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REGULATION OF GERMINATION
TIMING IN FACULTATIVELY FALLEMERGING GRASSES
Phil S. Allen
Susan B. Debaene-Gill
Susan E. Meyer
ABSTRACT
event initiates germination, there is a high probability the
seed zone will dry prior to radicle emergence. Therefore,
response of germinating seeds to dehydration regimes is
an important factor in determining whether autumn germination will occur.
In sagebrush-steppe ecosystems, precipitation is more
reliable in winter than summer or autumn, and most species have mechanisms restricting germination to winter
or early spring, when drought risk to seedlings is minimal. In contrast, many native and introduced perennial
grasses, as well as exotic annual weeds, are facultatively
fall-emerging. Seeds mature from midsummer to early
autumn and are either nondormant upon dispersal or become nondormant (afterripen) under summer conditions.
They either germinate in response to autumn rains, postpone germination until winter or early spring, or, as in
the case of exotic annuals like cheatgrass (Bromus tectorum), may carry seeds across years (Hull and Hansen
1974). Seedlings that emerge in autumn have a potential
advantage over spring-emerging seedlings because of
their headstart in growth, provided they survive risks
associated with autumn emergence. These risks include
the chance of death by desiccation if germination is triggered by rainfall inadequate to permit establishment.
Seeds of facultatively autumn-emerging species would
therefore be expected to possess mechanisms for sensing
the adequacy of soil moisture associated with autumn
precipitation events, and limiting germination to periods
with sufficient moisture for establishment.
This study was conducted to determine how germination timing is regulated in seeds of squirreltail CElymus
elymoides), bluebunch wheatgrass CPseudoroegneria
spicata), and cheatgrass. Multiple ecotypes of squirreltail and cheatgrass were included to determine whether
seed response to dry storage and intermittent hydration
varies as a function of habitat of origin.
Seeds of facultatively autumn-emerging grasses germinate in response to autumn rains or postpone germination
until the following winter or spring. This research was
conducted to determine how germination timing is regulated for seeds of cheatgrass (Bromus tectorum), squirreltail (Elymus elymoides), and bluebunch wheatgrass (Pseudoroegneria spicata), particularly when hydration is interrupted by dehydration episodes of varying duration and
severity. For each species, germination rate increased or
dormancy levels decreased as a function of time in dry
storage, indicating an afterripening requirement. Seeds
progressed toward germination with intermittent hydration, but showed sionificant between- and within-species
differences in response to dehydration episodes. Rate of
germination was markedly affected, particularly when dehydration occurred just prior to radicle emergence. These
findings suggest that germination timing is at least partially regulated by rate and degree of dehydration, which
provide a signal regarding adequacy of soil moisture for
seedling establishment.
INTRODUCTION
When seeds that are viable and nondormant are provided with adequate water, oxygen, and suitable temperatures, they germinate. Seeds, however, have no control
over the external environment to ensure that conditions
will remain favorable long enough to complete the germination process. Germination-directed physiology may be
interrupted by one to several episodes of conditions unfavorable for germination. Seed response to these episodes
mediates their germination behavior during later favorable periods.
Germination interruption due to soil drying is a frequent occurrence for autumn-emerging species in semiarid sagebrush-steppe plant communities, where summer/
autumn precipitation is unpredictable and often followed
by periods of rapid surface drying due to high insolation
and low atmospheric humidity (Evans and Young 1972;
Wight and Hanson 1987). After an autumn precipitation
METHODS
Seeds were wild-collected during the summer of 1991
(table 1). Preliminary experiments were conducted with
seeds from each collection to determine optimum temperatures for germination and afterripening patterns under contrasting temperature regimes. For experiments
involving interjected dehydration episodes, seeds afterripened for at least 6 months were incubated at 20 °C.
Control of water potential was achieved using the system
of Allen and others (1992), wherein seeds are alternately
exposed to liquid ('If =0) and vapor phase ('If =negative)
Paper presented at the Symposium on Ecology, Management, and Restoration of Intermountain Annual Rangelands, Boise, ID, May 18-22, 1992.
Phil S. Allen and Susan B. Debaene-Gill are, respectively, Assistant
Professor and Graduate Student, Department of Agronomy and Horticulture, Brigham Young University, Provo, UT 84602. Susan E. Meyer is a
Botanist, Intermountain Research Station, Forest Service, U.S. Department of Agriculture, at the Shrub Sciences Laboratory, Provo, UT 84606.
215
E'
Table 1-5eed source data for germination experiments
Seedlot
Elevation
January mean
Annual
temperature precipitation
oc
m
Pseudoregnaria
spicata
Provo Overlook
g
em
3oo
;
1Too
I
Hours
225
.5
1,970
-1.7
43
108
Elymus elymoides
1,250
South Price
South Santaquin 1,540
Strawberry
2,400
-4.9
-2.2
-8.3
16
34
65
84
144
164
Bromus tsctorum
Green River
Provo
Strawberry
-4.9
-2.0
-8.3
16
38
65
96
64
48
1,250
1,800
2,400
I
ft
.5
150
'E
~
J,
75
e
i
0
24
0
:E
T110 • time to 90 percent of total germination at 20 oc for each seedlol
1
means. Germination was slower at 10 °C, and was progressively inhibited at temperatures above 20 °C. Although Young and Evans (1977) state that neither squirreltail nor cheatgrass has an afterripening requirement,
our data show progressive changes in rate and percentage
germination across a range of temperatures when stored
under ambient laboratory conditions for a 14-week period
(fig. 2). Freshly harvested seedlots contained a fraction
conditionally dormant at suboptimal and superoptimal
RESULTS
Continuously Hydrated Seeds-Continuously hydrated seeds germinated most rapidly at constant 15 or
20 °C, or at alternating temperatures with 15 or 20
oc
I§
96
Figure 1-Example of triphasic pattern of water
uptake used to determine onset of dehydration
episodes. Early =end of imbibition; late =end
of lag phase, when first radicle was observed.
Data shown are for cheatgrass seeds incubated
at 20 oc on saturated blotters in sealed petri
dishes. Numbers above data points correspond
to percent germination at time indicated.
water. This approach facilitated the study of seed response
to variations in rate, degree, and timing of dehydration episodes initiated following the onset of imbibition.
Dehydration was timed to occur at an early· or late stage
of germination, based on the triphasic pattern of water uptake for each species (fig. 1). Early dehydration was initiated near the end of rapid imbibition, which was always
8 or 16 hours following the onset of hydration. Late dehydration was initiated near the end of the lag phase, and
varied between 19 and 72 hours depending on species and
ecotype. Dehydration episodes were either mild (-4 MPa)
or severe (-150 MPa), and lasted for a total of 48 hours.
r:1:
72
48
Time (hours)
Test Initiation date
-Q-713 -D· 815 -6- 8127
c
B
A
20
c1 0 ~15l
o
!§"=
7
14
I
21
28
Days at 5/15 •c
35
42
14
21
aa
Days at 10120 •c
35
42
14
21
28
Days at 15125 •c
35
21
28
35
G
42
Days at 10120 •c
14
21
28
Days at 15125 •c
Figure 2-Germfnation rate curves at four incubation temperatures for squirreltail (A-D) and cheatgrass (E-H) seeds after 2 to 14 weeks of laboratory dry storage. Day 7 differences among test dates within a temperature treatment were
significant at p < 0.05 level for both species.
216
1.
21
28
Days at 20130 •c
F
14
42
35
42
35
42
100
100
80
80
60
60
40
40
20
20
tcra.~11
0
0
~ 100
u
100
150
200
250
300
350
0
400
B
cG)
..l
50
0
50
100
150
200
250
300
350
400
0
50
100
150
200
250
300
350
400
100
80
80
c
60
60
1i
c
40
40
20
20
...._
0
..
E
CD
CJ
().
0
50
0
100
150
200
250
300
350
400
c
100
100
80
80
60
60
40.
40
20
20
0
0
0
50
100
150
200
250
300
350
400
0
50
100
150
Total Accumulated Hydration Hours
temperatures, and germinated slowly at optimal temperatures. Germination rate increased and dormancy levels
decreased as a function of time in dry storage.
For afterripened seeds subjected to continuous hydration, time to 90 percent relative germination (T90 ) (relative
germination =percent of total germination for a particular seedlot) varied considerably among species and ecotypes (table 1). Germination of cheatgrass seeds was
generally more rapid than for squirreltail or bluebunch
wheatgrass. Cheatgrass germination rate increased with
collection site elevation, while the rate for squirreltail decreased with collection site elevation.
200
250
300
350
400
Figure 3-Germinatlon rate
curves for squirreltail (A-C) and
cheatgrass (0-F) seeds subjected to continuous hydration or
an interjected 48- hour dehydration episode early or late during
germination. Seeds were from
low (A, D), middle (B,E), and high
(C,F) elevation sites listed In
table 1. Error bars Indicate the
maximum standard error for each
treatment.
normally upon rehydration. A slightly faster germination rate with early, mild dehydration suggests that
germination-directed physiology can continue to some extent during dehydration episodes that are not too severe.
When dehydration was initiated just prior to radicle
emergence, rapid dehydration (-150 MPa) resulted in
subsequent germination that was delayed and desynchronized; a mild gradient (-4 MPa) resulted in accumulation
of germination processes as with early dehydration. If
seeds were exposed to a 24-hour mild dehydration episode
prior to exposure to -150 MPa, delayed germination due
to severe dehydration was completely or partially avoided
(figs. 3, 4).
As with continuously hydrated seeds, there were significant differences in seed response to dehydration episodes depending on species and ecotype. Dehydration at
-150 MPa had less of a delaying, desynchronizing effect
on subsequent germination of cheatgrass seeds than on
squirreltail or bluebunch wheatgrass. Cheatgrass from
the low-elevation site had a slower germination rate with
Seeds Exposed to a Dehydration Episode-Seeds
exposed to early dehydration episodes required approximately the same number of hours in contact with liquid
water to germinate as did continuously hydrated seeds
(fig. 3). Thus, seeds of all species incrementally accumulated progress toward germination during periods when
water was not limiting. Early, mild dehydration appears to
act as a pause in the germination process, which resumes
217
studies were subjected to a single dehydration interruption, more extensive research on perennial ryegrass
(Lolium perenne) and annual bluegrass (Poa annua) (also
facultatively fall emerging) demonstrated that grass seeds
can progress toward germination across as many as 16
hydration-dehydration cycles (Allen 1990). With bluegrass, for example, germination could be delayed for more
than 200 hours by subjecting seeds to short (8 hour) hydration phases alternating with long (24 hour) dehydration phases at -10 MPa. Under these conditions, bluegrass seeds failed to germinate until exposed to at least
24 hours of continuous hydration. Multiple cycles also
had the effect of further synchronizing germination rates,
often to the point that all germination would occur during
a single hydration period of 16 or 24 hours.
Seed vigor and viability do not appear to be adversely
affected by rapid, severe dehydration. In related studies
using perennial ryegrass seeds, initiation of harsh dehydration just prior to radicle emergence had a negligible effect on seed viability and vigor as measured by electrolyte
leakage and seedling growth rate.
In cold-desert habitats, control of germination timing
in the field often operates through dormancy mechanisms
that restrict germination until late winter or early spring.
This minimizes the risk of seedling death due to inadequate moisture for establishment. However, germination
timing of facultatively autumn-emerging grasses is more
likely regulated through the rate at which germinationdirected physiology takes place. In contrast with crop
seed, where a slow germination rate is often considered
to be related to low seed vigor, slow germination in many
natural habitats may be adaptive as a deterrent to premature germination at permissive temperatures (Meyer and
others 1989). In the case of autumn germination, an afterripening requirement coupled with a slow rate that prolongs the germination process would expand the timeframe over which precipitation events leading to soil
moisture replenishment could occur. This would also help
limit autumn germination to periods with adequate soil
moisture, since a brief period of moisture availability
would be insufficient for seeds to complete germination.
Although we do not yet understand the important relationship between afterripening and hydration-dehydration
episodes that occur during summer conditions in the field
(especially superoptimal temperatures), our data support
the hypothesis that seeds can complete germination with
a noncontinuous moisture supply. The diurnal wave of solar radiation leads to considerable temperature and moisture fluctuation at the soil surface (Rose 1968), and seeds
may progress toward germination over a series of naturally mediated hydration-dehydration cycles. In the case
· of a precipitation event insufficient to allow completion
of germination, seeds at an early or intermediate stage
of germination would dehydrate, and progress toward germination would cease. With the following precipitation
event, seeds would resume normal progress toward germination or show a slight delay in germination rate (especially for seeds ofbluebunch and squirreltail), depending
on the rate at which the dehydration event occurred. Seeds
that dried rapidly at a late stage in the germination
100
-...8.
c
80
-&
60
ic
40
c
0
e..
CD
"
-
-
-
CarOIIous H)ldrallan
--e- Ealtl Delfjdral!on as -4 MPa
--e-- Ea1t1 Detr,Ga110n 111-150 MPa
~
20
Lell Dell)'lhllonll-150MPa
__.._. Lell Dqdra!!on:-4 MPatar
24h--150MPatar24h
0
I
0
50
100
150
200
250
®a
I
350
I
-.
400
Total Accumulated Hydration Hours
Figure 4-Germlnatlon rate curves for bluebunch wheatgrass seeds subjected to continuous hydration or an inte~ected dehydration
episode following an Initial hydration period of
16 hours (early) or 69 hours. Error bars indicate
the maximum standard error for each treatment.
continuous hydration, and was more strongly delayed/
desynchronized with late, harsh dehydration than seeds
of the same species from the high-elevation collection.
In contrast, seeds of squirreltail from the highest elevation source had the slowest germination rate for this species, while seeds from the low-elevation site germinated
faster. Still, the high-elevation squirreltail seed was more
delayed by dehydration interruptions than was cheatgrass
seed, as illustrated by the number of treatments with
slower germination than continuously hydrated seeds.
While only one collection ofbluebunch was available
for this study, its germination response was similar to
that of squirreltail. Continuously hydrated seeds had a
slow germination rate relative to cheatgrass, and most
dehydration treatments slowed germination appreciably.
Unlike the other two species, delayed germination of
bluebunch seeds due to late, harsh dehydration was only
slightly eliminated by a 24-hour pretreatment at -4 MPa.
DISCUSSION
In addition to incubation temperature and degree of
seed afterripening, seed response to fluctuating water
content is an important variable in determining whether
autumn germination will occur. For all grasses included
in these studies, seeds progressed incrementally toward
germination with intermittent hydration. This is in
agreement with findings ofMott (1974), McKeon (1985),
and Wilson (1973), who demonstrated that precipitation
events inadequate for completing germination can speed
subsequent emergence in the field during a later precipitation event. The ecological importance of cyclic wetting
and drying in the field has also been emphasized for
Chihuahuan desert grassland and Sahel species (Elberse
and Bremen 1990; Frasier 1989). While seeds in these
218
process would show considerably delayed germination following rehydration, thereby decreasing the probability
that autumn germination and emergence would occur.
Within- and between-species variation observed with
cheatgrass and squirreltail may provide important clues
into seedling establishment strategies. Cheatgrass seeds
tend to respond to intermittent moisture with minimal delay in germination rate, especially if collected from a highelevation site. In contrast, squirreltail seeds from the
same site required longer to afterripen, were slower to
germinate with continuous hydration, and were more substantially delayed following dehydration episodes. Thus,
the high-elevation cheatgrass seeds would have a higher
probability of germinating in autumn than would squirreltail seeds, given the same seedzone conditions. Cheatgrass seeds from the low-elevation site, presumably with
a greater probability of experiencing drought during establishment, are more likely to postpone germination until winter or spring. Again, this prediction is based on a
slower rate of afterripening, a slower germination rate,
and a greater delay in germination rate with extreme
moisture fluctuations. In contrast, the low-elevation
squirreltail seeds would have the highest probability for
autumn germination for this species.
Field retrieval studies, using both multiple ecotypes and
contrasting field site conditions, should make it possible
to validate these laboratory-based predictions pertaining
to seed behavior of facultatively autumn-emerging species. Currently, fall seedings are usually attempted late
enough to preclude the possibility of autumn emergence.
For facultatively autumn-emerging species, however,
early fall plantings may be advantageous in years with
sufficient soil moisture for naturally mediated hydrationdehydration cycles in the seedzone.
Elberse, W. T.; Bremen, H. 1990. Germination and establishment of Sahelian rangeland species. II. Effects of
water availability. Oecologia. 85: 32-40.
Evans, R. A.; Young, J. A. 1972. Microsite requirements
for establishment of annual rangeland weeds. Weed
Science. 20: 350-356.
Frasier, G. W. 1989. Characterization of seed germination
and seedling survival during the initial wet-dry periods
following planting. Journal of Range Management. 38:
187-190.
Hull, A C.; Hansen, W. T. 1974. Delayed germination of
cheatgrass seed. Journal of Range Management. 27:
366-368.
McKeon, G. M. 1985. Pasture seed dynamics in a dry
monsoonal climate. II. The effect of water availability,
light and temperature on germination speed and seedling survival of Stylosanthes humilis and Digitaria
ciliasi. Australian Journal ofEcology. 10: 149-163.
Meyer, S. E.; McArthur, E. D.; Jorgensen, G. L. 1989.
Variation in germination response to temperature in
rubber rabbitbrush (Chrysothamnus nauseosus) and
its ecological implications. American Journal of Botany.
76: 981-991.
Mott, J. J. 1974. Factors affecting seed germination in
three annual species from an arid region of western
Australia. Journal of Ecology. 62: 699-709.
Rose, C. W. 1968. Water transport in soil with a daily
temperature wave. I. Theory and experiment. Australian Journal of Soil Research. 6: 31-44.
Wight, J. R.; Hanson, C. L. 1987. The simulation of soil
temperature and water regimes associated with seedling establishment on rangeland soils. In: Frasier,
G. W.; Evans, R. A., eds. Proceedings of symposium on
seed and seedbed ecology of rangeland plants. U.S. Department of Agriculture, Agricultural Research Service.
Springfield, VA: National Technical Information Service: 93-96.
Wilson, A M. 1973. Responses of crested wheatgrass
seed to environment. Journal of Range Management.
26:43-46.
Young, J. A; Evans, R. A. 1977. Squirreltail seed germination. Journal of Range Management. 30: 33-36.
REFERENCES
Allen, P. S. 1990. Germination of perennial ryegrass and
annual bluegrass subjected to hydration-dehydration
cycles. Minneapolis: University of Minnesota. 100 p.
Thesis.
Allen, P. S.; White, D. B.; Russer, K.; Olson, D. 1992. A
system for controlling water potential in seed germination research. HortScience. 27: 364-366.
219