Problems with photosynthesis (part 2): Photodamage and the xanthophyll cycle

Right, so again a slightly late post (whatever happened to Monday updates?!) regarding more problems with photosynthesis.
Last time we dealt with problems arising from water stress, and how plants can deal with them. This time it's light, again an essential part of the photosynthetic process, but also one that if taken in excess will cause damage to the plant.

Photodamage occurs in the photosynthetic tissues of plants when light and/or heat is in excess and damages these delicate organs. Basically plants use a chain reaction to transfer energy from sunlight into carbohydrates (through an electron transport chain - a series of proteins that can pass on electrons and hence form a chain - a process known as the Calvin cycle - will discuss the exact mechanisms in a later post), however an excess of this energy can begin to damage the proteins that are used to convert this energy. If the amount of sunlight doesn't decrease (ie it's a hot summer day), the plants do need some way of releasing or absorbing this energy before they become photodamaged.
Luckily for them, they've evolved such a system.

A nifty piece of evolution known as Xanthophyll pigments evolved to deal with this problem. These are a series of 3 pigments that have the capacity to absorb some of this excess energy before it can damage the photosynthetic parts of the leaf, and these are found in the thylakoid membrane that makes up the surface of the chloroplast (where photosynthesis occurs - more on this next week). To explain how the system works, a little chemistry is needed (sorry).

The first pigment in the cycle is violaxanthin, and this contains two double-bonded Oxygen (for a good image of the pigments and their conversion process, go to wikipedia). When violaxanthin absorbs some of this excess energy, one of the oxygen atoms breaks away to form a water molecule, and the pigment becomes known as antheraxanthin. The same process can then occur again, and once the pigment has lost it's second Oxygen, it becomes known as Zeaxanthin. The whole process is reversible, which means during the day each violaxanthin molecule can absorb two excess electrons, and by loading the thylakoid membrane of each chloroplast with these pigments, the plant can prevent quite a lot of photodamage.
However there is a limit, and if excess light continues to fall on the leaf, photodamage will eventually result (and is more often than not  irreversible).
During the night is when these pigments usually convert back to the lowest energy state (violaxanthin), and the plant can prepare it's defenses for the next day.

The xanthophyll cycle is just one way plants can deal with light stress, but it is I think by far the coolest ;) Other ways can be to preferentially grow in shadier areas (although the plant will acclimatise and produce fewer xanthophyll pigments - afterall, why produce them if you grow in the shade and won't need them?) and also via leaf alignment (as in the Eucalyptus example in the previous post). Why this protective mechanism is so important will be the subject of the next post - how does photosynthesis work?