The poinsettia (Euphorbia pulcherrima) is a shrub native to Mexico and Central America, recognized for its striking, colorful foliage used widely in seasonal decorations. The plant transforms from a simple green shrub into a cascade of vibrant color during the winter months. Understanding this intense coloration requires examining its unique botanical structure and underlying biochemistry.
Identifying Bracts and True Flowers
The brilliant red structures commonly mistaken for the poinsettia’s flower petals are actually modified leaves called bracts. Bracts are structurally similar to typical leaves but serve a different biological purpose. Their primary function is to attract pollinators to the plant’s inconspicuous true flowers, an adaptation common in species that do not produce large, showy blossoms.
The actual flowers of the poinsettia are small, yellow or greenish structures grouped together in the center of the colorful bracts. These clusters of flowers are termed cyathia, and they are where the plant’s reproductive processes occur. The vibrant bracts create a highly visible target, guiding insects toward the tiny cyathia for successful pollination.
The Role of Anthocyanin Pigments
The spectacular red color of the bracts is a direct result of chemical compounds known as anthocyanins. These compounds are water-soluble pigments belonging to a larger class of molecules called flavonoids, which are responsible for the red, purple, and blue hues found in many fruits, vegetables, and flowers. In the poinsettia, these pigments accumulate in the epidermal layers of the bract cells, giving them their characteristic coloration.
The red appearance is a phenomenon of light reflection. The anthocyanin molecules absorb nearly all wavelengths of visible light except for the red spectrum, which is then reflected back to the observer’s eye. In the most popular red cultivars, the specific anthocyanins responsible include cyanidin and pelargonidin, with the proportions of these pigments influencing the final shade of red or pink. This mechanism contrasts with the green color of the lower leaves, which comes from chlorophyll reflecting green light.
How Photoperiodism Causes the Color Change
The formation of anthocyanins in the bracts is triggered by an environmental cue known as photoperiodism. This is a physiological response to the change in the relative length of light and dark periods within a 24-hour cycle. Poinsettias are classified as “short-day” plants, meaning they require a period of extended, uninterrupted darkness to initiate the transformation from green leaves to colored bracts and begin flowering.
To stimulate coloration, the plant needs a daily period of complete darkness lasting at least 12 hours. This long-night requirement must be maintained consistently over several weeks, typically six to eight, to induce the necessary biochemical changes. Even brief interruptions of light during the dark period, such as from a streetlamp or a quick switch of a room light, can disrupt the process and delay the production of the red pigments.