East To West And Back Again

Biological concepts – carbohydrates, heliotropism, monoecious, dioecious

I’m trying to think of a situation where quantity is better than

quality. Perhaps some could argue that since quality is subjective,

one person’s quality would be another person’s attempt for

quantity. In friends and experiences, I go with quality. You can

travel to every place on Earth, but if you don’t come back

changed, there was no quality. You can have many

acquaintances, but you really need only one true friend.

When it comes to the number of economically important plants, the Americas have not got many to show off. But what the two continents lack in number they make up for in quality. We have talked before about the biology of corn from North America and how it has been important for the development of molecular medicine.

Potatoes, cocoa beans, peanuts, and vanilla are also from the New World and deserve posts of their own. We’ll hear about vanilla later this summer. But one plant from the Americas has been important for food, oil, and decoration – the sunflower.

If we are going to talk about sunflowers, one question immediately comes to mind. Do sunflowers really turn to follow the sun?  The answer is more complicated than it would first seem, and the answer is just part of the amazing biology of this plant.

First things first – the sunflower (genus Helianthus, about 50 species), as named in Carolus Linnaeus in 1752, does not refer to their tendency to follow the sun. Instead, he called them sunflowers because, ”Who could see this plant….without admiring the handsome flower modeled after the sun’s shape.”

Analysis of nearly fossilized human waste from the caves of Arizona (4000 BCE) show that sunflowers were an important part of the Native American diet. Sunflowers were tough, so they could grow in the Great Plains and other environments that got little rain and lots of sun. They could also grow in temperate environments. Basically, all of North America was there home.

The buffalo would trample huge swathes of land in their migrations, and the torn up ground was perfect for germination of the sunflower seeds. Slowly, this rapacious weed became a cultivated crop. Hybrids were grown, crossing prairie species with forest species and such. In modern science, the sunflower has been used extensively to study genetics of hybrids, much of this work being done at Indiana University in Bloomington, IN – my alma mater, thank you very much.

Number two - the sunflower isn’t a flower, it’s an inflorescence. This is a scientific word for a group of flowers bunched together on the same stem. We talked long ago about the Philodendron selloum inflorescence that controls it’s own temperature and gets hot to attract pollinating beetles.

Sunflowers actually have two types of flowers, the rays and the

discs. The ray florets have a longer petal, they are yellow because

bees see yellow best. The rays are fertile, and have very small

stamens and pistils that provide pollen and ovules. The disc florets,

when male, may have a sterility gene, and this makes sunflowers

very good for studying hybridizations. They also have a naturally

occurring restorer gene, so that they can again make

functional pollen.

In the sunflower, there are two types of flowers, the ray florets around the edge that every one thinks are the only flower petals, and the disc florets, which everyone assumes are the seeds. The ray florets are sterile and therefore for show only; they attract the pollinators.

The ray florets are usually bright yellow, but the disc florets are different colors in different species. They can be yellow, maroon, or even red. The red varieties all stem from a single mutation, but that isn’t the weird part. The disc florets start out male, and produce stamens and pollen, but then turn female as they mature, with the stigma pushing its way up through the middle.

This makes the flowers “perfect” and the sunflower monecious, meaning that have both male and female structures on one plant, but it also makes them smart, as the different timing reduces the chances of self-pollination (pollen and stigma aren’t around at the same time).  For more discussion of monoecious (meaning “one house – male and female flowers on same plant, maybe even as the same flower as with the sunflower) and dioecious plants (male and female flowers on separate plants), see this post.

But even in this, the sunflower can be an exception. The florets mature from the outside discs to the inside discs over time. So while the inner ones may still be male, the outer ones may have become female. In times when pollinators are more rare, if a disc floret remains unpollinated, its stigma may bend down enough to touch the pollen of the still male florets more towards the center of the inflorescence! This is rare, but does occur in species that are annuals.

An achene is a type of fruit that has a hard shell and the seed is

inside. Strawberries are accessory fruits, where the accessory

organs from many achenes join together. The achenes are the

little pieces on the outside. The papery husk (exocarp) of the

sunflower achene is  made from the ovary wall and protects

the seed until it is ready to germinate, like being stuck in dry,

hard, cold ground, or in the belly of a bird.

And third, the disc florets each produce a single fruit (achene), which we call (incorrectly by the way) a sunflower seed. Inside the achene shell is the sunflower seed that we eat. A single sunflower inflorescence can have as many as two thousand disc florets, so that’s a lot of fruit. In species that have more than one inflorescence, each inflorescence will have many fewer than two thousand. Flowers are energetically very costly to produce. Incidentally, almost all the wild varieties have more than one inflorescence, the domesticated versions are bred to have one.

Now for the answer to today’s question – do sunflowers follow the sun? Well, yes and no. Young sunflower plants, including the very small, juvenile flowers, have the capacity to grow very quickly. This means lots of cell growth, and the need for lots of sunlight (to produce ATP and carbohydrates by photosynthesis).

The ability to follow the source of sunlight, called heliotropism (helio = sun, and tropic = loving) requires lots of cell growth. The flower stalks don’t turn so much as they grow in a different direction. As long as the cell growth is rapid enough and the stalk is small enough to respond to changes in cell size, the plant can appear to turn.

Heliotropism is seen in many plants; they need the sun for their

very lives, so it isn’t surprising that their biology would evolve to

maximize sun exposure. The reason the cartoon uses grass – that’s

the plant in which heliotropism was first studied. What scientist

discovered this marvel of nature? Charles Darwin.

The sunlight causes destruction of a plant hormone group called auxins, so they build up in the cells of the shady side. Auxins like indole acetic acid (IAA) promote cell growth and division, so there is much more growth (longer cells and more cells) on the shady side. The uneven growth pattern makes one side longer than the other and forces the stalk to turn (see picture).

So, immature flowers will face east in the morning and west in the afternoon. But that is only part of the answer. By morning, they’re facing east again. How does that happen? A current review (2014) suggests that there may be a diurnal rhythm of several plant hormones, or a natural easterly face that is altered by light signaling. The actual mechanism for the daily turning waits to be identified.

But even this is only half the story. As the stalk gets larger and the heavy inflorescence matures, there can’t be enough cell division or hormone action for the plant to move this massive flower. The mature flowers face east all the time. But why east? Maybe they just can’t bring themselves to move one morning, and since they start out facing east, they stay that way when they give up.

Maybe, but I would imagine there’s a more biologically reason than surrender. The 2014 review cites a study that hypothesizes that facing east protects pollen from the mature florets from sun damage. Final answer, sunflowers follow the sun until it’s time to make little sunflowers, then they settle down and face the rising sun.

So young sunflowers turn with the sun, but how about another question – Why? It’s an inflorescence, not the most efficient photosynthesizer (more about this soon), so why would that structure turn to keep facing the sun? It seems like it would keep the flower in one place and turn the leaves to the sun. Hmmmm.

Now that we’ve answered the question of the day and raised another, let’s talk about the sunflower and world history. But for some unfortunate biology, you might eat sunflower roots like French fries.

The Jerusalem artichoke tuber (top) looks a little like ginger root,

but it is sweeter and not so fibrous. See the text for why you almost

grew up eating McDonald’s sunchoke fries instead of potato fries.

One species of sunflower, Helioanthus tuberosus, has an edible tuber root that is often called a Jerusalem artichoke. Since the sunflower is from North America, you know that the Jerusalem part of the name is wrong. And it’s not an artichoke either.             How it got its name

Around 1600, the Jerusalem artichoke became a popular foodstuff. Easily grown and propagated, the sunflower tuber was a great source of carbohydrates and protein. It was easy to prepare, lasted a long time in storage, and didn’t taste like dirt or wood. Cultivation of the Jerusalem artichoke took off, and it became the primary food for many poor people and a delicacy for the rich.

The South American potato filled the same role, so who would win out as the food of the day? The Jerusalem artichoke (also called a sunchoke) had one big drawback, and it lost the battle. The potato won out, and 250 years later the great potato famine changed the immigration/emigration and ethnic patterns of the world.

What was this thing that cost H. tuberosus the war? It gives you gas. Among the many carbohydrate molecules produced by the Jerusalem artichoke is inulin. This polymer of six carbon sugars is one of those sugars that humans can’t digest, like cellulose. But our gut bacteria can.

Inulin is a branched chain of six carbon sugars. They come in several

varieties and together are called fructans. The “n” means there can be

any number of these units in the chain. They are a good source of

natural fructose, and chicory (right) is the most commercial source of f

ructans. Chicory has been used as a coffee substitute, a salad green

(endive and radicchio are types of chicory) and even in brewing beer.

In breaking down inulin, bacteria produce fructose monomers. They use these monomers as an energy source, and in doing so, produce carbon dioxide. In Central Europe, where the potato vs. Jerusalem artichoke battle was taking place, about 30-40% of the population have a genetic predilection for poor fructose absorption. This means more fructose stays in the gut….more bacteria food. This means much more carbon dioxide and …. flatulence. 

In U.S. finer restaurants and gastropubs, the sunchoke is making a comeback, mostly because Americans can usually absorb fructose just fine. And the fructose helps diabetics too. Many diabetics use the high fructose:glucose ration to even out their glycemic indices.

What’s more, a 2014 study found that mice fed a high fructose diet over time do develop type II diabetes and/or fatty liver. Preceding the disease development, many specific genes change their expression patterns. If their diet was supplemented with extract from Jerusalem artichoke, many of the genes showed normal expression, and the diseases did not develop. Not bad for a sun chasing flower.

Next week, another question to investigate - what/who makes the loudest noise in life?

Vandenbrink JP, Brown EA, Harmer SL, & Blackman BK (2014). Turning heads: The biology of solar tracking in sunflower. Plant science : an international journal of experimental plant biology, 224C, 20-26 PMID: 24908502

Chang WC, Jia H, Aw W, Saito K, Hasegawa S, & Kato H (2014). Beneficial effects of soluble dietary Jerusalem artichoke (Helianthus tuberosus) in the prevention of the onset of type 2 diabetes and non-alcoholic fatty liver disease in high-fructose diet-fed rats. The British journal of nutrition, 1-9 PMID: 24968200