Biology concepts – epigynous berries, seed dispersion,
scarification, drupe, endocarp
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Ocean
Spray alone sells 86.4 million cans of jellied cranberry
sauce
each year. No matter which sauce you prefer, I bet it
has
a lot of added sugar. Cranberries alone are tart enough
to
shrink your head.
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Cranberry (Vaccinium macrocarpon) is one of very few
commercially grown fruits native to North America. The vine needs cool
temperatures and acidic, sandy soil conditions, so New England, Southern Canada
and the Pacific Northwest are prime growing locations. Similar latitudes in
Europe also support growth of cranberries (Vaccinium oxycoccus) in their bogs. We have previously talked about bogs where the acid conditions preserve human remains and produce bog mummies.
But there is an exception in the Southern Hemisphere – Chile
in South America. In the northern part of Southern Chile, volcanic ash soils
mimic the sandy soils of peat bogs, both in consistency and acidity. Runoff
from the Andes Mountains allows for water, and the temperatures are similar to
those in Washington and Oregon - perfect for cranberry growing.
The Ocean Spray Company harvests berries in North America in
autumn, but it needs berries in the summer too. In January of 2013, Ocean Spray
bought the cranberry processing interests in Chile. The harvesting period in
Chile is March to May, just in time to supplement Ocean Spray’s dwindling
supplies.
Cranberries are tart compared to other fruits; they have
five times as much acid as their close cousins, the blueberries. Why? It may be
the acidic soils they grow in. In terms of evolution, growing in peat bogs was
a good choice. Not many things can grow in a bog, so competition is low. Competition
for what is the question – there is
very little nitrogen in the soil of a bog, and the water is acidic too.
Plants need fresh water and nitrogen to survive, so the
cranberry evolved better nitrogen tapping mechanisms, as well as leaves and
stems that can retain their fresh water very well. Not many other organisms have
adapted to these conditions, but the cranberry thrives, transferring the acids
to its leaves, stems and fruits.
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This
is the bog copper butterfly (Lycaena
epixanthe) that
lives
its entire life on a cranberry vine. It not only survives
the
acidic condition of the plant – it eats it up. It lays its eggs
on
the under side of the leaf, and the pupa and the larva can
survive
a flood that covers the plant for months.
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This acidity is also a help when it comes to pests. Several acidic
compounds have been isolated from V.
macrocarpon that stop insects from eating the leaves and stems. I’m
guessing insects don’t like Sour Patch Kids. The exception is the butterfly Lycaena epixanthe; it spends its entire
life feeding on the cranberry plant.
The second reason for the high acid content of the cranberry
is that it doesn’t need to be sweet.
The blueberry is much sweeter, but it has to be. Blueberry bushes spread their
seeds by having birds, rodents, or humans eat them one place and excrete them
in their feces somewhere else; sweetness promotes consumption.
Seed dispersal is
the most basic reason for any plant producing a fruit. If a seed falls directly
beneath the parent plant, no one wins. Both patent and child will require the
same nutrients, and they will end up competing for everything. Things would
also get very crowded.
Several mechanisms of seed dispersal have evolved. Wind is a
popular way to disperse seeds. You’ve seen those helicopter seeds from Maple
trees – they catch the air and twirl down vertically, but also move
horizontally. Sycamore trees have tufts on their seeds to catch the wind as well.
Fruiting is also a way to disperse seeds. Animals need
carbohydrates, and fruits are an important source for many animals. When they
eat the fruit, they also eat the seeds. Later on, the animal grabs a copy of
Sports Illustrated, locks the door, and deposit the seeds somewhere else.
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These
are some of the types of fruits. The peach is a drupe. It
has
an edible mesocarp. The coconut is also a drupe, but its
mesocarp
is more fibrous (flake coconut). The tomato is a true
berry.
It’s pericarp and locules or all edible. The raspberry is an
aggregate
fruit, many ovules and mesocarps held together. The
raspberry
is also a drupe, which you know when you get those
seeds
stuck in your teeth. Each little fruit is a druplet.
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The germinating embryonic plant isn’t strong enough to break
through the drupe endocarp on its own. Something must be done to weaken the endocarp.
The weakening (scarification) may
come from scratching the surface, freeze/thaw, fire (for the Ponderosa Pine),
or perhaps from the digestive enzymes of an animal. Many berries, like blackberries, currants, and raspberries require digestive
scarification in order to germinate. But the cranberry isn’t one of these
berries.
Why don’t cranberries need to be eaten for seed dispersal?
Because they float! When the bog (or similar sandy wetland) floods, the berries
are carried away from the parent plant, away to some far off place that may or
may not be suitable for cranberry vine growth. That’s the problem with
floating; you gotta go with the flow.
Cranberries float because they have air pockets trapped within
them. Floating fruit isn’t that exceptional, apples float too. It’s a good
thing; think how may lives this has saved during bobbing for apples season!
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On
top we see the coconut – it’s a drupe with a tough exocarp.
You
can see the germinating plant coming through one of the
eyes.
Seed dispersal for the coconut is shown on top right. We
don’t
know where palms come from originally, because they
could
spread around the world in just one generation. The
cranberry
also floats, because of the air pockets shown on the
bottom right. The frog is just a bonus – cute, huh?
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Given their bouyancy, it amazes me that it wasn’t until the
1960’s that someone thought of flooding the bogs in order to harvest the
cranberries. They have machines that shake the vines and release the ripe
berries.
Cranberry plants grow very low to the ground, they have long
runners (rhizomes), that can extend six or more feet from the parent vines, and
these can sink roots to become new plants. Because of their short stature, it
only takes about 18 inches of water to flood a cranberry bog for the wet
harvest. So those commercials with the two goobers standing waist high in water
in their waders are a bit of a stretch.
The cranberry was probably at the first Thanksgiving; they
are hearty and ready to be harvested just about the time we are sitting down to
our turkey and stuffing. But, the
pilgrims misled us – the cranberry isn’t a real berry! And don’t say it was
because the pilgrims were from across the ocean. The cranberry is closely
related to the European lingonberry,
so the mistake had already been made.
The cranberry is a false
berry, also called an epigynous
berry (epi = in addition to, and gynous = ovary). A berry is a fleshy
fruit derived from a single ovary. False berries develop from an inferior ovule
and contain tissues from parts of the flower other than the ovary, while true
berries develop from superior ovary tissue only (see picture). Other examples
of epigynous berry-producing plants are bananas, coffee and cucumbers.
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Here
is one difference between real and false berries. All true
berries
are hypogynous, where the ovary (in red) is above
where
the petals and pistil come out. False berries have an
inferior
ovary. Another difference is that the true berry is
made
from only the ovary, while the false berry incorporates
other
parts of the flower. Below on the left is the red currant,
and
on the right is the cranberry. As a berry, the currant is true
and the cranberry is false. But really, can you tell the difference?
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Most people who know about medicinal cranberries have had
a urinary tract infection (UTI). For a hundred years or so, old wives (and
young wives) have espoused the virtues of cranberry juice in preventing or
treating UTIs.
Recent years have seen many studies try to validate the home
remedy. As for if cranberries work,
there is evidence on both sides. Hundreds of published reports say it’s the
best thing since sliced bread, and hundreds say it doesn’t do a darn thing.
Such is science – and that’s a good thing. Argue away so we know we get it
right in the end.
One 2013 study found that sweetened dried cranberries added
to the diet made a real difference in women who were susceptible to UTIs. Half
the women in the study didn’t have even one UTI while on the study, and they
all had reduced numbers of incidents.
As for why caranberries may work, scientists
first thought it was the acid that killed the UTI-causing bacteria. Then it was
believed that cranberry compounds prevented the attachment of the bacteria to
the wall of the urogenitial epithelium via the bacterial fimbriae (appendages
for attachment). This may actually be true, but other actions are also
possible.
Another 2013 study showed that for the UTI causative agent Proteus mirabilis,
eating powdered cranberry was very effective for preventing UTI. In this
experiment, the researchers found that the organisms did not swim well or swarm
when exposed to cranberry compounds. In fact, the gene that expresses proteins
for their flagella (for motility) were inhibited by cranberry powder.
In addition, their urease virulence
factor was also suppressed. A virulence
factor is any molecule that helps an infectious organism to colonize and/or
obtain nutrition from a host, or helps it to evade or suppress the host immune
system.
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This
is a dividing bacterium showing the fimbriae that help it
attach
to surfaces. You can see the difference between these
and
the flagella that help in the motility of the organism. It
may
be that cranberry compounds mess with both to
prevent UTIs.
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Not to be a downer, but a different group carried out a meta-analysis (an organized compilation of many studies involving a lot of statistical math) of many cranberry/UTI studies in 2013 and determined that cranberry compounds have no effect on the prevention or treatment of UTIs. So, all that talk about
just how cranberry molecules suppress UTIs (fimbriae, acid, down regulation of
host molecules) can be ignored if you don't believe they work.
The news is
better on other fronts. In obese men, cranberry juice was able to inhibit the stiffening of blood vessels, an important factor in development of
cardiovascular disease (CVD). The effect was greatest in men with metabolic syndrome – a combination of high
blood pressure, blood glucose, and cholesterol, as well as obesity.
A second study confirmed this by showing that 1 cup of cranberry juice each day reduces
blood glucose levels and CVD risk in men with type II diabetes. And this is
just the beginning; 2013 studies also show how cranberry compounds may help you age well – this makes sense, some vines have been producing cranberries since
before the American Civil War. Other studies show that cranberry is a potent anti-viral agent as well as preventing bacterial UTIs. Respect the berry – uh,
false berry!
Next week,
let’s talk about another symbol of Thanksgiving, the indian corn that you think is just decorative is actually a fascinating story of discovery.
Burleigh AE, Benck SM, McAchran SE, Reed JD, Krueger CG, & Hopkins WJ (2013). Consumption of sweetened, dried cranberries may reduce urinary tract infection incidence in susceptible women -- a modified observational study. Nutrition journal, 12 (1) PMID: 24139545
McCall J, Hidalgo G, Asadishad B, & Tufenkji N (2013). Cranberry impairs selected behaviors essential for virulence in Proteus mirabilis HI4320. Canadian journal of microbiology, 59 (6), 430-6 PMID: 23750959
Lorenzo AJ, & Braga LH (2013). Use of cranberry products does not appear to be associated with a significant reduction in incidence of recurrent urinary tract infections. Evidence-based medicine, 18 (5), 181-2 PMID: 23416416
Ruel G, Lapointe A, Pomerleau S, Couture P, Lemieux S, Lamarche B, & Couillard C (2013). Evidence that cranberry juice may improve augmentation index in overweight men. Nutrition research (New York, N.Y.), 33 (1), 41-9 PMID: 23351409
Shidfar F, Heydari I, Hajimiresmaiel SJ, Hosseini S, Shidfar S, & Amiri F (2012). The effects of cranberry juice on serum glucose, apoB, apoA-I, Lp(a), and Paraoxonase-1 activity in type 2 diabetic male patients. Journal of research in medical sciences : the official journal of Isfahan University of Medical Sciences, 17 (4), 355-60 PMID: 23267397
For
more information or classroom activities, see:
Seed
dispersal mechanisms –
Scarification
–
Different
types of fruits –
Fimbriae
and flagellae –
