A Northumberland Alpine Gardener's Diary
This entry: 26 January 2014 by John Richards
Leaf chloroplast variegation. Number 262.
Leaf chloroplast variegation
Its a horrible late January day outside, wet, cold and windy, so I thought I would spend some time explaining the basis of leaf variegation. Although I am not on the whole a huge fan of variegated plants, I have to admit that they have their place in the garden, especially now in winter where some can shine like a beacon through the murk. What does seem to be true is that the basis of leaf variegation is not widely understood, and indeed it can be difficult to find a convincing explanation of what causes the various types of variegation, even in many standard textbooks, or on the web. There is however a good account in a web-published set of lecture notes under www.generalhorticulture.tamu.edu/
The first point to understand is that the green pigment of leaves, chlorophyll, is carried on a number of organelles, chloroplasts, within the plant cell. These chloroplasts, which were originally derived from free-living bacteria back in evolutionary time, have their own DNA, which partly controls whether or not a chloroplast can make chlorophyll. A mutation in this DNA might mean that the chloroplast cannot make chlorophyll (leucoplast). Chloroplasts replicate independently of the cell, and when the cell itself divides, some pass to one daughter cell, and some to the other. If a cell contains both chloroplasts and leucoplasts, its daughter cells might contain a mixture, and so remain green, or one might lack chloroplasts completely (by chance), so that it, and tissues that derive from it, will become white.
As an example, here is a twig of Ilex 'Silver Queen' which was formed from an initial that completely lacked chloroplasts, so that the whole twig can only make white leaves.
Of course, once a whole tissue, stem or branch, lacks chloroplasts, it has the potential to form a new individual which is completely lacking chloroplasts. Normally this will fail, for being separated from nurturing green bits, it will have no way of making sugars and storing energy from sunlight. Here is a rare exception, a whole (large!) plant of the orchid Epipactis helleborine which is completely achlorophyllose. This stands as a striking testament to the importance of the saprophytic mycorrhiza in providing sugars to orchid roots.
Equally, if the daughter cell in a variegated plant should by chance only contain green chloroplasts, tissues arising from that daughter cell will have no leucoplats and will 'revert' to a 'normal' green tissue, as in the Euonymus fortunei 'Emerald 'n Gold' shown below. Often, as here, the resulting green tissue produces larger leaves than the variegated tissue, presumably because most metabolites are only translocated locally.
Leucoplasts result from mutations to the chloroplast DNA, and in some cases the mutation rate is unusually high. It seems that such high levels of mutation can be caused by a 'mutator', a gene which causes adjacent genes to mutate frequently. If the mutation rate is not very high, then it is typical to find blocks of tissue which are mutant, in this case lacking chlorophyll. These variegations are known as 'sectoral', because a sector of the leaf is variegated, lacking chlorophyll. Here is a well-known example in Aspidistra elatior 'Variegata'. Here, the mutation has only occurred once, at the first division of the leaf initial, so that one half of the leaf is variegated, and the other green.
In the next example, the mutation has occurred several times during the formation of the leaf, sometimes in both the mesophyll layers (explanation below), giving a white sector, and sometimes only in one, giving a pale green sector.
The same explanation holds true for this variegated bamboo outside, for which I regret I have no name.
The last three examples have been monocots, with parallel veins, the leaves of which develop from a basal meristem. However, the same phenonemon also occurs in dicots. A familiar example is the variegated sycamore, Acer pseudoplatanus 'Variegata' for which I don't have a photo. For a change, here is a sectoral chimaera, not for chlorophyll in a leaf, but for anthocyanin pigment in a petal, of Ranunculus asiaticus.
When a mutator causes a really high rate of mutation for a chloroplast gene, the leaf may become 'spotty' rather than 'sectoral'. This is the same phenonemon, differing only in degree. Perhaps the most familiar example is Aucuba japonica 'Variegata', shown below. One supposes that another frequently seen variety 'Crotonifolia', with larger yellow blotches rather than speckles, has a rather lower mutation rate.
Incidentally, there is another phenonemon which can also cause a 'spotty' leaf, which is where some of the chloroplasts become infected with a virus. This can sometimes, but not always, be diagnosed by distortion of the leaf surface, caused by the virus. It is important to know whether the variegation is viral in origin rather than mutational,as in this case it will be infectious and has the potential to infect, and weaken' 'normal' plants. Perhaps the most familiar example of a viral variegation is in Abutilon megapotamicum 'Variegatum'. If this is grown alongside green abutilons, it is often found that the latter will become variegated. Another example, again in petal pigments rather than chlorophyll, are streak-petalled tulips, which are also caused by viruses. It is perhaps ironic that these diseased individuals, which were traded for huge sums of money in the 17th century, were probably doomed to early demise!
We move on to the commonest forms of variegation, in which the variegation takes a fairly consistent repeated pattern from leaf to leaf. To understand these, it is important to realise that plant tissues are usually made up of three distinct layers which rarely if ever mix, but remain distinct through the history of a plant individual. L1 forms the epidermis alone, and usually lacks chlorophyll except in the stomata. In leaves, L11 forms the outer mesophyll, and L111 forms the inner mesophyll (and the pallisade). Here a bud initial is shown, with the three independent layers coloured and labelled.
Therefore, if a chloroplast mutation occurs so that one of the layers L11 or L111 lacks chloroplasts, the resulting white segments of the leaf will form a distinct pattern. In the commonest (or at least most easily detected) condition, where L11 lacks chloroplasts, the leaf will form a white rim, because L111 (which has chloroplasts) does not reach the edge of the leaf. This is shown in the following diagram.
The width of the green sector, and its regularity, will depend merely on the position, and thickness, of the L111-originating mesophyll, as shown in the left hand diagram. Here are some familiar examples of such 'periclinal chimaeras', firstly Daphne x transatlantica 'Summer Ice'.
Pieris japonica 'Little Heath' is one of my favourite variegated plants which makes a fantastic statement in the winter garden. It too is a L11 periclinal.
We have already seen Euonymus fortunei 'Emerald 'n Gold' and Ilex 'Silver Queen', but here is Ilex 'Golden Queen', yet another L11 periclinal..
Compared to L11 mutants, L111 chloroplast mutants are much more tricky to detect, because L111-derived mesophyll is often wrapped in green L11-derived tissue, so that L111 white tissue cannot be seen. However, as we can see in the right-hand diagram below, in many plants the L11-derived tissue does not completely encapsulate the L111-derived tissue, so that the inner mutant layer can be seen.
Such an example of a L111-derived chloroplast mutant in my garden is Euonymus fortunei 'Blondy' (so it is writ, but surely it should be 'Blondie'?).
So this covers the main types of chloroplast leaf variegation, which can be summarised as:
I hope this lends a bit of fun, next time you choose a variegated plant, and say to the nurseryman 'of course its a L111 periclinal you know'. (!)