Oecologia (1990) 83:171-175
Oecologia
9 Springer-Verlag 1990
Ecological significance of light controlled seed germination
in two contrasting tropical habitats
C. Vfizquez-Yanes and A. Orozco-Segovia
Centro de Ecologia,UNAM, Apartado 70-275, Deleg. Coyoacfin,04510 M6xico, D.F.
Received August 12, 1989 / Accepted January 23, 1990
Summary. The effects of temperature, photoperiod, phytochrome photoreversion and the response to a R/FR
ratio gradient were investigated in seeds of four species
from two contrasting tropical habitats; two species from
a rain forest (Cecropia obtusifolia and Piper umbellatum)
and two from a high altitude lava field covered by low
vegetation (Buddleja cordata and Chenopodium ambrosioides). In the rain forest seed species the photoblastic
response seems to be adapted to light quality changes
due to canopy destruction, on the other hand, the lava
field seed species seem to be adapted to instantaneous
light stimulus such as would be produced by the sudden
exposure of a buried seed to the soil surface light environment.
Key words: Buddleja - Cecropia - Chenopodium - Phytochrome - Piper
The role of positive photoblastism in seeds has been
related with the preservation of dormancy of buried
seeds and/or that of unburied seeds of heliophile plants
laying beneath green canopies and litter (Frankland
1976). It is possible that seed responses to temperature,
to Red/Far Red (R/FR) ratios, to photoperiod and to
the environmental control by light quality are modulated
by the relative importance of photoblastism in survival
of seeds in either or both situations described above.
Conditions for germination and establishment are
basically different for buried seeds and seeds laying beneath a green canopy. In the first case the condition
is the change from a dark to an illuminated microenvironment with wide temperature fluctuations near the soil
surface (Wesson and Wareing 1969). In the second case,
canopy destruction produces a different light environment which is not always marked by temperature fluctuations (Vfizquez-Yanes and Smith 1982, Vfizques-Yanes
and Orozco-Segovia 1982). The distinction between sunOffprint requests to: C. Vazquez-Yanes
flecks and openings in the canopy can only be detected
by having a mechanism of germination that requires
consecutive phytochrome changes to the inhibited condition when the light stimulus is not long enough to correspond with the presence of a light gap in the canopy
(Vfizques-Yanes and Orozco-Segovia 1987).
On basis of these differences it would be expected
that photoblastic seeds which are mainly adapted to remain dormant in buried conditions will have a shorter
photoperiod requirement, low R/FR ratios would be
enough to start germination and a greater dependence
on temperature fluctuations. On the other hand, seeds
adapted to survival beneath green canopies would have
a longer photoperiod requirement, high R/FR ratio for
germination, greater photoreversion capability and less
dependence on temperature.
In this paper we have selected four species from two
very contrasting tropical habitats: a rain forest and a
lava field shrubland (Table 1), in order to test the differential response to light that these seeds would have in
the field. In the forest mainly the pioneer plants produce
the photoblastic seeds. This characteristic has been related with the maintenance of dormant seeds in the soil
for some time until a canopy destruction takes place
(Valio and Joly 1979; Whitmore 1983). In the lava vegetation photoblastism in seeds is very frequent and this
characteristic may be necessary for survival of seeds
when they are buried in the crevices of the rock or in
accumulations of soil.
A series of experiments were devised to test seed response to light and darkness in different temperature
conditions, number of daily expositions to light to trigger germination, response to photoperiod of white light
and speed of activation of phytochrome after a high
R/FR ratio stimulus and the response of seeds to a gradient of R/FR ratios.
Materials and methods
In the tropical rain forest the species selected were an abundant
pioneer tree from light gaps and secondary vegetation, Cecropia
172
Table 1. Characteristics of the areas of procedence of the seeds, a tropical rain forest (Bongers et al. 1988) and a lava field (Rzedowski
1954; Jauregui 1971)
Area
Vegetation
Climate
Soil
Reserve of the tropical Biology
Station at Los Tuxtlas, Veracruz,
S.E. M6xico. 160 m. a. s. 1.
High evergreen
rain forest
Warm and humid. Annual
precipitation 4639 ram.
Mean annual temperature 27 ~ C
Deep soil of recent volcanic ash origin
Reserve of the Pedregal de
San Angel at the south of Valley
of M6xico (M6xico City).
2240 m.a.s.1.
Open shrubland
Seasonal, temperate and
subhumid. Annual precipitation
1000 ram.
Mean annual temperature 16~ C
Underdevelop soil on lava of around
2500 years of age
~oC
--20-50~
?
I00 15 - 3 5 o
15 " 3 5 ~
t
C
C
80
80-
60-
B,
A
40-
40
r
r
20-
20
25 o C
2 0 - 30 ~ C
25-35 o C
I0
o
rom
2
E
4
6
8
I0
12
14
16
18
20
2 2 23
i
I
I
I
I
4
6
8
I0
12
I,..
(.9
25 ~ C
100-
I00"
25"38~
~ , . iS- 35 ~ C
o<
I00-
i0-50 ~ C
20.300(
25 ~ C
80-
80--
t
~25 -3 5 o C
25 ~ C
..,.15-35 o C
80] ~ &
20- 50 ~ C
5-35 o C
60-
C
60,
60.
13
B2
40-
40-
20-
20-
40-
20-
OI
2
I
4
15-35 ~ C
|
4
I
8
I
I0
~
3
!
4
i
5
days
Fig. 1 A-D. Effect of constant and fluctuating temperatures on seed germination. A Cecropia, BI Chenopodium without prechilling, B2 prechilled, C Piper and D Buddleja
173
obtusifolia Bertol. (Moraceae), and a perennial herb from disturbed
places, Piper umbellatum L. (Piperaceae). Characteristics and germination have been described in detail before by Vfizquez-Yanes
and Smith 1982; Martinez-Ramos and Alvarez-Buylla 1986; Orozco-Segovia and VAzquez-Yanes 1989. In the pedregat the two species selected were a small tree, Buddleja cordata H.B.K. (Loganiaceae), and a perennial herb Chenopodium ambrosioides L. (Chenopodiaceae). The four species selected produce massive amounts
of seeds the year round which in all the cases are small and photoblastic.
Mixed samples of fresh seeds from several plants from each
species were used for the experiments. Seeds were seeded in groups
of 50 in plastic petri dishes with 1% agar in distilled water in
all the experiments. Three dishes were used for each experiment.
The dishes were placed in incubators with temperature controls
(G-30 Conviron, Winnipeg, Canada). To study the effects of a
gradient from 1.3 to 0.1 of R/FR ratio (Toledo et al. 1990). The
seeds were placed on the surface of agar placed in rectangular
plastic boxes. Two fifty seed sets were used for each R/FR ratio
value. Values of R/FR ratio were measured with a spectroradiometer for R/FR (Skye, Scotland). The germination experiment
along the gradient lasted 15 days under the light treatment at 12 h
photoperiod and 10 days in total darkness. Both conditions at
25~ constant. The results are final germination obtained after
the dark period.
To test the effect of temperature on the response to tight, six
dishes were placed in germinators with 12 h of light each day at
the followings temperatures: one constant temperature of 25~
and three fluctuating temperature treatments of 20-30~ C, 2535~ C and 15-35 ~ C. Half of the petri dishes were wrapped in aluminium foil to keep them in permanent darkness. In each fluctuating temperature treatment, the highest temperature lasted four
hours with a gradual increase and decrease and the lowest temperature was reached during the dark period. During these experiments
it was discovered that imbibited seeds of Chenopodium required
a low temperature pretreatment to obtain maximum germination
response for all the temperatures with the exception of 15-35~ C
fluctuation. The chilling temperature pretreatment of 5~ C was used
for later experiments.
Consecutive days of light needed to obtain maximum germination were calculated for the constant temperature of 25~ C, a condition which proved favourable for all the species included pre-chilled
Chenopodium. Seeds at 25~ C were also exposed to consecutive
periods of light of 1, 5, 15, 30, 60 and 120 min per treatment
daily up to 40 days.
To produce phytochrome photoreversions by the low R/FR
light (R/FR ratio = 0.06) boxes made with red and blue Plexiglass
(serials Nos. 2423 and 2424, Rohm and Haas, M~xico, D.F.) were
placed inside the incubators under yellow incandescent light. The
seeded petri dishes were placed inside the boxes and were given
to white light for different periods in each treatment during
30 days. The treatments were 5, 15, 30, 60, 120, 240, 360, 480 min
of white light in the middle of twelve hours of FR treatment per
day. As control samples were placed in continuous 12 h of white
or FR light.
Table 2. Germination in darkness at different temperatures (SD)
Temperature
(~
% germination (SD)
25
20 30
25-35
15-35
Buddleja
Cecropia Chenopodium~
Piper
0
0
0
0
3 (2)
32(11)
17(2)
25(5)
0
0
0
0
similar results were found with and without chilling
A
~Bz~ c
T~"
*
&
D
C.9
~0
/r
--
/
T/
I
2
3
4
Fig. 2. Effect of treatments of 1, 2, or more consecutive days (12 h)
of exposure to white light on germination after a dark period.
A Chenopodium, B Cecropia, C Buddleja and D Piper
A
I00.
yJ "
•
c
0
80
0
C
60
E
G
r
40
v
I0
A ~ A
O"
I n the t e m p e r a t u r e e x p e r i m e n t s m o s t o f the seeds rem a i n e d d o r m a n t in total darkness. O n l y p a r t of this
p a r t i c u l a r seed crop o f Cecropia a n d a small fraction
o f Chenopodium seeds showed responses to t e m p e r a t u r e
f l u c t u a t i o n in d a r k n e s s in the first case a n d g e r m i n a t i o n
in d a r k u n d e r every c o n d i t i o n in the second case (Table 2). T h e o p t i m a l t e m p e r a t u r e s (either c o n s t a n t o r fluct u a t i n g ) for the r a i n f o r e s t species were high, a n d a period
at 15 ~ C in the t r e a t m e n t greatly reduced g e r m i n a t i o n .
5
Doys
C l
Results
3 (2)
5(3)
5(3)
2(1)
[
I
15
Minutes
=
30
610
120
Fig. 3. Effect of
treatments of a given
number of minutes of
daily exposition to
white light during seven
days on final seed
germination.
A Cbenopodium,
B Buddleja, C Cecropia
and D Piper
Uogl
Buddleja h a d a wide r a n g e o f t e m p e r a t u r e tolerances
b u t g e r m i n a t i o n was slower at lower temperatures.
Chenopodium g e r m i n a t e d in all c o n d i t i o n s with prechilling with the exception o f 15-35 ~ C c o n d i t i o n , which was
the o p t i m a l w i t h o u t chilling (Fig. 1).
174
The number of consecutive days required for germination was different for each especies but with the exception of Piper. They had more than 50% germination
with just one day of light (Fig. 2).
The effect of different amounts of exposure to white
light on germination showed that short time of light
was enough for the lava species but not so for the forest
species (Fig. 3).
Both forest species clearly showed phytochrome inactivation by dayly exposure to FR light. The lava species had a completely different response as they did not
present marked photoreversion (Fig. 4).
Along the R / F R ratio gradient there was a clear cut
difference between the forest and lava species. The first
two were inhibited by low ratios and the second two
could germinate in ratios below 0.1 (Fig. 5). The controls
indicated that Buddleja and the prechilled seeds of Chenopodium had partial germination in permanent FR conditions.
40-
C
0
I0-
8 TJl
0
r
Ioo-
E
IB
T
80-
T
c
T
TT/#
60-
Discussion
M i n u t e s
The two tropical rain forest species are essentially similar
in their response to light. Both species require long lasting periods of high R / F R ratio light to prevent further
phytochrome inactivation and both are inhibited by R/
FR ratios that can be found in the diffuse light of dense
forest (Lee 1987).
Buddleja is a case in its own because it can germinate
in very low R / F R ratio and with short bouts of light
so its photoblastism is an unsophisticated light detector
without much dependence on temperature. The thresh-
(log)
Fig. 4. Effect of daily exposures to white light in seeds placed in
12 h photoperiod of far red light. The percentage of germination
indicates where photoreversion is taking place. A Chenopodium,
B Cecropia, C Buddleja and D Piper
B
/
/
60G
0
4O-
/
0
20
C
9- -
0
i
,
I
i
i
,
~
i
E
I00
~,
60
C
I
/
/
Fig. 5A-D. Germination along a R/FR
ratio gradient after 15 days of exposure and
10 days in total darkness. A Chenopodium,
B Piper, C Buddleja and D Cecropia
/
O[
02
O~
0.4
0.5
0.6
07
08
0s
/O
I.I
12
1.3
R/FR
01.1 0 2
ratio
0 5i
04
0.5 06
07
Og 09
i
1.0
[iI
11.2 115
175
old of activated p h y t o c h r o m e for germination is reached
in almost pure F R as it is also the case in Chenopodium.
In an environment like a lava field this might prevent
germination in the rock cavities and crevices. However,
given the enormous production of seeds by Buddleja,
light sensitivity m a y not play an important survival role
in this environment and this species m a y be mainly moisture limited.
Chenopodium has a completely different behaviour
because of its prechilling requirement which must interact with light to p r o m o t e germination. D r y seeds are
insensitive to temperature and light stimulus and low
temperature in moist conditions with light promotes germination. Seeds require a chilling period to induce germination but when the light stimulus is given at low
temperatures germination can be achieved in dark at
high temperatures (unpublished data). This trait has
been found in other species of the same genus (Cumming
1963, Roberts and Benjamin 1979). In this case there
should be an interaction a m o n g light, temperature and
moisture for germination. This requirement lasts for at
least a year in stored seeds. The seeds germinate with
a short treatment of light and in low R / F R ratios. This
combination of factors is probably the m o s t typical of
photoblastic seeds that are adapted to survival in buried
conditions a m o n g the species studied here (Roberts and
Totterdell 1981).
The two forest species show an adaptation to remain
d o r m a n t under low R / F R ratio conditions with an efficient p h y t o c h r o m e activation mechanism that m a y act
as an indicator of the degree of canopy opening or of
a litter layer disturbance. The opposite results were
found in the lava species where R / F R ratio for germination is very low and photoreversion does not take place
even when the seeds are exposed only to very short lasting shots of white light. This indicates that they can
only detect a change from a condition of total darkness
(burial) to light as would be produced by the exhumation
of the seeds. The effect of temperature is less clear as
optimal conditions are similar although Chenopodium
requirement for low temperatures m a y indicate a geographical limitation to temperate areas or high altitudes
in the tropics. On the other hand Piper is very inhibited
by relatively low temperatures indicating its tropical lowland filiation.
Although different kinds of responses to light a m o n g
seeds of tropical plants have been described before (Rooden et al. 1970), the two ecophysiological roles covered
in this paper seem to have been overlooked until now.
Acknowledgments. We thank Biol. Jorge R. Toledo for assistance,
Dr. Emmanuel Rinc6n for his critical review of the manuscript
and MC. Nelly Diego for help with the identification of the pedregal species. This project was partially financed by the Mexican
Council of Science and Technology (CONACyT grant: Ref.
P220CCOR880134).
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