Phytochemistry in Bloom

Have you ever wondered what makes blueberries blue? Well, I have. In fact, I’m pretty much always wondering about the phytochemicals that make our world such a wonderful kaleidoscope of color. My front yard in the springtime is no exception. The magnolia tree is covered with plump gray fuzzy buds, the forsythia bush is exploding with golden yellow blooms and the quince branches are slowly unfolding their delicate pale red blossoms. But there is one flower in my front yard that epitomizes the arrival of spring -- the Crocus. These hardy beacons of springtime are emerging in all shades of purple and white and I want to know what gives them their beautiful range of colors.

Variation in Crocus color in my front yard

Not being much of a horticulturist, I set out to familiarize myself with the type of “Crocus” growing in my yard. From what I can tell, the cultivars in my yard belong to the species, Crocus vernus, otherwise called “Spring Crocus” or “Giant Dutch Crocus”. This species can be found in a range of bluey-purplish hues from light powdery blue to deep velvety purple. The white variety and the striped variety are also quite common.

Colchicine-the toxic alkaloid found in
Colchicum autumale

There are two types of “Crocus” that I’m NOT referring to here, but are nonetheless worth pointing out due to their interesting phytochemistry. First, I want to mention the highly toxic Autumn Crocus (Colchicum autumnale), which is actually not a true Crocus and is a member of the Colchicaceae family. Colchicum autumnale blooms in the fall (hence the name, duh) and is extremely poisonous due to it’s content of colchicine, an alkaloid with a long history of use in the treatment of gout. But don’t let its medicinal merit fool you. Colchicine poisoning has been reported in humans, dogs, cattle and other livestock and is often compared to arsenic poisoning. 



Secondly, the exotic and expensive spice, saffron is obtained from the stigmas of Crocus sativus, another fall-blooming species that has been used for centuries in folk medicine as an antispasmodic, aphrodisiac, expectorant, narcotic and sedative. Saffron’s characteristic bitter taste is due to the presence of the monoterpene glycoside, picrocin and a related derivative, safranal. The golden yellow color of saffron, which has been used as a natural dye for centuries, results from the presence of the carotenoid, crocin.

Crocin, the carotenoid pigment that gives saffron its golden yellow color

Saffron, the hand-harvested stigmas of Crocus sativus, is the most expensive spice by weight in the world.

So what pigments are responsible for the color variation in Spring Crocus?

The short answer is anthocyanins. But for all you nerds out there, I will also provide the longer version of this exciting phytochemical story.

In nature, there are two main classes of red-violet producing pigments: the anthocyanins and the betalains. The latter group was first discovered in the late 1960’s, by Dr. Tom Mabry, who at the time was working as a post-doc in the lab of Dr. André S. Dreiding at the University of Zurich. Mabry was assigned the difficult project of isolating and characterizing the elusive structure of the red pigments found in beets, which were originally thought to be a member of the well-known and widespread pigments, the anthocyanins (roses, red wine, etc.).

The red-violet color of beets is due to the content of betalains.

Several researchers had previously attempted to isolate the pigments but had failed because of the harsh isolation procedures that degraded the fragile compounds. Instead, Mabry employed a very mild methylation procedure using diazomethane in diethyl ether to derivatize the pigments allowing him to finally establishing their structures. Although this type of derivatization helped Mabry to avoid the harsh conditions employed by previous workers, this method is extremely dangerous. It is well known that in pure form at room temperature, diazomethane is highly explosive- even the slightest contact with a ground-glass joint or even a scratch on a piece of glassware can lead to an explosion. Its not surprising that Mabry chose to carry-out this experiment after hours when everyone had left the lab!

His findings startled the plant world because the compounds were not at all related to anthocyanins. Through the combined efforts of Mabry and other members of Dreiding's research group, they had discovered a completely new class of pigments, which they named the “betalains”. Mabry and coworkers eventually established that betalains were restricted to nine plant families in the Order Caryophyllales, (e.g. the Cactaceae) and some groups of fungi. A chemical survey of the Caryophylallales resulted in no anthocyanins (pigments that are found in over 95% of flowering plants) with the exception of the Caryophylaceae and Molugenaceae, which interestingly, contain only anthocyanins and no betalains (2)!

Delphinidin, an abundant anthocyanadin found in Crocus spp

Unlike the indole-derived betalains, anthocyanins belong to a larger class of compounds known as flavonoids. These water-soluble, pigments, which are the sugar- substituted form of the anthocyanidins, are responsible for the range of blue and purple colors of Crocus flowers. The anthocyanidins, delphinidin, petunidin and malvinadin, are especially abundant in Crocus species. These pigments were named after the flowers in which they were first discovered (Delphinium in the case of delphinidin, Petunia for petunidin and Malva for malvanadin) Interestingly, unlike all other monocotolydons, Crocus species do not produce flowers with red pigments. This may be due to modifications of the anthocyanin structures in Crocus, like malonylation of the attached glucoside units and hydroxylation or methoxylationon the B-ring of the anthocyanidins that causes a shift to more bluish hues (3). These types of anthocyanin modifications are distinguishing chemotaxonomic features in Crocus.Other flavonoids pigments are also present in Crocus, but do not contribute to the purple color of the flowers. For example, kaempferol and its glycosides make up between 70 and 90% of the flavonoids content in Crocus spp. (1). These compounds can act as co-pigments, enhancing the purple color of the anthocyanins. And how about the white variety of Crocus? The low levels of anthocyanin pigments and the high levels of colorless flavonoids give the appearance of a white flower to human visual perception. However, the petals of almost all white flower species absorb in the ultra-violet spectrum, creating special UV patterns called nectar guides that are visible to bees but not humans. Bees and other pollinators use these patterns to find the flower's nectar in return for pollinating the flower.

Photograph showing the visible and ultraviolet light view of a nectar guide on Crocus

I would love to hear your comments and your own phytochemical stories. Thanks for reading!

References:
(1) Flower pigment composition of Crocus species and cultivars used for a chemotaxonomic investigation. Biochemical Systematics and Ecology, Volume 30, Issue 8, August 2002, Pages 763-791R. Nørbæk, K. Brandt, J. K. Nielsen, M. Ørgaard and N. Jacobsen

(2) Pigment evolution in the Caryophyllales: a systematic overview. Clement, J.S., and Mabry, T.J. 1996. Botanica Acta. 109: 360-367.

(3) Anthocyanins from flowers of Crocus (Iridaceae). Nørbæk, R., Kondo, T. 1998. Phytochemistry 47 (5),861–864.
(4) A bees-eye view: How insects see flowers very differently to us. By MICHAEL HANLON

More on anthocyanins pigments:

• Behavior of 3-deoxyanthocyanidins in the presence of phenolic copigmentsFood Research International, Volume 41, Issue 5, 2008, Pages 532-538 Joseph M. Awika

• Blue flower color development by anthocyanins: from chemical structure to cell physiologyKumi Yoshida, Mihoko Mori and Tadao Kondo. Nat. Prod. Rep., 2009, 26, 884 - 915

Photo credits: the "Crocus vernus nectar guide" courtesy of Bjorn Roslett, Science Photo Library.