Plant Physiol. (1973) 51, 543-548
Evidence for Two Photoreactions and Possible Involvement
of Phytochrome in Light-dependent Stomatal Opening'
Received for publication July 21, 1972
HELEN M. HABERMANN
Department of Biological Sciences, Goucher College, Baltimore, Maryland 21204
ABSTRACT
Leaves of the xantha mutant of Helianthus annuus have a
higher rate of transpiration and a lower diffusive resistance in
the light than in the dark. Stomates of this nonphotosynthetic
mutant open in the light and close in the dark.
Comparative studies of tobacco, xantha mutant, and wildtype sunflower stomatal opening over a range of light intensities in isolated portions of the spectrum reveal two patterns of
response: (a) a low intensity opening in the green and far red
characterized by partial opening, absence of a threshold, and
saturation of the response at low light intensities; (b) a high
intensity response in the blue characterized by a threshold
(intensities greater than 100 microwatts per square centimeter
needed for opening) and a linear opening response at higher
incident light intensities. In xantha mutant stomates only the
low intensity system appears to be operational, while both low
and high intensity systems are present in the wild-type sunflower and tobacco.
Red light has an inhibiting effect on stomatal opening in
both mutant and wild-type sunflowers. They require prior exposure to far red for opening to occur in red light. This redfar red antagonism suggests the involvement of phytochrome.
The stomatal apparatus controls not only transpirational
water loss but also the exchange of gases between the interior
and exterior of leaves. Stomatal opening or closing is a consequence of turgor changes in the guard cells that are influenced
by a number of environmental factors including CO2 and 02
tension, water supply, temperature, and light. Of these controlling factors, the influence of light is perhaps the least
understood.
Intensity, quality, and periodicity of light influence stomatal
opening (9, 10, 13). Most hypotheses concerning the mechanism of light regulation of stomates postulate the involvement of the photosynthetic apparatus, an implication of the
almost universal presence of chloroplasts in the guard cells
and their absence in other epidermal cells. This circumstantial
evidence based on the pattern of chloroplast distribution is
supported by reports of the unresponsiveness to light of stomates in nonphotosynthetic mutants of higher plants and in
etiolated leaves (19-21). However, some aspects of the photoresponse of stomates are inconsistent with, or even contradic-
'This work was supported by National Science Foundation
Grant GB 14816.
tory to, the hypothesis that photosynthesis is involved. Most
studies of wavelength effects indicate that blue light is many
times more effective in promoting stomatal opening than red
light (9, 12, 13) while these regions of the spectrum are equally
efficient in promoting photosynthesis. There is no question
about the controlling effects of stomatal aperture on the rates
of photosynthesis through regulating the movement of atmospheric CO2 to the leaf mesophyll cells. Evidence that photosynthesis controls stomatal aperature, on the other hand, is far
less convincing and involves secondary effects such as depletion of CO2 from the interior of the leaf, condensation or
hydrolysis of the carbohydrate products of photosynthesis (interconversion of sugar and starch), or utilization of ATP
formed during photosynthesis as a source of energy for the
movement of solutes across membranes against concentration
gradients.
Two mutants of Helianthus annuus, the yellow xantha and
the white albina (22), unlike albino materials previously examined, do exhibit light-dependent increases in transpiration
rates and opening of stomates. In the dark, their transpiration
rates decrease and stomates close. The response to light of
these nonphotosynthetic mutants argues against the involvement of photosynthesis in light-dependent stomatal opening.
The experiments summarized here suggest that phytochrome is
involved in the stomatal response to light and that opening is
a consequence of two photoreactions that have distinctly different wavelength dependence and kinetics.
Many recent studies have presented overwhelming evidence
that light-dependent stomatal opening is accompanied by a flux
of potassium ions into the guard cells (1, 2, 7, 8, 14, 18). If
phytochrome mediates the change in membrane permeability
that must be associated with such an ion flux, then it appears
that light-dependent stomatal opening is but one of the turgor
phenomena in plants which have a common mechanism. The
other major category of responses with this mechanism is the
nyctinastic movement of leaves (16, 17). In the latter system,
light-dependent turgor changes in specific cells are accompanied by movements of potassium ions and are mediated by
phytochrome.
MATERIALS AND METHODS
Plant Materials. The xantha and albina mutants of Helianthus annuus were propagated by grafting seedlings onto
wild-type stocks according to procedures previously described
(6, 22). Grafted mutant and control sunflowers were grown in
pebbles irrigated with Hoagland's solution supplied with an
automatic nutrient cycling system.
Because large amounts of leaf material were needed, the
xantha rather than the albina mutant was utilized for most of
the experiments to be described. Xanthas can be grown in our
greenhouse in any season, but the albina mutant must be grown
543
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HABERMANN
544
(22 C) after dark equilibration and under a bank of
Sylvania Gro-Lux lamps (approximately 500 ftw/cm2 incident
light energy).
Stomatal aperture was estimated by means of the silicone
impression technique(15). In all experiments in which apertures were measured, leaf discs were floated on a 1-cm layer of
water. Samples exposed to the light (as well as dark controls)
were maintained in constant temperature rooms (22-23 C). The
room
5-15
E
l00
3~
-X
400
500
600
Wavelength (nm)
700
FIG. 1. Spectral distribution of incident light intensity for lamp
and filter combinations used to control light quality. la: Blue light:
General Electric blue fluorescent lamps plus 1 cm 10% CuSO4 filter;
lb: blue light: Sylvania Gro-lux lamps plus 1 cm 10% CuS04 filter.
2: Green light: General Electric green fluorescent lamps plus 1
10% CuS04 and one layer yellow cellophane filters. 3: Red
cm
light: Sylvania Gro-lux lamps plus one layer red cellophane. 4: Far
red light:50 w incandescent lamps plus one layer each red, blue,
and green cellophane filters.
under filters (4) and therefore can be propagated only during
winter months.
Tobacco plants (Nicotiana tabacum L., variety Broadleaf
John Williams) were grown in pots containing a soil, sand,
and peat mixture. Some experiments with two additional varieties of tobacco (Florida Green and Florida Gold) indicated
that there are no detectable varietal differences in stomatal
response.
The guard cells of all experimental materials have been
examined by phase contrast and fluorescence microscopy of
freshly prepared epidermal strips from lower leaf surfaces.
The presence or absence of chlorophyll fluorescence and structural characteristics were noted. Observations were documented by photomicrographs in which the same fields were
photographed first under phase contrast and then under
fluorescence optics.
harvested late in the afternoon on the day bewere transferred (with petioles in water) to a
constant temperature (22 C) dark room. The next morning
leaf discs (1.5 cm diameter) were cut with a No. 11 cork borer.
There is no true "safelight" for this operation. A green filtered
lamp was placed at least 1 m away from the work surface and
discs were floated on water in 50-ml beakers and transferred
to a light-tight box immediately after cutting.
Measurements. Transpiration rates were estimated as weight
loss per unit time of single excised leaves mounted in vials of
water. Leaves were equilibrated in the dark for 45 to 90 min
before weighing was begun. The glass doors of a Mettler
analytical balance were covered with several layers of black
greenhouse cloth during the dark periods. Daylight was supplemented with a single 150-w incandescent bulb aligned with
the level of the leaf but placed 1.5 m away. Weights were recorded at 5-min intervals. Between readings in the light, the
glass doors of the balance were opened. There were no measurable changes in temperature within the balance chamber
during the light period. At the end of the sequence of readings
which included an initial dark, a light, and final dark periods,
the leaf blade was severed at the level of the stopper holding it
in the vial. It was weighed, and its area was determined with a
planimeter. Transpiration rates could thus be expressed on a
weight or an area basis.
Diffusive resistance was measured with a Lambda diffusive
resistance meter. Measurements with either the tubular or
horizontal sensors generally agreed. Readings were taken on
leaves of intact plants under the following conditions: in the
greenhouse (full sunlight) and in a constant temperature darkLeaves
fore
use.
Plant Physiol. Vol. 51, 1973
were
They
standard duration of illumination was 2 hr. Stomatal aperature
was measured under oil immersion with a calibrated ocular
micrometer. Samples examined were cellulose acetate replicas
of silicone impressions. For estimates of per cent open stomates, stomates with apertures of 2 or more were scored as
2 as closed.
open, those with apertures of less thantp
Control of Light Quality and Intensity. Light intensity was
varied by controlling the distance between samples and the
source of illumination. The range of distances available was
approximately 10 to 100 cm. Light quality was controlled by
selecting appropriate lamp and filter combinations. Spectral
distribution of incident light intensity was measured for every
experimental condition with an ISCO model SR spectroradiometer. Representative curves for each of the lamp and filter
combinations used to control light quality are shown in Figure
,u
1.
RESULTS
Guard Cell Structure and Chlorophyll Content. The xantha
and albina mutants of Helianthus annuus L. are nonphotosynthetic (22). The complete absence of detectable chlorophyll
and carotenoids in the albina and the presence of only trace
levels of chlorophyll in the xantha (4, 22) are consistent with
the absence of light-dependent changes in 02 uptake and CO2
evolution. But are the trace amounts of chlorophyll in the
xantha mutant distributed through all of the normally photosynthetic cells and tissues of the leaf or localized within a
single cell type such as the guard cells? Fluorescence microscopy, which provides a sensitive test for the presence of chlorophyll, and observations of the same fields under phase contrast
to answer this critical question and to make
optics were
general comparisons of cell structure in the epidermal layers of
all experimental materials used in these studies.
The guard cell chloroplasts of both wild-type sunflower and
tobacco leaf epidermal strips exhibit the bright red fluorescence
characteristic of chlorophyll. Under fluorescence optics, albina
guard cells exhibit no traces of red fluorescence. In the xantha,
there is a barely perceptible pink fluorescence associated with
the poorly developed plastids in some preparations and an
absence of detectable fluorescence in others. Thus, fluorescence
microscopy is consistent with chemical analyses of the pigment
content of mutant leaves and provides no evidence that the
persistent traces of chlorophyll in the xantha mutant are localized within the guard cells.
Structural peculiarities were evident only in the epidermal
cells of the albina mutant. In this complete albino, there is
wide variation in size of the stomatal apparatus with unusually
small to exceptionally large pairs of guard cells. Xantha mutant
and wild-type stomates are structurally almost identical, the
most distinguishing feature being the absence or presence of
strong chlorophyll fluorescence in guard cell plastids.Mutant and
Transpiration Rates and Diffusive Resistance of and dark
Sunflowers. Transpiration rates in light
measured as weight loss from single excised leaves are summarized in Table I. In both xantha and albina mutants there
is a measurable response to illumination. Although individual
leaves exhibit wide variations in rate, there is clearly a more
rapid weight loss in the light than in the dark. Mutant leaves
utilized
Wild-type
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Plant
Physiol. Vol. 51, 1973
TWO PHOTOREACTIONS IN STOMATAL OPENING
545
Table I. Summary of Transpiration Rates anzd Diffusive Resistantce Measutrements for Mutantt and Wild-type Helianthus
Experimental
Conditions
Mfaterial
Units
Albina
A. Fresh weight per unzit area
mg cm-2
33.8 it 7.6
B. Transpiration rates
Excised leaves
Dark
Light;(room light plus incandescent
supplement)
Leaves on intact plants
Light (greenhouse)
Dark (22 C)
Light (22 C, Gro-Lux lamps)
annftuus L.
mg g-1 min-'
mg cm-2 min-'
mg g-1 min-'
5.8 i 3.0
0.20
8.6 ±4 4.7
0.29
mg cm72 min'1
C. Diffusive resistance
cm sec-1
cm sec-1
cm sec'1
tend to have more rapid rates of transpiration in the dark than
do wild-type leaves because their stomates tend to remain partially open in the dark.
Transpiration rates have been expressed on a weight and on
an area basis. There are differences between mutant and wildtype leaves in weight per unit area that are related to differences in leaf thickness. The albina has an abnormal leaf
development with little differentiation of the mesophyll and inhibited lamina expansion (4), while the xantha more nearly
approximates the wild type in leaf development (22). When
transpiration rates are expressed as weight loss per cm2 leaf
area, the albina has the highest rate of transpiration in both
the light and dark with a smaller difference in rate between light
and dark than exhibited by the xantha and wild type.
As would be anticipated, there are measurable differences in
values for diffusive resistance measured in the light and the
dark (Table I). Only data for the xantha and wild type sunflower are available because leaves of the albina mutant are too
small and irregular to fit in the sensors of the diffusive resistance meter.
The comparative data summarized in Table I indicate that
the guard cells of mutant leaves do respond to light and that a
photoactive stomatal opening can occur in the absence of
Xantha
Wild-type
26.1 i 6.0
18.0 + 5.5
2.5 ± 1.4
0.06
5.0 4 2.1
0.13
3.3 i 2.2
0.06
8.5 ± 3.8
0.15
2.8 i 0.5
11.0 4± 1.3
3.1 ± 0.3
2.7 4i 0.2
11.7 ± 1.9
5.9 ± 1.2
3.0
E
i2.0
aw
|
-.
.o
..
b...
)800
0
200 400 600
Light intensity (,iW/cm2)
FIG. 2. Light intensity response curves to white light. Sylvania
Gro-lux lamps (unfiltered). Xantha mutant sunflower (0); wildtype sunflower (0); tobacco (A).
tobacco stomates responded to low light intensities, as did the
xantha mutant. In the range of 200 to 300 puw/cm' both exhibited a plateau in their response with greater opening at
higher intensities. Without knowledge of the pattern of response in the xantha mutant, the nonlinearity in wild-type sunflower and tobacco light intensity-response curves might have
photosynthesis.
Responses to Light Intensity. Preliminary studies with leaf been attributed to experimental error. However, a similar patdiscs exposed to low intensity monochromatic light obtained tern of response has been described by Liebig (10) in which
by means of interference filters presented a very confusing pic- opening occurred at very low intensities, stomates closed parture of stomatal response to light as a function of wavelength. tially at higher intensities, but at still higher intensities opened
With xantha mutant and wild-type sunflower leaf discs, sto- further. With comparative data for the nonphotosynthetic mumatal opening was observed not only at 460 and 600 but also tant available, a plausible hypothesis for this strange behavior
at 720 nm. Parallel runs with tobacco leaf discs revealed is that in wild-type stomates two light-dependent reactions are
negligible opening at 460 and 600 nm with only partial open- involved in opening. Only one of these reactions (a low ining at 720 nm. It seemed possible that the light intensities used tensity response) is operational in the mutant. By next running
were too low for significant opening to be observed and there- light intensity-response curves in isolated portions of the
fore a series of light intensity-response curves were run be- spectrum, it was possible to demonstrate that one or the other
ginning with "white" light from Sylvania Gro-Lux lamps.
of the two postulated photoreactions could be activated selecWhite Light. The spectral output of Gro-Lux lamps is pre- tively by utilizing the appropriate light quality.
Blue Light. The intensity-response curves of wild-type sundominantly in the blue and red, regions of the spectrum that
are most effective for photosynthesis. In "white" light obtained flower and tobacco in blue light exhibit a pattern that will be
from these lamps without filters, two patterns of stomatal re- referred to here as the "high intensity" photoreaction of stosponse were evident (Fig. 2). With xantha mutant leaf discs, mates, a response that is not seen in the xantha mutant (Fig. 3).
maximal stomatal aperture and the highest per cent of open There is a distinct threshold with a minimal light intensity of
stomates were found in leaf discs exposed to 300 ,uw/cmn. At approximately 100 ttw/cm2 needed for opening. At higher inhigher intensities, stomates closed. Wild-type sunflower and tensities there is a fairly linear increase in stomatal aperture
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546
HABERMANN
E0E 80
c
60
Ck-
0
-c 40
a-
20
O'0
Light intensity (pW/cm2)
100
c)
E
U)
4.0
80
c 30
o
0
60
E
c
CL
0
-
2.0
40
jl
1.0
i.----I
__
-
f
.
<
X¶ 20
is
0
I
200
400
600 800
Light intensity (pW/cm2)
10)0
5
U)
20 8
E
c
v
CL
0
c
4D
u
-V
a.
-~3.02
30
.2
6
A
E
4'0
A
A-A0 4
2
.
O2
20
0
0
200
400
600 800
0
Light intensity
200
400
600
800
600
800
(p.W/cm 2)
4.0
1
-~3.0-
E
0
0
E
0
la!
2.0
-8
-A--
.0
0
Light intensity
200
400
(PW/cm2)
FIG. 3. Light intensity response curves to blue light. Sylvania
Gro-lux lamps plus 1 cm 10% CuS04 filter. Symbols as defined
in Figure 2.
FIG. 4. Light intensity response curves to green light. G.E.
green fluorescent lamps plus one layer yellow cellophane and 1 cm
10% CUS04 filters. Symbols as defined in Fig. 2.
FIG. 5. Light intensity response curves to red light. Sylvania
Gro-lux lamps plus one layer red cellophane. Symbols as defined
in Figure 2.
FIG. 6. Light intensity response curves to far red light. Fiftywatt incandescent lamps plus one layer each blue, green, and red
cellophane filters. Symbols as defined in Figure 2.
Plant Physiol. Vol. 51, 1973
and per cent open stomates. This pattern of response has
been reported by Kuiper (9). The maximal light intensity that
was available under the conditions of blue light used was less
than 4 times the threshold level. The stomatal response to
higher light intensities can be estimated by extrapolation of the
relatively linear curves shown in Figure 3. The estimated light
intensity needed for 100% open stomates is approximately
450 /Aw/cm2 for tobacco and 700 fUw/cm2 for wild-type sunflower. The estimated stomatal apertures at these light intensities are 5 ,u for tobacco and 7 ,u for wild-type sunflower. The
term "high intensity" response has been used for this pattern of
stomatal opening because the estimated levels of light energy
needed for maximal response are of the order of 5 to 20 times
those needed for saturation of the "low intensity" response
exhibited by stomates in green and far red light. The unresponsiveness of xantha stomates to blue light suggests that the
high intensity system is inoperative in the mutant and that the
photoreceptor for the low intensity system does not absorb
significantly in the blue region of the spectrum.
Green Light. In green light, xantha mutant and tobacco
stomates exhibit practically identical patterns of response (Fig.
4). The wild-type sunflower also follows this pattern in which
there is no threshold, a partial opening, and saturation of the
opening response at relatively low light intensities. The maximal aperture in green light is about 2 [,u which is the dividing
line between the arbitrary definitions of open versus closed
stomates. Thus, the apparent difference in response of the wildtype sunflower stomates is a consequence of the way that open
and closed have been defined.
From these data it appears that the low intensity response is
operational in the stomates of all three kinds of leaf material
used and that the photoreceptor for this response absorbs in
the green portion of the spectrum. It is difficult to imagine that
this stomatal response can be linked in any way to the photo-
synthetic apparatus.
Red Light. It has been recognized for some time that red
light is less effective than blue light in promoting stomatal
opening (9, 12). Nevertheless, the patterns of response observed in the red were not anticipated. Tobacco stomates exhibited the expected high intensity response with its characteristic threshold, but both xantha and wild-type sunflower stomates
remained closed (Fig. 5). The unresponsiveness of the xantha
mutant can be explained by its lack of the high intensity photosystem. There are two possible explanations for the inactivity
of the wild-type. If it is assumed that the low intensity response
must precede the high intensity response in sunflower stomates,
then opening does not occur either because the low intensity
response is not activated in red light or because the low intensity response is inhibited by red light.
Far Red Light. All three kinds of experimental material responded identically to far red light (Fig. 6) with a pattern
characteristic of low intensity stomatal opening. From estimates of the initial slopes of the light intensity-response curves,
far red light appears to be approximately three times as effective as green light in promoting low intensity opening.
Hypotheses Based on Light Intensity Response Curves. The
light intensity response patterns of stomates seen in isolated
portions of the spectrum support the hypothesis that two
photoreactions are involved in stomatal opening. The response
to white light by stomates with both systems operational is precisely what one would anticipate from the additive effects of
the two systems working in sequence. The active response of
the low intensity system to far red light and the strange behavior of stomates in the red suggest that phytochrome may be
involved in the low intensity response.
Red-Far Red Antagonism. As a test for the involvement of
phytochrome, the effects on stomatal opening of red and far
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Plant Physiol. Vol.
TWO PHOTOREACTIONS IN STOMATAL OPENING
51, 1973
red light in sequence were investigated. The experiments summarized in Table II were done with xantha mutant and wildtype sunflower leaf discs. The protocol involved exposure to
red or far red light for a variable time (0-120 min) followed by
the standard 2-hr period in light of the other color. The conditions of illumination used (204 ,ew/cm2 red and 140 ,uw/cm2
far red) were saturating for the low intensity opening in far
red but only twice the threshold level of the high intensity response to red light exhibited by tobacco stomates. The results
of these experiments can be summarized briefly as follows. (a)
Red preceding far red inhibits opening. The greatest inhibition
is achieved by relatively short exposures to red (5-10 min)
preceding 2 hr in far red. Longer preillumination (which significantly lengthens the total light period) is less inhibitory but
maximal opening is never significantly greater than in far red
alone. (b) Far red preceding red promotes opening. The extent
of opening after the final 2 hr of red light is proportional to the
length of prior exposure to far red. The greatest opening response occurred in 2 hr far red plus 2 hr red.
DISCUSSION
The results of these experiments clearly demonstrate a redfar red antagonism in the light-dependent opening of both
xantha and wild-type sunflower stomates. They suggest the involvement of phytochrome and also imply that there may be a
single mechanism for turgor movements in plants. The nyctinastic movements of leaves were first shown to be under
phytochrome control when Fondeville et al. (3) demonstrated
that a period of red prior to darkness hastens the closing moveTable II. Effects of Red and Far Red Light in Sequenzce on Stomatal
Openin2g in Xanttha anid Wild-type Sunlflowers
Stomatal Response'
Length of
First Light,
Xantha
0
5
10
15
30
60
90
120
0
1
2
Wild type
A. Red preceding 2 hr far red2
0.1
+ 3.3 33.8 i 2.3 1.8
i 4.7 4.8 i 1.3 1.2 ± 0.2
i 5.9 6.2 -+- 2.8 1.3 ±0.2
± 7.6 10.8 + 6.3 1.3 4 0.2
± 12.6 11.0 4 6.0 1.5 + 0.2
± 10.8 14.2 i 9.4 1.3 ± 0.2
± 2.6 38.5 ±4- 13.5 1.6 ± 0.1
+ 8.5 34.5 + 14.9 2.0 + 0.3
B. Far red precedinzg 2 hr red2
±
3.4 10.7 4 0.3
8.0 5.5
8.8
1.6 1.240.2
16.0± 6.31 5.2
5.7 1.3 + 0.2
9.5 10.0
21.8
38.8 i 8.9 18.2 ± 9.1 1.6 ± 0.2
43.5 it 9.9 26.8 == 3.6 1.7 - 0.2
54.0 -- 10.6 26.2 it 1.9 1.9 = 0.3
61.0 -- 13.6 34.8 ±= 9.4 2.2 ± 0.3
76.5 == 5.2 51.2 ±t 15.7 2.7 ± 0.4
C. Dark conttrols
4.8 -- 0.8 0.9 ±t 0.4
11.7 ±= 6.6
39.8
14.8
18.2
22.2
30.5
23.8
36.8
52.0
Acknzowledgments-The author gratefully acknowledges the technical assistance
of Mrs. Penelope IMcKellar Wolkow, Mrs. Jennifer Chambers, MIrs. Doreen B.
Sekulow, and 'Miss Shannon Beatty.
1.5
0.8
0.8
0.9
1.0
'1.0
1.5
1.5
4 0.1
i 0.1
± 0.2
i 0.3
4
0.2
i 0.3
±4- 0.2
i 0.3
0.1
0.9
0.8±0.1
0.9 4 0.2
1.1 ± 0.2
1.3 i 0.1
1.3 - 0.1
1.5 -- 0.2
1.8 ± 0.3
0.9±0.1
±
MAw/cm2;
ment of Mimosa pudica leaves while far red prior to darkness
delays this movement. Satter et al. (16, 17) have shown that
in the nyctinastic movements of Albizzia leaflets opening is accompanied by a flux of potassium ions from dorsal to ventral
pulvinule cells while in closing there is a movement of potassium ions in the opposite direction.
Both the nyctinastic movements of leaves and stomatal opening and closing are complicated by the fact that they exhibit
endogenous rhythms which are synchronized to diurnal cycles
of light and dark but can persist through periods of experimentally imposed constant light or constant dark conditions. It was
from studies of such rhythmic stomatal behavior that Mansfield
(11) first demonstrated a low intensity light effect which caused
a phase shift in the rhythm of stomatal opening ability. The
most effective wavelength for induction of such phase shifts
was found to be 703 nm, while comparable effects with red
light could be obtained only by continuous low intensity or
intermittent high intensity interruption of the dark period.
Mansfield considered the involvement of phytochrome in the
low intensity phase shift phenomenon, but he rejected this
hypothesis because of the absence of a reversal of red effects
by far red and the requirement for frequent interruptions of
the dark period for a phase shift response to be elicited.
The interactions of red and far red on the low intensity response are complex. There is no simple way to explain why a
long exposure to red preceding far red should be less inhibitory
than shorter exposures. However, sufficient interaction has
been demonstrated to support the proposed involvement of
phytochrome.
The experiments reported here provide little information
about the photoreceptor for the high intensity photoresponse
of stomates. Yet they do provide an explanation for the generally poor response to red as compared to blue light. If the
function of the low intensity response is, as Mansfield (11) has
suggested, one of effecting a "readiness to open" or, as we prefer to view it, to serve as an "unlocking mechanism," and if
this response is inhibited by red light, then the generally
greater response of stomates to blue light is understandable.
I Wild-type
p|
SD for four runs.
Mean
Intensity of illumination: red, 209
cm2.
Xantha
%
min
0
5
10
15
30
60
90
120
Aperture
Open stomates
Period
547
far red, 140 4w/
LITERATURE CITED
1. FISCHER, R. A. 1971. Role of potassium in stomatal opening in the leaf of
Vicia faba. Plant Physiol. 47: 555-558.
2. FISCHER, R. A. AND T. C. HsIAo. 1968. Stomatal opening in isolated epidermal
strips of Vicia faba. II. Response to KCl concentration and the role of
potassium absorption. Plant Physiol. 43: 1953-1958.
3. FONDEVILLE, J. C., H. A. BORTHWICK, AND S. B. HENDRICKS. 1966. Leaflet
movement of Mimosa pudica L. indicative of phytochrome action. Planta
69: 357-364.
4. HABERMANIN, H. MI. 1966. Light-inhibited leaf development in a white
mutant: resemblance to effects of 2-thiouracil in normally pigmented
Helianthus annuus. Physiol. Plant. 19: 122-127.
5. HABERNIANN, H. M. 1960. Spectra of normal and pigment-deficient mutant
leaves of Helianthus annuus L. Physiol. Plant. 13: 718-725.
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