Question: Explain Variegation in Ivy (xvi): leaf initiation

My post on may 27th discussed the SAM and how it contributes cells to a leaf. Then, I didn't get into detail on how a leaf initiates from the SAM. However, to be able to explain the irregular pattern of streaked variegation you first need to understand leaf initiation.

All three layers of the SAM are involved in the following process. At regular intervals an increased rate of cell division occurs on the flank of the meristem. The developing bulge is called a leaf buttress. A leaf primordium is generated by local apical growth in the leaf buttress. At some point during apical leaf growth the leaf development begins.

Imagine the SAM as a dome-shaped structure. Imagine the population of mutated (albino) cells in the SAM as a 'moving' and 'changing' stain in that structure. When leaf initiation occurs on the flank of the structure the stain could be involved. Perhaps 1) the initiation occurs in the centre of the stain. Or 2) the initiation occurs only partly in the stain. However, it's also possible 3) the initiation doesn't occurs at all near the stain.

In situation 1) and 2) it's impossible to predict the outcome. The structure of the leaf primordium depends on the proliferation of the cells involved. The leaf colour could be yellow or the leaf pattern could be streaked. In situation 3) it's clear the colour of the leaf will be green (without the occurrence of another mutation).

Next: trying to predict a speckled pattern.

Question: Explain Variegation in Ivy (xv): testing the theory: predicting streaked variegation

Streaked variegation, as opposed to the two former patterns, leaves a more irregular marking on a leaf. The almost mathematical patterns of a periclinal chimera tells us a streaked pattern must have a different cause.

When a layer has normal cells and a population of mutated cells (developing albino chloroplasts) the plant is called a mericlinal chimera. When the population of mutated cells covers more than one layer (still having normal cells) the plant is called a sectional chimera.

Entire yellow or green shoots are more likely to develop (as opposed to periclinal chimeras). The amount of mutated cells contributing sections of the leaf is unpredictable. To understand this you have to know a little about leaf initiation. This is the subject of the next post.

Question: Explain Variegation in Ivy (xiv): testing the theory: predicting the white margin

In this post I will try to explain the white margin. As opposed to a yellow centred cultivar having a mutated L3 layer, a white margined cultivar has a mutated L2 layer. Because the L2 layer is only sandwiched between the colourless epidermis, the margin will be white (eventually). The visible green patch is the actual section of cells the L3 layer contributed.

When you look closely at the green blotch you will often see the colour isn't uniform. Some regions are more of a lighter green or grey. There is no word about the cause of this by my sources. It is only mentioned (by Rose) that sectional chimeras can show interesting colour patterns. I think the way this pattern is constructed is similar to the construction of the mentioned colour ranges.

The explanation can be found in the distribution of the contributed cells of the L2 and L3 layers. Cells contributed by the L3 layer reside mainly in the regions around the larger veins. The L2 cells reside predominantly at the margin of the leaf. L3 cell layers are always sandwiched between L2 cell layers. At the centre of the leaf, where cells originated from L2 and L3 reside, one of these layers has the upper hand.

The palisade mesophyll of a Ivy leaf is about 3 cell layers thick. At places where the L3 cells reside there's only one layer of colourless L2 cells. This gives the darkest green colour possible. At the marginal region between the L2 and L3 layer it is possible the L2 layer occupies 2 or 3 layers of cells. This gives the lighter colours.

The distinct borders between the green blotches is caused, like the border between the green and white or yellow, by the vertical orientation of palisade cells. According to Rose Hedera helix 'Chester' is a cultivar with white margined leaves.

Leaves me only to predict the streaked and speckled patterns with the theory.

Question: Explain Variegation in Ivy (xiii): testing the theory: predicting the yellow centre

I would like to start the test of the theory with the prediction of the yellow centre (according to Zonneveld). After that I will try to explain the white margin and work my way through the more complicated types of variegation: the streaked and speckled patterns. See my other site 'Mere Ivy' (july 1st, 2006) for pictures of my favourite variegated cultivars.

One cell in the L3 layer of the apical meristem mutated and divided so all cells of the layer have the same mutation. Contributed cells of this layer develop albino chloroplasts. Because the L3 layer is responsible for the centre of the leaf, the centre would be white. However, the cells originated from the L2 layer are not mutated and therefore green. The mutated cells are sandwiched between these green cells so the white becomes yellow. Hedera helix 'Goldheart' is a cultivar with this pattern.

There's no word about the cause of the distinct border between the two colours. I think the cause is found in the vertical orientation of palisade cells in the palisade mesophyll. The border between L3 layer cells and L2 layer cells is therefore abrupt and the border between the colours clearly defined.

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

Next time I'll try to explain the white-margined variegation, the streaked pattern and the speckled pattern.

Question: Explain Variegation in Ivy (xii): intermezzo

Intermezzo
The Japanese admire their maples (e.g. Acer palmatum) especially the variegated cultivars. They named several leaf patterns (according to Vertrees). Some of them concern the leaf margin pattern, others a stain, blotch or stripe pattern.

Interesting to note there's no name for variegation with a green margin and yellow centre. There's is no explanation given for this. I checked my books and I couldn't find one cultivar. When a green margin occurred the centre was mottled with stains or netted with conspicuous veins. Perhaps the centre section in the leaf isn't as clearly defined as with other species.

Source: Japanese maples, Momiji and Kaede, J.D. Vertrees, Peter Gregory, 2001, Timber Press

Question: Explain Variegation in Ivy (xi): recap

Basically an Ivy leaf is a green mesophyll layer netted with veins sandwiched between the colourless epidermis. The mesophyll in turn is composed of the green palisade mesophyll on top of the lesser green spongy mesophyll where the veins run through.

SAM
Leaf cells, like all plant cells, originate from a meristem. Distinct layers of the meristem contribute specific sections in a leaf. L1 contributes the epidermis, L2 the margin of the mesophyll and L3 the venation and centre of the mesophyll. The L3 layer cells are sandwiched between the L2 layer cells. This is important because this construction conducts the colour pattern in a variegated leaf.

Chimera
Because of having meristem layers genetically different from each other the contributed cells in the leaf section are too. So, when one layer mutated to have albino chloroplasts the leaf section is colourless or shows the colour remaining in the cells.

Variegation
Chimeral variegation is the most common type of variegation, but stil very rare. In nature variegation is short lived because the green parts are usually stronger. It is man who keep variegated specimens alive.

It's time to test the theory. The next few posts I'll try to explain the leaf patterns with the information I gathered until now.

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.