Bonito GeraniumsOnline  Bonito

                                               

 

BOTANY 102—PLANT CHIMERAS  

SOURCES OF WONDER IN PELARGONIUMS    

By Wayne L. Handlos, Ph.D.

“All the variegated plants that I have seen amongst pelargoniums are chimeras, with

the exception of those that are caused by virus infection.” (Tilney-Bassett, 2008)

In Greek mythology, there was a fire breathing dragon called Chimaera. Its body

consisted of lion, goat and dragon parts.

Since at least 1644, occasional plants have been known which appeared to consist of

very different parts/tissues. One such plant was called the ‘Bizzarria’ orange. On

this grafted plant there were leaves, flowers and fruit identical with an orange or

identical to a citron and sometimes the fruits consisted of a compound structure that

was half orange and half citron (or some other proportions of the two species).

Studies have been conducted over the decades in an attempt to understand these unusual

plants. Through experimental grafting techniques, H. Winkler (a German) in

1907 created a plant composed of both tomato and black nightshade tissues. He

called this plant a “chimera.” About the same time (1909), E. Baur (another German)

experimented with zonal pelargoniums with green and white variegated leaves

(plants frequently called fancy leaf by pelargonium fanciers). Through further examination

of the cells and tissues of pelargonium and breeding experiments, he determined

that some of these unusual plants were composed of genetically different

cells. Depending on the arrangement of these genetically different cells within a

plant he described these plants as periclinal chimeras.

 

1    2 3 4 5

1. Guard cells with normal green chloroplasts  

2. ‘L’Elegante’ -normal leaf -GWG                   3. ‘L’Elegante’ -white leaf -GWW       

4. ‘L’Elegante’ -all green leaf -GGG                  5. ‘L’Elegante’ - variegated leaf with green marginal cells from LI

 

In the variegated leaf (or fancy leaf) zonal pelargonium, studies have shown that the

chlorophyll-bearing structures (called plastids or chloroplasts) are found in abundance

in the green portions of the leaf. But in the white, yellow or ivory areas of the

Page 2

leaf, the plastids are abnormal or do not develop normally; in either case they do

not contain a functional form of the green pigment chlorophyll.

Through a variety of experimental techniques, it has been determined that in the

green and white variegated fancy leaf pelargonium leaves, the various layers of

the tunica and the corpus are genetically different from each other. In the typically

variegated leaf with a whitish margin and a green center, the layer LI of the

tunica produces cells which contain normal green plastids/chloroplasts. But in

the LII layer of the tunica, the plastids do not produce green chlorophyll and the

part of the leaf derived from this layer (that is, the margin or edge of the leaf) is

white (ivory or yellow). The central area of the leaf develops from cells of the

LIII layer (that is, the corpus). If the plastids are genetically normal, they produce

chlorophyll and this tissue in the leaf is green.

In order to understand the variations in the tunica-corpus and their derivative

cells, the layers are designated either G (for green) or W (for white). In the case

of ‘L’Elegante’ which has a green and white variegated leaf, it would be designated

GWG for the LI, LII and LIII layers respectively. The G nature of the epidermal

cells (guard cells) can be seen microscopically or the margin of the leaf

can be screened/examined for the presence of small green areas where an occasional

periclinal division in LI pushes some cells into the region normally produced

by LII (that is the mesophyll) and they then produce normal green chloroplasts.

These plants are referred to as “sandwich chimeras” because the white

tissue is sandwiched between two layers of green tissue. Occasionally

‘L’Elegante’ will produce all white leaves. In these plants the corpus, LIII, layer

has been replaced by white cells of the LII layer through periclinal divisions of

cells of this layer. If there are green marginal flecks such a plant is described as

being GWW. On the other hand, occasionally ‘L’Elegante’ produces all green

leaves. In such plants the LII and LIII layers have been replaced (supplanted) by

repeated periclinal divisions of LI giving the composition GGG. Upon microscopic

examination of the epidermis it has been found that a certain few cultivars

(‘Caroline Schmidt’, ‘Wilhelm Langguth’, ‘Lady Cullum’ and ‘Mrs. Parker’)

have a genetically white epidermis and are designated as WWG.

Occasionally it is possible to obtain Pelargonium plants from adventitious buds.

These are buds that develop from structures other than the apical meristem. In

Pelargonium such buds develop frequently from the roots. This is particularly

common in the regal and scented leaf pelargoniums. However, such buds develop

from cells of the inner layers of the root (usually in areas of the vascular

system which would have been derived from the LIII layer). In such plants the

chimeral nature of the plant is lost and the genetic nature of the core tissue is re

revealed.

1  2  3  4  5

1. White shoot of ‘Mrs. J.C. Mappin’ - GWW

2. Marginal green tissue in ‘Mrs. J.C.Mappin’ showing the green potential of LI

3. Butterfly variegation pattern in ‘A Happy Thought’    4. Butterfly variegation in ‘Crystal Palace Gem’

5. All gold leaf of ‘Crystal Palace Gem’

 

Now, if you want to breed variegated leaf pelargoniums, remember that the reproductive

cells are usually derived from the LII layer. So if LII is white (W), then

the seedlings from selfed plants will be white. (And unfortunately all white

plants die because they lack chlorophyll and cannot produce their own food supply

to sustain life.) Similarly, if you decide to propagate your variegated leaf.

Pelargonium by tissue culture, the component cells become separated

(disassociated) so the G cells will produce only green plants and the W cells will

produce only white plants. In either case, no variegated plants result because

each individual plant and its tissues are derived from a single cell and therefore

all cells of that individual are genetically identical.

Most variegated leaf plants will at some time produce aberrant shoots. In zonal

pelargoniums the stems and their leaves will either be all green when LI replaced

LII and LIII (GWG to GGW to GGG), or all white when LII replaces LIII (GWG

to GWW).

The sandwich chimera, GWG, is the most common type giving white-margined

leaves. The green nature of the epidermal cells (and therefore of the cells of LI)

can be determined by examining the guard cells in the lower epidermis. In a genetically

green epidermis the guard cells contain green plastids; in a genetically

white epidermis the guard cells lack green plastids. The presence of small green

segments along the margin of the leaf due to periclinal division of cells of LI also

indicates the genetically green nature of LI and the epidermis. The LII layer is

responsible for the white margin. (Its presence is also confirmed by the appearance

of non-green seedlings from the self pollination of variegated plants.) The

green nature of LIII is confirmed by the green color of the stem and the production

of green shoots from adventitious buds from the roots. Sandwich chimeras

frequently give rise to all green (GWG to GGW to GGG) shoots or all white

shoots (GWG to GWW).

Up to this point, all the chimeras described have been related to the plastids of the

cells. The plastids contain their own DNA and are inherited independently of the

nuclear DNA which is contained in the chromosomes. However, there are chimeras

that are under the control of the nuclear DNA. The “butterfly” leaf pattern in

zonal pelargoniums is a case in point. In these plants, there is a central area of the

leaf that has a different pigmentation from the remainder of the leaf. ‘A Happy

Thought’ illustrates this chimeral type. These leaves have a white central area

with a darker green marginal area. In these plants the LI and LII layers are green

and the LIII layer is mutant and does not produce chlorophyll. From a genetic

point of view, LII is heterozygous for a nuclear mutation and produces chlorophyll;

LIII is homozygous recessive for a nuclear mutation which suppresses

chlorophyll production and is, therefore, white. It has been characterized as

GGPale for LI, LII and LIII, respectively. In genetic terms this is designated as

Pp, Pp, pp, where P is the dominant gene and p is the recessive gene.

In ‘Crystal Palace Gem’, also a nuclear chimera, the leaf is light, yellowish green

with a dark green center. The layers are designated as gold gold and green for LI,

LII and LIII respectively. When the green center is “lost” it would appear that

periclinal divisions have occurred in LII which then replaces the green layer of

LIII and gives the composition gold gold gold.

    1    2    3

1. ‘Paton’s Unique  2 ‘Spanish Lavender’    3. Leaf of ‘Golden Lilac Gem’ and the green leaf of ‘Lilac Gem'

    4         5  

        4. ‘Rober’s Lemon Rose’                     5. ‘Madame Thibaut’

Tilney-Bassett also notes that other characteristics of pelargoniums are due to chimeral

arrangements of the cell layers of the apical meristem. Besides the variegation

of leaves, these include layers giving rise to different flower color and differential

growth in various cell layers giving rise to cupped leaves.

Changes in flower color are commonly found in regal geraniums (P. x domesticum)

indicating that there is a genetic difference for pigmentation between different layers.

Amongst the zonals, color chimeras are found in such cultivars as ‘Miss

Burdette-Coutts’, ‘Hills of Snow’, and ‘Mrs. J.C. Mapping’.

Amongst the ivy geraniums (P. peltatum), the cupping of leaves is commonly seen

(as in ‘Golden Lilac Gem’, ‘Evka’, ‘Silver Crown,’ ‘Silver Cascade Pink’) The

differential growth of the different layers appears to be under genetic control.

Variegated leaves that are flat indicate that cupping is not due simply to the differential

production of food in the green versus the white cells.

In the scented geraniums, in particular, P. ‘Rober’s Lemon Rose’ the leaf shape of

plants derived from the apical meristem differs markedly from the leaf form of

plants growing from the roots.

The zonal pelargonium, ‘Skelly’s Pride’ or ‘Madame Thibaut’, with glossy leaves

and fringed petals produces a plant with matte leaves and normal petals when

propagated from root tissue. Fringed petals are characteristic of a number of regal

geraniums and loss of this characteristic would indicate that this characteristic is

also related to a chimeral situation. Similarly, the carnation type of flower in certain

ivy geraniums may be of a chimeral nature.

                           'Mr. Wren'             

Plant of ‘Mr. Wren’ showing a normal branch with

patterened flowers

and an aberrant branch with all red flowers

 

 

‘Mr. Wren’ with white-edged, red petals, when propagated from tissue culture, produces

flowers which are all red. When tested genetically it breeds as a heterozygous

red; no white offspring are produced. The white edge is determined by a gene

for pattern and gives offspring which are either red or patterned. The “New Life”

series of plants would also appear to be due to a chimeral arrangement and these

plants are notorious for changing color

Trichimeras. It can be imagined that with a three layered apical meristem, the genetic

nature of each layer theoretically could be different. In fact, this is true for

several pelargoniums that have been studied.

In the variegated zonal ‘Mrs. J.C. Mappin’ which has a green and white leaf variegation

and whitish flowers, it has been found that LI is genetically green and produces

white flowers. LII is genetically white and produces red flower; LIII is genetically

green and produces red flowers. In crossing experiments this cultivar

breeds as if it had red flowers because the reproductive cells are derived from LII

(which is genetically red) not LI (which is genetically white).

‘Madame Salleron’ another variegated zonal which rarely flowers has been determined

to have LI genetically green and produces long internodes; LII is genetically

white with the ability to produce long internodes; and LIII is genetically green with

short internodes. The loss of flowering ability appears to be related to the loss of

one chromosome so that 2n = 17 (rather than the expected 2n = 18).

           

        Miscanthus - GGG               GGW                        GWG

          

  Poinsettia chimeras                          Carnation chimeras                                  Explain this!!!

Many of our common ornamental and food plants appear to be of chimeral nature.

These include the many plants with green and white variegated leaves. For example,

variegated Fuchsias, ivies, Hydrangeas, Dracaena, Miscanthus. A number of

apple varieties are chimeral. For example, the color forms of ‘Green Delicious’,

‘Delicious’, ‘Northern Spy’, ‘McIntosh’ and ‘Cortland’. The same is true for many

of our potato cultivars where skin color (brownish-red, red, purple) and surface texture

(smooth vs. russet) and leaf form (simple to compound) are affected. The

common Christmas poinsettia is an interesting case. Here the color is produced in

bracts which are modified leaves which surround the small flowers at the tips of the

stems. Normally the bracts are red (R) and the LI, LII and LIII layers are designated

as RRR. When the plants produce pink bracts, the LI layer does not produce

any pigment and the epidermis is white (W). The genetic composition of the layers

is WRR. The red pigment is seen through the white layer and the bracts are seen

as pink rather than red. In cultivars that have a white edge and a pink center, the

genetic composition of the layers is WWR. Plants with all white bracts have the

composition WWW and no cell layer produces a colored pigment. A further variation

is seen where there are pink bracts with red streaks. In this case the red cells

break through the white epidermis and show their full red color. A similar situation

has been found in the florist carnation. ‘William Sim’ is red (RRR); ‘Pink Sim’ has

a pink epidermis over a red core (PRR); ‘White Sim’ has a white epidermis over a

red core (WRR); ‘Jacqueline’ is orange and red striped (YRR) and ‘Jacky” is orange

(YYR).

So we see that chimeras are more common than one might have assumed and contribute

significantly to the variations we see and cherish in our plants.

                                     Echidna and Typhon - parents of Chimaera

N.B. Much of this information comes from the following volumes:

R. Tilney-Bassett. 1986. Plant Chimeras. Edward Arnold, London. 199 pp.

R. Tilney-Bassett. 2008. Variegated Zonal Pelargoniums. Serendipity, Darlington, U.K

 

This heavy-duty article is one in a series of newsletters concerned with the structure of the plants of Pelargonium.

July 2009—Basic Plant Structure; Jan. 2010—Variegated Leaves; Dec. 2009—Chloroplast DNA;

Dec. 2007—Trichomes; Feb. 2009—Chromosomes; Sept. 2012—Stem Apex; June 2008—Flowers

 

Links to our website will help to refresh your memory.

www.geraniumsonline.com/trichomes.htm                   www.geraniumsonline.com/fancyleaf.htm

www.geraniumsonline.com/apicalmeristem.htm         www.geraniumsonline.com/flowers.htm

www.geraniumslonline.com/mitosis.htm                      www.geraniumsonline.com/anatomy.htm

www.geraniumsonline.com/chloroplast.htm

.