulating dormancy and blooming in temperate fruit trees. Some possible mechanisms are also proposed that might potentially guide future studies attempting to decipher the dormancy regulation or searching for methods to alleviate frost damages.
Fruit production in temperate and boreal regions is often threatened by spring frosts. Deciduous trees lose their hardiness to cold temperature after bud break in response to rising temperatures in the spring, and frost damage may occur if the temperature drops below or near freezing again.
Trees can suffer from two types of frosts: advection and radiation, with the former resulting from large cold air mass intruding an area, and the latter due to the rapid heat loss to the atmosphere, which typically occurs on calm and clear nights. In the context of global climate change, the risks of frost damage and the resultant economic losses on the global scale have been increasing at a steady pace Augspurger,The existing methods in protecting against frost can be classified into active and passive types.
Active approaches are ad hoc and can exhibit immediate and direct effects. Such methods usually involve the use of wind machines and helicopters to downdraft the warmer air aloft in the inversion layer 10—50 m above ground, surface irrigation by sprinklers to take advantage of the fusion heat in the water, and heaters e.
These approaches mitigate, instead of avoiding frost damage, and their efficacy is highly dependent on external factors such as frost types, wind speed, dew point, wet-bulb temperature, etc.
Tsipouridis et al. In addition, active methods are generally labor-intensive, cost-ineffective, and environmentally unsustainable. In contrast, passive methods are pre-emptive and have relatively long-lasting effects. In particular, the application of PGR holds great potential in frost protection due to their high efficiency, low cost, and ease of implementation. Endodormancy of buds is an adaptative mechanism that temperate species have evolved to survive the adverse conditions during wintertime.
During endodormancy, plant regrowth is repressed by intrinsic signals and remain unresponsive to environmental cues Rohde and Bhalerao,In this review, endodormancy will be used interchangeably with dormancy, unless specified otherwise. After entering dormancy, plants track the number of chilling hours 0—7. Chilling requirements are tightly controlled by genetics and are highly variable across species and varieties.
Upon completion of dormancy, resumption of plant growth is inhibited by unfavorable environmental conditions, rather than internal cues, and this period is termed as ecodormancy Lang,To reach full bloom stage, deciduous fruit species need to satisfy heat requirements HRs , a period of warm temperature, which is also a genotype-dependent trait Fan et al.
Bud dormancy and regrowth cycle in deciduous woody perennials are genetically programmed, highly regulated, as well as subject to the influence of many internal and external factors. Recent studies have also indicated that global climate change exerts profound impacts on the phenology of dormancy and flowering and the increasing risk of spring frost. Alterations in plant phenology, such as shortened dormancy and early budburst and flowering, have been observed in many woody perennials in the Northern Hemisphere Augspurger, ; Ma et al.
For example, apple blooming in Europe has advanced for a total of 6—9 days over the last 30 years Vitasse et al. Combined with other consequences of the climate change such as the number of frost days during the growing season Liu et al. Ethylene is a gaseous plant hormone that plays important roles in a plethora of physiological aspects, especially in responses to environmental stresses and biotic attacks Bleecker and Kende,Ethylene is derived from methionine, which undergoes a series of conversions catalyzed sequentially by S-adenosyl methionine SAM synthetase, aminocyclopropanecarboxylic acid ACC synthase, and ACC oxidase Wang et al.
The conversion of SAM to ACC is a rate-limiting step, and ACC has been recognized to have similar functions as ethylene in mediating plant development and defense responses Nascimento et al. In the ethylene signaling pathway, the endoplasmic reticulum ER -located ethylene receptors ETR , upon activation by ethylene, releases Ethylene Insensitive-2 EIN2 , a positive regulator of ethylene signaling, from the repression by the protein kinase Constitutive Triple Response 1 CTR1.
Both ethylene biosynthesis and signaling have been found to mediate in the regulation of dormancy, as application of ethylene antagonist 2, 5-norbornadiene NBD accelerates dormancy break in potato micro-tuber, Suttle, , and mutation with impaired ethylene reception prevents the initiation of dormancy in chrysanthemum Chrysanthemum morifolium , even after treatment with high dosage of ethephon Sumitomo et al.
The finding that many genes associated with ethylene biosynthesis and signaling e. Ethylene has been used to delay bloom and avoid frost damage on fruit trees for decades. Ethephon is a plant growth regulator that degrades and releases ethylene once entering plant cytoplasm Pahwa and Ghai,Many studies have reported that fall application of ethephon can effectively delay the blooming time in the following spring in many fruit species, especially stone fruits Table 1.
According to these studies, ethephon-induced bloom delay can range from 3 up to 18 days, depending on the concentrations and application time. In general, early application with higher concentration is more effective. The effectiveness of ethephon appears to be limited to the pre-dormancy stage, as serious flower bud abscission was induced when application was made after the satisfaction of CR Durner and Gianfagna, , and little or no effect was found when ethephon was applied in apricot during the dormancy stage Grijalva-Contreras et al.
In addition to delaying bloom date, fall application of ethephon was also found to enhance cold hardiness of dormant buds. In peach, a frost ofSimilarly, Durner showed that ethephon treated peach flower buds were 0.
Practically, the consequences of delaying bloom and improving cold hardiness are significant, as lessening the effects of winter injury and spring frost damage can greatly contribute to the increase of crop yields.
Despite the significant success in using ethephon to delay bloom and prevent spring frost, some studies have indicated beneficial effects of ethephon can be comprised by the occurrence of detrimental effects such as gummosis, leaf yellowing and abscission, terminal dieback, flower abscission, floral bud failure, low fruit set, and yield reduction Table 1.
Furthermore, fall application of 75— ppm ethephon on almond caused up to 3-fold yield reduction, and the yield reduction was in proportion to the ethephon concentrations Grijalva-Contreras et al. Such linear yield decrease caused by increasing ethephon concentrations was also documented in peach Crisosto et al.
The formation and exudation of gums on the trunk or limbs of fruit trees, known as gummosis, is another major problem that pesters fruit growers when using ethephon. Tree gums are mainly composed of polysaccharides and are induced by various environmental stresses, mechanical or chemical injury, insect attack, or infection Saniewski et al.
Ethylene has been implicated as the leading factor that induces gummosis Li et al. Ethylene can act synergistically with jasmonic acid to cause the breakdown of cell membranes and cell disintegration, which is the first step of the gum formation Saniewski et al.
Application of ethephon can induce gummosis in the bulbs of grape hyacinth Muscari armeniacum within several days Miyamoto et al. Ethephon-induced gummosis in fruit trees, especially of stone fruit species, has been reported in many studies Moghadam and Mokhtarian, ; Miyamoto et al. Though literature lacks the data on the yield reduction due to ethephon induced gummosis, growth retardation and value loss caused by gummosis in fruit trees can be substantial Beckman, ; Ezra et al.
Recently, ethephon has been suggested to be potentially hepatotoxic Bhadoria et al. Limited information is available on how the effect of a fall application of ethephon is carried over to influence the floral behavior during the following spring. The time of full bloom depends on the fulfillment of both chilling and HR; the former dictates endodormancy break and the latter, ecodormancy release Campoy et al. Accordingly, the delayed bloom may result from an inadequate accumulation of chilling or heat.
Indeed, previous studies have indicated that chilling accumulation negatively correlates with the number of days to full bloom and the HR Durner and Gianfagna, ; Li et al. Durner and Gianfagna proposed that ethephon can reduce the effectiveness of chilling in breaking flower bud dormancy, as ethephon treated flower buds needed about 3 additional weeks of chilling exposure to reach the CR compared to controls. By examining excised peach branches, Coston et al. Some early studies also supported this finding in which ethephon leads to altered bud responses to warm temperatures after endodormancy release, manifested as delay in the differentiation of flower buds and the growth rate Crisosto, ; Gianfagna et al.
These findings indicate that ethephon may prolong both endo- and eco-dormancy duration, and influence the springtime phenology of floral buds, which appear to develop slower responsiveness to seasonal changes. Particularly, if the ethephon increases the CR to the point that exceeds the local chilling hours, significant bloom delay will likely occur. Though many studies have gained some insights into the physiology of fruit trees influenced by ethephon, none of them have explored the underlying mechanisms of how ethylene affects bud ontogeny during endodormancy and their growth after dormancy break, especially at the molecular level, and more critical studies are still needed to answer these questions.
Considering the essential role of ethylene in plant defense and stress signaling pathways, the ethylene-induced bloom delay may be a result of the activated or amplified stress responses. If exogenous ethylene is sensed by plants as a stress signal, longer dormancy, and late flowering would be advantageous for perennial plants to survive the unfavorable conditions, since in terms of fitness, survival outweighs reproductive success in perennials Shefferson,Some evidence has emerged to support this hypothesis.
The aforementioned gummosis evidently links ethylene to plant defense network, as gummosis has a strict connection to defense response against biotic attacks Saniewski et al. Moreover, a study in Arabidopsis showed that treatment of ethylene precursor ACC rapidly decreases cell proliferation rates by arresting the cell cycle, in a similar way as osmotic stress Skirycz et al. Such ACC-induced arrest of cell cycle was shown to cause dwarfism in leafy spurge that mimics the dormant phenotype Dogramaci et al.
The high resemblance between stress and dormancy with regard to cell cycle arrest and growth inhibition suggests that they may share common or overlapping networks acting downstream of ethylene signaling pathway.
Growth arrest proceeds, and is a prerequisite for the dormancy induction, and accelerated growth cessation has been suggested to be associated with greater dormancy depth and late bud burst Kalcsits et al. In light of this, early growth arrest induced by exogenous ethylene may be responsible for the extended dormancy and bloom delay.
Nevertheless, how precocious growth cessation intensifies dormancy and delays bud burst still needs further examination. In a recent review, Beauvieux et al. The findings that ethylene enhances the production of ROS in response to stress Zhang et al. Thus, if the fall-applied ethephon causes the ROS levels to be higher than what would be induced by natural environmental cues, such as short photoperiod Karpinski et al.
It has been shown that stress can induce post-translational modifications. Increasing evidence has indicated that histone acetylation is necessary for the transcriptional regulation of ethylene signal transduction elements, i. Therefore, if such ethylene mediated chromatin modifications are retained, they may likely affect the subsequent events of dormancy and flowering. The phenology of plant dormancy and flowering are of great agricultural and economic importance. Ethylene exerts strong control over the progression of dormancy and flowering, and elucidation of its regulation mechanisms, especially its interaction and crosstalk with other signaling networks holds the key to formulate effective approaches for frost mitigation and avoidance.
This review intended to summarize the current knowledge of ethephon-mediated bloom delay in temperate fruit trees and highlight the possible mechanisms by which ethylene interferes with the initiation, maintenance, and release of bud dormancy. As ethylene is at the hub of crossroads to defense- and stress-related signaling pathways, it might be meaningful to examine ethylene in the context of stress responses.
Emerging evidence has suggested that pathways of stress response and dormancy may converge at the stages of growth cessation, and bud break. However, due to the wide range of climates under which these studies are conducted and to facilitate comparative analyses, it would be highly advisable that future investigations use chill units CU and growing degree hours GDH , rather than Gregorian calendar, to time sample collections during the bud dormancy cycle.
JL and SS have contributed equally to the writing, editing, and preparation of this mini-review. The authors declare that no conflicts of interest exist regarding the completion and publication of this research. Achard, P. The plant stress hormone ethylene controls floral transition via DELLA-dependent regulation of floral meristem-identity genes. Askari, E. Bloom, maturity, and fruit set of pistachio in response to early season application of ethephon.
Augspurger, C. Reconstructing patterns of temperature, phenology, and frost damage over years: spring damage risk is increasing.
Because of our climate, Wisconsin always has a risk of spring frosts. This sheet will briefly outline critical temperatures at different stages of flower development, and describe how a gardener might protect against late season frosts. As temperatures warm in the spring, bud scales in fruit trees begin to open and new leaves and flowers emerge. During this process the ability to withstand cold temperatures decreases. Tables have been developed that show these relationships very clearly.
Protect young trees by wrapping their trunks with burlap, blankets or cardboard, but inspect regularly for ant or other insect infestation. For young or small.
When you use sprinklers to reduce frost damage, you are simply using the internal latent energy of water molecules as a source of heat to your trees. Simply, water exists in three forms liquid, solid ice and gas vapor , and the transition among these forms can produce energy exothermic or consume energy endothermic as appears in the diagram below. Where is the problem? There are three things you have to understand and consider in order to get this positive effect of sprinklers and not to harm your plants. First, you have to watch the wind. The Wind encourages the transition of the water from the liquid phase into the gaseous phase vapor and while this happens, heat in the air and around your plants will be consumed in an endothermic reaction. U of Florida Ext. Second is the dew point: without putting complication to the topic, low dew points implies low humidity and if the dew point is too low, the water you add through sprinklers will be soon evaporated to compensate for the low air humidity and consequently cools down air around your plants.
When trying to protect orchards from frost damage, growers should think in terms of frost management, Dr. Robert Evans, an agricultural engineer with the U. Frost management begins with site selection, said Evans. Cold air flows downhill under the force of gravity.
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First, before planting fruit trees of any kind, choose the location carefully. Avoid planting at the bottom of a slope? If possible, plant on a north-facing slope to help delay blooming and thus avoid frost damage. The Alabama Cooperative Extension System ACES suggests checking seed catalog descriptions and choosing fruit varieties less susceptible to frost damage in order to find varieties that bud and bloom later, when frost is less likely to occur. For existing fruit trees, ACES recommends putting off pruning until late winter to early spring to stall budding and blooming. If frost is in the forecast when trees are in bloom and the soil has been dry, water the soil a day or two beforehand to a depth of 1 foot wet soils radiate more heat than dry soils do.
Local Local. Peach, nectarine, apricot, cherry, and plum trees have two types of buds — vegetative buds responsible for shoots and leaves, and flower buds. Each flower bud on stone fruit trees contains one blossom. Apple, pear and other pome fruits have vegetative and mixed buds. The mixed buds produce both shoots and leaves, and a cluster of five flowers. Apricots, at least down here in the valley, are already starting to pop their flower buds. As the flower buds expand, they become more sensitive to frost. And if not provided some protection, the crop of apricots will be lost.
The primary methods to protect fruit crops from injury are heat, the area around the trees and are effective under both advection and radiation freeze.
Most fruits need several weeks, if not months, to reach their juicy, mouth-watering peak. To do that they need to be quick off the mark, pushing out flower buds earlier than many other plants would dare. But sometimes spring can really kick you in the teeth! Remember all that mulching, feeding , watering and pruning you did?
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Protect young frost-sensitive trees by wrapping the trunks and branches with insulating material such as palm fronds, cornstalks, cardvoard, or fiberglass.
Protecting citrus trees from frost helps to guarantee your annual crop. Certain citrus — citron, lemon, lime, and Satsuma mandarins among them — are more sensitive than others. Want more? Two comments. That means sun hits the rocks, adding F of radiating energy at night.
Space to play or pause, M to mute, left and right arrows to seek, up and down arrows for volume. Sophie pulls back the mulch from all the citrus trees when the weather cools. This allows the soil access to the sun and to warm up during the day.