13th January 2018
Joe said the organisation of variegated tissues in plant cells can be seen through a microscope. This is where variegation starts from.
It can be caused through environmental conditions, viruses, pattern gene variegation, chimera, of which there are several kinds, and transposons, ( or switching genes).
Joe showed a range of slides of plants illustrating variations on variegation sources and patterns.
Variegation starts in individual cells, at an early stage of cell division, and the same variegation multiplies as that cell divides.
White variegation contains no chlorophyll and these cells are smaller than cream or yellow ones, containing a little chlorophyll, and these are a little smaller than green ones, so that when a white layer of tissue, overlays a green layer, or a green layer is bordered by a white layer, as in variegated leaves, the leaves may be puckered or twisted.
Each layer of tissue in a leaf may be differently pigmented from the others, and this will show as different shades at the surface. The absence of chlorophyll may also affect the proper shape of the leaf because of the cell size differential.
Frost, sunburn, chemical effects and access to specific mineral nutrients can also cause superficial variegation. This will alter in new leaves when those conditions change and wouldn’t be passed on to seeds, or remain in cuttings.
Viruses in cells can cause a range of variegation, some of which is considered decorative. This is subjective. The existence of a virus can be proved if it transmits to another plant.
Pattern gene variegation is controlled by chromosomes in the cell nucleus. This is a source of silver variegation, when the layers of the leaves become separated, and is most often seen in shade-loving plants.
Red and purple colours come from anthrocyanins, when present in the cells. These colours are not linked to the chlorophyll variegation, so are selected separately by nurserymen.
A chimera is composed of more than one form of DNA, Joe compared this to cancer in humans, but in plants it is not necessarily life-threatening. This does not necessarily cause variegation in plants.
Variegation from complete layers of cells is more stable than when the variegation has started from wedges of colours, as in stems.
Examples of sectorial chimeras were Euonymous and Acer negundo ‘ Flamingo’. This creates an unstable chimera, in which reversion to green is likely, and will grow at the expense of the variegated tissues.
Monocotyledons plants produce their variegation from a linear row of cells, which, as they divide to lengthen the leaf will maintain the variegation that each started with.
Transposons are genes which can be ‘ switched’ on or off, and will cause one of two colours at any one time. They are capable of switching during the cell’s lifetime. He showed a plant with finely speckled all over variegation as an example.
Another example was a dahlia with some petals entirely white whilst the others were entirely red, due to a gene switch in relevant petals when they were composed of only one cell.
Joe explained what seeds of different variegations were capable of.
Transposons always produce seedlings with transposons.
A seed from a variegated plant will be capable of producing a seedling of the colour carried by it. Seeds carrying no chlorophyll- producing capacity may grow an embryonic root, but not a viable plant.
Joe said that in Britain and in Japan, variegated forms of wild plants could always be found.
This was our longest talk ever 2hr 10min.