Download DISCUSSION Weight Fresh and dry weight data similar to that

DISCUSSION
Weight
Fresh
and dry weight
data similar to that reported here
were reported by Popp
(1926) and Shirley
plants in greenhouses
glassed with Corning Glass filters trans-
mitting
separate
spectral
full sunlight house
(1929), who grew many
regions with equal energies.
(289-720 nm) was more efficient
duction of dry matter than was their skylight
which gave "satisfactory"
skylight
and canopy shade treatments,
weight were produced.
no wavelengths
However,
spectral band permitted
comparable
concluded
plants of nearly equal
Shirleyrs
canopy shade house had
present.
This more natural extended
efficiency
in weight production
in photosynthesis
of the HIR, or High Irradiance
will be considered
Reaction
Shirley
was a result of
under various
spectra;
conclusion.
The role
(Mohr, 1962) in growth
in later discussion.
During the pre-emergence
stage of seed germination,
major energy source is endosperm-stored
independence
synthesizing
plants to have dry
to plants under skylight conditions.
data from this research reinforce Shirleyts
Seedling
(529-
In our corresponding
canopy shade-grown
that the decrease
the decreased
(374-585 nm) house,
shorter than 529 nm, while our canopy shade treat-
ments had blue wavelengths
weights
for the pro-
results, or their canopy shade
720 nm) house, which gave poor results.
Their
seedlings
materials
the
(Friend, 1966).
is assumed when the dry weight of photo(minus seed coats) is equal to the initial
50
weight of the embryo
all treatments
and endosperm.
At week 3, plants from
had just passed this weight of 0.44 mg per Golf-
rood red fescue seed
(Fig. 5).
Although germination
at the same time in all treatments,
lation had greater dry weights
dence slightly
occurred
plants under the sun simu-
and probably
reached indepen-
sooner than those under skylight and canopy
shade treatments.
Plants grown in sun-simulated
weights
and had a higher growth rate even at 3 weeks.
growth rate may indicate
photosynthesis,
running
that physiological
transpiration,
respiration)
are
are better integrated
plants do
rate at weeks 3 and 4, and lower
points and respiration
plants from other treatments,
organized
(i.e.,
Since sun simulation-grown
have a higher photosynthetic
light compensation
A higher
processes
translocation,
faster or that these processes
and hence more efficient.
better
spectra had higher dry
rates compared to
we conclude that the processes
are
and hence the plants are more physiologically
mature.
Fresh Weight/Dry
Higher
reported
ings.
Ratio
fresh weight/dry
weight ratios in shade leaves were
as early as 1917 by Hanson.
(1929) confirmed
ditions.
Weight
Popp
(1926) and Shirley
this finding under more controlled
Data presented
Hanson concluded
in this report substantiate
growth conthese find-
that the lower fresh weight/dry
weight
ratios in sun leaves showed that more dry matter was laid down
51
in sun leaves.
His sun leaves had 2 layers of compact palisade
cells, compared
to a single palisade
rangement
in shade leaves.
The higher fresh weight/dry
ratio of the fescue is probably
tercellular
layer and a loose cell ar-
a result of larger and more in-
spaces holding more water.
useful under conditions
This would be particularly
of soil moisture
stress where sun leaves,
by virtue of having a higher osmotic pressure
experience
stomatal
2
than shaded leaves.
plants have a lower photosynthetic
synthesis
reserves
Although
shade
compensate
for the re-
Since sun plants usually have higher carbohydrate
(Daubenmire,
1959), the sun environment
logically more desirable,
survival
uptake at high-
rate, the time of active photo-
may be longer and may partially
duced rate.
(Marsh, 1941), may
closure and hence reduced CO
er soil water potentials
weight
but mechanisms
may be physio-
appear to exist allowing
and growth under less than optimum conditions.
Leaves
Mitchell
(l953b) studied leaf initiation
in ryegrass and
found that no new leaf primordia were initiated in darkness
the rate of leaf appearance was proportional
thetic activities
of the plants.
to the photosyn-
In red fescue, however, plants
grown in the skylight or in the canopy shade simulations
same photosynthetic
the skylight
rates.
simulation,
and
had the
There were more leaves per plant in
the individual
in the canopy shade simulation,
leaf weight was greater
and total leaf biomass
(essen-
tially total plant weight) was the same under these two spectra.
52
As seen in Figs. 2 and 3, the skylight spectrum was proportionally higher
in blue wavelengths
than was the canopy shade spec-
truro and also had higher red/far-red
lations contained
The work of Mohr
that both the classical
in regulating
Tentatively,
(1972) supports the hy-
red/far-red
system and the blue High Irradiance
involved
Canopy shade simu-
lower absolute radiant fluxes in the blue re-
gion of the spectrum.
pothesis
ratios.
phytochrome
Reaction
leaf initiation
reaction
(HIR) system are
and leaf development.
it would appear that leaf number is more
likely to be under the control of the HIR, blue system than the
red/far-red
phytochrome
system because
the high red/far-red
ratio in the skylight would tend to negate the formative effects of inducing
in the skylight
red radiation
spectrum
(Kasperbauer,
weight may be more influenced
canopy shade.
Leaf biomass,
the result of interactions
red systems.
Obviously,
and the blue HIR flux is high
1971).
Individual
leaf
by the relatively high red in
on the other hand, appears to be
between both the HIR and the red/far-
such conclusions
are highly speculative
and tentative.
Leaf length was higher in all simulated radiation
than under full sunlight with the exception
treatment.
in beans.
and red light inhibited,
that far-
stem elongation
In full sunlight the far-red is counterbalanced
the red component,
lectively
of chamber skylight
Downs et ale (1957) and others demonstrated
red light stimulated,
spectra
by
but under conditions where red light is se-
reduced while
far-red is less modified
(as in the
53
simulated
canopy shade treatments)
be expected.
increased elongation would
This is supported by the observation
that in the
growth chamber, the sun treatment had a greater ratio of red
to far-red
light than did the skylight treatment,
leaves were longer.
treatments
Similarly,
transmitted
the greenhouse
and the sun
sun and skylight
nearly the same red to far-red ratio
and the plants showed comparable
leaf lengths.
Tillering
The tillering
response also appears to be controlled
by the availability
of photosynthate
(Mitchell, 1953a).
Leopold
and by other factors
(1949) showed that tillering
ley and teosinte is controlled by auxin.
ported that increased
ler production
Although
both
Friend
light intensity enhanced
in balance with increased
in bar-
(1966) re-
the rate of til-
leaf production.
fescue seedlings were used and not mature plants,
sun-simulation
plants in the growth chamber had approximately
4 leaves before tillering while those in the sun-simulation
house had only 3 leaves before tillering.
expanded
lar environment
Ryle (1964) showed that for a particu-
and species, the number of leaves which are
actively growing and contributing
photosynthate
(1964) studied distribution
of Lolium perenne
panded
At least 2 fully-
leaves were required before any seedling tillered.
In related studies,
Williams
green-
and Phleum pratense
leaves steadily contributed
is constant.
of assimilates
in leaves
and found that fully ex-
less photosynthate
to the
54
rest of the plant until at senescence
Maximum
photosynthate
auxin distribution
expanded,
probable
and minimum
they were unessential.
auxin production
and maximum
would occur when new leaves are almost fully
but before old leaves have begun to abscise.
that spectral
and/or distribution
control of growth regulator synthesis
would operate in tillering.
leaf area may control tiller development
ergy substrate
the spectral
It is
and growth regulator
flux and quality
Leaf number or
by adjusting both en-
concentrations
available.
relative to
New plants
(tillers)
will be fonned only under conditions where the parent plant is
well established
and able to compete with other plants
for avail-
able radiation.
stomates
Spectral effects
Mansfield
on stomata were reviewed by Meidner
(1968), who discussed briefly
the occurrance
mates under spectral shade and also the physiology
induced
stomatal movement.
more effective
and
of sto-
of wavelength-
Blue light was approximately
7 times
in causing full stomate opening than was red light.
There seems to be limited data on spectral intensity or
quality
on stomatal action or development,
environmental
ferentiation
entiation
influenced
conditions
existing
in the embryonic
(Zucker, 1963).
at the time of stomatal dif-
leaf bud greatly affect differ-
Zucker found that both lAA and kinetin
one or both divisions
gested that gibberellin
but it is known that
in stomate formation,
might also be involved.
and sug-
He concluded
55
that there is no absolute
ment because
plying
light requirement
they are differentiated
for stomata develop-
in complete darkness, im-
that any light effect is secondary.
If light plays only a secondary
ergy substrate
synthate
and growth regulator
is expected
in diffuse
regulator
skylight
and canopy shade-grown
ment had the least.
treatments
synthetic
regulation
plants and least
plants.
Growth
or a balance of these
from plants in both the
and the growth chamber were of equal size, but difplants had the highest
of the three treatments,
therefore,
or kinetin,
In fescue., stomates
fuse skylight-grown
skylight
Most photo-
are less certain because it is not
known if auxin, gibberellin,
greenhouse
concentrations.
in sun simulation-grown
concentrations
is regulatory.
role, it may control en-
and plants
from the canopy shade treat-
The plants under canopy shade and diffuse
had nearly equal rates of photosynthesis
stomate size was not determined
process.
density of stomates
The light-influenced
only by the photo-
mechanism
of stomate
is not clear, but it could be influenced
action involving phytochrome
and
and the High Irradiance
by a similar
Reaction.
Photopigments
with the exception
of skylight simulation
in the growth
chamber, no significant
differences were noted in either chloro-
phylls or carotenoids.
This exception
in the Results section.
to synthesize
to the rule has been noted
We can only conclude that the capacity
photosynthetic
pigments
is independent
of the
56
tested spectra
and one would not expect chlorophyll
ing under natural
Kramer,
Oosting
shaded conditions.
and Korstian
be at a selective
be synthesized.
adequate
which
at a more rapid rate than they could
and maintenance
serve a photo-protective
tations on photosynthesis
The synthesis
permitted
(Fig. 15)
(Krinsky, 1968), limi-
due to alterations
in chlorophyll
con-
role in the economy of the
plant.
of the photopigments
specific wavelengths
major activation
of carotenoids
function
do not play a major
shaded or non-shaded
chlorophyll,
in full sunlight if their chloro-
Since all of the spectral simulations
synthesis
tent probably
This point was stressed by
(1952) who stated that plants might
disadvantage
phylls were photo-oxidized
to be limit-
is under the control of
of visible and near-visible
of chlorophyll
at the absorption
i.e., in both the blue and the red.
radiation, with
peaks of protoBlue wave-
lengths are low in canopy shade relative to sunlight or skylight, but the red end of the visible spectrum is relatively
high.
The reverse is true for the skylight spectrum.
Since
either
red or blue can activate chlorophyll
the tested
shade spectra would not limit chlorophyll
synthesis,
production
greatly.
In red fescue, the plants may be considered to be genetically
pre-adapted
to permit chlorophyll
diant fluxes, which,
full sunlight.
periments
synthesis even under low ra-
under our conditions, were 5% and 10% of
This is not true for all plants; indeed, ex-
from many laboratories
have shown that neutral
shade
at 10% or even 25% of full sunlight results in plants that are
57
pale green, and which were assumed to be low in chlorophyll.
However
as discussed
in the visual observations
canopy shade plants
section,
also appeared pale green but contained the
same amount of chlorophyll
on a dry weight basis.
In addition,
the canopy shade and skylight plants had equal weights
canopy shade plants had significantly
Therefore
the
but the
fewer but longer leaves.
it is assumed that the canopy shade plants had less
chlorophyll
per cell, although the cells were larger.
Energy Metabolism
In these studies,
it was noted
fairly constant decrease
iod of measurement
(Fig. 16) that there was a
in respiratory
with a threefold
intensity
over the per-
drop over the three week
period.
In both the growth chamber and in the greenhouse
lations,
the respiratory
simulations
ditions.
intensities
of the plants in full sun
were lower than for either canopy or skylight
Since respiration
biosynthetic
the decreased
purposes
simu-
con-
provides ATP and reducing power for
and carbon chains for such biosyntheses,
respiratory
intensities
suggest that the develop-
ment of new biomass had passed its peak and that respiratory
energy
demand.
and precursors
for biosynthesis
was no longer in high
The fact that the respiratory
intensity of the sun-
grown plants was always lower than for canopy-grown
grown plants
or skylight-
suggests that the sun-grown plants were,
time measurements
from the
were started, more mature.
The production
of photosynthate,
as measured
by CO2 fixation
58
appeared
to be fairly constant in plants grown under sun-simu-
lation conditions.
decreased
These data, together with the data on the
rate of respiration
over the same time period suggest
that in plants grown under sun-simulation
be an accumulation
on leaf weights
of dry matter.
and leaf number in sun-simulation
This formulation
that were measured
a somewhat
maturity
showed fairly constant
skylight
point
(Fig. 17).
and canopy shade conditions,
different pattern was noted.
tory intensities
In both sets, respira-
decreased with time, indicating
of the plants, but the rates of CO
2
creases over the period of measurement.
perrnitting the accumulation
necessary
for the synthesis of cell components.
in production
showed in-
We consider this to
an adaptation
increase
increased
fixation
represent
adaptive
conditions
can also be seen in the changes
in the compensation
Under the simulated
there will
This is borne out by the data
(Figs. 5 and 8), where the leaf biomass
increments.
conditions
of materials
However,
of photosynthate
the
did not reach
a level where growth
(dry weight, etc.) caught up with the plants
under sun-simulation
conditions.
to whether,
The question
had the study been continued
to five weeks,
still remains as
for an additional
the plants under canopy shade and skylight
four
con-
ditions would have reached the level of maturation
seen in 6 weeks
(Fig. 9) for full sun-grown plants.
the trends in
both respiration
sation points
a physiological
and photosynthesis
Nevertheless,
and the converging
(Fig. 17) suggest that the catching-~p
adaptation, was well under way within
compen-
process,
5 weeks.
59
In view of the photosynthetic
biochemical
Obviously,
basis for the findings
the adaptation
photopigments,
nor to 'structural modifications
One must look, therefore,
Bensen
cycle enzymes.
a Calvin-Bensen
Bjorkman
in stomate num-
(1968a) measured
the activity of
enzyme, carboxydismutase
carboxylase),
higher plants grown in habitats
in the
at the activity of the Calvin-
cycle photosynthetic
(ribulose-I, diphosphate
from leaf extracts
The comparatively
in the shade plants indicated
zymes other than carboxydismutase
that low carboxydismutase
capacity
difference
synthesis
plants in weak light
He concluded
in shade plants.
capacity cannot explain
of utilization
of the photosynthetic
process.
the
This was commonly thought to
(Daubenmire, 1959).
probably
How-
of weak light in photo-
for high photosynthetic
that shade adaptation
Practical
that their content of en-
photosynthesis
(Bjorkman, 1968b).
have been an explanation
low content of soluble
may also be low.
in light-capturing
in efficiency
com-
activity is a factor that limits the
for light-saturated
ever, differences
of
of various light intensities.
sun plants had higher levels of enzyme activity
pared to shade plants.
protein
requires some evaluation_
is not due to an alteration
bers.
Ecotypic
adaptation noted above, the
rates of shade
Bjorkman
concluded
involves several component
steps
Applications
The importance
of these formulations
in shade adaptation
is unknown, but results of this study have practical
application
60
in regard to densities
planted,
the practice
is frequently
of cultivated
proper plant densities
will cause the many seedlings
an optimum number of seedlings
is that "seeding heavy"
in that area.
receiving
receive virtually
trum, resulting
in poor leaf initiation.
where
canopies
overstory
ing provides
Thus, instead of
a complete spectrum,
a canopy shade spec-
are not extremely
short periods
A justification
in an area to compete for and also
the light environment
the crowded seedlings
When seed is
of "seeding heavy to insure a good stand"
used with no apparent rationale.
for maintaining
to modify
turfgrass.
In natural
situations
dense, and sun fleck-
of complete sun spectra, excessively
high seedling densities with resultant mutual shading might negate the beneficial
effects of the sun flecks.