Wednesday, November 24, 2010

Startling Yellows

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During autumn, how do the Ginko trees (Ginkgo biloba),come up with the brightest yellow foliage?

Botanists refer to this phenomenon as "leaf senscence in deciduous trees"  and is linked to the changing seasons (a "phytogerontological phenomenon"). The change in leaf coloration is generally due to the progressive loss of chlorophyll cells coinciding with the partial retention of carotenoids.

The big difference between the Ginko and other leaf-shedding trees like Maples and Lindens is that, as the green chlorophyll disappears, as much as half of total carotinoids originally present in the leaf is retained by the time of shedding, which give it a bright yellow color. In the other tree species, carotenoid retention is less pronounced but still high enough to cause a golden appearance of the autumnal foliage.

Now, if you were to ask the why question, explanations are not so easy to find...

It has always intrigued me that as far as plants are concerned, the green color - so soothing to inhabitants of concrete jungles - belongs to the  a wavelength that is practically useless for photosynthesis. And so plants & trees absorb all other wavebands and rejects the green color.

Much the same way, carotinoids appear yellow-red in color but they absorb the blue light. So why is there a longer retention of carotinoids in the Ginko leaves??



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REFERENCES:

Matile, Philippe (2000), Biochemistry of Indian summer: physiology of autumnal leaf coloration. Experimental Gerontology 35 (2000). pp. 145-158
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