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Rising CO2 and pollen production of common ragweed (Ambrosia artemisiifolia L.), a known allergy-inducing species: implications for public health.



Although environmental factors such as precipitation and temperature are recognized as influencing pollen production, the impact of rising atmospheric carbon dioxide concentration ([CO2]) on the potential growth and pollen production of hay-fever-inducing plants is unknown. Here we present measurements of growth and pollen production of common ragweed (Ambrosia artemisiifolia L.) from pre-industrial [CO2] (280 mol mol–1) to current concentrations (370 mol mol–1) to a projected 21st century concentration (600 mol mol–1). We found that exposure to current and elevated [CO2] increased ragweed pollen production by 131 and 320%, respectively, compared to plants grown at pre-industrial [CO2]. The observed stimulations of pollen production from the pre-industrial [CO2] were due to an increase in the number (at 370 mol mol–1) and number and size (at 600 mol mol–1) of floral spikes. Overall, floral weight as a percentage of total plant weight decreased (from 21% to 13%), while investment in pollen increased (from 3.6 to 6%) between 280 and 600 mol mol–1 CO2. Our results suggest that the continuing increase in atmospheric [CO2] could directly influence public health by stimulating the growth and pollen production of allergy-inducing species such as ragweed.
Volume 27,2000
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Since the start of the Industrial Revolution, atmospheric
[CO2] has risen from ~280 µmol mol–1 to ~370 µmol mol–1
(Houghton et al. 1996). This rise in atmospheric [CO2] has
not been linear. Approximately two-thirds of the observed
increase in [CO2] has occurred since the 1950s (Keeling and
Whorf 1994). Although the projected rate of future atmos-
pheric [CO2] increase varies by model, it is generally
acknowledged that atmospheric [CO2] should reach
600 µmol mol–1 sometime during the 21st century (Houghton
et al. 1996).
Because CO2supplies the carbon for all terrestrial
biology, research efforts have focused on determining the
impact of rising atmospheric [CO2] on the growth and repro-
duction of native species and crops (e.g. Kimball et al. 1993;
Poorter 1993; Curtis and Wang 1998). Reproduction is an
especially important parameter since it affects both ecologi-
cal fitness for native species, and economic production for
The impact of [CO2] on allocation of resources to flower-
ing and changes in reproductive phenology appears to be
highly species-specific (Ackerly and Bazzaz 1995).
Enhanced [CO2] has resulted in earlier flowering (Lawlor
and Mitchell 1991) and increased flower and fruit number
for a number of agronomic plants (Deng and Woodward
1998). In contrast, flowering of some native species has been
unaffected or delayed with increased [CO2] (Garbutt and
Bazzaz 1984; Reekie et al. 1997), but this is by no means a
universal response (e.g. Datura stramonium, Garbutt and
Bazzaz 1984).
While specific attention has been given to aspects of
reproductive biology which could influence seed develop-
ment and yield, almost nothing is known concerning the
impact of [CO2] on pollen production per se. Aside from the
obvious consequences for fertilization, fecundity and eco-
logical fitness, [CO2]-induced changes in pollen production
could have a direct impact on atmospheric pollen concen-
tration, with subsequent effects on human allergic disease
and public health. For example, in a recent survey among the
general population in the US, approximately 70% of respon-
dents indicated pollen as the principal agent producing
symptoms of allergies, with ragweed pollen cited as the indi-
vidual plant species eliciting the greatest response (Meggs
et al. 1996). In addition to ragweed, other weedy species (e.g.
lambsquarters, Chenopodium album L.) and pigweed
(Amaranthus retroflexus L.) as well as native trees
Aust. J. Plant Physiol., 2000, 27, 893–898
10.1071/PP00032 0310-7841/00/100893
Abbreviations used: A, rate of CO2assimilation; Ca, ambient CO2concentration; Ci, internal CO2concentration; DAS, days after sowing;
PPFD, photosynthetic photon flux density; RGR, relative growth rate.
© CSIRO 2000
Rising CO2and pollen production of common ragweed
(Ambrosia artemisiifolia), a known allergy-inducing species:
implications for public health
Lewis H. ZiskaAand Frances A. Caulfield
Climate Stress Laboratory, Bldg 046A, USDA-ARS, Beltsville Agricultural Research Center,
10300 Baltimore Avenue, Beltsville MD 20705, USA.
ACorresponding author: email;
Abstract. Although environmental factors such as precipitation and temperature are recognized as influencing
pollen production, the impact of rising atmospheric carbon dioxide concentration ([CO2]) on the potential growth
and pollen production of hay-fever-inducing plants is unknown. Here we present measurements of growth and pollen
production of common ragweed (Ambrosia artemisiifolia L.) from pre-industrial [CO2] (280 µmol mol–1) to current
concentrations (370 µmol mol–1) to a projected 21st century concentration (600 µmol mol–1). We found that expo-
sure to current and elevated [CO2] increased ragweed pollen production by 131 and 320%, respectively, compared
to plants grown at pre-industrial [CO2]. The observed stimulations of pollen production from the pre-industrial [CO2]
were due to an increase in the number (at 370 µmol mol–1) and number and size (at 600 µmol mol–1) of floral spikes.
Overall, floral weight as a percentage of total plant weight decreased (from 21% to 13%), while investment in pollen
increased (from 3.6 to 6%) between 280 and 600 µmol mol–1 CO2. Our results suggest that the continuing increase
in atmospheric [CO2] could directly influence public health by stimulating the growth and pollen production of
allergy-inducing species such as ragweed.
Keywords: allergens, elevated carbon dioxide, photosynthesis, pollen, relative growth rate.
L. H. Ziska and F. A. Caulfield894
(e.g. Quercus and Acer spp.) and grasses (e.g. Setaria) are
recognized as influencing seasonal allergies through pollen
production (Gergen and Turkeltaub 1992; Emberlin 1994).
In the current experiment, our principal objective was to
test whether the increase in atmospheric [CO2] since the
Industrial Revolution, and projected future increases in
[CO2], may alter growth and pollen production of known
hay-fever-inducing plants using common ragweed
(Ambrosia artemisiifolia L.) as a model species. Pollen pro-
duction of ragweed is generally acknowledged to be a major
source of air-borne allergens and a public health concern in
North America. In general, ragweed production peaks
between late August and November in North America, and is
the principal pollen associated with fall allergies in the US
(Frenz et al. 1995; Meggs et al. 1996). CO2-induced changes
in the life cycle and pollen-producing capacity of aero-aller-
gen plants such as ragweed could have significant implica-
tions for public health.
Materials and methods
Experiments were conducted using a controlled environment chamber
located at the Climate Stress Laboratory, USDA-ARS, Beltsville, MD,
USA. An environmental chamber was used rather than field chambers
or Free-Air CO2Exchange (FACE), in order to maintain constant pre-
industrial [CO2] at a given temperature and consistent light and humid-
ity for 24-h periods.
[CO2] was controlled by flushing the chamber with CO2-free air
using a Ballston 75-60 type CO2scrubber (Ballston Filter Products,
Lexington, MA, USA), then re-injecting CO2to the desired [CO2].
Injection of CO2was controlled by an infrared gas analyser (WMA-2,
PP systems, Haverhill, MA, USA) in absolute mode that sampled
chamber air continuously. The set points for [CO2] control were 280
(pre-industrial), 370 (current) and 600 (future) µmol mol–1 CO2. Actual
CO2concentrations determined at 10-min intervals over 24 h for each
[CO2] treatment were 281.5 ± 23.4, 374 ± 14.1 and 603 ± 12.9 µmol
mol–1, respectively. In all chambers, plants received 14 h of
1.0 mmol m–2 s–1 photosynthetic photon flux density (PPFD) from a
mixture of high-pressure sodium and metal halide lamps for the first
35 days after sowing (DAS). After 35 DAS, PPFD was altered to 12 h
of 1.0 mmol m–2 s–1 PPFD to induce flowering. Day/night temperature
was 28/22°C and average daily humidity exceeded 60%. Temperature,
[CO2] and relative humidity were monitored and recorded at 1-min
intervals by an EGC network data logger (EGC Corp., Chagrin Falls,
OH, USA) in conjunction with a PC.
Seeds of common ragweed (Ambrosia artemisiifolia L.) were broad-
cast in pots of different sizes ranging from 10 to 30 cm in diameter
(1.8–21.2 L in volume). Smaller pots were elevated so that the height of
the plants was uniform. Seed was obtained from the Valley Seed
Company (Fresno, CA, USA). Plants in all pots were thinned to one
plant per pot within 48 h after emergence. Pots were filled with vermi-
culite and watered daily to dripping point with a complete nutrient solu-
tion containing 13.5 mMnitrogen (Robinson 1984). For each
experiment, 32 pots were assigned to a given [CO2], with pots arranged
to avoid mutual shading.
To determine potential changes in photosynthesis as a function of
the growth [CO2], single leaf photosynthesis (A, the rate of CO2assimi-
lation) was determined as a function of short-term changes in internal
[CO2] (Ci) twice during the vegetative growth at each [CO2] treatment.
Assimilation was determined on the uppermost, fully expanded leaf for
four plants of each [CO2] between 30 and 40 DAS using a differential
infrared CO2analyser (model 6252, Li-Cor Corp., Lincoln, NE, USA)
in an open configuration attached to two single-leaf cuvettes.
Temperature, humidity and [CO2] were set to approximate values main-
tained in the growth chamber. The gas stream was humidified by
bubbling through a temperature-controlled water bath to obtain a given
dew point, and humidity was monitored with a dew point hygrometer
(Hygro M-1, General Eastern Corp., Cambridge, MA, USA). Mass
flow controllers were used to mix dry CO2-free air with 100% CO2to
obtain a desired [CO2] within a cuvette. Supplemental lighting was sup-
plied by a 150-W cool-beam floodlight (GE Corp., Cleveland, OH,
USA) attached to a variable transformer to obtain a desired PPFD.
Photosynthesis was determined initially as the CO2assimilation rate
at the growth [CO2] (Ca) at the growth PPFD (1.0 mmol m–2 s–1), then
re-measured at saturating light intensity (1.6 mmol m–2 s–1). Cawas then
reduced to 90 µmol mol–1 and increased in steps to 180, 360, 720, 1080
and 1450 µmol mol–1. Sufficient time (usually 20–30 min) was given
after Cawas changed, to allow equilibration. At the end of the measure-
ment, leaf laminae contained within a cuvette were cut, and leaf area
determined with a leaf area meter (Li-Cor Corp.).
For each [CO2] treatment, six plants were harvested at 21, 25, 29 and
35 DAS and again at seed maturity. Seed maturity was defined as occur-
ring when seed set exceeded 90%. At each harvest, all plants for a given
[CO2] treatment were cut at ground level and separated into leaf
laminae, stems (including petioles) and roots. Smaller-volume pots
were harvested first to avoid root-binding effects. Total leaf area was
determined photometrically as described previously. Dry weights were
obtained separately for leaves, stems and roots. Material was dried at
65°C for a minimum of 72 h or until dry weight was constant, and then
weighed. For the maturity harvest, leaf area was estimated based on the
regression analysis between leaf area and weight obtained from previ-
ous harvests (R2= 0.99).
Before pollination, 10 terminal staminate floral spikes (catkins)
were selected on each of five plants from each [CO2] treatment, and
labeled. A 5 ×25 cm polyethylene bag was placed over each spike to
collect pollen. Each bag had a 2.5-cm slit cut approximately 2 cm from
the bottom of the bag, into which the floral spike was placed with the
peduncle of the raceme located at the bottom of the slit. After placement
of the bag, the slit was taped so the floral spike was inside the bag with
at least 5 cm of space from the top of the open bag. Tops of bags were
left open for air circulation and ventilation. Floral heads were tapped
gently each day, and pollen was allowed to fall to the bottom of the bag.
After flowers were dehiscent and heads had completed pollen produc-
tion, each bagged floral spike was cut immediately below the first
flower and the floral structure was removed from the bag after tapping
any residual pollen. Each spike was measured for length along with
fresh and dry weights. Total pollen for a given spike was calculated by
subtracting the initial bag weight from the bag and pollen weight. At
maturity, for each plant from each [CO2] treatment, the total number of
floral spikes was recorded, spikes were harvested and the dry weight
(without pollen) recorded. The ratio of pollen collected to dry weight of
the floral structure resulted in a consistent ratio that was used to esti-
mate pollen production per plant.
Because of potential differences in microclimate between chambers,
the same growth chamber was used for all three CO2levels.
Adjustments to PPFD, humidity and temperature control were made
prior to the start of each [CO2] treatment to maintain consistency in
microclimate. In addition, the entire experiment (i.e. all three [CO2]
treatments) was repeated. A two-way ANOVA (SuperANOVA, Abacus
Concepts, Berkeley, CA, USA) was used to test for differences between
the two runs and between treatments. Because no significant run effect
was detected, treatment effects were compared with a one-way analysis
of variance on the combined data. Final biomass for a given [CO2] treat-
ment differed by <10% between runs. Three separate post-hoc tests
(Student–Newman–Keuls, Duncan New Multiple Range and Fisher’s
protected LSD) determined differences at the 0.05 significance level as
a function of [CO2] treatment.
Pollen production increased significantly with rising [CO2].
The observed increase was 132% from pre-industrial to
current CO2levels, and ~90% from current to future CO2
levels of 600 µmol mol–1 (Fig. 1). Sensitivity of pollen pro-
duction to increasing [CO2] was greater from pre-industrial
to current CO2levels (0.7 g of pollen per 10 µmol mol–1
increase in [CO2]), diminishing as CO2increased to
600 µmol mol–1 (0.4 g of pollen per 10 µmol mol–1 increase
in [CO2]). Floral spike number did not change from pre-
industrial to current [CO2], but pollen production per
spikelet increased significantly (Fig. 1). From 370 to
600 µmol mol–1 CO2, no further change in pollen production
per spikelet was noted, but the number of floral spikes
approximately doubled (Fig. 1). Analysis of the diameter of
200 individual pollen grains for each [CO2] using a SEM
(15 kV, ×1.5k) indicated no change in average pollen size
(data not shown).
Small (non-significant) changes in total biomass (i.e.
roots, stems and leaves) were observed by 21 DAS, and sig-
nificant differences were observed by 29 DAS (Fig. 2).
Significant differences in relative growth rate (RGR) also
occurred by 29 DAS among [CO2] treatments (0.174, 0.209
and 0.220 g g–1 day–1 for the 280, 370 and 600 µmol mol–1
[CO2] treatments, respectively).
At seed maturity, total plant biomass was directly propor-
tional to [CO2]. From pre-industrial to current atmospheric
[CO2], leaf weight and stem weight increased by 36 and 49%,
respectively, with no significant change in root or floral
weight (Table 1). Leaf area, however, almost doubled in size
for this same increase in [CO2] (Table 1). At 600 µmol mol–1,
significant increases in all growth parameters at maturity
were observed relative to the 370 and 280 µmol mol–1 treat-
ments. The largest relative increase was observed for root
weight which increased ~ 4-fold from 280 to 600 µmol mol–1
[CO2]. No significant changes in stem to root ratio or
specific leaf area occurred among [CO2] treatments (data not
Leaf photosynthesis, measured at the growth [CO2],
increased significantly with [CO2], rising 170 and 250%
from pre-industrial to current and future [CO2], respectively,
and 30% from current to future [CO2] (Table 2). Values of
assimilation at a measurement [CO2] of 280 µmol mol–1 did
not differ among [CO2] treatments. However, at higher mea-
surement CO2levels, leaves grown at 280 µmol mol–1 [CO2]
had significantly lower photosynthetic rates — an indication
of down-regulation (Table 2). Acclimation was not observed
between the 370 and 600 µmol mol–1 [CO2] treatments.
Analysis of the response of assimilation to internal CO2indi-
cated no significant change in the initial slope of the
response curve as a function of [CO2] treatment. However,
the internal CO2concentration where leaf assimilation is
equal to zero (Γ*, the CO2compensation point), and the
maximum observed assimilation rate was lower for the 280
than the 370 and 600 µmol mol–1 treatments (data not
For many years, both botanists and health workers have been
interested in those climatic and/or meteorological factors
that influence atmospheric pollen concentration (e.g.
Gregory 1973; Buck and Levetin 1982). Abiotic factors that
influence pollen productivity such as rainfall, temperature
and light also determine pollen amounts and severity of
Rising CO2and pollen production of common ragweed
Pollen production (g plant–1)
25 280 µmol mol–1
370 µmol mol–1
600 µmol mol–1
Pollen spike–1 (mg)
Floral spikes (plant–1)
1890 2000 2050?
Fig. 1. Pollen production, pollen per floral spike and number of floral
spikes in common ragweed grown at pre-industrial CO2concentrations
(280 µmol mol–1), current concentrations (370 µmol mol–1) and a pro-
jected 21st century concentration (600 µmol mol–1). Bars are ± s.e.
Student–Newman–Keuls was used to determine differences among the
[CO2] treatments at the 0.05 significance level (a, b or c).
L. H. Ziska and F. A. Caulfield896
allergies among susceptible populations during a given
allergy season. Consequently, there has been a great deal of
interest in modeling changes in pollen type and abundance
associated with global climate change (Emberlin 1994).
However, less is known concerning the direct stimulation of
growth and pollen production of allergy-inducing species by
rising [CO2], one of the principal ‘greenhouse’ gases.
The observed stimulation in growth and photosynthesis
observed here for ragweed is consistent with results seen
elsewhere for C3species grown with enhanced [CO2]
(Kimball et al. 1993). Increasing the [CO2] to 600 µmol mol–1
increased ragweed RGR within 4 weeks following emer-
gence, and stimulated biomass at maturity almost 3-fold
above pre-industrial [CO2] values. Final vegetative biomass
values obtained in the current experiment at ambient [CO2]
are consistent with those observed for single ragweed plants
in abandoned agricultural fields at maturity (D. Patterson,
pers. comm.). In the current study, both leaf area and weight
were particularly sensitive to [CO2]. The continued stimula-
tion of single-leaf photosynthesis (at least through anthesis)
and the observed increase in leaf area have obvious implica-
tions for maintaining a continued stimulation of photo-
synthesis and growth at the whole plant level with future CO2
If growth of ragweed is indeed stimulated by increasing
[CO2], how does this alter subsequent reproductive effort?
Floral spikes of ragweed contain both staminate and pistil-
late flowers. Pollen is wind-directed from numerous stami-
nate flowers to pistillate heads, which are fewer in number
and occur at the base of the floral spike (Bianchi et al. 1959).
Because wind is the primary means of transport, large
amounts of pollen are necessary to achieve seed set.
Increasing vegetative growth provides both a structural plat-
form for floral production and the carbon assimilate needed
to produce flowers. In the current experiment, floral weight
increased 70%, but floral weight as a percentage of total
plant weight decreased (from 21% to 13%) from 280 to
600 µmol mol–1 CO2. However, investment in pollen
increased (from 3.6 to 6%) from 280 to 600 µmol mol–1 CO2.
Because of the role of ragweed pollen in inducing aller-
gies, the reproductive response of ragweed to rising atmos-
pheric [CO2] is of obvious interest. In the current study, the
response of ragweed pollen production to rising [CO2] was
2-fold. Increasing [CO2] from pre-industrial to current levels
increased the amount of pollen produced by an individual
floral spike. As [CO2] increased further to 600 µmol mol–1,
no additional increase in pollen per floral spike was
observed, but the number of floral spikes rose significantly.
The net result was an approximate doubling of pollen
production capacity from pre-industrial to present day [CO2]
and a further doubling to a projected [CO2] of
600 µmol mol–1.
Interestingly, interpolation of the potential pollen
response of ragweed to [CO2] from the 1950s
(~315 µmol mol–1) to current levels shows a percentage
Days after sowing
20 22 24 26 28 30 32 34 36
Total biomass (g plant1)
280 µmol mol1
370 µmol mol1
600 µmol mol1
growth concentration
Fig. 2. Change in total plant biomass (g plant–1) as a function of days
after sowing (DAS) for ragweed grown at pre-industrial, current and
future atmospheric [CO2]. Bars are ± s.e. Relative growth rate (RGR)
was determined between 21 and 29 DAS. No further change in RGR as
a function of [CO2] was observed after 35 DAS. * indicates a signifi-
cant increase in total biomass relative to the 280 µmol mol–1 [CO2]
Table 1. Changes in measured (leaf area, leaf, stem and root dry
weights) vegetative parameters at maturity for common ragweed
(Ambrosia artemisiifolia L.) grown at pre-industrial, current and
future levels of atmospheric [CO2]
Different letters within a column indicate statistical differences between
[CO2] treatments at the 0.05 level according to Student–Newman–
Keuls. Data are given on a per-plant basis
[CO2] Area Weight (g) Total
(µmol mol–1)(m
2) Leaf Stem Root Floral weight (g)
280 1.15 c 65.1 c 30.7 c 11.3 b 28.9 b 135.1 c
370 2.17 b 88.7 b 45.7 b 13.5 b 35.7 b 183.6 b
600 3.41 a 178.9 a 97.1 a 50.3 a 49.2 a 372.4 a
Table 2. Photosynthesis (as CO2assimilation rate, µmol CO2
m–2 s–1) for ragweed (Ambrosia artemisiifolia L.) grown and
measured at pre-industrial, current and future atmospheric [CO2]
Different letters within a column indicate statistical differences between
[CO2] treatments at the 0.05 level according to Student–Newman–
Keuls. Additional details are given in ‘Materials and methods’
Growth [CO2] Measurement [CO2] (µmol mol–1)
(µmol mol–1) 280 370 600
280 15.1 23.3 b 33.1 b
370 19.6 40.7 a 53.0 a
600 14.8 35.6 a 52.9 a
increase in ragweed pollen consistent with the recent
reported percentage increase in allergies and allergy-induced
asthma among the general population (Platt-Mills and Carter
1997; Woolcock and Peat 1997). However, has the actual
amount of ragweed pollen in the environment increased
within the last 40 years? Because the rise in atmospheric
[CO2] has been so rapid, traditional 14C dating techniques to
determine in situ increases in pollen production since the
mid-1950s are not applicable. The earliest pollen studies are
based on some 13 000 gravity slide samples from 22
American cities summarized from 1916 to 1928 (Durham
1929). Unfortunately, direct comparisons between gravi-
metric and volumetric devices (e.g. Rotorod sampler) are dif-
ficult to perform. Differences in pollen recovery cannot be
quantified, even roughly (see Frenz 1999a). Even if different
sampling techniques were comparable, changes in land use
and nitrogen deposition in industrial areas could not be sep-
arated from any direct atmospheric CO2effect. Current
regional, on-site estimates of ragweed pollen production do
not date back more than a few years (Frenz et al. 1995, Frenz
1999b), although it is hoped that these data could be used to
verify potential increases in ragweed pollen with future
increases in atmospheric [CO2].
Will similar increases in pollen with enhanced [CO2] be
observed for other known allergy-inducing species?
Projected increases in [CO2] have been shown to stimulate
the photosynthesis and growth of C3species such as lambs-
quarters (Carlson and Bazzaz 1982) and oak (Bunce 1992),
as well as some C4species such as pigweed (Tremmel and
Patterson 1993) and foxtail (Ziska and Bunce 1997).
However, the reproductive response to [CO2] cannot always
be elucidated from observed increases in vegetative biomass
(Jablonski 1997).
Critics of the role of CO2in climate change correctly
point out that rising [CO2] could result in a lush plant
environment (Idso and Idso 1994). However, it should also
be emphasized that the rise in [CO2] is indiscriminatory with
respect to the stimulation of both useful and noxious plant
species. Furthermore, elimination of noxious weedy species
by chemical means cannot always be assumed as atmos-
pheric [CO2] increases (Ziska et al. 1999). Consequently, the
role of rising atmospheric [CO2] with respect to distribution,
growth and pollen production of weeds impacting human
health should be of growing concern.
The authors thank Dr JA Bunce of the Climate Stress
Laboratory for his suggestions to the manuscript, Dr Dave
Patterson, weed specialist at North Carolina State University
for his advice and Dr Linda Ford of the National Allergy
Bureau for her insightful comments.
Ackerly DD, Bazzaz FA (1995) Plant growth and reproduction along
CO2gradients: non-linear responses and implications for commu-
nity change. Global Change Biology 1, 199–207.
Bianchi DE, Schwemmin DJ, Wagner WH Jr (1959) Pollen release in
the common ragweed (Ambrosia artemisiifolia). Botanical Gazette
120, 235–243.
Buck P, Levetin E (1982). Weather patterns and ragweed pollen pro-
duction in Tulsa, Oklahoma. Annals of Allergy 49, 272–275.
Bunce JA (1992) Stomatal conductance, photosynthesis and respiration
of temperate deciduous tree seedlings grown outdoors at an elevated
concentration of carbon dioxide. Plant, Cell and Environment 15,
Carlson RW, Bazzaz FA (1982) Photosynthetic and growth response to
fumigation with SO2at elevated CO2for C3and C4plants.
Oecologia 54, 50–54.
Curtis PS, Wang X (1998) A meta-analysis of elevated CO2effects on
woody plant mass, form and physiology. Oecologia 113, 299–313.
Deng X, Woodward FI (1998) The growth and yield responses of
Fragaria ananassa to elevated CO2and N supply. Annals of Botany
81, 67–71.
Durham OC (1929) Cooperative studies in ragweed pollen incidence:
atmospheric data from twenty-two cities. Journal of Allergy 1,
Emberlin J (1994) The effects of patterns in climate and pollen abun-
dance on allergy. Allergy 49, 15–20.
Frenz DA (1999a) Comparing pollen and spore counts collected with
the Rotorod sampler and Burkard spore trap. Annals of Allergy
Asthma & Immunology 83, 341–349.
Frenz DA (1999b) Volumetric ragweed pollen data for eight cities in the
continental United States. Annals Allergy Asthma and Immunology
82, 41–46.
Frenz DA, Palmer MA, Hokanson JM, Scamehorn RT (1995) Seasonal
characteristics of ragweed pollen dispersal in the United States.
Annals of Allergy, Asthma and Immunology 75, 417–422.
Garbutt K, Bazzaz FA (1984) The effects of elevated CO2on plants: III.
Flower, fruit and seed production and abortion. New Phytologist 98,
Gergen PJ, Turkeltaub PC (1992) The association of individual allergen
reactivity with respiratory disease in a national sample: data from
the second National Health and Nutrition Survey (NHANES II)
Journal of Allergy and Clinical Immunology 90, 579–588.
Gregory PH (1973) ‘The microbiology of the atmosphere.’ 2nd edn,
453 p. (John Wiley and Sons: Chichester, UK)
Houghton JT, Meira-Filho LG, Callander BA, Harris N, Kattenburg A,
Maskell K (1996) ‘IPCC climate change assessment 1995. The
science of climate change.’ (Cambridge University Press:
Cambridge, UK)
Idso KE, Idso SB (1994) Plant responses to atmospheric CO2enrich-
ment in the face of environmental constraints: a review of the past
10 years’ research. Agricultural and Forest Meteorology 69,
Jablonski LM (1997) Response of vegetative and reproductive traits to
elevated CO2and nitrogen in Raphanus varieties. Canadian Journal
of Botany 75, 533–545.
Keeling CD, Whorf TP (1994) Atmospheric CO2records from sites in
the SIO air sampling network. In ‘Trends ‘93: a compendium of data
on global change’. (Eds TA Boden, DP Kaiser, RJ Sepanski and
FW Stoss) pp. 20–26. (Carbon Dioxide Information Analysis
Center, Oak Ridge National Laboratory: Oak Ridge, TN)
Kimball BA, Mauney JR, Nakayama FS, Idso SB (1993) Effects of
increasing atmospheric CO2on vegetation. Vegetatio 104/105,
Rising CO2and pollen production of common ragweed
L. H. Ziska and F. A. Caulfield898
Lawlor DW, Mitchell RAC (1991) The effects of increasing CO2on
crop photosynthesis and productivity: a review of field studies.
Plant, Cell and Environment 14, 807–818.
Meggs WJ, Dunn KA, Bloch RM, Goodman PE, Davidoff AL (1996)
Prevalence and nature of allergy and chemical sensitivity in a
general population. Archives of Environmental Health 51, 275–282.
Platts-Mills TAE, Carter MC (1997) Asthma and indoor exposure to
allergens. New England Journal of Medicine 336, 1382–1384.
Poorter H (1993) Interspecific variation in the growth response of
plants to an elevated ambient CO2concentration. Vegetatio 104/105,
Reekie JYC, Hicklenton J, Reekie EG (1997) The interactive effects of
carbon dioxide enrichment and daylength on growth and develop-
ment in Petunia hybrida. Annals of Botany 80, 57–64.
Robinson JM (1984) Photosynthetic carbon metabolism in leaves and
isolated chloroplasts from spinach plants grown under short and
intermediate photosynthetic periods. Plant Physiology 75, 397–409.
Tremmel DC, Patterson DT (1993) Responses of soybean and five
weeds to CO2enrichment under two temperature regimes. Canadian
Journal of Plant Science 73, 1249–1260.
Woolcock AJ, Peat JK (1997) Evidence for the increase in asthma
worldwide. In ‘The rising trends in asthma, Ciba foundation sym-
posium 206’. (Eds D Chadwick and G Cardew) pp. 123–125. (John
Wiley and Sons: Chichester, UK)
Ziska LH, Bunce JA (1997) Influence of increasing carbon dioxide
concentration on the photosynthetic and growth stimulation of
selected C4crops and weeds. Photosynthesis Research 54, 199–208.
Ziska LH, Teasdale JR, Bunce JA (1999) Future atmospheric carbon
dioxide may increase tolerance to glyphosate. Weed Science 47,
Manuscript received 25 February 2000, accepted 3 July 2000
... Pollen production has been studied for numerous species, such as the tree species Betula (Jato et al. 2007;Piotrowska 2008;Jochner et al. 2011;Katz et al. 2020;Kolek 2021;Ranpal et al. 2022), Quercus (Tormo Molina et al. 1996;Gómez-Casero et al. 2004;Charalampopoulos et al. 2013;Kim et al. 2018;Fernández-González et al. 2020;Katz et al. 2020), Alnus (Moe 1998), Fraxinus (Tormo Molina et al. 1996;Castiñeiras et al. 2019), Acer (Tormo Molina et al. 1996;Katz et al. 2020), Corylus , Cupressus (Hidalgo et al. 1999;Damialis et al. 2011), Olea (Tormo Molina et al. 1996;Damialis et al. 2011), Platanus Katz et al. 2020), Juniperus (Pers-Kamczyc et al. 2020), Pinus Charalampopoulos et al. 2013), and Cedrus or for grass species (Subba Reddi and Reddi 1986;Prieto-Baena et al. 2003;Piotrowska 2008;Aboulaich et al. 2009;Tormo-Molina et al. 2015;Jung et al. 2018;Romero-Morte et al. 2018;Ali et al. 2022;Severova et al. 2022) and herbaceous plants such as Artemisia (Piotrowska 2008;Bogawski et al. 2016), Rumex (Piotrowska 2008), Plantago (Hyde and Williams 1946;Sharma et al. 1999;Piotrowska 2008;González-Parrado et al. 2015), Parietaria and Ambrosia (Ziska and Caulfield 2000;Wayne et al. 2002;Rogers et al. 2006). ...
... Pollen production can be influenced by environmental factors (Ziska and Caulfield 2000;Rogers et al. 2006;Damialis et al. 2011). While Barnes (2018) reported an increase in pollen production under warmer conditions, another study found the opposite . ...
... While Barnes (2018) reported an increase in pollen production under warmer conditions, another study found the opposite . Furthermore, increased carbon dioxide (CO 2 ) concentrations have been reported to lead to an increase in pollen production (Ziska and Caulfield 2000;Jablonski et al. 2002;Wayne et al. 2002;Ziska et al. 2003;Rogers et al. 2006;Kim et al. 2018). For air pollutants, nitrogen dioxide (NO 2 ) might reduce pollen production ), but also the opposite has been documented (Zhao et al. 2017). ...
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Pollen production is one plant characteristic that is considered to be altered by changes in environmental conditions. In this study, we investigated pollen production of the three anemophilous species Betula pendula , Plantago lanceolata , and Dactylis glomerata along an urbanization gradient in Ingolstadt, Germany. We compared pollen production with the potential influencing factors urbanization, air temperature, and the air pollutants nitrogen dioxide (NO 2 ) and ozone (O 3 ). While we measured air temperature in the field, we computed concentration levels of NO 2 and O 3 from a land use regression model. The results showed that average pollen production (in million pollen grains) was 1.2 ± 1.0 per catkin of Betula pendula , 5.0 ± 2.4 per inflorescence of Plantago lanceolata , and 0.7 ± 0.5 per spikelet of Dactylis glomerata. Pollen production was higher in rural compared to urban locations on average for B. pendula (+ 73%) and P. lanceolata (+ 31%), while the opposite was the case for D. glomerata (− 14%). We found that there was substantial heterogeneity across the three species with respect to the association of pollen production and environmental influences. Pollen production decreased for all species with increasing temperature and urbanization, while for increasing pollutant concentrations, decreases were observed for B. pendula , P. lanceolata , and increases for D. glomerata . Additionally, pollen production was found to be highly variable across species and within species—even at small spatial distances. Experiments should be conducted to further explore plant responses to altering environmental conditions.
... Moreover, A. artemisiifolia is a pioneer plant that emerges from a durable, dense soil seed bank, especially if the ground is disturbed, such as in agricultural contexts (Simard et al. 2020); it is also observed to grow outside agricultural areas, colonizing cities and causing serious human health problems. Numerous studies have drawn attention to the accelerated invasion of this species, its increased pollen production (Ziska & Caulfield 2000, Wayne et al. 2002, Bullock et al. 2012, Chapman et al. 2016, as well as its late pollen production, dependent in turn on the habitats in which it occurs (Fumanal et al. 2007), with related negative effects on human well-being stemming from its earlier and longer pollen seasons (see Beggs & Bambrick 2005). In addition, evidence for the long-distance dispersal of A. artemisiifolia pollen suggests its remarkable capability for extensive transportation (and, thus, pollinosis outbreaks) and concurrent great seed dispersal, with subsequent establishment in new areas (Šikoparija et al. 2013(Šikoparija et al. , Grewling et al. 2019. ...
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Invasive alien species represent a multifaceted management problem in terms of threats to biodiversity and ecosystems and their impacts on agriculture and human well-being. Ambrosia artemisiifolia is an invasive alien plant in Europe that affects the human population as its already highly allergenic pollen can interact with air pollutants, resulting in detrimental effects on health. In this context, the invasive beetle Ophraella communa was proposed as a biocontrol agent of A. artemisiifolia, as it feeds on its leaves, leading to a decrease in pollen production. This paper takes advantage of the different co-occurrence classes obtained by the ecological niche models inferred for both of these species based on current and future climatic conditions. We integrate them with spatial data regarding major air pollutants (nitrogen dioxide and fine particulate matter). We couple this information with European human population density data at a narrow territorial scale to infer current and future statistically significant hotspots of health risk. The Netherlands and the UK host the widest hotspots within their national territory for both current (7.09% and 3.54%, respectively) and future (15.04% and 6.70%, respectively) scenarios. Considering the alarming results obtained for some areas, the monitoring and biocontrol of A. artemisiifolia should be applied as a European strategy.
... On the other hand, northern Europe locations such as Poland registered higher annual values of 7050 pollen grains [65]. The start, end and length features of the MPS and even the pollen concentration are highly influenced by meteorological factors; hence, in recent years, different authors have studied how climate change will influence plant behaviour and its impact on people's health [67,68]. In the present study, we calculate the trends for the main MPS characteristics, registering statistically significant linear trends (p < 0.01) for the end of MPS, the annual pollen integral (SPIn) and the pollen peak. ...
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Black alder (Alnus glutinosa (L.) Gaertn.) is a species of tree widespread along Europe and belongs to mixed hardwood forests. In urban environments, the tree is usually located along watercourses, as is the case in the city of Ourense. This taxon belongs to the betulaceae family, so it has a high allergenic potential in sensitive people. Due to the high allergenic capacity of this pollen type and the increase in global temperature produced by climate change, which induces a greater allergenicity, the present study proposes the implementation of a Machine Learning (ML) model capable of accurately predicting high-risk periods for allergies among sensitive people. The study was carried out in the city of Ourense for 28 years and pollen data were collected by means of the Hirst trap model Lanzoni VPPS-2000. During the same period, meteorological data were obtained from the meteorological station of METEOGALICIA in Ourense. We observed that Alnus airborne pollen was present in the study area during winter months, mainly in January and February. We found statistically significant trends for the end of the main pollen season with a lag trend of 0.68 days per year, and an increase in the annual pollen integral of 112 pollen grains per year and approximately 12 pollen grains/m3 per year during the pollen peak. A Spearman correlation test was carried out in order to select the variables for the ML model. The best ML model was Random Forest, which was able to detect those days with medium and high labels.
... Based on climate change and pollution different plant invasion scenarios have predicted ragweed dispersal further north and towards higher altitudes resulting in prolonged pollen season and increased exposure and sensitization (Lake et al., 2017;Vogl et al., 2008). Furthermore pollution and higher temperatures have been found to increase ragweed allergenicity by increasing pollen production and influencing allergen content (Rogers et al., 2006;Wayne et al., 2002;Ziska and Caulfield, 2000). Currently, eleven allergens have been identified in ragweed, two are major allergens, and three are known pan-allergens: profilin, lipid transfer protein (LTP), and polcalcin (Chen et al., 2018). ...
Background: Ragweed is an invasive plant in Europe, causing hay fever and asthma in allergic patients. Climate change is predicted to increase expansion and allergenicity. Elevated NO2 induced upregulation of a new allergen in ragweed pollen, an enolase, Amb a 12. Objective: of this study was producing ragweed enolase as a recombinant protein and characterizing its physicochemical and immunological features. Methods: Amb a 12 was designed for E. coli and insect cell expression. Physicochemical features were determined by mass spectrometry, circular dichroism measurements and enzymatic activity assay. Immunological characteristics were determined in ELISA, in a mediator release assay and by investigation of association with clinical symptoms. Common allergen sources were screened for similar proteins. Results: Ragweed enolase was produced as a 48 kDa protein forming oligomers in both expression systems, showing differences in secondary structure content and enzymatic activity depending on expression system. IgE frequency and allergenicity were low regardless of expression system. Enolase-specific serum bound to similar sized molecules in mugwort, timothy grass and birch pollen, as well as food allergen sources, while highest IgE inhibition was achieved with peach pulp extract. Conclusions: Amb a 12 had high sequence similarity and comparable IgE frequency to enolase allergens from different sources. 50 kDa proteins were found in other pollen and food allergen sources, suggesting that enolases might be pan-allergens in pollen and plant foods.
... Their results showed that increasing atmospheric carbon dioxide would preferably increase the growth and spread of young trees. In addition, similar studies have suggested that ragweed quantitative and qualitative spread during the twentieth century has been due to increased concentrations of atmospheric carbon dioxide (Singer et al., 2005;Ziska & Caulfield, 2000). ...
... The temporal trend shown in Fig. 6, although qualitative by its nature, suggests that its importance may have increased in the past decade for the Baltic Sea and, after logical extension, possibly for global water bodies under similar pollen influence. Indeed, the increased pollen coverage is in line with several earlier studies which showed more pollen production under higher CO 2 concentration and higher temperature (Ziska and Caulfield, 2000;Albertine et al., 2014), with pollination season becoming earlier and longer in North America and increases in tree pollens being more prominent than in grass or weed pollen (Anderegg et al., 2021). The increased pollen coverage is also in line with Kahru et al. (2016), where more surface incoming shortwave irradiance and warmer waters were found in recent years than in the 1980s. ...
Multi-sensor satellite images often show recurrent image slicks in the Baltic Sea. Based on historical reports, laboratory experiments, and spectral diagnostics, these image slicks are determined to be caused by pine (Pinus sylvestris) pollen aggregations as opposed to surface scums of cyanobacteria or other floating matters. This attribution is because the reflectance spectral shapes of these image slicks resemble those of pollen grains, which are all different from other floating matters. They all show rapid reflectance increase from ∼400 to ∼500 nm, beyond which the reflectance shape appears to plateau. During the 22-year period of 2000–2021, satellite images indicated large amounts of pollen grains in 14 years, with the earliest day being May 10 (in 2000) and the latest day being June 16 (in 2006). The longest duration in a single year is 22 days, from May 12 to June 2 of 2018. The waters containing pollen grains have expanded significantly in recent years, encompassing nearly the entire Baltic Sea. Because pollen grains contain a significant amount of carbon and have distinctive optical properties from other particles, these findings have significant implications on carbon sequestration, marine ecology, bio-optics, ocean color calibration/validation, and remote sensing of marine debris.
... However, despite these beneficial effects on crops, the rising level of atmospheric CO 2 may have negative consequences for yield losses due to weed-crop competition (Ziska et al., 2019). Observations have shown that while vegetative growth of C 3 crops is favored over C 4 weeds under elevated CO 2 (Ziska and Caulfield, 2000), C 3 weeds may be favored over C 3 crops (Ziska et al., 2019). It has recently been suggested that the effect of drought may be overcome by some compensatory mechanism within the plants (Qi et al., 2021). ...
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Rising atmospheric CO 2 concentrations are known to influence the response of many plants under drought. This paper aimed to measure the leaf gas exchange, water use efficiency, carboxylation efficiency, and photosystem II (PS II) activity of Datura stramonium under progressive drought conditions, along with ambient conditions of 400 ppm (aCO 2 ) and elevated conditions of 700 ppm (eCO 2 ). Plants of D. stramonium were grown at 400 ppm and 700 ppm under 100 and 60% field capacity in a laboratory growth chamber. For 10 days at two-day intervals, photosynthesis rate, stomatal conductance, transpiration rate, intercellular CO 2 concentration, water use efficiency, intrinsic water use efficiency, instantaneous carboxylation efficiency, PSII activity, electron transport rate, and photochemical quenching were measured. While drought stress had generally negative effects on the aforementioned physiological traits of D. stramonium , it was found that eCO 2 concentration mitigated the adverse effects of drought and most of the physiological parameters were sustained with increasing drought duration when compared to that with aCO 2 . D. stramonium , which was grown under drought conditions, was re-watered on day 8 and indicated a partial recovery in all the parameters except maximum fluorescence, with this recovery being higher with eCO 2 compared to aCO 2 . These results suggest that elevated CO 2 mitigates the adverse growth effects of drought, thereby enhancing the adaptive mechanism of this weed by improving its water use efficiency. It is concluded that this weed has the potential to take advantage of climate change by increasing its competitiveness with other plants in drought-prone areas, suggesting that it could expand into new localities.
... They are both included on the list published by the Ministry of Ecology and Environment of the People's Republic of China [29]. Common ragweed, an annual weed in crop fields, usually forms dense mono-specific stands and produces a considerable amount of pollen [30], and this weed is one of the most problematic aero-allergens [31]. It was introduced into China in the 1930s and, since then, it has spread across twenty provinces [32]. ...
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As multiple invaders often co-occur, understanding the interactions between different invasive species is important. Previous studies have reported on invasional meltdown and neutral and interference relationships between invasive species. However, interspecific interactions may vary with environmental change owing to the different responses of interacting invaders. To better understand the interaction of notorious invasive alien plants under CO2 enrichment and N deposition, the growth characteristics of common ragweed (Ambrosia artemisiifolia) and redroot pigweed (Amaranthus retroflexus) were studied when they were planted in monoculture (4Rag and 4Pig) or mixture (1Rag:3Pig, 2Rag:2Pig, 3Rag:1Pig) under four environmental treatments: elevated CO2, increased N, elevated CO2 + increased N and a control. Increased N positively affected almost all the traits (basal stem diameter, height, shoot biomass, root biomass and total biomass) of common ragweed, except for branch number and root-shoot ratio. But increased N only promoted redroot pigweed’s height and basal stem diameter. interspecific competition promoted basal stem diameter and number of branches but decreased root biomass of common ragweed, and the basal stem diameter was significantly higher in 1Rag:3Pig and 2Rag:2Pig compared to the other two treatments. interspecific competition inhibited almost all the characteristics of redroot pigweed. The interaction between elevated CO2 and increased N also increased the biomass characteristics (shoot biomass, root biomass and total biomass) of common ragweed. However, elevated CO2 inhibited the root biomass of redroot pigweed. The results indicated that common ragweed was a superior competitor under conditions of elevated CO2 and increased N. Moreover, environmental change might strengthen the super-invasive plant common ragweed’s competitive ability.
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Global climate change and its consequences such as rising temperatures, carbon dioxide (CO2) levels, variable rainfall and soil salinity has an impact on weed species, their population dynamics, reproduction and competitive ability; eventually crop productivity. Among these environmental factors, rising CO2 levels will benefit C3 plants, but in the near future, plants with C4 pathways will be benefited more from rising temperatures and changing rainfall patterns around the world. Climate change can alter the weed life cycle, community composition, distribution, phenology and infestation. Over the course of the year, some weed species go extinct while others develop into more noxious invaders and evolve genetic artefacts in response to intensifying climatic and non-climatic selection pressures. Due to unfavourable climatic change, weed species release allelo chemicals along with increase the growth of below ground part such as root or rhizome, particularly in perennial weeds, making the weed more competitive over crops. On the solution side, variations in temperature, CO2 levels and rainfall alter stomatal conductance, cuticle viscosity, transport, uptake, leaf retention duration and herbicide efficacy. Therefore, to adapt and mitigate; it is important to review how climate change can influence the crop-weed interaction, competition, herbicide efficacy and resistance.
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Plants of six weedy species (Amaranthus retroflexus, Echinochloa crus-galli, Panicum dichotomiflorum, Setaria faberi, Setaria viridis, Sorghum halapense) and 4 crop species (Amaranthus hypochondriacus, Saccharum officinarum, Sorghum bicolor and Zea mays) possessing the C4type of photosynthesis were grown at ambient (38 Pa) and elevated (69 Pa) carbon dioxide during early development (i.e. up to 60 days after sowing) to determine: (a) if plants possessing the C4photosynthetic pathway could respond photosynthetically or in biomass production to future increases in global carbon dioxide and (b) whether differences exist between weeds and crops in the degree of response. Based on observations in the response of photosynthesis (measured as A, CO2assimilation rate) to the growth CO2condition as well as to a range of internal CO2(Ci) concentrations, eight of ten C4species showed a significant increase in photosynthesis. The largest and smallest increases observed were for A. retroflexus (+30%) and Z. mays (+5%), respectively. Weed species (+19%) showed approximately twice the degree of photosynthetic stimulation as that of crop species (+10%) at the higher CO2concentration. Elevated carbon dioxide also resulted in significant increases in whole plant biomass for four C4weeds (A. retroflexus, E. crus-galli, P. dichotomiflorum, S. viridis) relative to the ambient CO2condition. Leaf water potentials for three selected species (A. retroflexus, A. hypochondriacus, Z. mays) indicated that differences in photosynthetic stimulation were not due solely to improved leaf water status. Data from this study indicate that C4plants may respond directly to increasing CO2concentration, and in the case of some C4weeds (e.g. A. retroflexus) may show photosynthetic increases similar to those published for C3species.
The increasing atmospheric CO2 concentration probably will have significant direct effects on vegetation whether predicted changes in climate occur or not. Averaging over many prior greenhouse and growth chamber studies, plant growth and yield have typically increased more than 30% with a doubling of CO2 concentration. Such a doubling also causes stomatal conductance to decrease about 37%, which typically increases leaf temperatures more than 1 °C, and which may decrease evapotranspiration, although increases in leaf area counteract the latter effect. Interactions between CO2 and climate variables also appear important. In one study the growth increase from near-doubled CO2 ranged from minus 60% at 12 °C to 0% at 19 °C to plus 130% at 34 °C, suggesting that if the climate warms, the average growth response to doubled CO2 could be consistently higher than the 30% mentioned above. Even when growing in nutrient-poor soil, the growth response to elevated CO2 has been large, in contrast to nutrient solution studies which showed little response. Several studies have suggested that under water-stress, the CO2 growth stimulation is as large or larger than under wellwatered conditions. Therefore, the direct CO2 effect will compensate somewhat, if not completely, for a hotter drier climate. And if any climate change is small, then plant growth and crop yields will probably be significantly higher in the future high-CO2 world.
We tested whether the efficacy of chemical weed control might change as atmospheric CO2 concentration [CO2] increases by determining if tolerance to a widely used, phloem mobile, postemergence herbicide, glyphosate, was altered by a doubling of [CO2]. Tolerance was determined by following the growth of Amaranthus retroflexus L. (redroot pigweed), a C4 species, and Chenopodium album L. (common lambsquarters), a C3 species, grown at near ambient (360 μmol mol-1) and twice ambient (720 μmol mol-1) [CO2] for 14 d following glyphosate application at rates of 0.00 (control), 0.112 kg ai ha-1 (0.1 x the commercial rate), and 1.12 kg ai ha-1 (1.0 x the commercial rate) in four separate trials. Irrespective of [CO2], growth of the C4 species, A. retroflexus, was significantly reduced and was eliminated altogether at glyphosate application rates of 0.112 and 1.12 kg ai ha-1, respectively. However, in contrast to the ambient [CO2] treatment, an application rate of 0.112 kg ai ha-1 had no effect on growth, and a 1.12-kg ai ha-1 rate reduced but did not eliminate growth in elevated [CO2]-grown C. album. Although glyphosate tolerance does increase with plant size at the time of application, differences in glyphosate tolerance between CO2 treatments in C. album cannot be explained by size alone. These data indicate that rising atmospheric [CO2] could increase glyphosate tolerance in a C3 weedy species. Changes in herbicide tolerance at elevated [CO2] could limit chemical weed control efficacy and increase weed-crop competition.
Four populations of Phlox drummondii and one population each of Datura stramonium and Abutilon theophrasti were grown in growth chambers at 300, 600 and 900 mu l l-1CO2, all other environmental variables remaining constant. Changes in timing and numbers of flowers produced were species- and population- dependent. In general, P. drummondii and D. stramonium flowered earlier under high CO2; A. theophrasti was not affected. Significant population X CO2 interactions were found for several flower production characters in P. drummondii, indicating differential response to elevated CO2 levels even within a species. In D. stramonium, increased biomass in high CO2 caused significantly larger fruits to be formed, but there was no significant increase in seed number. In A. theophrasti, individual seed weight increased with increasing CO2, but total seed weight per plant remained constant. Results are discussed in relation to their possible implications to plant community structure, and the effects on higher trophic levels (eg pollinators and plant predators). -from Authors
Rising atmospheric CO2 levels could affect plant growth both directly, through effects on physiology, and indirectly, through the effects of possible CO2-induced temperature increases. In this study we examined the interacting effects of CO2 enrichment and temperature on the growth and allocation of soybean and five weeds. Individual plants of soybean [Glycine max (L.) Merr. ’Braxton’], johnsongrass [Sorghum halepense (L.) Pers.], quackgrass [Elytrigia repens (L.) Nevski], redroot pigweed (Amaranthus retroflexus L.), sicklepod (Cassia obtusifolia L.), and velvetleaf (Abutilon theophrasti Medic.) were grown in growth chambers in all combinations of two temperatures (avg. day/night of 26/19 °C and 30/23 °C) and two CO2 concentrations (350 and 700 ppm) for 35 d. Leaf area and plant biomass were greater at higher temperatures, regardless of CO2 level, in all species except quackgrass. Quackgrass (C3) produced its greatest leaf area and biomass at elevated CO2, whereas johnsongrass (C4) showed little response....
Plants were grown at either 350 or 1000 μl l−1CO2and in one of three photoperiod treatments: continuous short days (SD), continuous long days (LD), or short switched to long days at day 41 (SD–LD). All plants received 9 h of light at 450 μmol m−2s−1and LD plants received an additional 4 h of light at 8 μmol m−2s−1. Growth of SD plants responded more positively to elevated CO2than did LD plants, due largely to differences in the effect of CO2on unit leaf rate. High CO2increased height and decreased branching under SD conditions, but had no effect under LD conditions. Elevated CO2also increased the number of buds and open flowers, the effect for flower number being greater in short than in long days. The specific leaf area of plants grown at 1000 μl l−1CO2was reduced regardless of daylength. High CO2also decreased leaf and increased reproductive allocation, the magnitude of these effects being greater under SD conditions. Bud formation and flower opening was advanced under high CO2conditions in SD plants but bud formation was delayed and there was no effect on flower opening under LD conditions. The effects of CO2on plants switched from SD to LD conditions were largely intermediate between the two continuous treatments, but for some parameters, more closely resembled one or the other. The results illustrate that daylength is an important factor controlling response of plants to elevated CO2.
This paper presents a detailed analysis of several hundred plant carbon exchange rate (CER) and dry weight (DW) responses to atmospheric CO2 enrichment determined over the past 10 years. It demonstrates that the percentage increase in plant growth produced by raising the air's CO2 content is generally not reduced by less than optimal levels of light, water or soil nutrients, nor by high temperatures, salinity or gaseous air pollution. More often than not, in fact, the data show the relative growth-enhancing effects of atmospheric CO2 enrichment to be greatest when resource limitations and environmental stresses are most severe.
Six early successional plant species with differing photosynthetic pathways (3 C3 species and 3 C4 species) were grown at either 300, 600, or 1,200 ppm CO2 and at either 0.0 or 0.25 ppm SO2. Total plant growth increased with CO2 concentration for the C3 species and varied only slightly with CO2 for the C4 species. Fumigation with SO2 caused reduced growth of the C3 species at 300 ppm CO2 but not at the higher concentrations of CO2. Fumigation with SO2 reduced growth of the C4 species at high CO2 and increased growth at 300 ppm CO2. Leaf area increased with increasing CO2 for all plant species. Fumigation with SO2 reduced leaf area of C3 plants more at low CO2 than at high CO2 while leaf area of C4 plants was reduced more at high CO2 than at low CO2. These results support the notion that C3 species are more sensitive to SO2 fumigation than are C4 species at concentrations of CO2 equal to that found in normal ambient air. However, the difference in sensitivity to SO2 between C3 and C4 species was found to be reversed at higher concentrations of CO2. A possible explanation for this reversal based upon differences in stomatal response to elevated CO2 between C3 and C4 species is discussed.
1. A study of staminate flowers of common ragweed (Ambrosia artemisiifolia L.) revealed six more or less defined stages, which are described: (a) maturation of the flower; (b) extension of pollen sacs; (c) opening of pollen sacs and dropping of pollen clumps; (d) flotation of pollen by wind; (e) extension of the pistillodium; and (f) closure of the flower. Stages b and c were examined in relation to environmental factors-temperature, humidity, light, and water availability. 2. Actual pollen release is accomplished mainly by enlargement of the filaments of the stamens which extends the pollen sacs and pushes open the corolla lobes, and by dehiscence of the sacs by pleating and separation of their walls and extension of the anther appendages. The pistillodium may function as a "sweeper" of pollen grains still remaining in the anthers. 3. Field studies showed the occurrence of a definite diurnal periodicity: extension and opening of the anthers occurs between 6:30 and 8:00 A.M. This is correlated with rise i...