Scientist probes light effect on plants
BELTSVILLE, Md. - At
precisely 8 a.m., plant physiologist
Steven Britz opened the door of the
growth chamber, a large metal
box somewhat smaller than a
walk-in freezer, and switched on
the lights.
Then he opened wooden boxes
inside the chamber. In the pots
inside the boxes, grass grew 14
inches high.
Within an hour, Britz began
harvesting Pangola grass, a forage
crop grown in lyarm climates
worldwide. This is part of his
research work at the Beltsville
Research Center of the U.S.
Department of Agriculture.
Britz wanted to find out how each
grass plant handled the internal
distribution of soluble sugars and
starch, the food plants
manufactured by photosynthesis,
using energy from sunlight. To do
this, Britz measured the size of
each plant’s leaves and then the
amount of starch stored in each
leaf.
A plant stores some of the
products of photosynthesis as
starch in the leaves and moves
some, as sugar, into other parts of
the plant. Britz knew that the
length of daylight controls the
amount of starch stored in leaves
in other words, that starch
storage is photoperiodic. Many
changes in plants, such as
flowering, are controlled by the
length of daylight.
But the USDA Agricultural
Research Service scientist wanted
to know:
“Is there a particular day length
that affects the plant’s starch
storage machinery? ’ ’
Britz had chosen to seek the
answer in Pangola grass because
that plant has the same rate of
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photosynthesis no matter what the
day length. The amount of starch
which is stored in leaves, however,
would depend on the length of the
previous day.
The scientist had exposed
several pots of the grass to seven
hours of light the previous day,
some to 16 hours, and others to
varying times in between.
Now, as the time came to har
vest the grass and measure its
starch content, he removed the
first potted plant and placed it on a
table. It was 9:03 a.m. Carefully,
he selected just the youngest
mature leaves, then picked them
off by tend and piled them neatly
beside a small box which
measured the exact surface area
of each leaf.
“I’m sorry, but I won’t be talking
for awhile,” Britz interrupted his
concentration to explain. “In
experiments, there usually are
concentrated bursts of activity like
this.”
As two research assistants fed
the individual leaves past a photo
electric cell in the measuring
device, a digital dial on the box
blinked out the informaiton that
the first plant’s leaf surface totaled
215.19 square centimeters. Another
registered 242.24 square cen
timeters, still another 163.69.
Seldom have grass clippings
been handled with such care. None
it seemed, could be mislaid before
or after their measurement and
storage in a nearby container of
liquid nitrogen. A few days after
the harvest and freezing, the
starch content of the leaves would
be determined.
Perhaps realizing that such are
with grass clippings might seem
unreasonable to an observer, Britz
said; “If there is one thing that
makes me unhappy, it is fuzzy data
numbers that don’t provide a
clear cut result.”
By 10:24 a.m., half of the grass
had been picked, measured and
frozen. The other half would be
harvested in the afternoon,
beginning precisely at 2 p.m. Britz
then could calculate the rate of
starch storage in its leaves for the
intervening five hours.
For now, there was time for
Britz to take a coffee break and
discuss his career.
Curiosity about the effect of light
on plants had burned inside him for
more than half of his 32 years.
Striving to find nuggets or truth
about die relationship of light to
plants began for the scientist with
a high school science experiment
in Cherry Hill, N. J.
“After that,” he recalled, “I
considered medical research, but
plants held my interest. Their
dependence on light and the many
different ways they react to light
fascinated me.”
The plant kingdom unfolded
many of its secrets to him.
Plants, as Britz came to know
them, are not simply room or
naments or something to eat but
are complicated organisms with
electrons shuttling back and forth
between molecules; enzymes
digesting some compounds and
creating others, and live “plum
bing” which carries plant food
manufactured in the leaves to all
parts of the plant, such as roots
and seeds. They are organisms of
boundless complexity and
challenge.
Back m his office, Britz was
pleased to find a letter from
friends in Germany, where he had
studied for a year. Also, he put
aside some newly arrived scien-
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tific journals for browsing later.
“Keeping up on experimentation
around the world is an important
part of the job,” he said; Later that
day, he would be briefing a
Japanese colleague on his ex
periments.
Before beginning another ex
periment that day, the sixth he
would be conducting
simultaneously, Britz explained
his current goals.
“I want to know more about
partitioning, that is, how plants
decide where to send the products
of photosynthesis,” he said.
Such curiosity, he believes, will
benefit people in the years ahead
as he and other plant investigators
uncover important secrets of
nature which will lead to more and
better food.
Extensive experiments in
partitioning in potatoes were
conducted at the University of
Wisconsin, where a famous type of
laboratory called a biotron
provides a controlled environment
for experiments for both plants
and animals. In North Carolina,
phytotrons (a “biotron” for plants
only) provide controlled en
vironments for other experiments,
such as the effect of variable light
periods and temperatures on
potatoes. The phytotrons are
located at North Carolina State
University and Duke University.
If a food producer knew the
secrets of partitioning, Britz
believes, he or she could cultivate
plants in such a way, say, that ears
of com would become bigger, or
cassava roots (a major source of
good in developing countries)
would grow larger, or forage
grasses such as Pangola grass
would grow more nutritious
leaves.
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A half million acres of Pangola
grass are grown in the
southeastern United States alone.
Anything that improves Pangola
grass could affect beef supplies
worldwide.
In other words, the energy of the
sun and the seemingly endless raw
materials of ordinary air could be
targeted to those parts of plants
which humanity finds most useful.
Already, Britz has found that if
day length is increased by two
hours, the storage of starch in
leaves is reduced by one-half.
* “The more starch plants retain
in the leaves, the less there is to
transport to other parts of the
plant,” said Britz. “That may be
beneficial in a crop where the
leaves are eaten but disad
vantageous in a crop where some
other part is harvested.”
Britz wants to find those parts of
the plant those photoereceptor.,
which act as switches in the
partitioning process under the
influence of light and darkness.
Light, as we generally see it, is a
combination of colors ranging
from red to violet. Each kind of
photoreceptor in a plant responds
to a particular color of light.
“Knowing which colors of light
activate which processes, such as
starch storage, helps us un
derstand the chemical make-up of
receptors and how they work,”
said Britz as he turned to a
separate experiment on the effect
of light on pea plants.
Once again, as the interviewer
left, Britz was absorbed in his
world of plants and light.
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