During flowering, with the passing of summer, days become shorter. The phytochromes, the light-sensing mechanisms in leaves, recognize the shorter day lengths. The shorter days and lower temperatures arrest chlorophyll production. Chlorophyll breaks down faster than it is replaced, allowing the yellow and orange pigments to be unmasked.
The molecules reflecting red wavelengths, anthocyanins, are water-soluble pigments that occur in the cell sap, creating the red, pink, and purple hues. These pigments may not be present during the summer, or vegetative cycle, but their formation is encouraged during a succession of cool nights and sunny days. During these days when photosynthesis and chlorophyll production are decreasing, an abundance of sugars accumulates in the leaf. The cool nights promote a separation layer of cells in the petiole—where the leaf attaches to the stem—that prevents sugar from flowing out of the leaf, and also arrests the flow of nutrients into the leaf. The formation of anthocyanin requires bright light, a diminishing water supply, and the accumulation of sugars trapped in the leaf.
Another factor that can cause purpling is nutrient deficiency, generally phosphorus. Although these stunted plants may bedazzle the novice, they are typically quite distinguishable from naturally occurring anthocyanin expression, due to the other visible adverse side effects of nutrient deficiency, such as leaf and bud malformation and low calyx-to-leaf ratios.
