Blue flowers are uncommon in the natural world, unlike reds, yellows, or whites. Their scarcity stems from complex evolutionary and biochemical factors. Understanding how plants achieve these striking colors reveals intricate biological processes.
The Rarity of True Blue
True blue is a challenging color for plants to produce, appearing in fewer than 10% of flowering species. This rarity is due to the absence of a true blue pigment in most plants. Instead, plants often employ biochemical “trickery” to display blue.
The evolutionary path for plants did not strongly favor blue pigmentation. Producing the specific molecules required to reflect blue light is metabolically demanding. Despite these challenges, blue flowers offer an evolutionary advantage by attracting specific pollinators, such as bees and butterflies, which are highly sensitive to blue light.
The Science Behind Blue Hues
Blue coloration primarily involves anthocyanins, a class of flavonoids. Specifically, the anthocyanin delphinidin is largely responsible for blue hues. However, delphinidin alone is not enough to guarantee a blue flower; its appearance is heavily influenced by the cellular environment.
pH levels within plant cells play a significant role. An alkaline environment often contributes to a bluer shade, while more acidic conditions can result in red or pink tones from the same anthocyanin. For instance, morning glories shift from purple to blue as their vacuolar pH increases. The interaction of anthocyanins with co-pigments like flavones and metal ions, such as aluminum, also modifies the final color. Beyond pigments, some flowers achieve blue through structural coloration, where microscopic structures on petal surfaces scatter light, similar to a blue morpho butterfly’s wings.
Examples of Nature’s Blue Blooms
Despite their rarity, several naturally occurring blue flowers exist. Delphiniums are well-known for their tall spikes of intensely blue flowers, owing their color to delphinidin. Morning glories (Ipomoea tricolor and Ipomoea nil) display vibrant blue, though their color can shift with cellular pH. Bachelor’s buttons (Centaurea cyanus) are recognized for their deep blue blooms.
Hydrangeas (Hydrangea macrophylla) famously exhibit blue flowers, their shade often depending on soil acidity and aluminum ions. Other examples include ‘Forget Me Not’ (Myosotis scorpioides) with sky-blue petals, and bell-shaped bluebells (Hyacinthoides non-scripta). Lobelias, petunias, and irises also come in shades close to true blue.
Achieving Blue Artificially
The quest for blue flowers has led to various artificial interventions. A common method involves dyeing flowers, particularly white roses or carnations, by introducing blue dyes through their stems. This results in superficially blue flowers that are not naturally pigmented.
Biotechnology has focused on creating “true blue” varieties of traditionally non-blue flowers, such as the elusive blue rose. Conventional breeding methods have largely failed to produce blue roses, resulting in lilac or mauve shades like ‘Blue Moon’. However, genetic engineering has made progress. Researchers have introduced genes from other blue-flower-producing plants, like the butterfly pea and Canterbury bells, into roses to enable delphinidin production. While these genetically engineered roses, such as Suntory’s ‘Applause’, produce delphinidin, their color often appears more lavender or pale mauve due to petal acidity. Similar genetic efforts have led to blue chrysanthemums.