Late in the season, white or very pale chrysanthemums (mums) often develop streaks or washes of pink and purple on their petals. This change is not a sign of disease or a genetic mutation, but rather a natural and temporary biological response. The plant is reacting to shifts in its environment, specifically the cooling weather, by activating a dormant color-producing pathway. This phenomenon is explained by the fundamental chemistry and protective physiology of the plant.
The Role of Anthocyanin Pigments
The colors red, purple, and blue in nearly all plants are due to water-soluble compounds known as anthocyanins, which are stored within the cell vacuoles of the flower petals. White chrysanthemum varieties appear white because they genetically lack the ability to produce a high concentration of these pigments, or the pigments are masked by other compounds. The white appearance is essentially a lack of color pigment, where the light is simply reflected by the cell structure.
However, many white varieties still possess the latent genetic machinery to create anthocyanins, a capability that is suppressed under normal growing conditions. When this machinery is activated, the resulting hue is highly dependent on the acidity level, or pH, of the cell sap inside the vacuole. An acidic pH tends to produce red or pink shades, while a more neutral pH results in purple or blue colors. The purple coloration is a direct result of these newly synthesized anthocyanin molecules accumulating in the flower tissue.
The Primary Trigger: Cold Temperature Stress
The activation signal for this pigment production is primarily cold temperature stress, particularly the sharp drops and light frosts that characterize late autumn. Cold weather acts as a stressor that disrupts the plant’s normal metabolic processes, specifically how it handles carbohydrates. When temperatures fall, the movement and utilization of sugars produced during the day’s photosynthesis slow down dramatically.
This cold-induced slowdown causes these sugars to accumulate within the plant’s tissues. The plant then converts this excess sugar into anthocyanins through a specific metabolic pathway. Producing this purple pigment serves as a protective mechanism for the plant, essentially acting as an internal sunscreen. The darker pigment absorbs excess light energy, shielding the vulnerable flower cells from photo-oxidation, which occurs when cold slows growth but sunlight remains intense.
Is the Color Change Permanent?
For most white chrysanthemum cultivars, the purple or pink hue is not a permanent change in the plant’s color. This phenomenon is a reactive, stress-induced response, which means the color will only persist as long as the environmental stressor—the cold temperature—is present. The new blooms that develop under the cold conditions will display the purple tint.
If the plant is moved to a warmer environment, such as indoors, or if the weather warms up significantly, the stress response ceases and the production of anthocyanins slows down. Any new flowers that subsequently open will revert to the original white color. Only a limited number of white varieties are genetically predisposed to this specific color change, as it requires the presence of regulatory genes that can be switched on by the cold. This temporary display of color is a transient adaptation, not a stable genetic change like those seen in hybrid varieties.