Home > Begonian > Volume 70 (March/April 2003)

Red Pigments
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 coleodies 'Variegatus'.

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 plant.

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 months.