70 (March/April 2003)
by Arlene Hoskins
Pink, red or purple color in leaves is caused by the
presence of pigments called anthocyanins. They are glucosides
(carbohydrates) that are dissolved in the cell sap, which is the fluid in
the cell vacuole. While not vital to the plant, they do absorb light rays
that are a different wave length than those absorbed by chlorophyll and
are used in food production. This may be a function of the red pigments in
the layer of cells on the underside of some begonia leaves.
An abundance of red pigment is part of the genetic
makeup of certain plants. Iresine and some Cord vline terminalis
(ti plants) are completely red in color. The anthocyanins mask the
chlorophyll and other pigments.
1. In the spring when temperatures rise and light
intensity increases, the red pigment forms on the leaf edges of many
plants. It acts as a sunscreen to protect the plant from an increase in
ultraviolet rays. Ex: Pelargonium 'L' Elegente', Plectranthus
2. In the fall certain succulents produce an
abundance of anthocyanin. Echeveria and Kalanchoe thyrsiflora
exhibit a color change from a dull summer gray to a vibrant winter red
especially if they are growing in full sun. This occurs when the
temperatures drop and the days are shorter. The cooler temperatures
inhibit the movement of sugar from the leaves so it accumulates and plays
a role in the production of anthocyanin.
3. Red pigments also form when autumn colors
appear in the leaves of deciduous plants. Again anthocyanin formation is
thought to be a protective mechanism. It acts as a sunscreen by absorbing
sunlight when food production slows down and the chlorophyll
disintegrates. To much light energy when it is not needed can damage the
Some scientists also believe that this protective
mechanism may allow nitrogen to move more efficiently from the leaves to
other areas of the plant before leaf drop. The nitrogen can be used the
following spring in new leaf production.
Yellow and orange pigments are already present in the
leaves. When chlorophyll disappears, they are no longer concealed and join
the newly produced anthocyanins to give autumn color.
4. Red pigments are also sensitive to pH.
Differences in pH may cause the red pigment to take on many different
hues. Acidification of the soil will greatly increase the intensity of the
color. Use of fertilizers that contain sulfur will lower the pH and the
color will be much more vibrant. This is evident in the cultivation of Rex
begonias, ti plants, coleus, crotons and many other tropical plants.
A slightly higher pH turns the anthocyanins more
purple. In growing Solandra the pigment was not visible until I
used a more a more acidic fertilizer, which resulted in the appearance of
the brilliant purple color in new leaves. For many years the leaves
displayed only a green and white variegation.
In B. valida increased light intensity probably
triggers red pigment formation. I grow B. valida in a greenhouse
where it almost reaches the top of the structure. Last year my plant had
mostly green leaves even during the summer. Then a fellow begonia lover
viewed my plant, pondering the absence of red pigment and suggested using
less nitrogen in my fertilizer. Following his advice, the red pigments
soon appeared in the upper leaves. Too much chlorophyll was probably
masking some of the red pigments. At the present time some of the upper
leaves still show some red color, but pigment production is decreasing as
the duration and intensity of light changes during the summer