Plant Cell Physiol. 38(12): 1346-1353 (1997)
JSPP © 1997
Gravitropism of Oat and Wheat Coleoptiles: Dependence on the Stimulation
Angle and Involvement of Autotropic Straightening
Yutaka Tarui and Moritoshi lino '
Botanical Gardens, Faculty of Science, Osaka City University, Kisaichi, Katano-shi, Osaka, 576 Japan
Gravitropism of oat (A vena sativa L.) and wheat (Triticum aestivum L.) coleoptiles was investigated in relation to
the displacement angle or to the initially set stimulation
angle (SA). We measured curvature rates at the early phase
of curvature, before it was affected by the drop in SA
resulting from the curvature response itself. The plot of the
rates against the sines of initial SAs revealed similar curves
for oats and wheat, which approached saturation as the
sine increased to unity. The two species and previously analyzed rice [lino et al. (1996) Plant Cell Environ. 19: 1160]
appeared to have similar gravisensitivities. Initial SAs
below and over 90° yielded comparable rates when the sine
values were the same, indicating that the extent of gravitropism is determined by the gravity component perpendicular
to the organ's long axis. Long-term curvature kinetics at
different SAs indicated that the net curvature rate dropped
sharply before the tip reached the vertical position and
then the tip approached the vertical slowly, with clear
oscillatory movements in. the case of wheat. During this
late curvature phase, the coleoptile straightened gradually,
although none of its parts had yet reached the vertical.
When rotated on horizontal clinostats or displaced upwards to reduce SA in the late curvature phase, coleoptiles
bent in the opposite direction. These results demonstrated
that autotropism counteracts gravitropism to straighten coleoptiles.
Key words: Autotropism — Coleoptile — Gravitropism —
Oat (Avenasativa L.) — Stimulation angle — Wheat (Triticum aestivum L.).
In studies of gravitropism, experimental determination of the sensitivity of plants to a stimulus is limited by
the constant and uniform gravity on the earth. Dependence
of gravitropism on the angle of displacement, however,
may be used as a measure of the sensitivity to gravity. Such
angular dependence has been shown with different experimental approaches (Fitting 1905, Rutten-Pekelharing 1910,
Metzner 1929, Audus 1964, Pickard 1973). One of the approaches used was to rotate plants on a horizontal clinostat
Abbreviation: SA (SAs), stimulation angle(s).
Corresponding author: Fax +81-720-91-7199, E-mail iino@sci.
osaka-cu.ac.jp
1
after a relatively short period of displacement and to measure gravitropic curvature at a denned time (Larsen 1969,
Pickard 1973). In our preceding study of gravitropism of
maize and rice coleoptiles (lino et al. 1996), the angular dependence was explored further as a measure of gravisensitivity. We measured the rate of coleoptile curvature in
plants left as displaced initially. The curvature rate was determined at an early phase of curvature in which the rate
was directly related to the displacement angle or the initially set stimulation angle (SA), without being influenced by
the decrease in the actual SA resulting from the curvature
response itself. It has been demonstrated that the angular
dependence determined by this method is distinct between
maize and rice, whereas it is similar in air-grown and submerged rice. In the present study, we applied the same
method to coleoptiles of oats and wheat to extend the comparison of the relationship between SAs and curvature
rates.
Studies of the angular dependence of gravitropism
originated in the hypothesis of Sachs (1882) that the
magnitude of the gravitropic response depends on the component of gravity which is perpendicular to the organ's
long axis. The condition of this hypothesis that the gravitropic response is maximal at the SA of 90° was supported
by Fitting (1905) and Rutten-Pekelharing (1910). However,
later workers reported that the optimal SA was greater
than 90° in roots and stems (Metzner 1929, Audus 1964,
Larsen 1969, Iversen and Rommelhoff 1978, Legue et al.
1994, Kiss et al. 1997). Larsen (1969) conducted a detailed
study using cress roots to find that the optimal angle was
90° when plants were rotated on a clinostat after a stimulation of shorter than 0.5 min. When stimulated for 1-15
min, however, the optimal angle shifted to 120-140°. To
evaluate the problem of the optimal SA, we included SAs
greater than 90° in the present analysis of angular dependence.
Measurements of curvature development over a long
period may provide information useful in understanding
the overall control of gravitropism. As shown previously
(lino et al. 1996), rice and maize coleoptiles overshot
the vertical before regaining the vertical orientation and
straight appearance. This overshooting was especially
marked in submerged rice coleoptiles. The results appeared
to support the view that straightening is achieved primarily
by a gravitropic response at each locus of the coleoptile
(Kohler and Daum 1979). In the present study, analyses of
1346
Gravitropism kinetics in oat and wheat coleoptiles
1347
long-term curvature development were extended with oat
and wheat coleoptiles. We found that, unlike in maize and
rice, these coleoptiles do not overshoot the vertical, and
that autotropism (Simon 1912, Dolk 1936, Firn and Digby
1979) participates in the process of straightening.
Here we report the results obtained by analyzing curvatures initiated at different initial SAs in oat and wheat
coleoptiles. The curvature kinetics at an early phase of gravitropism were first analyzed with respect to their relationship to SAs. The curvature kinetics during the late phase
were next analyzed with respect to the response elements involved in the process of straightening.
(lino 1996, lino et al. 1996). Curvature was quantified by drawing
a line tangent at the tip to the long axis of each coleoptile and
measuring the angular deviation of the tangent from the one obtained at the onset of gravistimulation (lino et al. 1996). In one set
of experiments (Fig. 6), the angle of the tangent from the horizontal was determined. (When the initial SA is 90°, this angle is equivalent to the angle of curvature.) •
In some .experiments, plants were rotated on horizontal
clinostats at 2.4 rpm. For clinostat treatment, the original growth
direction of the coleoptile was set parallel to the rotation axis. The
distance between plants and the rotation axis did not exceed 20
mm.
Materials and Methods
The relationship between SAs and curvature rates—According to the method described in lino et al. (1996), we
first analyzed the period of curvature development in which
curvature rates are directly related to initial SAs. Figure 1
shows the results of experiments planned for this purpose.
In these experiments, the curvature responses were compared at initial SAs of 10, 30 and 90° between plants kept
Results
Plant material—Plants were raised and experiments were conducted at 25°C under continuous irradiation with red light (2-3
yumol m~2 s~') as described in lino et al. (1996). It has long been
known that pretreatment of dark-adapted oat and maize seedlings
with red light considerably affects the gravitropism of their coleoptiles (Blaauw 1961, Huisinga 1964, Wilkins and Goldsmith 1964,
Wilkins 1965, Hild 1977). Therefore, when dark-adapted plants
are gravistimulated, one has to be careful with the nature of the
safelight used to handle and observe them (for general consideration, see lino and Briggs 1984, lino 1990). The red-light growth
condition was adopted here to maintain plants at the steady-state
with respect to the effect of red light, which is probably mediated
by phytochrome, and also to ease experimental handling and photographic recording of plants.
Caryopses of oats (A vena saliva L. cv. Almighty) were purchased from Snow Brand Seed Co., Ltd., Sapporo, Japan, and
those of wheat (Triticum aestivum L. cv. Shiroganekomugi) were
kindly donated by Osaka Prefectural Agricultural and Forestry
Research Center, Habikino-shi, Osaka, Japan.
a>
••••.
Dehusked caryopses of oats were floated, embryo side up, on
o>
water and incubated for 40-42 h. Germinated caryopses were
transplanted to acrylic cuvettes (top area: l l x l l mm2, height: 43
mm; Elkay Products Inc., Shrewsbury, MA, U.S.A.) filled with
\% agar (Nakarai Tesque Inc., Kyoto, Japan). Wheat caryopses
2
were surface-sterilized in 70% ethanol for 1 min, soaked in running tap water for 11-14 h, and incubated, embryo side up, on wet
u
paper towels (Kimtowel; Jujo Kimberly Inc., Tokyo, Japan). At
22-26 h of incubation, germinated caryopses were transplanted to
acrylic cuvettes filled with \% agar.
The transplanted caryopses of oats and wheat were incubated
for an additional day. Seedlings were then selected for uniform,
straight coleoptiles (oats, 18-22 mm; wheat, 16-20 mm) and used
20
for experiments. After transplantation, plants were kept in a box
made of red plate acrylic (No. 102, 3 mm thick; Mitsubishi Rayon,
Tokyo) until the end of gravitropism experiments, except when
0
30
60
90
120 0
X
60
90
120
handled for selection and experimental treatments.
Time
(min)
Gravitropic stimulation and curvature measurement—Coleoptiles were stimulated for gravitropism by displacing the plants
by a selected angle from the vertical. Plants were oriented for gravFig. 1 The effect of the drop in SA on the development of curistimulation in such a way that the direction of gravity was parallel
vature in coleoptiles of oats (A-C) and wheat (D-F). Plants were
to the plane passing through the two vascular bundles of the coledisplaced by 10° (A and D), 30° (B and E) and 90° (C and F) at
optile. The displacement angle (the initial SA) was determined
time 0. At 25 min of displacement (vertical line), plants were
with a digital clinometer (DL-1000; Suehiro Tool Co., Ltd,
returned to the original vertical position (o). Control plants were
kept displaced for the entire period (•). Mean measurements
Hyogo, Japan).
(±SE) obtained from 16-24 plants are shown. The SE is not distinPhotographs of coleoptiles were taken, usually at 5 min interguished when smaller than the symbol size.
vals after the onset of gravistimulation, to monitor their curvature
1348
Gravitropism kinetics in oat and wheat coleoptiles
as initially displaced and those displaced for 25 min and
returned to the original vertical position.
As is apparent in Fig. 1, the angle of curvature induced
in oats and wheat during the 25-min stimulation was less
than one-tenth of each displacement angle. Therefore, the
change in the SA which resulted from curvature of the coleoptile could be regarded as negligible in this period (see
lino et al. 1996 for detailed considerations). The oat and
wheat coleoptiles stimulated for only 25 min returned gradually to the original vertical position after showing a curvature similar to that in the coleoptiles stimulated continuously. In both oats and wheat, the curvature response up to
a period of at least 45 min after the onset of stimulation
was nearly identical between the coleoptiles stimulated for
25 min and those stimulated continuously. Therefore, it
was concluded that the curvature which had developed during the 45 min period in continuously stimulated coleoptiles was determined by the SA set initially and was not
affected by the decrease in SA resulting from the curvature
of coleoptiles.
The relationship between SAs and curvature rates
were analyzed in light of the results described. For oat coleoptiles, curvature rates were measured in the period from
25 to 40 min of gravistimulation. It is preferable to measure curvature rates at a phase of linear curvature development (lino et al. 1996). This condition was met for oats,
since oat coleoptiles showed a nearly linear increase of curvature from about 25 min (Fig. 1A-C, and many other
time-course data not shown). However, in wheat coleoptiles, the curvature rate increased gradually over a longer period and a linear phase of curvature could not be established before 35-45 min of gravistimulation (Fig. 1D-F,
and many other time-course data not shown). Since it was
more critical to determine the curvature rate before it was
influenced by the decrease in SA, the rate for wheat coleoptiles was determined in the period from 30 to 45 min.
Figure 2 shows the relationship between initial SAs (0180°) and curvature rates measured for oats (A) and wheat
(B) using the conditions described above. The rates were
plotted against the sines of SAs. The open circles indicate
the data for the SAs from 0 to 90°, and the closed circles,
for the SAs greater than 90°. For both oats and wheat, the
plots yielded non-linear curves. The slope of the curve decreased with the increase in the sine value, and the curvature rate approached saturation, although full saturation
did not appear to be established at the sine of unity (SA of
90°). The curves for oats and wheat were similar. The data
in Fig. 2 demonstrated further that oat and wheat coleoptiles show similar curvature rates between initial SAs
smaller than 90° and those larger than 90° when the sine
values were the same.
Exponential and hyperbolic functions were fitted to
the data of Fig. 2 (0-90°, open circles) as described in lino
et al. (1996). Both functions could be fitted to the data rea-
0.6
A
6
0.4.
O
Q
B
o
3
8
(
0
•
^\^O
1
•
(0 0.2
<D
25)
S
o
©
2
•
B
<D 0 . 6
1
0
0
2
0 ^
= 0.4
IK
0.2
o X°
^8
0
1
•
ri
0
o
'
V
-
•
fi/lo
•
0.2
0.4
0.8
0.8
1.0
sin 9
15
30
45
90
135
90
o0 (degrees)
180
165
150
• 0 (degrees)
Fig. 2 The relationship between SAs (0) and curvature rates in
oat (A) and wheat (B) coleoptiles. The curvature rate between 25
and 40 min (A) or 30 and 45 rain (B) after the onset of gravistimulation was calculated by linear regression from the mean measurements obtained at 5-min intervals, using 8-12 plants. O: initial SAs
between 0 and 90°. •: initial SAs larger than 90°. Curvature rates
are plotted against the sines of SAs (sin 9). Non-linear scales of
SAs are also indicated. The function y=A(l —e'8*) was fitted to
the data for 90° and smaller SAs with the least squares method to
obtain the solid curves (y, curvature rate; x, sin 8; A and B, constants).
sonably well. The solid lines in Fig. 2 are those generated
with the exponential function.
Long-term time courses of gravitropism—Gravitropic
curvatures were monitored over long periods. Figure 3
shows examples of such time courses obtained at three initial SAs. Oat coleoptiles showed a rapid upward curvature
from about 20 min to 2.5 h (Fig. 3A). After this period, the
curvature rate decreased sharply; the coleoptile then continued to bend at a much reduced rate, approaching the vertical slowly.
Wheat coleoptiles showed a rapid upward curvature
from about 20 min to 2 h and ceased to bend abruptly
Gravitropism kinetics in oat and wheat coleoptiles
1349
B
Oh
1h
2h
3h
4h
10
Fig. 3 Long-term time courses of gravitropic curvature in oat
(A) and wheat (B) coleoptiles. Gravitropic stimulation was initiated at time 0. The initial SAs were 30° (•), 60° (•) and 90° (•).
The coleoptile tip regains vertical orientation when the net curvature reaches these angles (indicated by dashed lines). Mean measurements (±SE) obtained from 12 plants are shown. The SE was
smaller than the symbol size in many cases.
(Fig.3B). After this cessation, the coleoptile showed
oscillatory movements. When stimulated at 30°, the coleoptile tip reached the vertical position at the conclusion of the
initial rapid curvature and then showed slight oscillation
around the vertical. When stimulated at 60° and 90°, the coleoptile concluded its rapid curvature before reaching the
vertical and then oscillated while gradually approaching
the vertical. These oscillatory movements were highly synchronized among plants (note that the data in Fig. 3 represent the mean curvatures obtained from 12 plants).
It appeared that oat and wheat coleoptiles develop
gravitropism through early and late phases. The early
phase is characterized by a rapid curvature, and the late
phase, by a slow curvature. In wheat, the late phase is additionally characterized by marked oscillatory movements.
Such oscillatory movements were not observed in oats, but
the data obtained at 90° (Fig. 3A) suggested that weak oscillation might occur in oats.
When the initial SA was 90°, the curvature rate dropped when the coleoptile tip was about 30° from the vertical
5h
6h
Fig. 4 Development of gravitropic curvature in oat (A) and
wheat (B) coleoptiles after 90° displacement. The photographs
shown are those of the same plants obtained at the indicated times
after displacement. The arrow indicates the direction of gravity.
(Fig. 3 A, B). (Because the coleoptile was arc-shaped at this
time, lower parts of the coleoptile had even greater angles
from the vertical.) It was noted that the rate of the ensuing
slow curvature was much smaller than the rate of the curvature induced at an initial SA of 30° (Fig. 3A, B). Therefore, the decrease in SA resulting from upward curvature
of the coleoptile could not explain the observed drop in curvature rate.
Coleoptile straightening in the late phase of gravitropism—During the late phase of gravitropism, coleoptiles of
oats and wheat straightened gradually. This is depicted in
the photographs of Fig. 4. Following gravistimulation at
90°, an upward curvature developed along the length of the
1350
Gravitropism kinetics in oat and wheat coleoptiles
0
Fig. 5 The effect of clinostat treatment on curvature during the
late phase of gravitropism in oat coleoptiles. Plants were displaced
by 90° at time 0. From 5 h after the displacement (indicated by a
dotted vertical line), they were rotated on horizontal clinostats at
2.4 rpm (o). Control plants received no clinostat treatment (•).
Each point is the mean (±SE) obtained from 16 independent
plants.
coleoptile, resulting in an arc shape (1-3 h). After this early
phase of gravitropism, the base of the coleoptile continued
to bend upwards, but the apical to middle parts straightened gradually. Evidently, this straightening response took
place before the coleoptile tip, or any part of the coleoptile,
gained the vertical orientation. Therefore, the gravity
perceived at any part of the coleoptile cannot be the cause
of the straightening response.
It is possible that the coleoptile straightened because it
reached the final stage of growth with the same maximal
length being attained in its two sides. To investigate this
possibility, oat and wheat coleoptiles were displaced by
90°, and, when the upper half of the coleoptile became
straight at 6 h after showing curvature (see Fig. 4), they
were displaced downwards again in such a way that the
straight part was nearly horizontal. This part showed clear
upward curvature in response to the second displacement
(data not shown), demonstrating that the part which had
straightened had enough growth capacity for curvature.
We conclude that the straightening of the coleoptile is
an autonomic curvature response that does not directly depend on gravity but results somehow as a consequence of
preceding gravitropic curvature. We choose the term autotropism (Simon 1912, Dolk 1936) to describe this autonomic curvature response.
The process of coleoptile straightening was investigated further using oat coleoptiles. The data in Fig. 5 were
obtained by stimulating oat coleoptiles at 90° and rotating
them on horizontal clinostats when they were in the late
curvature phase (Fig. 5). The clinostat-treated coleoptiles
not only ceased the upward curvature but developed a
1
2
3
4.
Fig. 6 The effect of an upward displacement of oat coleoptiles
on their curvature during the late phase of gravitropism. Plants
were displaced by 90° from the vertical at time 0. After 3.5 h, the
coleoptiles were displaced upwards by 15°. The angle between the
direction of the coleoptile tip and the horizon was determined. As
a result of the upward displacement, the angle between the direction of the coleoptile tip and the horizon was enhanced, as indicated by the arrow. Mean measurements obtained from 12
plants are shown. The SE was smaller than the symbol size in all
cases.
downward curvature. The results demonstrated that when
the gravitropic stimulus for an upward curvature is terminated by rotation on a clinostat, the coleoptile actually
develops a curvature in the opposite direction. It appeared
that autotropic response actively counteracts the gravitropic response to straighten the coleoptile in the late curvature
phase.
In the next experiments, oat coleoptiles were stimulated similarly at 90°, and when the coleoptiles had just reached the late curvature phase, they were displaced upwards in
such a way that the tip of the coleoptile became nearly vertical. As shown in Fig. 6, the displaced coleoptiles began to
bend downwards, and the direction of these coleoptiles approached that of the non-displaced coleoptiles. Thus, when
the gravitropic stimulus for upward curvature is weakened
by an upward displacement, the coleoptile develops a curvature in the opposite direction. The results substantiated
the conclusion that the autotropic response actively counteracts the gravitropic response in the late curvature phase.
Discussion
Angular dependence of gravitropism—The plot of curvature rates against the sines of SAs yielded a straight line
for maize and a non-linear saturation curve for rice (lino et
al. 1996). The corresponding plots for oats and wheat were
similar to the one obtained for rice (Fig. 2), indicating that
the non-linear relationship is more common in Gramineae
coleoptiles and that the coleoptiles of rice, oats, and wheat
Gravitropism kinetics in oat and wheat coleoptiles
have similar gravisensitivities. In fact, the high gravisensitivity represented by a non-linear saturation curve may a
property prevailing among plants (Barlow et al. 1993,
Myers et al. 1995; see discussion in lino et al. 1996).
In both oats and wheat, initial SAs below and above
90° resulted in similar curvature rates when their sine
values were the same. This supports the view that the extent
of gravitropism is determined by the component of gravity
that is perpendicular to the organ's long axis (Sachs 1882).
Our results appear to represent a simple and straightforward relationship between SAs and gravitropic responses.
As discussed below, however, further study is necessary to
clarify whether or not such a relationship is a general property of plant gravitropism.
Pickard (1973) previously investigated the angular dependence in oat coleoptiles. She subjected coleoptiles to
different SAs for 25 min and subsequently rotated them on
horizontal clinostats to obtain curvature responses. At SAs
smaller than 90°, a linear relationship was found between
the sines of SAs and curvatures. Our method yielded such a
linear relationship for maize (lino et al. 1996), but not for
oats (Fig. 2A). Therefore, the results disagree for the same
material. Pickard (1973) found, however, that the relationship between the sines of SAs and curvatures was nonlinear at stimulation angles greater than 90°; the curvature
response was nearly constant at SAs from 90° to about
120°. This part of her results is similar to ours. More recently, the relationship between accelerations and gravitropism
in oat coleoptiles was investigated in the microgravity environment of a space laboratory (Brown et al. 1995). In this
study, the coleoptile was exposed at the right angle to its
long axis to three different accelerations for different periods. For a given stimulation period, the extent of the gravitropic response was similar at 0.4 and 0.6 g, whereas 0.4 g
produced a greater response than 0.2 g. These results indicate, in agreement with ours, that the gravitropic response is nearly saturated at 0.4 g or at a sine SA value of
0.4 under 1 g. Although it is not clear why the SA-response
curve obtained by Pickard (1973) in the 0-90° range was
different from ours, it is perhaps worthwhile to consider
the possibility that the clinostat treatment practiced by
Pickard had an additional effect on the angular dependence, as reported for other aspects of gravitropism (Chapman et al. 1994, Volkmann and Tewinkel 1996, Brown et
al. 1996).
Although we found a straightforward relationship between the sines of SAs and curvature responses for oat and
wheat coleoptiles, it has been concluded more commonly
that the relationship is different between SAs smaller than
90° and those larger than 90°, the maximal response occurring at a SA of 120-140° (see Introduction). Such a shift of
the optimal SA to an angle greater than 90° has been
reported, for example, in roots of Lupinus albus (Metzner
1929), cress (Larsen 1969, Iversen and Rommelhoff 1978)
1351
and rapeseed (Legue 1994) and in hypocotyls of Arabidopsis thaliana (Kiss et al. 1997) (for other organs and
materials, see Metzner 1929 and Audus 1964). It is possible
that the clinostat treatment used in the experiments had
some uncontrolled effect on the SA-response relationship
(e.g., Metzner 1929, Larsen 1969), but this cannot be
the sole reason for the shift of the optimal SA, because
the shift was also identified without clinostat treatment
(Iversen and Rommelhoff 1978, Legue 1994, Kiss et al.
1997). When clinostats were not used, however, the greater
gravitropic response at a SA greater than 90° was demonstrated by measuring curvature response over a relatively
long period of gravistimulation (Iversen and Rommelhoff
1978, Legue 1994, Kiss et al. 1997). Therefore, the early curvature kinetics studied in the present study remain to be resolved in other materials. For further clarification of the
SA-response relationship in those materials in which the optimal SA could not be identified at 90°, it is necessary to investigate whether or not the optimal SA is modified by the
clinostat treatment and to evaluate the SA-response relationship by measuring the early curvature kinetics in continuously stimulated plants.
The method used in the present study allows determination of the relationship between SAs and gravitropism
without involving classical clinostat treatment. The relationship can be easily applied as an experimental criterion
in studies of the components or the reactions that underlie
the mechanisms of gravity perception.
Autotropicstraightening—Plant organs usually regain
their straight appearance after showing gravitropic curvature. Each part of the organ changes angular position as
it develops gravitropism. This change in angular position
appeared to be a cause of the straightening, especially when
an organ bent until its upper part overshot the vertical
(Kohler and Daum 1979, lino et al. 1996). However, there
have been experimental results which could not be explained simply by the change in angular position and the response to gravity. Dolk (1936) showed that oat coleoptiles
straightened even while rotated on a horizontal clinostat.
Firn and Digby (1979) showed that sunflower hypocotyls
and maize coleoptiles began to straighten in the parts which
had not reached the vertical position. Pickard (1973) provided shadowgraphs of an oat coleoptile indicating that
straightening progressed before any part of the coleoptile
reached the vertical position. More recently, it was shown
that oat coleoptiles straightened even in the microgravity
environment of a space laboratory (Chapman et al. 1994).
These results indicate that the straightening is achieved by
autotropism.
The present study has further substantiated the occurrence of an autotropic straightening response. As shown by
Pickard (1973), oat coleoptiles began to straighten before
any part of the coleoptile reached the vertical position
(Fig. 4A). A similar result could also be obtained with
1352
Gravitropism kinetics in oat and wheat coleoptiles
wheat coleoptiles (Fig. 4B). As described in the Results, the
straightening occurred in the part which had the capacity to
develop a further gravitropic curvature. More importantly,
we were able to demonstrate that autotropism actively
counteracts gravitropism to straighten the coleoptile (Fig. 5,
6).
Coleoptiles of oats and wheat sharply reduced the rate
of their gravitropic curvature before the tip reached the
vertical position (Fig. 3). Thereafter they developed net curvature slowly and the tip gradually approached the vertical
position. The wheat coleoptiles subjected to small initial
SAs seemed to be somewhat exceptional (Fig. 3B), but their
curvature still dropped sharply when the tip reached the
vertical position. In light of the results described in the
preceding paragraph, it is most probable that the observed
sharp decrease in curvature rate reflects the initiation of
autotropism. In fact, the time-lapse photographs (Fig. 4,
and many other photographs obtained at shorter time intervals) indicated that the straightening of the coleoptile in its
upper part began at the same time as the measured sharp
drop in net curvature rate.
Reported results have often indicated that gravitropic
curvature progresses continuously until the apical part of a
responding organ overshoots the vertical position. Such
overshooting responses were recorded in sunflower hypocotyls (Kohler 1978, Firn and Digby 1979), maize coleoptiles (Firn and Digby 1979, lino et al. 1996), rice coleoptiles
(lino et al. 1996), and maize roots (Ishikawa et al. 1991,
Barlow et al. 1993). Autotropism is less apparent in these
cases, but it may still be an important component of the
process of organ straightening (Firn and Digby 1979).
During the late curvature phase, wheat coleoptiles
showed marked oscillatory movements. This oscillation
was probably caused by the interaction between gravitropism and autotropism. Oscillatory movements during gravitropism were also observed in rice and maize coleoptiles
(lino et al. 1996) and maize roots (Ishikawa et al. 1991,
Barlow et al. 1993). Rice coleoptiles oscillated after overshooting the vertical, and the response to gravity appeared
to explain this oscillation (lino et al. 1996). The oscillation
of maize coleoptiles also occurred after overshooting the
vertical, although it could not be explained only in terms of
the response to gravity (lino et al. 1996). The oscillation of
maize roots accompanied nutational movements that could
also be observed without gravistimulation. (Barlow et al.
1993). No such nutational movement was apparent in
non-gravistimulated wheat coleoptiles (our observation).
Perhaps the wheat coleoptile provides the first clear case in
which oscillation is purely based on autotropism, free from
complications due to overshooting and nutational responses.
The mechanism of autotropism is not clear. Gravitropic organs may detect the mechanical distortion of tissues
caused by curvature. However, autotropic straightening
did not take place during phototropism of oat coleoptiles
(Dolk 1936), suggesting that autotropism is a specific attribute of gravitropism. During the straightening process,
the tip orientation is finely adjusted so that it slowly approaches the vertical (Fig. 3,4). This observation suggests
that the plant somehow uses gravity information to regulate the expression of autotropism and to accomplish the final straightening response.
This work was supported by a fund for basic experiments
oriented to space station utilization from the Institute of Space
and Astronautical Science, Japan.
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(Received June 27, 1997; Accepted October 3, 1997)