Germination of C0,-enriched Pinus taeda L. seeds and
subsequent seedling growth responses to CO, enrichment
M . H U S S A I N , " M . E . K U B I S K E * ? and K . F. C O N N O R S
rMz,trr\lppr Sfare fJtzctler\rfy, Departr7irrzt of Fore\trv, BOY9681, Mz\\r\\rppc S f o f ( :MS39762-9681, USA, und
f LIS Fore\r Scrlvr e 110 Thon1ptor7 Hirll Box 9681, M I \ S Z \ S L ~Sfate,
J I I M S 39762-9681, U S A
Summary
1. Pinus tuedli seeds, developed under ambient or elevated (ambient + 200 pl1 I) [CO,],
were collected from Duke Forest, North Carolina, USA in October 1998. Seeds were
germinated in nutrient-deficientsoil in either ambient or elevated [COJ (ambient + 200 p11 ')
greenhouse chambers and allowed to grow for 120 days.
2. Seeds that developed in elevated [COJ had 91 and 265% greater weight and lipid
content, respectively, and three times the germination success, compared to those
developed in current ambient [CO,].
3. Seedlings from the elevated [CO,] seed source had significantly greater root length
and more needles regardless of greenhouse chamber, but there were no treatment effects
on tissue or total biomass.
4. Severely limiting nutrient conditions resulted in significant photosynthetic downregulation by seedlings grown in greenhouse chambers with elevated [CO,], regardless
of seed source.
5. Our hypothesis that greater seed reserves from CO, enrichment would synergistically
affect seedling growth responses to elevated [CO,] was not strongly supported. Nonetheless, seeds produced in a C0,-enriched environment may have fundamental changes in
their viability, chemistry and germination that may affect reproduction.
Kel~-ivoitlt Elevded CO,, Plnu, tcredc~,seed germination, seed lipids, seed p r o d u ~ t ~ o n
Furx rroncrl E i o f o ~ (2001)
v
15, 3 4 4 3 5 0
Introduction
Increasing atmospheric carbon dioxide concentration,
[CO,], has profound effects on growth and development of trees. A doubling of [CO,] generally stimulates
photosynthesis (Murray 1995; Saxe, Ellsworth & Heath
1998) and can lead to a substantial increase in tree
growth (Poorter 1993). For example, doubling [CO,]
increased the rate of photosynthesis per unit leaf area
(A) of Pinus tiiec/c~by 1 4 146'%)(Ellsworth 1999; L ~ L&I
Teskey 1995; Thoinas, Lewis & Strain 1994; Tissue
er trl. 1996; Tissue e f (11. 1997), and inci-eased relative
growth by 40-233'%1 (DeLucia ct ul. 1999; Sionit et crl.
1985; Tissue et crl. 1996; Tissue ct (11. 1997). While it is
well known that nutrient availability can greatly
affect the CO, response (Kubiske rt crl. 1998; Pregitzer
c f crl. 1995; Thomas c.f (11.1994), we suggest that in very
young seedlings, elevated [CO,] effects may interact
with internal factors sucll as seed N or C reserves. Thus
internal and external factors that regulate growth at
the seedling stage may act collectively to regulate
11 2001 British
Ecological Society
tAut11or to whom correspondence should be addi-essed. E-mail:
rnkubiskc(r2:cfr.111sst:tte.edu
subsequent seedling growth (Corbineau & Come
1995).
Virtually all elevated [CO,] work on trees has been
conducted on seedlings grown from seed that developed
under current ambient CO, conditions. A parental
environment of elevated [CO,] may produce seeds with
increased amounts of carbohydrates, lipids and proteins
to support early seedling growth. The contents of these
substances in seed have profound effects on gern~ination,
because they are utilized as both substrate and energy
source for the growing seedlings (Kernlode 1995).
For the same reasons, the reserve content of seeds
may also affect seedling responses to atmospheric
[CO,]. For example, Miao (1995) grew plants from
Qurrcus ruhrc~acorns and found that seed size acted
synergistically with atmospheric [CO,] to enhance
seedling growth. Since developing seeds are usually
very strong C sinks (Zamski 1995), it is very likely
that enhanced photosynthetic production will improve
s important synerthe energy reserves of seed. T h ~ t an
gistic effect may be missing from virtually all elevated
[CO,] studies on trees: that of seed energy reserves
in combination with atmospheric [CO,] enrichment.
Such studies are essential to fully understand growth
345
Gerr?~rr~iitrorz
and qro~t117 of
CO2-1311r21hrd
P taeda \et>iA
at the time of collection, and only the seeds of open cones
and development of t ~ e eseedltngs 111 1' futule, CO,were used in this experiment. Seeds were de-winged.
cni~chedecosystem
floated in water for 2 days, and stratified for 120 days
Untrl now, ~t hds been i ~ n p ~ ~ t c tto
~ cstudy
~ i l seed
ptoduct~on'ind development of forest tiees 7 I1e developat 10 "C; seeds that remained floating were discarded
nlent of flee-mr CO, eniicl~ment(FACE) technology
(Schopmeyer 1974).
Following stratification, seeds were sown in 0.16 1
for f o ~ e s c'tnoples
t
now makes such s t u d ~ c sposs~blc
(flendrey ef trl 1999) Sevei a1 FACE 1e5enrch f ~ l c ~ l ~ t i e s (4 cm diameter x 21 crn long) tubes in a greenhouse at
Mississippi State University, USA, on Mal-ch 19, 1999.
are piesently oper'ittng 111 forest coinrnur~~t~es
't~onnd
the wo~ld.~ncluct~ng
PIMLI\tae(/lr In Noith C'irol~nd,
The tubes were filled with eclual an~ountsof coarse, sterile
silica sand. Seecls were placed on the sand surface
IJSA (DCLLICI'I1~ (11 1999, Ell\wo~th 1999, Myers,
T11ornas & D e L ~ l c ~1999),
a
Poptrllrr frer~rzrlorcl~~,
Bcflrb
=2 cm below the rim of the tubes. All seeds were watered
for 2 nlin every 2 h by an automatic inisting system. No
/~trpii r f c ~ r r'ind At cr rac t htri 11/11 111 W~sconsin,USA
( K a ~ n o s k yet (11 1999a, Knrnoskq rr a1 1999b),
additional fertilizers were used, so that early seedling
de\clopment was ln~gelyI' funct~onof seed storage
L/cllrro'crr7zhrn \rr.rtrc~flir(iin Tennessee, USA (Gunderson
The tubes wele then placed 111to stx c11,lnibers
c f 111 1999). Poptr/lr\ spp 'it V ~ t e ~ b'ind
o S~ena.Itnly
(1 2 x 1 2 x 0 9 in), constructed In the g~eenhouse,that
(Tognett1 el rrl 1999), ,tnd tiop1c~11f o ~ e s tspeclej 'tt
were enclosed on the top and four s ~ d e by
s transpn~ent
S a r d ~ n ~ l l PanamLt
a,
We collected P I I I ~ f(/e(kr
O
seed
p l a s t ~and
~ by plywood on the bottom E'ich ol the six
that developed uncte~ FACE to study the subsequent
c l ~ d n l b e ~cso ~ ~ t ~ i ~28n eseeds
d
14 froin FACE rings
~
[CO,]
response of seedl~ngsunder s i t n ~ l nelevdted
(refel red to CIS the CO,-enriched seed source), and 14
conditrons It wns hypotl?esized that apply~ngelcv,~ted
result In greater early growth of seedlings. Tins expectat ~ o nwai based on the premlse th'tt the increased seed
reserves would provide additional substrates for growth,
and nlso reduce feedback inhtbttion of seedl~ngphotosynthes~sunder elevated [CO?]
Materials and methods
SI:I:D
I'ROI>U('TION,
C'OLLLC rION AND
GERMINATION
(: 2001 Ht.itish
I3cologic~1lSociety,
I;irr~c,tio~itrl
G.olo,qy.
15, -344350
The Duke Forest (Orange County, North Carolina,
USA) FACE experinlent was constructed in 1996 in a
14-year-old I? trrctr'o.plantation. It consisted of a randoinized block design of six 30-nl-diarnetei- open-air gas
fumigation arrays in three replicate blocks. Three of
the arrays fumigated the forest c;inopy with arnbient
air enriched by 200 yl I ' CO,, and three with ambient
air. The elevated [ C 0 2 ]plots were fumigated for 81
and 79'1/;1of 1997 and 1998, respectively; the annual
~ivcragc[CO?] at the centre of each FACE plot was
199 2 0 3 i: 84 yl 1 ' above the ambient concentration.
Pi~zlrsrrrctltr is an important timber tree species in
the south-east USA, and its reproductive cycle is
typical of North American pines. it ~iormallyreaches
reproductive maturity at age 5 1 0 years (Schopmcyer
1974). Pollination occ~li-sbetween February ~ i n dApril.
Fertilization takes place the year following pollination,
and cones nature by October of that same year (t3~1rns
& Honkala 1990). In 1997, at age 15, a few of the canopy
trees had begun to produce cones. The seeds that were
collected in the present stucty underwent their entire
developmental cycle, fi 0111pollin,ition th~oughnlittul ~ty,
~lnderexpel ~~llent'il
tre,itments
A tot'll of 39 concs t1l'it ~n~it~iiecl
In 1998 wetc collected
l i o ~ nthe Duke Foiest experl~nclitbetween Octobci 29
'~ndNovembe~1. 1998 Most 01 the cone\ w c ~ eopen
and treatment 111 each chamber, for 1' totnl of 168 ~ e e d s
in the experiment The seedling tube? were drrdnged
randomly w ~ t h ~each
n chdmbei Three blowers, each
connected to two chambers (one nmbient chamber and
one CO,-enr~chedchamber), circulated alr through the
chambers at n rdte of two chamber volumes per n11n
Volume-How regulators were used to dispense pure
CO, g'is ~ n t othe blower air \tredln of one chamber rn
each palr The concentrntion of CO, 111 each chmnber
WLIS C O I I ~ I I I L I ~non~torcd
~I~~~
w1tl1a17infrrlied g'i~'kn'ilyser
(LI-Coi model 6252, L~ncoln,NB, USA) 'tnd logged
to n ~ o r n p u t e r Thus three of the chainbers wele
conttnuously supplied w ~ t h'tmb~ent+ 200 y1 1 CO,
(517 759 p1 1 at 9S1X1conf~dence~ n t e ~ v ~refericd
tl,
to
as elevated), \tmilar to the iuinig,lt~onpiotocol used at
Duke Fo~est,while the otller tl-t~eech'imbcrs (letelled
to 1' s 'i~nbient)were suppl~edeq~idlvolumes of amb~ent
,111 (365 481 y1 1 ', 24 11, 95'1/;1 conf~dence~nterv~il)
Amh~entch,imbers were vented into the g~eenhou\e
whe~e'lselevated [CO,] ch'unbezs wele vented to the
outs~dc,ttr Ch,tinbe~and \ o ~ teinpe~
l
. I ~ L I I ~( \~ ~ l c , t s u ~ e d
in two tubes per chamber with copper-constantan
thermocouples) zivei-aged 34.3 and 33.9 "C, respectively,
throughout the experiment.
'
'
The seeds in all cha~nberswere monitored twice daily
for enlergcnce of radicles. Upon racliclc emergence, the
date was recorded and seeds were immediately cove]-ed
with 1 cln of sand. fiypocotyl height and cotylcclorl
length were measured wl1e1-ithe seedlings re;tched their
first resting stage, which was identified by the appearance
of 21 terminal bud. Subsecluent height-growth ineasureinents from the sand surface to the top of seedlings were
conducted on May 17, June 2 and 27, and July 1 1 and 16,
for all seedlings. At the end of the experiment seedlings
had not yet beg~lnto prod~lceneedle fascicles. Mean
relative height-growth incre~nentfor each measurement
il~tervalwas calc~ilatedaccording to Hunt (1 982):
where lnI2, and InH, are the natural logs of heights at
times T2and 7',,respectively.
Seedlings were harvested on July 17, 1999 and scparated into leaves (all leaves were j~lvenileleaves), stems
and roots for additional analysis. The number of
needles per seedling were counted and total projected
leaf areas werc measured with a leaf-area meter (LiCor n~odelL1-3100. Lincoln, NE, USA). The length of
the first-order root (tap root) from the root collar was
measured by carefully extending it along a cm scale.
Nitrogen concentrations were deterlnined using a CNS
analyses (Fisons Model NA-1500, Milan, Italy) on
oven-dry, grotlnd tissues. Biomass was determined
following oven drying at 70 "C for 48 h.
Shoot net assin~ilationefficiency was measured as
the initial slope of a photosynthesis - [COJ response
f~lnctionon two plants from each chamber at the time
of harvest. Seedlings were randomly selected from
each seed source (ambient and C02-enriched seed) in
each greenhouse chamber for a total of 12 seedlings.
A portable infrared photosynthesis system (ADC,
Model-LCA2, Hoddesdon, UK) was used. A gas cylinder
supplied C0,-enriched air to the systeln and a soda-lime
scrubber was used to modify the cuvette [CO,]. The
entire needle-bearing epicotyl was enclosed in a conifer
cuvette and exposed to 1000 pi1101 111 s ' photosynthetically active radiation, which is saturating for
Pinzi.s tcrr(i'cl (Liu & Teskey 1995). C 0 2and H,O vapour
exchange were recorded at cuvette [CO,] ral~gingfrom
50 to 500 121 1'. A period of 10- 15 miti was allowed
for leaf equilibration at each cuvette [C02].Seedlings
were considered equilibrated when a steady-state CO,
differential was observed for 3 min. Photosynthesis
( A ) and leaf internal CO, concentration (C,) were
cnlculated based on total projectcd leaf area (measured
with a leaf-area metet; Li-Cor model LI-3100) in the
cuvctte according to Von Caemmcrer & Rirquhar (198 1).
'
A subset of seed was used for total lipid determination
using a petroleum ether Soxhlex extraction (1-lorwitz
1965). Seeds were lyophilized and stored in a vacuuin
desiccator ~intilbeing ground with a mortar and pestle.
Ground seed material (98 215 mg) fro111 each Duke
Forest treatment ring was placed in a separate cellulose
thimble (total of six), and lipid was extracted with
boiling petroleum ether at 100 OC in a recirculating
boiler condenser. Additional petroleum etlier was added
periodically to m;lintain volu~ncat 6 0 ml. After S 11,
XOOI British
the hot petroleum ether-lipid extract was collected into
Ecological
flasks and evaporated overnight. Total
~ l t r , c . ~ ; o r l r r ~ ~ ~ ~ Oprcwcighed
~O,y~~,
lipid mass was then detcrlnined by weighing the flasks.
15, 344 350
S T A T I S T I C A L ANA1,YSIS
The relationships between individual seedling height
and age were co~uparedusing least-squares regression.
All other data were analysed as a randomized complete
block design with a split-plot treatment arrangement
with subsampling. The whole-plot effect was greenhouse
chamber, and the split-plot effect was seed source
within chambers. For each measurement the mean
t
used in analysis
values within each split-plot ~ l n iwere
of variance. Significant differences between treatment
means were determined using Fisher's protected LSD
(least significance differences) at P < 0.05.
Results
The CO,-enriched seed source had 91% greater weight
(I;,,, = 10.6, P < 0.05) than the ambient seed source
(Table I). In addition, the C02-enriched seed source
had 128'%1greater lipid concentration (F,,, = 54.0, P <
0.05) and 265'%) greater lipid content (F,,, = 26.78,
P < 0.05). Seed source also significantly affected the
rate and success of germination (F,,, = 46.37, P < 0.05)
regardless of chamber C 0 2 treatment (Fig. 1). The
total germination success of the CO,-enriched seed
source was more than three times that of the ambient
source (mean of 95 versus 28'%),respectively). In addition, the CO,-enriched seed source began germinating
up to 5 days earlier than ambient seed. Greenhouse
chamber treatments had no effect on germination.
Seed source had no significant effect on seedling
relative height growth rates (data not shown). However, at 98 days the relative height growth increment
for the final 5 days of the study was significantly higher
for the C02-enriched seed source grown in the elevated
[CO,] chambers (0.01 cm cm day ' compared to
0,005 cm cm ' day for all other treatment combinations; F,,, = 12.55, P < 0.05). Consequently, the CO,enriched seed source produced significantly greater
(F,,, = 0.68, P < 0.05) seedling bio~nassin the elevated
[COJ greenhouse chambers, but those seedlings were
not significantly larger than any seedlings grown in the
ambient [CO,] chambers (Table 2). The C02-enriched
seed source produced seedlings that had more needles
(F,,, = 14.77, P < 0.05), and longer first-order roots
'
'
Table 1. Dry mass, total lipid conceiltration and content
(mean i SE) of Pinrrs rcrcdir seeds collected froin a free
atmosphericCO, enrichment experiment,North Carolina, USA
Parameter
Seed mass (nig)
Lipid concentration ('5))
Lipid content (mg)
Alnblent
1 1.6i 2.25
5.30 i 0.50
0.43 i 0.06
Elebated
22.0 i 2.31
12.10i 0.80
1.57 It 0.34
Secds werc produced in either ambient or elevated
(ambient + 200 yl I ') atmospheric C02conce~itration
(n = 3, and five to tcn seecis per replicate). The CO? effect
was significant for all three paranletcrs ( P < 0.05).
.
Seedlings grown in greenhouse chambers with elevated
[COJ had significantly lower slioot net assimilation
efficiency (shallower initial slope of A-C02 response
curve) compared to those in the ambient chambers for
both the ainbient seed source ( t = 0.01 9, P < 0.05) and
the elevated seed source ( r = 0.022, P < 0.05; Fig. 2).
Seed soul-ce had no effect on whole-shoot net assitnilation efficiency.
Gerrniriution
a n d g r o ~ v tqf
l~
C0,-enriched
P. taeda seecis
Discussion
)
Days since sowing
Pig. 1. Cumulative gerlninatioli perce~itversus days since
(DSS; based on date of radicle emergence) of Pinu,y
trrrrlLl seed sources that developed ullder
(solid
syiiibols) or elevated (ambient + 200 p1 1 ',open symbols and
crossed circles) atmospheric CO, concentration. Seeds were
germinated in either ambient (triangles) o r elevated (circles)
greenhousc chambers. Points represent three replicates of 14
seeds in each seed source by greenhouse chamber combination.
Regression equations are significantly different (P < 0.05)
(ambient seed source: %I germination = -52.1 5 + 3.71
DSS 0.02 DSS', r' = 0.59; elevated seed source: '%I germination
= -1 50.22 i13.80 DSS 0.19 DSS2, r k 0.84. One elevated
seed source by elevated greenhouse chamber replicate (crossed
circles) was not incl~tdedin the regression).
-
(F,,, = 13.98, P < 0.05) than the ambient seed source
regardless of chamber growth conditions. Howevel;
treatment effects on the biomass of those organs were
not significant. There were no significant treatment
effects on seedling allonietry in terms of leaf area ratio
(overall mean of 0.20 1: 0.02 cm2 projected leaf area
mg ' stem mass) or root : shoot dry mass ratio (overall
mean of 1.1 5 F 0,231, or on tissue N concentrations
(leaves = 5.89 rt 0.02 mg g '; steins = 4.46 f 0.03 ing g I;
roots = 4.86 F 0.02 ing g I).
Elevated [COJ didmaticnlly ~ncieasedthe stze , ~ n d
Iip~dcontent of Pznut tuerki seeds Herbaceous and
clop speclec often exhlblt incleases In seed slze (Baker
er nl 1989, Kiinball & Maulley 1993) or lipid content
(Rogers, Thomas & Bingh'im 1983) undet elevated
[CO?].We know of only one other study that examined
such effects in trees: Connor o r ul. (1998) found a
significant increase in Cornus.fiori(k~fi-uit mass under
elevated C 0 2and temperature using branch chambers.
Lipids comprise the largest portion of C storage in
Pinzi.s seed, four times that of carbohydrates (Bewley &
Black 1994). Various lots of R taedu seed contained
7.6-9.7'Yi total fatty acids, in agreement with lipid
concentrations reported here (Marquez-Millano 1989).
Storage lipids are synthesized from photoassiniilated
sucrose translocated to seeds via phloem (Slack & Browse
1984). Developing fruits and seeds are high priority C
sinks, and carbohydrates are usually allocated to favour
reproductive versus vegetative structures (?%rdlaw 1990).
The capacity of vegetative struct~tresto utilize assiinilates depends in part on the efficiency of sy~ilplastic
transport at the site of phloem unloading (Tl~orne
1985). In contrast, high sink strength of developing
fruit and seed results lai-gely from a low-resistance,
apoplastic phloem-unloading pathway; the ability of
developing seed to utilize photosyntliate is more strongly
dependent on carbohydrate supply compared to vegetative sinks (Cannell & Dewar 1994; Tliorne 1985).
Increased assimilation under elevated [CO?],such as
that reported for P trrccr'o at the Duke FACE experiment (DeLucia el (11. 1999; Ellsworth 1999; Myers (11.
Table 2. Growth characteristics (mean i SE) of Pinus tric~rlcrseedlings grown in ainbieiit and elevated C 0 2(ambient
greenhouse chambers from seed pmdnced under ambient and elevated (ambient + 200 p1 1 ') CO?
Anibiclit greeiiti~usechambers
+ 200 pl 1 ')
Elevated greenhouse cl~ainbers
p.dl-.dllleter
0 2001 British
F c o l ~ g i c Society,
~~l
Fzrnctiontrl Eco/o,qj,.
15, 3 4 4 350
Total seedling height (cm)
Total scedliiig biomass (mg)
Leaf Illass (i~ig)
Stein inass (mg)
Root mass (iiig)
Leaf area per seedling (cin')
Number of leaves
Taproot leiigth (cm)
Means are of thl-ee rcplicatcs ( 1 1 = 3) consisting of three to 14 seedlings each depending on seed germination success. Means
within a row not followed by thc same letter arc sig~iificantlydifl'erent ( P< 0.05).
("201 British
L:.cologicalSociety,
, , ; ~ , , ,
15, 344 350
of P rtrc~tltrdeclined in proportion to the loss of fatty
acid content with seed ageing (Marquez-Millano 1989).
During germination. nlost lipid reserves are broken
down into their constituent fatty acids, converted to
sugars. and transported to the cotyledons and axis for
dry weight gain and inaintenance (Doman (11. 1982;
(iori 1979; Shewry & Stobart 1973: Stone & Giff'orcl
1999; Vanni, Vincenzini 8i Vincieri 1975). Doinan cJr (11.
(1982) pointed out that axis ctry weight gain in the first
48 11 of germination was allnost exclusively root growth.
which would agree with our findings of earlier radicle
emergence and generally longer roots of the COzenriched secd source (Fig. I : Table 2).
Because of its relatively sinall seed size, Pirizc.c ttrerkr
seedlings begin to depend upon current photoassiinilates during cotyledon expansion ( D. J. Gifford, personal
communication). Both seed sources showed significant
down-regulation of shoot assitnilation d'ficiency when
grown under CO, enrichment. (It is not likely that stem
respiratiot~diffel-entially afected our estimates of shoot
assirnilation. because leaf-area ratios did not difrer
arnong treatments.) Photosynthetic down-regulation
under elevated [CO,] is often reported where limited
rooting environlnent restricts the belowground C sink,
or where severe llutrient limitations restrict overall plant
Fig.2. Shoot net C 0 2 assimilation rate of Pir111.strretkii
growth (Arp 1991; Eainus & Jarvis 1989; Sage 1994;
seedlings grown in either ambient (triangles)or elevated CO,
Stitt 1991; Thomas & Strain 1991: Tissue e t (11. 1996).
(ambient + 200 pl 1 ', circles) greenhouse chambers, and
from sced that developed under ambient (solid symbols, a) or
Our seedlings were not constrained by rooting volurne
elevated C 0 2 (open symbols, b). Regression lines were littect
at the time of harvest, but they were severely nutrientto data for three seedlings from each treatment combination.
limited, which probably contributed to the overall
Regression equations are: ambient CO,-grown seed in ambient
decrease in assimilation efficiency (cf. Kubiske r t (11.
CO, greenhouse chambers (A). A = 0.416 + 0.01 1 C;, r 2 = 0.97:
1998; Medlyn cJrtlI. 1999; Thomas r / ( I / . 1994) and the
ambient CO2-g1-ownseed in elevateci CO, greenhouse chambers
(e).A = -0.237 + 0.007C7,.r L = 0.94; CO1-enriched sced in
overall lack of a CO, effect on growth.
;tmbient C02greenhouse chambers (A), A = -0.414 + 0,012C;,
GI-owthincreases of up to 233'%,with COzenriclnnent
r 2 = 0.94: COL-enrichedseed in elevated C O , greenhouse
hiwe been reported for P t ~ r e r l t r(Gebauer e t i l l . 1996;
chambers (0).
A = 0.787 + 0.006C;. 1.' = 0.98.
Tissue cJ/ (11. 1996, Tissue r t (/I. 1997). DeLucia PI NI.
(1999) reported a sustained growth increase for 2 years
for I? trocvltr at the Duke Forest FACE experitnent.
1999). should generally increase C supply to developing
Strong growth responses to elevated [CO?] were not
fruit and seed. Moreover, this I-esponsc should occur
observed in our experiment, and our hypothesis that
even where no vegetative growth response of the maternal
the elcviltcd C 0 2seed source would enjoy a synergistic
growth increase under [CO,] enrichnlent was not strongly
parent occurs.
The elevated [CO?] sced source in our study had
supported. The sniall sample size, severe nutrient
three ti~nesthe gernlination success (9O'%,gern-rination)
limitations iund photosynthetic down-regulation probably contributed to unimpressive growth responses.
and earlier germination times compared to the arnbient
seed source. Both rest~ltswere probably related to the
Our objective in this study was to understand the
26j1%,diil'erence in seed lipid content. During sced
potential interactive responses of elevated [CO,] on secd
performance of
development lipids in the nucellar layer, which lies
development and subsequent seedli~~g
between the secd coat and the megagametophyte. appear
L I I important foi-est tree species. Seedling respo~~ses
to provide metabolic energy for maturation of the
play an important role in tree regeneration and succesembryo (Tillman-Sutela cr i l l . 1996). Also, the lipid
sion, because germination and initial seedling growth
content of that layer. co~nposedprimarily of polar glycoset the pattern for future growth (Miao 1995). For
example, in I? trrc(/ri, laiger seed had significantly shorter
and phospholipids that d o not restrict the passage
of water, was related to imbibition and germination of
germination times than snlaller secd, which influenced
seedling height after 28 days (Dunlap & Barnett 1983).
I? .sj.li~r.sfri.\.(Tillman-St~tela (11. 1996). Howevel-. the
bulk (85'%,)of I? rrreiltr seed energy reserves lie in the
The growth advantage gained froill larger seed inass
was obsel-ved in standing tree volumes 15 years after
~~~cg;igail-retopl~yte
lipitl pool (Stone & Giflbrcl 1999).
sowing
(Robinson & van Buijtcnen 1979). Thus a small
Gern~ination
is
an
energy-demanding
p
r
o
m
s
that
is
,
li~clledby respiration of firts. 'l'he germination s~~ccess gronltll advantage due to earlier germination time 01-
trrltl gro~c.thof'
C'O,-c~ririchrd
P. taeda srrtls
seedling vigour often compounds over tinie to affect
tree-growth characteristics years later.
Our results have profound i~nplicationsfor reproductive strategies of trees if elevated [COZ]has dissimilar.
effects on seed develop~iietitamong species. For example,
P ttce,(tkr seecl reserves directly influence seedling growth
for only a very short period of time, after which new
photo,~ss~iiiiIntes
suppoi t gionth It 1s ersti~clyposs~ble
that ln~gel-seeded competltol s, s~tclias Qlrer c ur or
Cur 1 N , w o ~ l drespond In n d~ffe ~ e nwdy
t
Although the
reedllng glowth phase of tlns study dtd not coticluslbely suppoi t oui mcilli hypothesis, there were striking
effects of elevdted [CO,] 01s the s i x , ~hernistiy and
gernnnnt~onot the seeds themselves As our seed wele
extracted fro111open Lolies, i t is I I C ) ~clenr IS our findlngq
nle representntive of '111 P rcr'thr seed produced nt the
is thnt
Duke F'oiert experiinent in 1998 Oiie posstb~l~ty
i ~ i d ~ v ~ dcones
u n l under clcvdted CO, c o ~ i t d ~ ~al elarge1
d
propol tion of l~pld-i~cli.
vi'lble seed, and the populntlon of v~ableseed fio~lithe dmb~entseed sou1c.e hdd
shed prlol to our collect~onA n o t h e ~poss~b~llty
IS thnt
the manner of \eed sliedd~ngvnr~edbetween CO, tredt~nentsAdd~tionalwork 1s needed to ~esolvethese questions, so that the ecologicnl ramlficat~onsof elevated
[CO,] eil'ects on Pznlrr seed product~oncan be assessed
Acknowledgements
The Duke FACE project 1s s~tpportedby the US
Department of Eneigy The g~eenliousework was
f~tildedIn part by compet~tiveg~,ttit no 97-351065056, US Depart~iientof Ag~~cult~tle,
Nnt~olinlKesedrch
Iinttntlvc JetTery Hutclitnson dideci in construction
and mainteliance of the greenho~~se
chambers. and
Jua~iitaMobley ltelped witli the N analysis. Sam Land,
Elllily Scliultz a11t1Jack Vozzo pi-ovided critical reviews
of the tnanuscript. Approved for sub~nissionto peer
review as manuscript number F0145 of the Forest and
Wildlife Research Center, Mississippi State University.
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