Question: Explain Variegation in Ivy (x): possible changes roadmap

There are no records of Hedera helix cultivars which originated from mitotic recombination. Hedera helix 'Buttercup' has entire yellow leaves. Peter Rose states that 'Buttercup' is a nuclear mutation because this cultivar comes true to type. This could mean that 'Buttercup' is a yellow leaved mutation as described by Ben Zonneveld. There is no record of a variegated sport that originated from 'Buttercup' (caused by mitotic recombination).

Hedera helix 'Sterntaler' is another cultivar with entire yellow leaves. According to Ingobert Heieck yellow shoots of 'Goldstern' can be propagated vegetatively. He named this new cultivar first 'Goldstern 2' and finally 'Sterntaler'. Only yellow leaved mutations will survive vegetative propagation. This could mean that 'Sterntaler' is a yellow leaved sport of the variegated cultivar 'Goldstern' which in turn originated from an unnamed yellow leaved mutation by mitotic recombination. Pictures of 'Sterntaler' show a light green leaf colour so I'm not sure.

Green leaved species roadmap to variegation:

* albino chloroplasts
** green chloroplasts
*** lethal type of chloroplasts in a chimera with viable chloroplasts

Sources: Hedera sorten, Ihre Entstehung und Geschichte dargestellt am Sortiment der Gaertnerei Abtei Neuburg, Brueder Ingobert Heieck OSB, 1980, Gaertnerei Abtei Neuburg; The Gardener's Guide to Growing Ivies, Peter Q. Rose, 1996, Timber Press; Variegated trees & shrubs, Ronald Houtman, 2004, Timber Press

The next time it's time for a recap.

Question: Explain Variegation in Ivy (ix): mitosis

Mitosis is the process by which the nucleus and cytoplasm in a cell divide and form two identical cells. The division starts when DNA replication already has takes place. DNA contain genetic information vital for proper cell function. Chromosomes, present in the nucleus when mitosis takes place, are complexes of tightly-coiled DNA (loosely bundled DNA are called chromatins). Diploid plants, like Hedera helix, have two versions of each chromosome (homologous chromosomes), one from the seed parent and another from the pollen parent.

The first phase (prophase) is when the chromatins condense together into chromosomes, the nuclear membrane disintegrates and a spindle appears. The second phase (metaphase) is when the chromosomes line up on the equator of the spindle. The stage where the replicated chromosomes split by the threads of the spindle and are pulled apart to the opposite poles is called the third phase (anaphase). During the fourth fase (telophase) a new nuclear membrane forms around the separated chromosomes and a cell plate is formed to divide the two cells (cytokinesis).

If a mutation arises only in one copy of homologous chromosomes and the gene is dominant, like the colour yellow for a leaf, each cell has a dominant gene for yellow and a recessive gene for green chloroplasts resulting in a yellow sport. If mitotic recombination occurred, it's possible the following will happen: one of the cells may be lost, and the surviving cell can lead to an entire layer.

Source: Variegated trees & shrubs, Ronald Houtman, 2004, Timber Press

I tried to create a 'roadmap' of the possible changes which I'll share in my next post.

Question: Explain Variegation in Ivy (viii): causes and possible changes

On very rare occasions a green plant develops a yellow or variegated shoot. These mutations are called a sport (a shoot with a different habit or leaf shape is also called a sport). Consecutive changes, leaf colour wise, are caused by chimeral rearrangement, back mutation or mitotic recombination.

As stated before a chimera originates from a mutation, usually as a sport. Chimeral variegation is not inheritable because only 1 layer (L3 for Hedera helix and other species) is responsible for contributing egg cells or pollen. Another sport originates from a mutation indirectly changing the development of chloroplasts. This feature is inheritable because it is caused by a nuclear mutation.

When a chimeral rearrangement occurs cells are exchanged between 2 layers of a chimera. When layers exchange cells a green or yellow sport may develop. A shift of cells from L2 to L1 may have no visible effect because L1 doesn't contribute cells that develop chloroplasts.

A back mutation is a mutation where a cultivar reverts to the wild type. Back mutation is as rare as a 'normal' mutation so a sport on a cultivar is usually caused by a, also rare, chimeral rearrangement or mitotic recombination, depending on the mutation.

Mitotic recombination is the exchange of part of a chromosome due to an error during cell division (mitosis). To understand mitotic recombination you need to know, to some extend, what stages a cell goes through during mitosis. In my next post I'll cover mitosis.

Source: Variegated trees & shrubs, Ronald Houtman, 2004, Timber Press

Question: Explain Variegation in Ivy (vii): addendum

I verified my previous posts with Zonneveld's essay. It's fairly complete. There are only two bits of information I'd like to add to my finds. However, Zonneveld's essay covers more than the development of variegation. He also explains the causes of the possible changes a plant, leaf colour wise, can go through! I can't resist to post that explanation. But first the two additions.

When I explored the SAM, I stated that L1 contribute, in some plants, tissues around the leaf margin. Well, Hedera helix isn't one of them. For Hedera helix L2 is the layer that, besides the mesophyll, contribute the margin of the leaf. Zonneveld uses Hedera helix 'Goldheart' to explain yellow-centered variegation.

When I covered chimeras I listed the three types of chimera in plants. Zonneveld writes that sectional and mericlinal chimeras can lead to periclinal chimeras.

The next few posts will cover the causes of changes -and the possible changes- a variegated plant can go through.

Question: Explain Variegation in Ivy (vi): new information source

I finally found an information source covering variegation in detail! It's a book called 'Variegated trees & shrubs: the illustrated encyclopedia', by Ronald Houtman for the Royal Boskoop Horticultural Society. It's published in 2004 by Timber Press, Inc (ISBN: 0-88192-649-3, hardback).

This book covers the most important variegated woody plants, mostly cultivars. Hedera helix is included with 30. A variegated plant in this book is defined as a woody plant with more than 1 leaf colour. For every cultivar there's a description of habit, origin and culture complemented with a picture. A valuable source when designing a garden or when starting a collection.

For me Appendix A is the most interesting. The title is 'Variegation in plants' by Ben J. M. Zonneveld. Zonneveld is Doctor at the Institute of Biology, Clusius Laboratory, Leiden, The Netherlands. In his essay Zonneveld discusses all types and causes of variegation.

At this point I feel I need to verify my gathered information with the information of Zonneveld. The next time I will post my finds.

Question: Explain Variegation in Ivy (v): chimera

Until now it was all biology class textbook material (except maybe the info about the SAM). Now the real stuff. The phenomenon chimera is very interesting, to say the least. A chimera is an animal or plant composed of genetically different cells.

Division of a mutant cell in the apical meristem is the cause for plant chimeras. Albino mutant cells lost the ability to produce chlorophyll (resulting in variegation in a plant), but other types of mutation are possible. A plant could be a chimera for virtually any trait, albeit often invisible to the naked eye.

For plant chimeras there are three categories based on the location and relative proportion of mutated to nonmutated cells in the apical meristem. A periclinal chimera is a shoot formed from an apical meristem in which at least one of the clonally distinct cell layers is genetically different. These mutations are fairly stable allowing propagation of distinct cultivars. This propagation occurs vegetatively because having more than one type of genetic material chimeras will not be true to type.

For mericlinal and sectional chimeras the mutant cells do not comprise a complete layer of the apical meristem, but only a portion. Mutant cells for mericlinal chimeras only cover a part of one layer. With sectional chimeras the mutant cells cover sections of several layers. Chimeras of these categories may develop chimeric shoots or leaves, but also normal or completely yellow ones. Generally they are not suitable for propagation, but particularly sectional chimeras with albino mutant cells display some interesting patterns.

Sources: Origin, development, and propagation of chimera, R.D. Lineberger et al, unknown, http://aggie-horticulture.tamu.edu/tisscult/chimeras/chimeralec/chimeras.html. Lecture 18: Shoot Apical Meristem (SAM), Phil Becraft, 2005, http://public/iastate.edu/~bot512/lectures/SAM.htm

Now we're able to further answer the question: explain variegation in ivy. That will be the topic of the next post.

Question: Explain Variegation in Ivy (iv): mesophyll cells

Like all cells, mesophyll cells consist of cytoplasm and a nucleus. Only plant cells contain plastids, structures with specialized functions. Plastids may take several forms, chloroplast being one of them. Mesophyll cells contain chloroplasts, in their turn containing chlorophyll consisting of four components: chlorophyll a (bluish green), chlorophyll b (yellowish green), xanthophyll (yellow) and carotene (yellowish orange). With the chlorophyll chloroplasts conduct photosynthesis.

Chlorophyll gives the leaf its colour only after exposure to sun light. Otherwise it's colourless. Vacuoles (compartments within the cytoplasm with several functions) sometimes contain a pigment called anthocyanin which gives a red, blue and purple colouration to most flowers and leaves (found in epidermis cells and peripheral mesophyll cells). The spongy parenchyma contains less chloroplasts than the palisade parenchyma. That's why the underside of a leaf looks less green then the top.

This is all you have to know about mesophyll cells to understand variegation. Next post: chimeras.

Question: Explain Variegation in Ivy (iii): leaf tissue

You probably know that most leaves have a top (adaxial side) and a underside (abaxial side) both with a distinct function and structure. The top is built to protect the leaf; the underside is for exchanging gasses for photosynthesis. The intermediate layer is where photosynthesis takes place.

The upper most cell layer(s) is the epidermis. These cells have thick impermeable cell walls and form a continuous layer, mostly covered with a coating (cuticula). Epidermis cells don't contain any chloroplasts.

The outer most layer(s) of the underside is also called epidermis. The difference with the upper side are the stomata (pores). These groups of cells allow the leaf to exchange gasses (and contain chloroplasts). Mostly there is a mechanism to open and close a pore.

The layers of cells between the epidermis is the mesophyll. These cells contain chloroplasts so photosynthesis can take place. The dense layer of cells underneath the upper epidermis is called the palisade parenchyma and collects the sun light.

The layers below the palisade parenchyma is called the spongy parenchyma. This airy layer is also connected to the stomata and the vascular bundles. Here the gasses for -and the products of- photosynthesis are transported.

The vascular bundles consist of two layers of cells: the xylem (the upper layer) and the phloem (the bottom layer). Through the xylem water -coming from the roots- is transported. Through the phloem the sugars are distributed to other parts of the plant.

Next post I will take a closer look at mesophyll cells before I cover chimeras.