Differences in Temperature Dependence of
Respiration Distinguish Subspecies and
Hybrid Populations of Big Sagebrush:
Nature Versus Nurture
Bruce N. Smith
Susan Eldredge
David L. Moulton
Thomas A. Monaco
Angela R. Jones
Lee D. Hansen
E. Durant McArthur
D. Carl Freeman
Abstract—Artemisia tridentata ssp. vaseyana grows at slightly
higher, cooler, and drier sites than does A.t. ssp. tridentata. Natural
hybrids between the two subspecies are found in Salt Creek Canyon
near Nephi, Utah where the parent populations are separated by
85 m in elevation and 1.1 km along the transect. In 1993, three
gardens were established with seedlings from five populations
along the transect planted in each garden. From 1995 to 1998,
physiological measurements were made using carbon isotope ratios,
chlorophyll a fluorescence and microcalorimetry. Significant differences were observed among the gardens whatever the source of
origin and among the plant sources in whichever garden they were
grown. Both nature and nurture have an influence. Microcalorimetry has the power to predict optimal growth for a given population
even within this narrow range of environments as the temperature
dependence of heat rate and carbon dioxide evolution differed for
each population. Sagebrush grows slowly at cool temperatures. The
plants are stressed and growth ceases at temperatures much above
30°C.
Artemisia tridentata Nutt. or big sagebrush is one of the
most widespread and economically important shrubs in
western North America. The species ranges from western
Nebraska to eastern California and from British Columbia
to northern New Mexico (USDA 1937, 1988). Big sagebrush
and its subspecies have been prominent since the late
Tertiary or early Quaternary (Axelrod 1950). The present
distribution of the subspecies and contact zones between
In: McArthur, E. Durant; Ostler, W. Kent; Wambolt, Carl L., comps. 1999.
Proceedings: shrubland ecotones; 1998 August 12–14; Ephraim, UT. Proc.
RMRS-P-11. Ogden, UT: U.S. Department of Agriculture, Forest Service,
Rocky Mountain Research Station.
Bruce N. Smith is a Professor; Susan Eldredge and David L. Moulton are
undergraduate students; Thomas A. Monaco and Angela R. Jones are graduate students in the Department of Botany and Range Science; Lee D. Hansen
is a Professor in the Department of Chemistry and Biochemistry, Brigham
Young University, Provo, UT 84602. E. Durant McArthur is a Project Leader
and Research Geneticist at the Shrub Sciences Laboratory, Rocky Mountain
Research Station, USDA Forest Service, 735 North 500 East, Provo, UT
84606. D. Carl Freeman is a Professor in the Department of Biological
Sciences, Wayne State University, Detroit, MI 48202.
USDA Forest Service Proceedings RMRS-P-11. 1999
them were probably established at the end of the last
glaciation (Freeman and others 1991). Basin big sagebrush
grows at lower elevations than mountain big sagebrush.
Differences in volatile compounds between the subspecies
have also been noted (Weber and others 1994). Natural
hybrids between the subspecies often occur when parent
populations are in close proximity. Hybrids may also be
produced by controlled pollination (Graham and others
1995).
Success of parental subspecies and hybrids between them
in the face of environmental stresses is often assessed by
growth measurements and changes in morphology. A more
sensitive assessment could be made using physiological
parameters. Stem water potential and gas-exchange respiration have been measured in parental and hybrid big
sagebrush grown in common gardens (McArthur and others
1998). Significant differences were found among habitats
and source populations.
Carbon isotopic fractionation associated with degree of
stomatal closure in some circumstances has been correlated
with productivity (Condon and others 1987). Chlorophyll
fluorescence has proven to be a sensitive indicator of plant
stress (Guidi and others 1997, Loik and Harte 1996). Calorimetric measurements of the respiratory heat rates of plant
tissues, made simultaneously with measurements of gas
exchange rates, allow calculation of plant growth rates as a
function of temperature (Hansen and others 1998, Criddle
and others 1997). The purpose of this paper is to demonstrate, using these techniques, adaptation of parental and
hybrid plants to environmental stresses in common gardens
on a single hillside.
Materials and Methods ___________
Mountain sagebrush (Artemisia tridentata ssp. vaseyana
grows at slightly higher, cooler and drier sites than does
valley sagebrush (A. tridentata ssp. tridentata). Natural
hybrids between the two subspecies are often found in
locations such as Salt Creek Canyon, located 10 km east of
Nephi, Utah where the parent populations are separated
25
85 meters in elevation and 1.1 km in distance on the eastfacing slope of the canyon. Previous studies (Freeman and
others 1991, Graham and others 1995, McArthur and others
1998) have shown that much of the hillside between the
parent populations is occupied by hybrids. Common gardens
were established at the mountain and valley locations and
about halfway between the two. Seed was collected from the
parent locations and from three areas in the hybrid population zone. The seed was germinated and grown in containers
in the greenhouse. In the spring of 1993, 60 seedlings were
randomly planted in each of the three fenced common
gardens, 12 from each of the five populations (mountain,
high-elevation hybrid, mid-elevation hybrid, low-elevation
hybrid, and valley).
Plant tissues were collected on: July 6, 1995 at air temperatures in the shade ranging from 24 to 30°C; August 8,
1995 at 18 to 24.5°C; and on September 28, 1995, at 18 to
24°C. From 1996 through 1998 plant tissues were collected
at several times. Samples were kept refrigerated for analysis in a Hart model 7707 microcalorimeter.
Metabolic heat rate (q) and respiration rate (RCO2) were
measured for each sample at 15 and 25°C (1995 samples).
Metabolic measurements in subsequent years were made at
nine temperatures: 0, 5, 10, 15, 20, 25, 30, 35, and 40°C.
Using a mathematical model developed by Hansen and
others (1994) metabolic response to temperature was calculated for each individual plant over the entire range of
growth temperatures for sagebrush.
Knowing the heat rate (q) and the respiration rate (RCO2),
the relative specific growth rate (RSG) can be predicted:
15
RSGΔ HB
10
5
0
–5
–10
–15
–20
0
10
20
30
Temperature (°C)
(2)
where ε is the substrate carbon conversion efficiency. Thus
growth rate is directly proportional to both respiration rate
and efficiency.
Clamps to darken shoot tips and leaves were placed on
sagebrush plants in the gardens on August 30, 1996, with an
air temperature of 28°C. After equilibration in the dark for
10 minutes, a pulse of 655 nm light was given from the
Morgan Scientific CF-1000 Chlorophyll Fluorescence Measurement System and the quantum yield of photosytem II
was measured as fluorescence at 695 nm and expressed as
the ratio of variable to maximal fluorescence (Fv/Fm). A
decrease in the quantum yield (lower Fv/Fm ratio) indicates
greater environmental stress on the plant.
Pooled samples of sagebrush shoot tips collected on each
of the three dates in 1995 were dried overnight at 65°C and
26
As shown in figure 1, both relative specific growth rate (A)
and efficiency (B) were predicted to be high at low temperatures and inhibited at higher temperatures. Growth rate
indicated stress at about 30°C while efficiency was decreased at 25°C. Differences between populations were noted.
Calorimetric data for all of the sagebrush plants (fig. 1)
indicate that sagebrush grows best at low temperatures and
always grows slowly. Plants that grow best at low temperatures generally do poorly at warm temperatures while those
plants that grow better at warmer temperatures do less well
than others at cooler temperatures. They never seem to have
it both ways.
Pooled metabolic and isotopic data from all dates, sites,
and sources (table 1) indicated a decline in heat rate and
respiration rate from early May until late September. An
approximation of efficiency is q/RCO2, predicting best growth
during June and July and in the basin garden. Transplantation itself may have been a slight problem for vaseyana but
differences between source populations were small. Carbon
isotopic values showed only small differences but indicated
stress in warmer weather and in vaseyana. Plant tissue
collected on the warmest date (July 6, 1995) was more
negative than that collected in August or September. This
confirms that sagebrush grows best in cool weather and is
(1)
where RSG is the specific growth rate in terms of moles of
carbon incorporated per gram of biomass, q is the specific
heat rate in μW/mg, RCO2 is the rate of CO2 evolution in the
dark at pmol mg–1 sec–1, and Δ HB is the enthalpy change for
structural biomass formation (as kJ/mol carbon). If photosynthate is stored as starch or sugars (which have chemical
oxidation states of zero), and assuming that ΔHB is constant
with temperature and among sagebrush plants, Thornton’s
constant (–455 kJ mole–1) may be introduced.
Since the method measures energy changes (q) as well as
gas exchange rates (RCO2), equation (1) can also be expressed
as:
RSG = RCO2[ε /(1–ε )]
Results ________________________
0.5
(ε /1-ε )Δ HB
RSGΔHB = (455RCO2 – q)
analyzed for carbon isotopes at the Stable Isotope Ratio
Facility for Environmental Research at the University of
Utah.
0.25
0
–0.25
–0.5
0
10
20
30
Temperature (°C)
Figure 1—Calorimetric measurements at 15 and
25°C extrapolated through the growth range of temperatures expressed as (A) relative specific growth
rates (RSGΔ HB) and (B) efficiency [(ε /1-ε )Δ HB] for
each of the three gardens and populations. Please
see text for details.
USDA Forest Service Proceedings RMRS-P-11. 1999
Table 1—Pooled metabolic and isotopic data from all dates, sites, and
sources in 1995 and 1996. The values are averages from
36 plants in each case.
Date
Qave@15°C
RCO2@15°C
Qave/RCO2
δ 13C
16.14
12.90
13.43
7.77
7.12
322.7
293.2
292.1
302.5
337.0
–29.0
–28.3
–27.9
13.35
10.54
9.55
254.3
347.0
349.5
–28.3
–28.7
–28.1
324.9
323.4
299.9
–28.0
–28.0
–29.2
Data pooled by date:
May 10, 1996
June 6, 1996
July 6, 1995
Aug. 8, 1995
Sept. 28, 1995
4.596
4.270
3.554
2.114
2.097
Data pooled by site (garden):
Basin
Hybrid
Mountain
3.646
3.362
2.853
Data pooled by source (origin of plants):
Basin
Hybrid
Mountain
3.596
3.341
3.046
11.61
11.66
10.65
stressed at moderately warm temperatures. Subspecies tridentata seems to withstand stress better than ssp. vaseyana
or hybrids between them.
Recent measurements of metabolic heat rates and respiration are expressed as predicted growth rate (RSG) plotted
against temperature (figure 2). Growth of hybrid plants was
slow but not inhibited at cool temperatures in all three
gardens but was inhibited at 30°C in the mountain garden,
at 35°C in the basin garden, and at 40°C in the hybrid
garden. This seems to indicate that hybrid plants are best
adapted to warm temperature extremes at their place of
origin.
Chlorophyll fluorescence measurements (table 2) indicated stress as a reduction in quantum yield only for vaseyana plants grown in the basin garden. Pooled carbon
isotopic ratios for all plants in each garden became more
negative with increased elevation, indicating greater stress
at the higher, cooler, drier sites (table 2). If one pools all the
plants from a common site of origin, no matter where they
were grown, isotopic ratios for A.t. ssp. tridentata were more
positive than hybrid plants, while ssp. vaseyana had the
most negative values at all sites (table 1).
Discussion _____________________
Perhaps our most surprising finding is that sagebrush
grows slowly in cold weather but is not stressed by cool
temperatures. On the other hand, warmer temperatures
produced evidence of stress and reduced growth. Sagebrush
is well-adapted to living in the Great Basin which has cold,
wet winters and hot dry summers. The winters are not
extremely cold and it is rare to find the ground frozen as a
blanket of snow usually insulates the ground surface. Nelson
USDA Forest Service Proceedings RMRS-P-11. 1999
Figure 2— Calorimetric measurements made at nine
temperatures for each garden and the central hybrid
population expressed as relative specific growth rates
(RSGΔHB).
27
Table 2—Sagebrush chlorophyll fluorescence (Fv/Fm) on August 30, 1996
and carbon isotope ratios on September 28, 1995.
Garden
Basin
Hybrid
Mountain
Basin
Hybrid
Mountain
Basin
Hybrid
Mountain
Basin
Hybrid
Mountain
Basin
Hybrid
Mountain
Population
Basin
Basin
Basin
Low elev. hybrid
Low elev. hybrid
Low elev. hybrid
Hybrid
Hybrid
Hybrid
High elev. hybrid
High elev. hybrid
High elev. hybrid
Mountain
Mountain
Mountain
and Tiernan (1973) found extensive winter injury to big
sagebrush in years with low snow cover and consequent
exposure to extreme cold. Sagebrush apparently can become
dormant and withstand high summer temperatures, but
may have no mechanism for slowing growth during very cold
conditions.
Since sagebrush retains leaves all year, Pearson (1975)
determined hourly and daily photosynthesis rates on six
Artemisia tridentata plants grown outside in Rexburg, Idaho.
He found the highest rates of photosynthesis to be in December, January, and February, while the lowest rates of photosynthesis occurred in July and August when the soil was
very dry. Summer drought and high temperatures coincide
in the Great Basin and the relative contribution of both
stresses must be addressed. In a controlled greenhouse
experiment, Booth and others (1990) concluded that for
three subspecies of big sagebrush, even under the most
severe conditions employed, water was not sufficiently limited to retard seedling growth. These results were supported
by work of Matzner and Richards (1996) who found that
sagebrush roots could maintain nutrient capacity even under water stress.
More negative carbon isotopic ratios may indicate increased discrimination against the heavy isotope during
diffusion through partially closed stomates (Condon and
others 1987). Greater water availability, more extensive
root systems, and cooler temperatures allow more open
stomates and less isotopic fractionation. Data from tables 1
and 2 support that hypothesis.
References _____________________
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28
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USDA Forest Service Proceedings RMRS-P-11. 1999