The poinsettia (Euphorbia pulcherrima) is synonymous with the winter holiday season, celebrated for its striking red and green display. The brilliant red structures are often mistaken for flower petals or standard foliage. This seasonal transformation is a precise biological mechanism involving specialized plant structures, specific pigments, and a strict dependence on environmental cues.
Bracts, Not Leaves: Understanding the Color Source
The spectacular red parts of the poinsettia are specialized, modified leaves called bracts, not true leaves. These bracts function similarly to petals, attracting pollinators to the plant’s true flowers. The actual flowers, known as cyathia, are the small, unassuming yellow-green structures clustered in the center of the brightly colored bracts.
The color change is localized within these bracts, not the dark green leaves found lower on the stem. A bract is botanically defined as a leaf that has a flower or flower cluster in its axil. In the poinsettia, the bracts are much larger and more colorful than the true flowers, making them the visual focus of the plant.
The Chemistry of Color: Anthocyanin Production
The vivid red hue in poinsettia bracts is due to the accumulation of water-soluble pigments called anthocyanins. These pigments are part of the flavonoid class and are stored within the cell’s central vacuole. Red coloration results from the plant shifting its resources from chlorophyll production to the synthesis of these genetically regulated pigments.
The primary anthocyanin compounds responsible for the red color are derived from cyanidin. The proportion of these compounds determines the exact shade of red or pink in different cultivars. The pH level within the cell’s vacuole influences the final visible color, with acidic conditions favoring brighter red tones.
The biosynthesis of these pigments is triggered by external signals, initiating a cascade of gene expression. This increases enzyme activity, leading to the rapid manufacture and accumulation of anthocyanins within the bract cells. This pigmentation occurs concurrently with a decrease in chlorophyll concentration, allowing the red color to become visible.
Environmental Requirements for Coloration
The biological cue that initiates anthocyanin production is photoperiodism, the plant’s response to the length of day and night. Poinsettias are classified as “short-day plants,” meaning they require a specific duration of uninterrupted darkness daily to trigger the color-changing process. This period of darkness, called the inductive phase, signals the plant to stop vegetative growth and begin forming colorful bracts.
To successfully induce the red color, the plant needs 12 to 14 hours of complete, continuous darkness daily for eight to ten weeks. Even a brief interruption of light during the dark cycle, such as a flash from a streetlight, disrupts the chemical signaling and prevents coloring. This strict light-exclusion must be started in early autumn to ensure the plant is fully colored by the winter holidays.
Temperature plays a significant role, complementing the darkness requirement. Optimal daytime temperatures for growth range between 65°F and 75°F. During the night, temperatures should be maintained above 60°F, as temperatures below 50°F can be detrimental.