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TEMPERATURE PROFILES FOR
GERMINATION OF CHEATGRASS
VERSUS NATIVE PERENNIAL
BUNCHGRASSES
Ellen Martens
Debra Palmquist
James A, Young
ABSTRACT
Profiles for germination in relation to temperature were
developed for seeds of cheatgrass (Bromus tectorum), other
introduced brome species (B. mollis, B. japonicus, B. brizaeformis, B. rubens, and B. diandrus), and three native bunchgrass species (Stipa thurberiana, Festuca idahoensis, and
Elytrigia spicata). Seeds were incubated at 55 constant or
alternating temperatures from 0 oc to 40 °C. Cheatgrass
seeds were highly germinable over a wide range of constant
and alternating incubation temperatures. The germination
of seeds of other brome species and of the native bunchgrasses was much more limited.
INTRODUCTION
Seeds (caryopses) and germination are obviously a key
portion of the ecology of an annual such as cheatgrass
(Bromus tectorum L.). There are three aspects of the seed
ecology of this species that are critical. These factors are
simultaneous and continuous germination and soil-litter
seedbanks.
The inherent potential of the physiologic systems of cheatgrass seeds to support germination must interact with the
potential of seedbeds to support germination. Cheatgrass
is very plastic phenotypically. The species is established
over an immense expanse of diverse habitats in North
America. Ecotypic variation has been demonstrated for
the species. Variable habitats and perhaps genotypes have
led to conflicts in interpreting the seed ecology of the species. Most of these conflicts can be easily accounted for and
interpreted if genetic and environmental differences are
understood.
In the western Great Basin, cheatgrass only germinates
in the fall once every 5 years. Germination normally occurs
in very early spring when seedbed temperatures are at or
near freezing during a portion of each diurnal fluctuation
(Evans and others 1970). Competition in such seedbeds is
primarily for available soil moisture. A method of partially escaping this competition would be to germinate
when temperatures are extremely low during the winter.
In the Great Basin environment most precipitation occurs
during the midwinter months, almost completely out of
phase with temperatures that permit growth (Houghton
and others 1972). Considering the limited potential of Intermountain area seedbeds to support germination and
establishment of seedlings, it has long been noted that
species that have seeds that germinate simultaneously
and at the earliest possible time have the highest and
most consistent opportunity for success.
Simultaneous germination is apparently a great advantage for cheatgrass, but the extreme variability in the frequency of moisture events in temperate desert environments presents the risk that all the seeds would germinate
in one flush and then die due to drought. The reciprocal
of simultaneous germination is continuous germination.
Cheatgrass accomplishes continuous germination through
seeds acquiring dormancy in the field (Young and others
1969). The duration of this acquired dormancy can be influenced by the duration of moist incubation and gibberellin enrichment of the substrate.
Remember that cheatgrass seeds are only going to acquire dormancy if they are not in a seedbed environment
that permits simultaneous germination. The position of
cheatgrass seeds in seedbeds in relation to litter and microtopography largely controls simultaneous germination.
Litter and microtopography influence extremes in temperature and relative humidity. Therefore, temperatures at
which cheatgrass seeds will or will not germinate become
important in defining simultaneous germination and partially in the induction of acquired dormancy and continuous germination. A second aspect of this research is how
germination of seeds of native perennial species and other
annual grasses compares to germination of cheatgrass
seeds.
Our purpose was to compare germination temperature
profiles for various sources of cheatgrass and other perennial and annual species adapted to similar environments.
METHODS AND MATERIALS
Paper presented at the Symposium on Ecology, Management, and Restoration of Intermountain Annual Rangelands, Boise, ID, May 18-22, 1992.
Ellen Martens is a Biochemistry Technician, Debra Palmquist a
Mathematician/Statistician, and James A Young a Range Scientist, Agricultural Research Service, U.S. Department of Agriculture, 920 Valley
Road, Reno, NV 89512.
We compared the germination of collections of seeds of
cheatgrass from Pyramid Lake and Edwards Creek Valley,
NV. Germination-temperature profiles were also obtained
238
Table 1-Definition and explanation of parameters calculated from germination profiles
Germination parameter
1. Mean germination
Purpose
Calculation
l: germination
Simple, single-factor parameter that can be compared
statistically.
55
2. Mean germination of
temperature regimes
with some germination
l: germination
No. regimes with germination
High value compared with overall mean germination
restricted to specific temperature regimes.
3. Regimes with some
germination
No. regimes with germination
Indicates b.readth of response in relation to temperature.
55
By Itself, this is a difficult parameter to interpret.
A large number of regimes with optimum germination Is
ideal, if the mean germination of the optima is high.
4. Regimes with optimum
germination
No. regimes with optimum germination
5. Optimum germination
max germination - (confidence Interval + 2)
6. Mean of optima
7. Maximum germination
8. Frequency of
optima
55
This definition of optimum relates statistically,
through the confidence interval, to means associated
with the maximum observed germination. We use the
narrow definition of one-half the confidence interval
to limit the number of optima.
Provides measure of temperature regimes that support
highest germination not markedly different from maximum.
optima
No. of optima
Gives measure of ultimate potential to germinate under
ideal conditions.
Mean with highest germination
Gives a precise measure of what is optimum temperature
for germination of cultivar or group of cultivars.
No. times temp. reg. supports optimum germ.
No. sources tested
for the annual brome species, rattlesnake grass {B. brizaeformis [F. & M.]), Japanese chess (B. japonicus Thurb.),
foxtail chess {B. rubens L.), ripgut (B. diandrus Roth.), and
soft chess (B. mollis L.). These annual grass seed germination profiles were compared to those of bluebunch wheatgrass (Elytrigia spicata [Pursh] D.R. Dewey), Idaho fescue
(Ji'estuca idahoensis Elmer) and Thurber's needlegrass (Stipa thurberiana Piper) from previously published sources.
Seeds of the brome species were collected from naturalized
stands and stored in the laboratory until tested.
Seeds of each source were placed on one thickness of germination paper (blotter paper without germicides) in closed
petri dishes and kept moist with tap water during incubation in dark germinators. The seeds were considered germinated when the radicle emerged 2 mm. Germination
counts were made after 1, 2, and 4 weeks of incubation.
· ~e used four replications of 25 seeds each, arranged in a
randomized block design. Seeds were incubated at 0, 2, 5,
and 5 oc increments through 40 °C, and at alternating temperature regimes consisting of 16 hours at each cooler temperature and 8 hours at each warmer temperature daily.
For example, 0 oc alternated with 2, 5, 10, 15, 20, 25, 30,
35, and 40 oc, but 35 °C alternated with 40 oc only (Young
and Evans 1973). Where multiple collections were available, we have taken the mean of the data for each species.
To present a broader spectrum, for the last section on
frequency of optima, we used published and unpublished
data from our files on other collections of each of the species discussed in this paper. We used a total of 24 collections of Bromus and 19 other grasses.
Quadratic response surface analysis was used to analyze
seed germination data in relation to constant and alternating temperatures (Evans and others 1982). Several
239
parameters were calculated from the response surfaces
to help interpret the germination response (table 1). Response surface comparisons, among species or collections,
were made using procedures developed by Palmquist and
others (1987). The response surfaces were compared at
different categories of seedbed temperatures (fig. 1), which
were derived from microenvironmental monitoring of seedbed conditions in the early spring in the western Great
Basin (Evans and Young 1970).
CHEATGRASS GERMINATION
PROFILE
The seeds of both collections of cheatgrass were highly
germinable at a wide range of constant and alternating
.
.c:
0
G
l
u
l,..
e
0
!l
E
~
I:.
!!
8
Figure 1-Average germination (percent) for
general categories of seedbed temperatures.
Table 2-Temperature-germination profile of estimated percent germination for cheatgrass from Pyramid Lake, 19891
Warm period 8 h oc
Cold period
16h
oc
0
2
5
10
15
20
25
30
35
40
0
281
(5)
2
5
10
15
20
25
30
35
40
83(4)
86(4)
85(4)
89(3)
92{4)
87(3)
91(3)
96(2)
97(4)
87(4)
92(3)
98(2)
100(3)
97(4)
85(4)
91(3)
98(3)
100(2)
100(3)
90(4)
82(4)
88(3)
96{3)
100(3)
100(3)
93(3)
78(4)
76(3)
83(3)
92(2)
100(2)
100(3)
94(3)
80(3)
59(4)
69(4)
n(3)
86{3)
96(3)
99(3)
94{3)
82(3)
62(3)
35(5)
59(6)
68(5)
79(4)
90(4)
95{4)
91(4)
81(4)
63(4)
37(5)
4(7)
1
Temperature-germination profile for Edwards Creek was very similar.
in parentheses are one-half the confidence interval for the germination estimate (p .. 0.05).
2 Numbers
Table 3-Germination of chaatgrass and three other native perennial bunchgrasses
Species
Optimum
germination
Temp. with
some germ.
Temp. with
opt. germ.
Mean of
optima
--------------------Peroent-------------------Cheatgrass
Bluebunch wheatgrass
Idaho fescue
Thurber's needlegrass
1Mean
1
97
90
81
25
100
95
87
80
22
13
7
5
96
86
77
24
of two sources (Pyramid Lake and Edwards Creek).
CHEATGRASS VERSUS NATIVE
BUNCHGRASSES
incubation temperatures (table 2). The Pyramid Lake and
Edwards Creek Valley collections of cheatgrass had maximum germination of 100 and 94 percent, respectively
(table 3). For seeds produced under semiarid wildland conditions these are both highly viable seed collections. Some
germination occurred at 100 percent of the 55 temperatures
tested for the two collections. Optimum germination (see
table 1 for definition of optimum) occurred at 22 percent
of the temperature regimes of both sources. The mean germination of the optima temperature regimes for cheatgrass
was 96 percent.
In all categories of seedbed temperatures the cheatgrass
collections had markedly higher estimated germination
than the three native perennial bunchgrasses (table 4).
The only statistical overlap occurred at moderate temperatures where the germination of seeds ofbluebunch was not
significantly {p 0.05 percent) lower than the cheatgrass
collections. At very cold, cold, cold fluctuating, fluctuating, and warm temperatures the estimated germination
of seeds of the native bunchgrasses is significantly lower
{p =0.05) than the cheatgrass collections.
=
PERENNIAL GRASS GERMINATION
PROFILES
OTHERBROMUSSPECmS
PROFILES
The germination profiles ofbluebunch wheatgrass and
Idaho fescue showed a maximum germination of90 and 81
percent, respectively, with some germination at 95 and 87
percent of the 55 temperature combinations (table 3). The
mean of the optimum temperature regimes for germination
was 96 percent for the cheatgrass selections and 86 and 77
for bluebunch wheatgrass and Idaho fescue, respectively.
The number of temperature regimes supporting optimum
germination dropped to 13 and 7 percent as compared to
the 22 percent for cheatgrass. Thurber's needlegrass was
the lowest germinator of the group tested, having a maximum germination of 25 percent. Although 80 percent of
the temperature combinations showed some germination,
only 5 percent of the temperature regimes supported optimum germination, with a mean of optima of 24 percent.
Rattlesnake grass and Japanese chess collections produced the highest maximum germination of the Bromus
species tested (table 5). The maximum observed germination for both the rattlesnake grass and Japanese chess was
100 percent for all collections tested and optimum germination occurred in 45 percent of the temperature regimes
for rattlesnake grass and 35 percent of the temperature
regimes for seeds of the Japanese chess collections. Some
germination occurred at 98 and 97 percent of all temperature regimes tested. The mean of optima for both was 99
percent.
Germination profiles for seeds offo~ chess were similar to those for collections of cheatgrass seeds (table 5).
240
Table 4-Comparison of response surfaces for germination-temperatures for seed of two collections of cheatgrass and three native perennial bunchgrass1
Collection
Very
cold
Cold
Cheatgrass
Pyramid Lake
Edwards Creek
Bluebunch wheatgrass
Idaho fescue
Thurber's needlegrass
84a
70a
13b
7b
4b
91a
79b
46c
32c
9d
Seedbed temperature catego!}
Cold
Fluctuating
Moderate
fluctuating
78a
76a
34b
17b
3c
90a
83a
95a
90a
78ab
SOb
SOb
18c
2d
16c
Warm
57b
73a
31c
19c
4d
1Estimated
germination means followed by the same letter within columns are not signifiCantly different at the 0.05 laval of
probability as determined by overlap of the confidence intervals.
Table 5--Germination of cheatgrass and five other annual brome species
Species
Cheatgrass
Rattlesnake grass
Japanese chess
Foxtail chess
Ripgut
Soft chess
1Values
Optimum
germination
Temp. with
some germ.
Temp. with
opt. germ.
Mean of
optima
--------------------Pe~nt-------------------197
100
1100
198
81
66
100
98
97
97
84
100
22
45
35
18
18
25
96
99
99
97
77
63
are means of multiple collections.
compared to cheatgrass. Estimated germination of seeds
of ripgut and soft chess was not significantly different from
cheatgrass at only one temperature category each. Ripgut
did comparably well at the cold and soft chess did comparably well at the warm category of seedbed temperature as
compared to the cheatgrass collection (table 6).
Foxtail chess seeds had a maximum germination of 98
percent. There was some germination at 97 percent of the
55 temperature regimes tested, but only 18 percent of the
temperatures regimes supported the optimum germination. The mean germination for the optimum temperature
regimes was 97 percent. In comparison, the cheatgrass
collections tested had an average maximum germination
of 97 percent, with 22 percent of the temperature regimes
supporting optimum germination. The mean of the optima
was 96 percent.
Ripgut had maximum germination of 81 percent with 18
percent of the temperature regimes supporting optimum
germination (table 5). Some seeds germinated at 84 percent of the temperatures tested. The mean optimum germination was 77 percent. The poorest germinator of the
Bromus species tested was soft chess. The maximum germination was 66 percent with 25 percent of the temperature regimes supporting optimum germination. The mean
optimum germination was 63 percent. Some germination
occurred at 100 percent of the temperature regimes tested.
OPTIMUM TEMPERATURE REGIMES
The most frequent optimum germination temperature
regimes, those regimes where optimum germination occurred 80 percent of the time, for all the plant material
tested (15 collections) were 10/20 and 15/20 oc (table 7).
For the two collections of cheatgrass tested for this paper,
the most frequent temperatures (occurring 100 percent
of the time) for optimum germination were 10/15 through
10/30 and 15/15 through 15/35 °C. For a broader spectrum
of cheatgrass collections (10 collections), the most frequent
optima, occurring 80 percent of the time, occurred at 10/20,
10/25, and 15/20 oc (table 8).
The species of Bromus besides cheatgrass (14 collections) had most frequent optimum germination, occurring
93 percent of the time, at 10/20 and 15/20 oc temperatures
(table 9).
One collection of each of the native perennial grasses
was tested. The most frequent temperatures for optimum
germination from these data were: bluebunch wheatgrass
10/20, 10/25, 15/20 through 15/30, 20/25, and 20/30 oc;
Idaho fescue 15/15, 15/20, 20/20, and 20/25 oc; and for
Thurber's needlegrass 15/15, 20/20, and 25/25 oc (table 10).
Bluebunch wheatgrass and Idaho fescue had an optima of
15/20 and 20/25 in common, and Thurber's needlegrass and
RESPONSE SURFACE COMPARISONS
Over all seedbed temperature categories, estimated germination of rattlesnake grass seed was not significantly
(p = 0.05) different from that of cheatgrass. There was no
significant difference between Japanese chess and cheatgrass at five of the six categories. Significant differences
occurred only under the fluctuating temperature regime
category, with cheatgrass higher. Seeds of foxtail chess
had no significant differences in germination at very cold,
cold, and moderate categories of seedbed temperatures
241
Table &-Comparison of response surfaces for biological areas of germination-temperatures for seed
of two collections of cheatgrass and five other Bromus species1
Collection
Very
cold
Cold
Cheatgrass
Pyramid Lake
Edwards Creek
Rattlesnake grass
Japanese chess
Foxtail chess
Ripgut
Soft chess
84a
70ab
87a
51 be
84a
18cd
t1d
91a
79bc
99a
86ab
95a
58cd
36d
Seedbed temperature category
cold
fluctuating
Fluctuating
Moderate
78a
76a
75ab
63abc
58bc
43c
49c
90a
83ab
74bc
63cd
52d
21e
52d
Warm
95ab
90ab
96a
96ab
85bc
57b
73a
49bc
51 be
34c
9d
45bc
65cd
61d
'Estimated germination means followed by the same letter within columns are not significantly different at the
0.05 level of probability as determined by overlap of the confidence intervals.
Table 7-Frequency of optima (percent) for 15 collections of plant material tested
Cold period
16h°C
0
2
0
2
5
10
15
20
25
30
35
5
10
7
7
27
33
27
13
Warm ~rlod 8 hOC
15
20
53
60
67
67
53
47
53
67
80
80
60
25
30
35
53
60
20
40
7
33
40
73
73
67
7
40
7
40
Table &-Frequency of optima (percent) for 10 collections of cheatgrass
Cold period
16h°C
0
2
5
10
15
20
25
30
35
0
2
5
10
10
10
10
10
20
20
20
20
Warm period 8 h oc
15
20
20
30
40
50
40
10
10
50
80
80
40
25
30
35
30
30
80
70
60
10
10
10
60
70
40
40
10
40
10
10
10
40
Table 9-Frequency of optima (percent) for 14 collections of Bromus, excluding cheatgrass
Cold period
16 hoc
0
2
5
10
15
20
25
30
35
40
0
2
5
10
7
7
36
50
36
7
Warm period 8 h oc
20
15
64
79
86
79
43
242
57
64
86
93
93
....M..
25
30
35
40
14
43
50
86
86
50
14
7
7
7
57
64
36
14
7
7
7
7
7
21
14
7
7
7
7
7
7
14
7
Table1G-Optimum germination for bluebunch wheatgrass,ldaho fescue, and Thurber's needlegrass
(1 = Bluebunch wheatgrass, 2 = Idaho fescue, 3 = Thurber's needlegrass)
Cold period
16 hoc
0
2
5
10
Warm period 8 h oc
15
20
25
30
35
40
0
2
5
10
15
20
25
1
2,3
1,2
2,3
1
1,2
3
30
35
40
Table 11-Frequency of optima (percent) for seven collections of bluebunch wheatgrass and 11 collections
of Idaho fescue (in bold)
Cold period
16h oc
0
2
5
10
15
20
25
0
2
5
10
18
Warm period 8 h oc
15
20
25
30
35
1427
4318
579
57 91 100 100 100 91
8691 100100
5755
10045
10036
43
18
18
40
30
35
40
Idaho fescue had 15/15 and 20/20 in common, but there were
no temperature regimes that gave optimum germination for
all three collections.
With data. from our earlier files, we compared seven collections ofbluebunch wheatgrass and found 100 percent of
them gave an optimum germination at five temperature regimes: 15/20 through 15/30, 20/25, and 20/30 °C. For Idaho
fescue, we found 100 percent of the 11 collections gave optimum germination at the 15/20 and 20/25 °C temperature
regimes. The most frequent optimum incubation temperatures for bluebunch wheatgrass and Idaho fescue are 15120
and 20125 oc (table 11).
CONCLUSIONS
The only temperature regime that seems to be the best
for all Bromus species as well as bluebunch wheatgrass
and Idaho fescue is 15/20 °C. All of the above species had
optimum germination occurring in one or more other temperature regimes, but not the same ones. Cheatgrass does
germinate at a wider range of temperatures than the other
plants and at a greater percent germination within that
range.
243
REFERENCES
Evans, R. A; Easi, D. A; Book, D. N.; Young, J. A 1982.
Quadratic response surface analysis of seed-germination
trial. Weed Science. 30:411-416.
Evans, R. A.; Holbo, H. R.; Eckert, R. E., Jr.; Young, J. A
1970. Influences of weed control and seeding methods on
the functional environment of rangelands. Weed Science.
18: 154-162.
Evans, R. A; Young, J. A. 1970. Plant litter and establishment of alien annual species in rangeland communities.
Weed Science. 18: 697-703.
Houghton, J. G.; Sakameto, C. M.; Gifford, R. 0. 1972.
Nevada's weather and climate. Spec. Publ. 2. Reno, NV:
Nevada Bureau of Mines and Geology, Mackay School of
Mines, University of Nevada, Reno. 78 p.
Palmquist, D. E.; Evans, R. A.; Young, J. A. 1987. Comparative analysis of temperature-germination response
surfaces. In: Frasier, G. W.; Evans, R. A., eds. Seed and
seedbed ecology of rangeland plants: Proceedings; 1987
April21-23; Tucson, AZ. Washington, DC: U.S. Department of Agriculture: 97-103.
Young, J. A.; Evans, R. A.; Eckert, R. E., Jr.1969. Population dynamics of downy brome. Weed Science. 17: 20-26.
Young, J. A.; Evans, R. A.; Kay, B. L. 1973. Temperature
requirements for seed germination in an annual type
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