Saturday, March 14, 2009

Why Autumn leaves are red (part II)

One of the hypotheses to explain the investment of anthocyanins in plants during autumn was very well detailed here previously (see the 14 Nov 2008 post). A recent TRENDS review by Archetti himself, with Doring and other experts in diverse fields lists the abiotic (photoprotection, osmotic regulation and warming) and other biotic factors (coevolution, fruit flag, direct defence, camouflage, anticamouflage and tritrophic mutualism). Here's a short note on the photoprotection hypothesis, as reviewed by Archetti et al., to complement the anti-herbivory theory.

As protection against photo-oxidative stress in autumn, the red anthocyanins may do one, two or all of three things:

(1) Directly shield leaf tissues from the sun's rays. There is some support for such a function in aging, young and evergreen leaves, as red leaves are less light stressed than non-red leaves in high light conditions.

(2) Indirectly protecting tissues from reactive oxygen species (ROS; i.e., antioxidant effect), hydrogen peroxide being the most likely as it is the only one known to penetrate both chloroplast (ROS produced) and vacuole (ROS stored).

(3) Enhancing nutrient (nitrogen) absorption through maintenance of light absorption function by anthocyanins.

The photoprotection hypothesis is significant due to the increased risk of damage in autumn as a result of: (i) lower photosynthetic capacity in the cold; (ii) increased light from thinning canopy, and (iii) decreased self-shading by chlorophyll due to its breakdown.

Some other untested hypotheses include:
-anthocyanins help decrease leaf osmotic potential, i.e., retain water to prevent drought stress when leaves start to be lost
-anthocyanins convert light to heat, protecting against cold temperatures
-anthocyanins make leaves unpalatable to herbivores
-anthocyanins inhibit fungal growth
-colours attract birds for seed dispersal
-colours expose camouflaged herbivorous insects
-colours attract aphids + ants (those that are symbiotic to each other) to defend the plant

1 comment:

Brandon said...

Fascinating.... of course it could be that all (or at least more than one) of these processes are operating at the same time - which could explain why the theory has been so difficult to explain. There may be some kind of trade-off involved which balances out the mutual benefits and costs.