[Grovenet] A bit of phun with physics . . . . . .

[Meredith Bliss]

Three cheers for those anthocyanins!

On Thursday 28 September 2006 17:32, Bob Browning wrote:
> The Science Behind Fall Foliage
>  By WeatherBug Meteorologist, Adam Bell
> After months of heat, humidity and ongoing drought, the time has come for
> summer to gracefully step aside. In its place the spotlight shines on
> autumn, a season renowned for cooler temperatures and the return of
> football. However, while attending a football game in the chilly weather
> can be invigorating after the stifling summer heat, that's not all the Fall
> season has to offer. Another popular feature of the post-summer months is
> the fall foliage. Starting in mid-September and lasting until early
> November, the changing leaf colors can be seen in the Pacific Northwest,
> the Rockies, Midwest, Ohio Valley, the eastern mountains as well as the
> mid-Atlantic and the Northeast. With these changes arriving very soon,
> let's take a look at what makes those leaves change color. First things
> first, why are leaves green during the spring and summer? One word:
> chlorophyll. This green pigment is abundant in leaf cells during the
> growing season. As a result, the chlorophylls' green color masks out any
> other pigments in the leaf. In order to understand the changing fall
> colors, we must learn how the leaf works. In addition to giving leaves
> their green color, chlorophyll also captures the sun's energy. Chlorophyll
> utilizes this energy to split water molecules, brought into the leaf via
> the roots, into hydrogen and oxygen atoms. Meanwhile, tiny pores on the
> surface of the leaf, called stomata, take in carbon dioxide from the air.
> Once the carbon dioxide reacts with sugars in the plant, it too is broken
> down to its simplest parts, carbon and oxygen. After all of the compounds
> have been simplified, the leaf begins the reconstruction process.
> Individual atoms of carbon, hydrogen and oxygen are joined together to make
> glucose, an energy-rich sugar that nourishes the plant. For every molecule
> of glucose that is produced, there are six molecules of oxygen leftover.
> The stomata release this gas into the air we breathe. This process, called
> photosynthesis, tends to destroy the chlorophyll. But the warm temperatures
> and strong incoming solar radiation during the spring and summer helps to
> create more chlorophyll to replace what is lost. Because the chlorophyll is
> constantly replenished, the green pigment is always present in the leaf,
> keeping the tree green. That's all well and good, but what gives the leaves
> that unmistakable fall color? During the autumn season, the Northern
> Hemisphere starts to tilt away from the sun. This shortens the window of
> daylight, which in turn lowers the temperatures. As this cooling takes
> place, a thin layer of cells, located at the junction of the leaf and stem,
> begin to swell and form a cork-like substance. This swelling cuts off the
> supply of water from the roots, thereby taking away one key ingredient of
> photosynthesis. Another important part of photosynthesis, the sunlight, is
> also in short supply due to the Earth's tilt. The lack of water and
> sunlight halts the photosynthetic process and therefore ceases chlorophyll
> production. With this absence of chlorophyll, other pigments in the leaves
> finally have a chance to come forward. One such pigment is carotenoid.
> Responsible for the yellows, oranges, browns and hues in between,
> carotenoids are also found in familiar foods. Corn, carrots and bananas are
> just a few that are colored by this pigment. Like chlorophyll, the
> carotenoids are present in the leaf cell during the entire growing season.
> The similarities stop there, however, as the concentration of carotenoids
> is much less than that of chlorophyll. Some species of trees that have
> higher amounts of carotenoids include hickories, birch, poplar, sugar maple
> and black maple. The reds and purples come from a pigment called
> anthocyanin. It too adds color to foods such as cranberries, red apples,
> cherries and strawberries. In contrast to chlorophyll and carotenoids,
> anthocyanins are not always present in the leaf cell. Their formation is
> dependent on low levels of phosphate in the leaf. Because there is a high
> level of phosphate during the growing season, the production of
> anthocyanins must hold off until that level drops, usually around late
> summer. A few trees that produce red and purple leaves in the Fall include
> the dogwood and the red maple. Weather plays a very large role determining
> how vibrant the fall color will be. A warm, wet period during the autumn
> will decrease the brightness of the foliage. A severe frost early in the
> season will likely kill the leaves, turning them brown and causing them to
> fall from the tree. Severe drought could delay the fall colors by two to
> three weeks. So what are the weather conditions for great fall foliage?
> While there is no magic recipe, the best colors tend to be seen when there
> is a warm, rainy spring, a summer that is not brutally hot and a fall
> season with sunny days and crisp, cool nights. Since a few of the northern
> states are already reporting signs of fall color, now is the time to start
> planning that foliage drive. For those who have never driven through the
> changing colors, you should make an attempt this autumn as the trip is
> truly scenic.

-- 
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Just happy to be here, but speaking 
only for myself!
Meredith Bliss --- www.rdrop.com/~mbliss
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