Re: Anthocyanin

["James Purchase" <jppurchase at rogers_com>]

Tom wrote:
"In the recent literature, the notion that Anthocyanins protect the plant
from excess light does not appear valid. Bioassays have indicated that it
might be an antifungal agent."

"I think some folks confuse it with the Xanophyll cycle which is used to
protect against excess light and gives heat, Anthocyanins don't give off
heat, as an early hypotheses suggested."
------------

I agree that the names of all of the various pigments can get confusing, but
I didn't bring anthocyanins into the discussion - Daniel did. What I did
bring in was the suggestion that in some cases, some of the auxiliary
pigments can serve to offer some protection against free radicals. It
certainly isn't an original idea (of mine) - I relied on several recent
books that I have. In Aquatic Photosynthesis, (published in 1997), on page
56, the authors state:

"In addition to promoting energy transfer to or from chlorophyll,
carotenoids play an important role in protecting photosynthetic organisms
from damage resulting from the photochemical generation of oxygen radicals
(Sandmann et al. 1993)."

Further on in the text, in a discussion on UV-b inhibition on page 295, they
discuss some of the possible ideas for why and how UV can damage plant
cells. They do say that the plant is often capable of repairing itself at
the same rate that it is bring damaged and quote Cullen, Neal 1993 for the
statement. They continue:

"Numerous strategies to screen UV-b radiation have evolved in photosynthetic
organisms. One of the most common is the production of pigments which absorb
the radiation and screen it from the photosynthetic apparatus. Some
carotenoids, such as beta-carotene, serve this purpose (Dohler, Haas 1995)."

In the 6th edition of Photosynthesis, by Hall and Rao (1999), in section 3.2
Chloroplast Pigments, they state:

"The carotenoids are yellow or orange pigments found in most
photosynthesizing cells. Their color in the leaves is normally masked by
chlorophyll,... They have a triple-banded absorption spectra in the region
from about 400 to 550 nm. The carotenoids are situated in the chloroplast
lamella, bound to proteins, in close proximity to the chlorophyll. The
energy absorbed by the carotenoids may be transferred to the chlorophyll a
for photosynthesis. In addition, the carotenoids protect the chlorophyll
molecules from too much photo-oxidation in excessive light."

From these, and from what both you and Karen have added, I am left with a
number of questions -

Does your statement above (regarding Anthocyanins) also apply to
Carotenoids, thus thowing out what was written in Aquatic Photosynthesis?

Can you please let the rest of us know what sources you base your dismissals
upon?

Has knowledge progressed THAT fast that a book published in 1999 is
hopelessly out of date in 2002?

Are you saying or implying that the non-green colors seen in some aquatic
plants are totally unrelated to the photosynthetic pigments that are in the
plants? Are we chasing the wrong rabbit? Which bunny do you chase?

Or are you saying that aquatic plants, on their way back into the pond,
picked up some tricks that their land plant ancestors didn't need and the
rest of us haven't figured out yet?

It can get bewildering for the rest of us when someone dismisses an idea out
of hand without offering an alternative hypothesis, either of your own or
from someone else.

James Purchase
Toronto