Unfortunately I'm not much of a plant
physiologist, but here's what I was able to dig
up. Lettuce seeds get activated for germination
through light signals.Light is sensed through a
receptor family called the phytochromes (similar
tothe chlorophylls.)Phytochrome can absorb light,
which changes its structure.There are two forms of
phytochrome - active (phytochrome that absorbs far
red light = Pfr) and inactive (phytochrome that
absorbs red light = Pr.) In red light,
phytochrome in the form of Pr (through absorbing
red light) changes to Pfr. This is the active
form which can translocate into the nucleus and
actually cause transcription of genes which
eventually result in germination. Thus phytochrome
acts both as a receptor that senses light and as a
transcription factor. It's important to remember
that light has both red and far red light, thus
will be playing with the ratios of Pfr and Pr. In
full light, there is more red than far red (thus
causing a higher amount of the active form Pfr.)
This is why seeds may need light to germinate. In
the soil this ratio is reduced, but there is
enough red light to convert enough inactive
phytochrome to active phytochrome (depends on how
deep the seed is planted; which is why it's
important not to bury your lettuce seeds to
Apparently depending on the seed
lot, there will be some lettuce seeds in the bunch
that will germinate in the dark because they are
not as sensitive to light (higher amounts of Pfr
compared to Pr ... I'm not sure how this
occurs....genetics and a bunch of hand
waving??)I'm also uncertain as to the effects of
other hormones (i.e. giberrelin, ethylene...) on
germination in lettuce.
occur in the dark with other species of plants.
Typically these are the hard seed plants (have
hard coat so aren't really light accessible until
after seed germination.) These would also be the
seeds that don't mind being buried deep in the
soil. I guess the lettuce seed coat is thin; thus
it can respond to light.
As far as water
goes, it seems that water does play a role in the
events downstream of the Pr => Pfr conversion.
What exactly it does, I'm not sure anyone knows.
Perhaps diluting out inhibitors of germination?
Water doesn't play exactly the same role as in
barley activating giberrelin.
Pubmed how higher water content enables the
response to red/far redlight, lower water content
makes the seed insensitive to light
seed does have appropriate stores to germinate
(found in the endosperm). The amount it has stored
away varies depending on the plant. Seeds that can
be buried far under ground would have more storage
than seeds that need to be buried close to the
surface. The farther they have to travel to reach
light and be able to photosynthesize, the more
food they need stored away.
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