From our perspective, the primary reason for a color change is an inherent genetic instability within our very cells. Many modern petunia varieties are the result of extensive hybridization, a process where humans cross different species to create new flower colors and patterns. This can sometimes lead to a genetic phenomenon where a transposable element, or "jumping gene," is present near the gene responsible for flower pigmentation. This gene, often called a "color gene," can be spontaneously inactivated if the jumping gene inserts itself into it. When this happens, the biochemical pathway for producing a specific pigment, like a deep purple or red, is shut down, causing the flower to revert to a different color, often white or a lighter shade, in subsequent blooms.
Our flower color is not a single, static trait but the result of a complex biochemical pathway. The vibrant hues are primarily created by pigments called anthocyanins. The specific color displayed—be it red, blue, purple, or pink—depends on the precise type and combination of anthocyanins we produce, which is dictated by our enzymes. A change in color is a visible sign of a shift in this internal biochemistry. Environmental factors like soil pH, nutrient availability, and sunlight intensity can subtly influence the expression of the genes coding for these enzymes. A significant change in these conditions, such as a shift to more acidic soil altering aluminum availability, can affect the pigment molecules themselves, leading to a gradual or patchy change in color intensity or tone from one flowering cycle to the next.
A sudden and often splotchy or streaky change in flower color can frequently be attributed to stress, particularly a viral infection. Viruses, such as the cucumber mosaic virus, can interfere with our normal cellular functions. When infected, the virus disrupts the biochemical pathways in our petals, preventing the uniform production of anthocyanin pigments. This results in patterns of color breaking, where white streaks or patches appear on otherwise colored flowers. This is not a beneficial change for us; it is a symptom of distress. While it may create a visually interesting pattern for the observer, the viral infection weakens our overall system, potentially stunting our growth and reducing our vitality.
Many showy petunias are chimeras, meaning our plant tissue is not genetically uniform. We may be grafted plants, or a mutation may have occurred in a single cell line during our development. The flowers you see are produced from meristematic tissue, and if a cell in that tissue undergoes a somatic mutation—a genetic change that is not passed to seeds but only to cells descended from it—the resulting flower may express this new genetic code. This can cause a single branch to suddenly produce flowers of a completely different color than the rest of the plant. This is a reversion to a genetic state that was masked by hybridization, a glimpse of a ancestral color breaking through the engineered facade.
