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GERMINATION ENHANCEMENT OF
PERENNIAL GRASSES NATIVE TO
THE INTERMOUNTAIN REGION
Stuart P. Hardegree
Heydecker 1977; Heydecker and Coolbear 1977). In almost all cases, positive priming effects are expressed to
the greatest degree at temperatures normally suboptimal
for germination (Heydecker and others 1975). Previous
studies on intermountain grass species have shown that
prehydration treatments have a beneficial effect on subsequent germination rates (Bleak and Keller 1972; Griswold
1936; Keller and Bleak 1968). These previous studies,
however, were not designed to control hydration at
subgermination water content.
The purpose of this paper is to outline the development
of seed priming treatments for enhancing native-plant
germination response to low temperature. These studies
include the determination of optimal priming conditions
for seed treatment, an evaluation of the effects of drying
back primed seeds, and a comparison of native seed germination response to that of untreated cheatgrass.
ABSTRACT
Seed priming can be used to enhance germination rates
of native perennial grasses relative to untreated cheatgrass. A matric-priming technique was used to enhance
low-temperature germination rates of seven native perennial grasses. The median germination time at 10 oc was
reduced by as much as 8 days. Priming increased germination rates of several species to a level comparable to
cheatgrass. Drying back after priming reduced, but did
not eliminate, the priming effect.
INTRODUCTION
Native perennial grasses and shrubs have been replaced by cheatgrass (Bromus tectorum L.) over large
areas of the Intermountain West (Young and others
1987). One of the mechanisms that may contribute to
the success of cheatgrass is its ability to germinate and
establish a root system at relatively low temperatures
early in the spring (Harris and Wilson 1970; Wilson and
others 1974; Young and Evans 1982). In an extensive
study of germination response to temperature, Young and
Evans (1982) predicted that germination advancement of
even a few days might make a difference in establishment
success of native perennial grasses that are in competition
with cheatgrass.
Seed priming is a technique by which seeds are partially hydrated to a point where germination processes
begin, but radicle emergence does not occur (Bradford
1986; Heydecker and Coolbear 1977). Seeds that have
been primed exhibit more rapid germination and emergence, greater germination uniformity, and sometimes
higher total percent germination (Brocklehurst and others
1984; Heydecker and Coolbear 1977). Germination enhancement has been variously attributed to metabolic repair processes (Bray and others 1989; Burgass and Powell
1984), a buildup of germination metabolites (Coolbear and
others 1980; Khan and others 1978), osmotic adjustment
(Bradford 1986), and to a simple reduction in the lag time
for imbibition (Bewley and Black 1982; Brocklehurst and
Dearman 1983; Heydecker 1977). The magnitude of germination enhancement depends on the priming medium,
priming water potential, priming duration, and whether
the seeds are redried after priming (Bradford 1986;
PLANT MATERIALS
Germination response to priming was determined for
bluebunch wheatgrass (Agropyron spicatum [Pursh]
Scribn. and Smith), thickspike wheatgrass (Agropyron
dasystachyum [Hook.] Scribn.), basin wildrye CElymus
cinereus Scribn. and Merr.), sheep fescue (Ji'estuca ovina L.),
canby bluegrass (Poa canbyi Scribn.), Sandberg bluegrass
(Poa sandbergii Vasey), and bottlebrush squirreltail
(Sitanion hystrix [Nutt.] J.G. Smith). These grasses have
been identified by the Bureau of Land Management, U.S.
Department of the Interior, as high-priority species for
restoring native plant diversity in the Great Basin and
on the Columbia River Plateau. Primed seeds were compared to nonprimed seeds of the same species and to nonprimed seeds of three accessions of cheatgrass collected
in southwestern Idaho.
PRIMING/GERMINATION SYSTEM
The most common osmotic and solid-matrix seedpriming systems involve intermixture of seeds with the
priming medium (Heydecker and Coolbear 1977; Taylor
and others 1988). This causes problems for subsequent
seed handling, seed water content determination, and
oxygen availability during priming (Hardegree and
Emmerich 1992). In our experiment, seeds were primed
in a priming/germination vial designed for matric-water
potential control (Hardegree and Emmerich 1992). In this
system, the seeds are separated from an osmotic solution
of high molecular weight polymer (polyethylene glycol
8000; PEG) by a cellulose membrane that has a low molecular weight exclusion limit. The membrane excludes
Paper presented at the Symposium on Ecology, Management, and Restoration of Intermountain Annual Rangelands, Boise, ID, May 18-22, 1992.
Stuart P. Hardegree is Plant Physiologist, U.S. Department of Agriculture, Agricultural Research Service, Northwest Watershed Research Center, 800 Park Blvd., Plaza IV, Suite 105, Boise, ID 83712.
229
PEG from contact with the seeds and, therefore, provides
a matric-potential control surface for seed equilibration.
The matric potential of the membrane surface is determined by the osmotic potential of the solution with which
it is in equilibrium. A detailed description of the priming!
germination vial and matric-priming procedure is given
by Hardegree and Emmerich (1992).
00
90
c:
80
0
iic:
·e...
-...
40
30
a..
20
CD
C!J
c:
OPTIMAL PRIMING CONDITIONS
CD
0
CD
The basis of all seed-priming treatments is to equilibrate seeds at a water potential that allows initiation of
positive germination processes but prevents radicle emergence (Heydecker and Coolbear 1977). Optimal priming
solutions are usually determined to be at the least negative water potential that does not result in germination
during treatment (Dell'Aquila and Tritto 1990; Evans and
Pill1989).
In the current experiment, optimal priming water potential at 25 oc was estimated by determining the germination response of each species to reduced water potential. PEG was mixed with water to yield seven solutions
over the water potential range of 0 to -2.5 MPa using
equation 4 of Michel (1983) as suggested by Hardegree
and Emmerich (1990). Seeds were deposited on the membrane surface in individual germination/priming vials and
allowed to equilibrate for 14 days. Seeds were considered
germinated and were removed and counted when they
exhibited radicle extension of ~2 mm. Germination vials
were maintained in a controlled temperature room at
25 oc under both fluorescent and incandescent light for
12 h/day. The cellulose membranes were treated with a
light dusting of fungicide powder (Daconil) at the beginning of the experiment. Thirty-five seeds comprised each
treatment replicate for the relatively small Poa and Festuca species. Thirty seeds per replicate sample were used
for all other species. Each treatment was replicated six
times with each vial in a different randomized block
within the controlled temperature room.
Priming water potential was estimated to be the least
negative water potential that did not result in germination after 14 days. Figure 1 represents the germination
response for canby bluegrass and thickspike wheatgrass.
Table 1lists the total germination percent and estimated
optimal priming water potentials for each species.
70
60
50
10
0
0.0
-o.s
-1.0
-1.5
-2.0
Water Potential (MPa)
Figure 1-Total percent germination for canby
bluegrass {e) and thlcksplke wheatgrass (o) as
a function of water potential. Optimal priming
water potential was estimated to be the least
negative water potential that resulted In zero
germination after 14 days at 25 °C.
Table 1-Estimated optimal priming water potentials determined
from seeds germinated for 14 days at 25 °C, and total
percent germination in the 0 MPa water potential
treatments
Species
Agropyron spicatum
Agropyron dasystachyum
Elymus cinsrsus
Fsstuca ovina
Poa canby/
Poa sandbsrgil
Sitanion hystrix
Priming water
potential (MPa)
-2.5
-2.0
-2.5
-2.0
-1.0
-1.0
-1.5
Total percent
germination
90
97
63
92
86
71
65
replicated five times for the native species and 10 times
for each cheatgrass accession. The priming experiment
was carried out in November 1991.
Days to 50 percent of total germination (D60) was determined for each germination vial as an index of germination rate. Means and standard error values of the
optimal-priming treatments are listed in table 2. Primed
seeds germinated as much as 8 days sooner than nonprimed seeds at 10 oc (table 2). Nonprimed native seeds
germinated more slowly than cheatgrass at 10 °C, but
several species germinated at a rate comparable to cheatgrass after priming (table 2).
PRIMING EFFECTS
Seeds were primed for 2, 4, 6, or 8 days at 25 oc at
the estimated optimal priming water potentials listed in
table 1. Date of priming initiation was staggered so that
all priming treatments terminated on the same day.
Primed seeds were switched to priming/germination vials
containing pure water and subsequent germination response at 10 oc was monitored for 21 days. A set of nonprimed control treatments were initiated at the same time
that primed seeds were switched to pure water. These
control treatments also included three cheatgrass seed
lots collected in July 1991 from three sites in southwestem Idaho: Ada County, near Orchard, Ten-Mile Creek,
and Kuna Butte. Priming and control treatments were
DRYING AFTER PRIMING
Germination response of freshly primed seeds is dependent on seedbed conditions at the time of planting.
Germination advancement of the magnitude fo11;Dd in the
current experiment assumes optimal field conditions at
230
Table 2-0ays to 50 percent of total germination for optimal
duration treatments and for conrol treatments of native
grasses and cheatgrass (standard error in parentheses)
Species
A. spicatum
A. dasystachyum
E. cinereus
F. ovina
P. canbyi
P. sandbergii
S. hystrix
B. tectorum
Treatment
duration
(days)
Control
seeds
(Dao)
Primed
seeds
(Dao)
8
6
8
8
6
4
8
7.8 (2.1)
7.9 (0.8)
14.8 (2.4)
8.7(1.1)
11.6 (1.8)
13.6 (1.1)
13.2 (1.6)
3.2 (0.2)
4.0 (0.5)
11.4 (2.0)
4.2 (0.3)
6.8 (1.4)
5.4 (0.3)
5.6 (0.5)
Kuna
Orchard
Tenmile
for thickspike wheatgrass. Drying back resulted in an
average loss of 35 percent of the priming effect on D60 •
This loss of priming effect ranged from 20 percent for
bluebunch wheatgrass to 65 percent for basin wildrye.
Some of the priming effect results from a reduction in the
lag time of imbibition, which is lost when the seeds are
dried back. Germination advancement of primed and
dried-back seeds over nonprimed control seeds, however,
indicates that metabolic seed processes are responsible for
the bulk of the priming effect.
DISCUSSION
Matric-priming has been shown to advance germination
in laboratory experiments, but development of a practical
application requires further study. One limitation of
matric-priming is that the technique as described by ·
Hardegree and Emmerich (1992) is unsuitable for scaling
up to handle bulk seed quantities. Matric-priming, however, is thermodynamically equivalent to simple water
addition to subgermination water content (Heydecker
and Coolbear 1977). We anticipate that matric-priming
will be used only to establish the optimal priming conditions of water potential, temperature, and treatment duration. The appropriate seed water content for optimal
priming response can then be achieved by gravimetric
techniques such as those described by Gray and others
(1990) and Heydecker and Coolbear (1977). We predict
that our current results do not necessarily reflect the
maximum obtainable level of germination enhancement
for these species. Other combinations of priming temperature, water potential, and duration may yield better
results, as our treatment conditions were semiarbitrary.
Seed priming of native perennial grasses may reduce,
somewhat, the competitive advantage of cheatgrass during early spring establishment. All of the experiments described here, however, are limited by the artificial nature
of the laboratory procedure. We have not yet determined
germination response under field conditions of variable
temperature and moisture. Germination rate is also only
one of many environmental factors affecting establishment success of native grasses. Measurable benefits from
seed treatment may be possible only in conjunction with
appropriate seedbed preparation. The most important
requirement of any priming system will be to coordinate
planting with appropriate conditions of seedbed microclimate. Planting under optimal conditions for germination
may not be possible as these conditions may preclude the
use of heaVY planting equipment. Field application of
priming treatments may, therefore, be limited to use of
seeds that have been redried before planting.
4.0 (0.3)
3.6 (0.3)
4.0 (0.8)
the time of planting. Since primed seeds are metabolically
active, it is not feasible to store them for long periods of
time if seedbed conditions are not suitable for rapid germination. It is, therefore, desirable to determine whether
primed seeds can be dried back without losing the priming
effect.
Seeds of each species were primed as before except that
only a 7-day priming duration was used. One set of seeds
was switched to pure water immediately and monitored
for germination for 21 days. A second set was air-dried for
1 week and then switched to pure water for a determination of germination response. A control set of nonprimed
seeds was initiated at the start of each germination run
and each treatment was replicated nine times. This experiment was carried out in April1992.
Figure 2 shows the cumulative germination response of
freshly primed, primed/dried back, and control treatments
1.0 , . . . - - - - - - - - - - - - - - - - - - - ,
c:
.2
0.8
·e....
0.6
m
c:
m
(!)
m
.~
a;
0.4
'3
E
::::J
0
0.2
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0
3
9
6
12
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15
Days
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231
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232