DlO—Lancaster Farming, Saturday, June 20,1981
BELTSVILLE, Md. -
Refrigeration of imbibed, dormant
seeds that require bght for ger
mination for 24 hours prior to
planting can greatly improve their
germination, USDA plant
physiologist William Van-
DerWoude reported at a USDA
symposium on plant reproduction.
VanDerWoude, a scientist with
USDA’s Science and Education
Admmsitration, reported that
chilling lettuce seeds, at 39 degrees
Fahrenheit, then warming them to
68 degrees Fahrenheit, made them
up to 10,000 times more sensitive to
light. So sensitive do the seeds
become that even far-red light,
which normally inhibits ger
mination, promotes germination.
VanDerWoude said the chilling
period followed by normal tem
peratures makes the seeds’ cell
membranes a better environment
for the action of phytochrome, a
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Light, temperature effect seed germination
complex molecule mai enables
plants to respond to light.
Phytochrome, a plant protein
first discovered at the Beltsville
Center in 1952 by H.A. Borthwick,
S.B. Hendrocks, M.W. Parker,
F.H. Toole, and V.K. Toole, was
believed until recently to be a
molecule composed of a single
subunit existing in either an
inactive form, red-light-absorbing
phytochrome (Pr) or in an active
form, far-red-light-absorbing
phytochrome (Pfr). Dr. Lee H.
Pratt, University of Georgia,
reported in 1980 that phytochrome
is in fact a molecule having a pair
of subunits—a dimer.
Working at the Seed Research
Laboratory at Beltsville, Van-
DerWoude compared the enhan
cement of light sensitivity m let
tuce seed germination by chilling
and warming treatments and
compared this to a mathematical
model of phytochrome’s action as
a dimer and identified the
physiological significance of, its
existence in this form. Van-
DerWoude observed that two of the
three possible photochemical
forms of the dimer (Pr:Pfr and
Pfr: Pfr) lead to germination
responses. His work indicates that
the action of Pr: Pfr in germination
and other plant growth responses
is controlled by the membrane
properties around the receptor in
the cell to which it binds.
VanDerWoude, who coordinated
a session on “Photo Control
Systems in Plant Reproduction”
during a recent Beltsville Sym
posium VI, STRATEGIES OF
PLANT REPRODUCTION, also
highlighted related presentations
by other speakers.
He said that Daphne Vmce-Prue
of Glasshouse Crops Research
Institute, West Sussex, England,
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would review' the impact of ar
tificaily manipulating day length
on plant growth, flowering, and
crop production. Then Gerald
Dietzer, of the Radiation Biology
Laboratory, Smithsonian Institute,
discussed ways of altering the
internal biological clocks of plants
through the use of red and far-red
light, VanDerWoude said.
VanDerWoude called attention
to work by Lee H. Pratt, of thie
Department of Botany at the
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University of Georgia, who
presented research on the dif
ferences between inactive and
active phytochrome and the
evidence for the existence of
membrane-bound receptors for
phytochrome. He said Pratt would
describe how phytochrome’s
photochemical properties control
plant growth and development in
the natural environment, and. how
shading by other -plants affects
germination and growth beneath a
canopy of leaves.
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