Since the late 2010s, the world of Aeonium collectors has undergone a true revolution. Dozens of new Aeonium variegata , with ever more spectacular variegation, have appeared on the market, mainly from China.
This explosion of new varieties naturally raises a question that many enthusiasts are asking:
How do variegated Aeonium appear?
Are these solely natural mutations selected by producers? Are there techniques to artificially induce these mutations? Why do the majority of new varieties originate from a few large Asian nurseries?
In this article, I offer a synthesis of currently available scientific knowledge, supplemented by my own research and observations carried out over several years.
I should point out, however, that I am not a biologist. Therefore, some interpretations may be incomplete or may change as scientific progresses. If you have any further information or would like to discuss this fascinating topic, please feel free to contact me.
Understanding the essential role of the meristem
To understand the origin of variation in Aeoniums , we must first look at the meristem .
The word meristem comes from the Greek meristos , which means « divided ».
This is a tissue made up of undifferentiated cells capable of constantly multiplying. In plants, this tissue is responsible for all plant growth.
The main distinctions are:
- primary meristems, which ensure the growth in length of stems, roots and leaves;
- secondary meristems, responsible for the thickening of stems and trunks in some species.
In Aeoniums, the apical meristem is located in the center of each rosette. It is this meristem that progressively produces all the new leaves, future branches, and new shoots.
In other words, the entire architecture of the plant is programmed from the meristem .
What we know today about Aeoniums variegata
Although many areas of uncertainty remain, several observations now seem to be largely established.
In variegated Aeoniums:
- the meristem contains the cells responsible for variation;
- New shoots inherit this cellular organization when they originate from a vegetative bud;
- Variety is extremely rarely transmitted by seeds.
This last characteristic is essential for understanding how new cultivars appear.
Unlike some plants in which variegation can be genetically inherited, variegated Aeoniums do not reproduce their variegation from seed.
How Does Variegation Arise? Spontaneous Mutations and Possible Causes
Scientific research has identified several mechanisms that can lead to the development of variegation.
Not all of them apply to Aeonium, but they help us better understand the different phenomena observed throughout the plant kingdom.
1. Differential Gene Expression
In some species, all tissues share the same genetic makeup, but certain genes are expressed only in specific areas of the plant.
In such cases, variegation is genetically programmed and can be passed on to subsequent generations through seeds.
This mechanism does not appear to correspond well with observations in Aeonium, where variegation is rarely inherited through seed propagation.
2. Structural Variegation
Some plants display silvery or white areas that are not caused by a lack of chlorophyll.
These effects are simply the result of the internal structure of the leaves: tiny air spaces alter the way light is reflected, producing a pale or silvery appearance.
This phenomenon is relatively uncommon and does not appear to explain the types of variegation observed in Aeonium.
3. Viruses
Several plant viruses can cause irregular chlorosis that gives the appearance of variegation.
These infections are generally unattractive and can significantly weaken the plant.
Although such cases are known in horticulture, this explanation does not appear to correspond to the variegated Aeonium cultivars grown today.
4. Genetic Mosaics
Genetic mosaics are currently one of the most widely studied explanations for variegation.
A plant becomes a genetic mosaic when different populations of cells, with different genetic makeups, coexist within the same individual.
These mutations may affect:
- the nuclear genome;
- the chloroplasts;
- the mitochondria.
When certain cells lose their ability to produce chlorophyll normally, they become white or yellow.
The juxtaposition of normal and mutated cells then creates the characteristic variegated patterns.
In most cases, these mutations are not transmitted through seeds.
Chimeras: The Most Plausible Explanation for Variegated Aeoniums
At present, the theory of meristematic chimeras appears to provide the most consistent explanation for the majority of variegated Aeonium cultivars.
A plant chimera occurs when several genetically distinct cell lineages coexist within the same meristem.
Depending on which cell layers are affected, the variegation may be more or less stable, more or less extensive, and may sometimes change over the years.
Spontaneous Mutations
The first possible origin is entirely natural.
A mutation may arise spontaneously in a small number of cells within the meristem.
If this mutation affects cells involved in chloroplast development or function, variegation may appear in a new rosette or offset.
This is probably how many cultivars that are now famous first originated.
However, in nature, such mutations have very little chance of surviving over the long term.
Tissues lacking chlorophyll carry out less photosynthesis, which slows the plant’s growth and reduces its ability to compete with fully green individuals.
Without human intervention, these mutations therefore often disappear naturally.
Human-Induced Mutations
A second hypothesis involves the deliberate use of mutagenic agents.
For several decades, ornamental horticulture has used various techniques to artificially increase the frequency of mutations.
These include:
- gamma radiation;
- X-rays;
- certain types of ion beams;
- chemical mutagens such as EMS (ethyl methanesulfonate);
- various inhibitors of chlorophyll synthesis.
These treatments can considerably increase the frequency of mutations.
They have long been used to create new horticultural varieties in many plant species.
However, there is currently no publicly available evidence demonstrating that these techniques are being used to create the variegated Aeoniums currently available on the market.
From a scientific perspective, this remains a possibility, but it should be regarded as a hypothesis unless and until it is properly documented.
Why Can’t Variegated Aeoniums Be Propagated from Seed?
This is a question that comes up very frequently.
In chimeric plants, the cells that produce the gametes generally do not have the same genetic organization as the cells responsible for the variegation.
The result is simple:
Seeds almost always produce entirely green plants.
This is why growers rely primarily on vegetative propagation.
Cuttings: The Standard Propagation Method
Stem and offset cuttings make it possible to faithfully preserve the organization of the meristem.
As a result, the new plants almost always reproduce the variegation of the mother plant.
This remarkable stability helps explain the extremely rapid expansion of variegated Aeonium collections in recent years.
Why Do So Many New Cultivars Come from China?
This is probably one of the most intriguing questions.
To date, there is no definitive answer.
Based on the many discussions I have had with several Chinese growers, new cultivars are said to arise primarily from spontaneous mutations, carefully identified among extremely large-scale horticultural productions.
This explanation appears entirely plausible.
When hundreds of thousands, or even millions, of Aeonium plants are grown simultaneously, the probability of encountering rare mutations increases considerably.
Once detected, these mutations are selected, propagated through cuttings, and then gradually introduced to the market.
It is also common for an initially unstable variegated cultivar to give rise, over the years, to new variants displaying different patterns or more pronounced variegation.
However, the possibility of deliberate human intervention aimed at increasing the occurrence of certain mutations cannot be completely ruled out. To date, however, no scientific publication or official statement has confirmed the use of such practices by Aeonium growers.
Conclusion
The exact origin of variegation in Aeonium still remains partly mysterious.
Current knowledge suggests that meristematic chimeras, resulting from spontaneous mutations in meristem cells, are the most likely mechanism.
Artificial mutagenesis techniques demonstrate that it is scientifically possible to induce this type of mutation in many horticultural species. However, there is currently no evidence that these techniques are responsible for the emergence of the many variegated Aeonium cultivars that have recently entered the market.
As is often the case in botany, every new discovery seems to provide as many new questions as answers.
This article will be updated as new scientific publications become available and as further information is gathered from growers and researchers. If you have additional information or would like to share your own experience, I would be delighted to discuss it and help build our collective knowledge of this fascinating subject.
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