The Romans decorated their houses with evergreen boughs in
December, and in particular, the Germanic and Norse pagans celebrated the jexla
or jol (respectively) festivals in late December until early January. In
English, this became Yule, or Yuletide (yule time).
Evergreens, as the name suggests, stay green all winter;
they don’t drop the majority of their leaves (needles) and they keep chlorophyll
in their leaves all year round. Chlorophyll is green, so there you have it -
evergreen.
Chlorophyll is energetically costly to make and maintain, so
if there isn’t enough sunlight to make photosynthesis a net gain (once you
subtract the energy needed to make and maintain leaves and chlorophyll); a good
strategy might be to just drop them and start over again in the spring.
So one reason for being an evergreen would be that there is
enough sun and moisture in a certain location to warrant chlorophyll production
and maintenance year round. But there might be a more pressing reason for
keeping leaves all year round - nutrients.
When deciduous trees lose their leaves, there are letting a
lot of nutrients blow away with the wind – usually into my yard. This is
costly, especially if you grow in nutrient poor soil like many evergreens do.
In cold climates, leaf litter doesn’t decay fast enough to allow nutrients back
into the soil, so holding on to your nitrogen is a good idea. Conifer needles
tend to be lower in nitrogen than deciduous leaves, so they hold on to their
nutrients better.
The result of all this is that evergreens, conifer trees and
some bushes, stay green year round. But they’re just fighting to stay alive, not growing
year round. It turns out that evergreens spend a lot of energy to avoid sun damage caused by
the very mechanisms that allow them to gather sunlight light year round. In winter: chlorophyll + sunlight = death. Just
like vampires, they have to protect themselves from the suns rays, or they burn
up.
The truth is, evergreen trees in cold climates do very little
photosynthesis in the winter, even if there is sunshine. They'll make
carbohydrates from sunlight, water, and CO2 (the three ingredients
needed for photosynthesis) when they’re each available, but available is a
relative term.
Air is always available, so CO2 isn’t problem.
The sun is at a lower angle in the winter, but it isn’t cloudy and gray every
winter day; it just seems that way. So sunshine is available at least part of
the time - usually daytime.
Even temperatures near
freezing can slow down water movement in plants, and since cold air holds less
water (humidity), more water can be lost from leaves - even the wax covered, thick
leaves of conifers. The end result is that photosynthesis is just not feasible
during most points of the winter.
Yet the evergreens don’t drop their leaves and keep their
chlorophyll in their chloroplasts. That means that every time the sun comes
out, some of the energy of the rays are caught and transferred to the
photosynthetic reaction centers, including chlorophyll. If photosynthesis can’t
be completed because of a lack of available water, then what happens to all
that energy? It is free to bounce around and damage plant tissues, usually in
the form of reactive oxygen species (ROS, see this post). Enough damage and the
plant will lose its ability to function and die.
To avoid the irony and embarrassment of becoming a dead midwinter symbol of life, evolution
has provided plants with certain photoprotective
mechanisms. Not just evergreen
plants, but all plants. It turns out that photosynthesis pathways are
saturable, only so much sunlight can be used to produce energy and then
carbohydrates.
The sun is unwilling to play the game; once the saturation
limit is reached, it just keeps on shining. On bright summer days, just about
any plant is susceptible to damage from excess energy absorption in
chloroplasts. Some plants have elaborate mechanisms to change the angle of
their leaves so that they receive less sunlight.
According to a 2007 study from Japan, yew cell chloroplasts congregate in the center of the cell volume in response to low temperatures, whereas in
summer they can be found along the edges closest to where the light comes in.
In this way, many chloroplasts can be shielded from sunlight in the winter, so
less energy will be harvested. Pretty smart.
For many evergreen plants, their chlorophyll is doomed to
harvest sunlight all winter without being able to use it for
photosynthesis, so what can they do with it - other than just let it damage them until they die?
It is important that the plant dissipates the light energy
before it reaches the reaction center.
Chlorophyll isn’t the molecule that changes sun energy to chemicoelectrical
energy. Chlorophyll is the pigment that absorbs the energy. That energy can be
transferred to an adjacent chlorophyll molecule and so on until it reaches the
reaction center. Here, the accumulated the energy is used to split a water
molecule and two electrons move into the electron transport chain. It's the
reaction center that actually transduces (changes) the energy from light to chemical
form.
2 and turn it into carbohydrate.
Those electrons are free to attack any nearby proteins or lipids and break them
down. In particular, they can attach themselves to oxygen and create ROS. These molecules are just itching to react with something, anything, and
this leads to damage to many structures of the cell.
Plants can try to stop this by producing more antioxidants,
which can absorb the electrons from ROS molecules. They might be destroyed in
the process, but at least they aren’t allowing the ROS to damage something
important. Evergreens ramp up antioxidant molecule production (especially
glutathione and alpha-tocopherol) in the winter to prevent ROS damage.
Notice that we said above that the reaction center splits a
water to generate the free electrons. Didn’t we also say that in winter,
freezing conditions wouldn't allow for available water? This true, but damage to the reaction center and/or chlorophyll is possible BEFORE the point
where water would be split. This energy has to be dealt with as well.
To dissipate the energy before it damages the chlorophyll or
the reaction center, plants use a technique called non-photochemical quenching
(NPQ). Demonstrated in a classic 1987 study, the physical positions of proteins in the
photosystem (chlorophylls + reaction center) can be shifted during NPQ so that
they create energy traps. In these traps, different pigments, called carotenoids
(see this post for plant pigments), can accept the energy of the light.
When the energy hits a carotenoid pigment called
violaxanthin, it converts it to another pigment, zeaxanthin. Zeaxanthin can’t
passed energy along to the reaction center, but is good at giving it up as
heat. When sunlight can be used for photosynthesis, zeaxanthin is turned back
to violaxanthin and the photosystem redistributes itself so that light energy
can be focused to the reaction center. This is called the xanthophyll cycle.
Cadmium
is used in batteries, paint pigments, metal plating
and
in the production of other metals, like copper and zinc.
Long-term
exposure can lead to kidney damage, but a new
study
shows the problem may be worse than that. The
experiments
showed that cadmium can interrupt non-
photochemical
quenching in barley. This can lead to damage
and
reduced barley harvests. Barley is used in making beer –
something
must be done!
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This would presumably create more oxygen radicals from the
chlorophyll since the energy can’t be transferred to the reaction center, but
since zeaxanthin is being kept all winter, that energy is dissipated too. It
sounds like a lot of work, but it requires much less energy than dropping
leaves in the Fall and then re-growing them in Spring. You wonder why all
plants aren’t evergreens – because then we wouldn’t have a special symbol to cram presents during the holidays.
So instead of being a vampire that has to stay out of the
sun, the evergreen is more like a superhero that can overcome the power of the
sun – his power of photoprotection saves Christmas for us. But maybe not - next
week we’ll talk about how many different ways your Christmas evergreens can
kill you.
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