Plant and Garden

Ethylene, Anthocyanins, “Pink” Succulents, and Man-made Plants

Most succulents are green in color, but some varieties can turn shades of red, pink, or purple when stressed. These succulents that can display vibrant colors other than green include some aloes, aeoniums, crassulas, echeverias, sedums, kalanchoes, sempervivums, and euphorbias.

Typical Echeveria that is naturally pink.

But what about these new “pink” succulents? Why can we not find them at our growers’ nurseries? 

Are they real? Some are.

Most succulent varieties begin with a green color and turn hues of red from some type of stress. Not the typical type of stress experienced by humans, plants experience stress that makes them more beautiful. These include water stress, sunlight stress, and cold stress.

The process of artificially pinkening the leaves of plants by gassing greenhouses with a chemical to stimulate ethylene production is how the pink leaves of some plants are “created”. 

Ethylene is a plant hormone that plays a key role in many naturally occurring processes like ripening fruit, but in this case, it’s to be used to make new leaves pink for some time… the plant eventually reverts.

Ethylene gas is a major plant hormone that influences diverse processes in plant growth, development, and stress responses throughout the plant life cycle. 

Responses to ethylene, such as fruit ripening, are significant to agriculture. The core molecular elements of the ethylene-signaling pathway have been uncovered, revealing a unique pathway that is negatively regulated. Practical applications of this knowledge can lead to substantial improvements in agriculture.

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The simple hydrocarbon ethylene (C2H4) is a tiny gaseous molecule of great significance. In addition to being the most widely produced organic compound in the world (used in manufacturing numerous products such as rubber, plastics, paints, detergents, and toys), ethylene is a major hormone in plant biology. 

This volatile molecule mediates many complex aspects of plant growth, development and survival throughout the plant life cycle, including seed germination, root development, shoot, and root growth, the formation of adventitious roots, abscission of leaves and fruits, flowering, sex determination, and senescence of flowers and leaves. Ethylene also mediates adaptive responses to a variety of stresses, such as drought, flooding, pathogen attack, and high salinity. 

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During flooding, for instance, ethylene induces the formation of aerenchyma tissue (consisting of air-filled cavities) for oxygenation. Ethylene is best known, however, for its essential role in the ripening of climacteric fruits, such as tomatoes, bananas, pears, and apples. Placing a ripe banana in a paper bag containing unripe avocados, for instance, will hasten the ripening of the avocados due to the accumulation of ethylene produced by the banana.

Interestingly, the discovery of ethylene as a plant hormone came about due to the unintended presence of ethylene in the environment. In the 1800s, illuminating gas (coal gas) was widely used for lighting, and its leakage from gas lines was known to cause extensive damage to plants, such as the defoliation of trees around streetlamps. 

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Near the end of the 1800s, Dimitry Neljubow observed that etiolated pea seedlings exhibited a peculiar growth consisting of a shortened and thickened epicotyl and horizontal bending was due to leaking illuminating gas in his laboratory.

Neljubow determined that ethylene was the biologically active component of illuminating gas. This finding led to numerous studies on the wide-ranging effects of ethylene. 

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In 1934, Richard Gane discovered that plants synthesize ethylene; the correlation of ethylene biosynthesis with biological activity was a major step toward convincing researchers that a gas could be a plant hormone. Ethylene was the first gaseous signaling molecule to be identified in any organism.

Ethylene is different from non-gaseous hormones in several ways. Ethylene moves within the plant by diffusion and is thought to be synthesized at or near its site of action, similar to the gaseous signal nitric oxide in mammals. 

Because ethylene can diffuse across membranes into nearby cells, there is no requirement for transporter proteins to deliver to target cells, and no such transporters have been identified, though there is the transport of the immediate precursor to ethylene, 1-aminocyclopropane-1-carboxylic acid (ACC). 

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Ethylene is also not known to be conjugated or broken down for storage or deactivation; ethylene simply diffuses away from the plant. While managing a gaseous hormone is simpler for plants, it is more complicated for researchers.

Ethylene experiments are generally carried out in contained environments, such as airtight chambers, although in some situations this can be circumvented by treating plants with ACC instead of ethylene.

Plants synthesize ethylene using a two-step biochemical pathway starting from S-adenosyl-L-methionine (SAM). SAM is converted to ACC by the enzyme ACC synthase (ACS). ACC is then converted to ethylene by the enzyme ACC oxidase (ACO). 

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The ACS and ACO enzymes are each encoded by a multigene family whose members are differentially expressed in response to internal developmental cues and environmental stresses, such as wounding, flooding, drought, mechanical pressure, and pathogen attack. Ethylene biosynthesis is also controlled by ACC synthase turnover, which is regulated by phosphorylation.

Ethylene is biologically active at very low concentrations of around 0.01 to 1.0 part per million (ppm). Lower or higher sensitivities have been observed depending on the species and the response. 

Some climacteric fruits, such as tomatoes and apples, can generate tens of ppm of ethylene. It is worth noting here that ethylene is a byproduct of partial combustion of organic fuels and is present,

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Therefore, in the atmosphere due to such things as forest fires, volcanic eruptions, and car exhaust. A study in 1973 detected up to 0.7 ppm of ethylene around the Beltway (the highway that circles Washington DC and the University of Maryland), and these levels harmed the surrounding vegetation.

Metazoans lack ethylene receptor homologs and do not perceive and respond to ethylene as plants do. High concentrations of ethylene (>1000 ppm) can, however, cause dizziness or light-headedness. For several decades in the 1900s, ethylene was used as a general anesthetic. 

In ancient Greece, ethylene emanating from geologic faults beneath the Oracle of Delphi’s underground chamber may have been responsible for the oracles’ trance-like states and prophetic hallucinations. The greatest danger in working with pure ethylene is the risk of explosion because ethylene is a flammable gas. However, there is far too little ethylene in tomato for it to explode!

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The ethylene effect on leaf growth and development may be independent or dependent on its interaction with other hormones.

Multiple receptors of one phytohormone might be involved in non-redundant responses, either in different tissues, at different developmental stages, or upon different environmental cues.

Anthocyanins are naturally occurring pigments belonging to the group of flavonoids, a subclass of the polyphenol family. They are common components of the human diet, as they are present in many foods, fruits, and vegetables, especially in berries and red wine.

There were more studies conducted on the effect of processing and storage on changes and stability of colors of anthocyanins in foods such as fruits and also for their use as natural colorants.

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Anacampseros rufescens, a succulent with CAM-cycling photosynthesis, grows on rock outcrops in South Africa and has purple leaves (abaxial and adaxial surfaces) as a result of high anthocyanin contents.

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Now, I am not saying that these new “pink” succulents are fake. I am just saying that man feels the need to mess with naturally occurring things and this may very well be the case.

Let’s do our research before falling for the “Pink Congo”. A plant that is sold to many enthusiasts as a rare pink philodendron but is just pumped with an artificial hormone, duping people into spending their money on a strange plant.

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