Diurnal patterns of gas-exchange and metabolic pools in tundra plants during three phases of the arctic growing season.

1.6 MB·English

by  Patankar, Rajit

Diurnal patterns of gas-exchange and metabolic pools in
tundra plants during three phases of the arctic growing
season
Rajit Patankar1, Behzad Mortazavi1,2, Steven F. Oberbauer3 & Gregory Starr1
1DepartmentofBiologicalSciences,UniversityofAlabama,Tuscaloosa,Alabama
2DauphinIslandSeaLaboratory,DauphinIsland,Alabama
3DepartmentofBiologicalSciences,FloridaInternationalUniversity,Miami,Florida
Keywords Abstract
Alaska,circadianclock,photoperiod,
Arctic tundra plant communities are subject to a short growing season that is
photosynthesis,respiration,sugars,total
nonstructuralcarbohydrates,tussocktundra. the primary period in which carbon is sequestered for growth and survival. This
period is often characterized by 24-h photoperiods for several months a year.
Correspondence To compensate for the short growing season tundra plants may extend their
GregoryStarr,Dept.ofBiologicalSciences, carbon uptake capacity on a diurnal basis, but whether this is true remains
1325S&EComplex,300HackberryLane,
unknown. Here, we examined in situ diurnal patterns of physiological activity
UniversityofAlabama,Tuscaloosa,Alabama
and foliar metabolites during the early, mid, and late growing season in seven
35401.Tel:[phone];Fax:+1
arctic species under light-saturated conditions. We found clear diurnal patterns
(205)-348-1786;E-mail:[email protected]
in photosynthesis and respiration, with midday peaks and midnight lulls
FundingInformation indicative of circadian regulation. Diurnal patterns in foliar metabolite concen-
ThisstudywasfundedbytheArcticNatural trations were less distinct between the species and across seasons, suggesting
ProgramwithintheNationalScience that metabolic pools are likely governed by proximate external factors. This
FoundationOfficeofPolarPrograms(NSF
understanding of diurnal physiology will also enhance the parameterization of
award806776).
process-based models, which will aid in better predicting future carbon dynam-
ics for the tundra. This becomes even more critical considering the rapid
Received:9July2012;Revised:5December
2012;Accepted:11December2012 changes that are occurring circumpolarly that are altering plant community
structure, function, and ultimately regional and global carbon budgets.
EcologyandEvolution2013;3(2):375–388
doi:10.1002/ece3.467
2003; Starr et al. 2008), which limits our understanding
Introduction
of arctic plant carbon acquisition to a narrow temporal
Over half a century has passed since the initial studies window during the diurnal time course. Furthermore,
on photosynthesis in arctic tundra vascular plants whether tundra plants maintain or depart from predicted
(Bliss 1956, 1962; Mooney and Billings 1961). However, circadian rhythms would influence estimates of key eco-
surprisingly, given the unique, short growing season char- system processes, such as primary productivity, carbon
acterized by 24-h photoperiods, a fundamental question balance, and the parameterization of process-based eco-
remains: Do arctic tundra plants show circadian rhythms system carbon models (such as GAS-FLUX, Tenhunen
in their photosynthetic cycles during the growing season? et al. 1994; SPA, Williams et al. 1996) that incorporate
This lack of knowledge is all the more extraordinary given species-level physiological parameters. Thus, the need to
the recent trend toward understanding linkages in carbon understand diurnal patterns in arctic tundra vascular
dynamics across multiple scales (leaf to ecosystem) within plants across diverse growth forms (evergreen, deciduous,
the context of arctic climate warming (Chapin et al. 2005; graminoids) remains crucial for several compelling rea-
McGuire et al. 2009; Post et al. 2009). To date, ecophysi- sons.
ological studies on arctic tundra plants have typically Anumberofstudieshaverevealedsomegeneralizedpat-
measured ambient or light-saturated midday values (e.g., terns in arctic plant carbon dynamics since Bliss’s (1956)
Tieszen 1973; Johnson and Tieszen 1976; Defoliart et al. seminal work. Photosynthesis in arctic plants generally
1988; Oberbauer and Oechel 1989; Starr and Oberbauer appear to have a nonlinear uni-modal relationship with
ª2013TheAuthors.PublishedbyBlackwellPublishingLtd.ThisisanopenaccessarticleunderthetermsoftheCreative 375
CommonsAttributionLicense,whichpermitsuse,distributionandreproductioninanymedium,provided
theoriginalstudyisproperlycited.
DiurnalPhotosynthesisinArcticPlants R.Patankaretal.
temperature, with increases up to, and decreases after, a In arctic tundra plants, exposure to high light and low
mean temperature of ~15°C (tundra grasses, Tieszen1973; temperatures during spring melt out cause highly stressful
dwarf deciduous and evergreen species, Johnson and Ties- conditions to the photosynthetic apparatus thus resulting
zen1976)indicatinganinabilitytophotosynthesizeatpeak in low photosynthetic efficiencies represented by low
rates beyond this range of moderate temperatures. Com- quantum yield values (Oberbauer and Starr 2002). Quan-
parison of midday light-saturated photosynthesis between tum yield of photosystem II measured as the ratio of
19 vascular growth forms (Oberbauer and Oechel 1989) variable to maximum chlorophyll fluorescence (Maxwell
revealed that evergreens show the lowest photosynthetic & Johnson 2000), is a common measure of plant “stress”
ratescomparedtoothergrowthforms(graminoids,forbes, associated with photosynthetic efficiency. Additionally,
and deciduous shrubs), a pattern consistent with prior plants (insulated) under snow show visibly lower stress
area-basedmeasurementsofphotosynthesisinarcticplants than those that are recently snow-free (Oberbauer et al.
(e.g.,TieszenandJohnson1975). unpubl. data). However, with increasing soil and air tem-
Seasonal patterns in midday plant physiology reveal peratures, photosynthetic efficiencies increase rapidly to a
that light-saturated photosynthesis is typically lowest right maxima that remain fairly constant through much of the
after melt out in the spring, peaks during the middle of summer and into fall, when rates again drop as a result
the growing season (early July), and decreases approach- of leaf hardening and/or deteriorating leaf health
ing fall (Defoliart et al. 1988; Starr et al. 2000, 2008). (Oberbauer and Starr 2002).
Moreover, simulated warming and extended growing Carbon metabolites produced as a direct result of
season do not appear to have measurable effects on pho- photosynthesis are regulated by physical, photochemical,
tosynthetic rates in several species (Starr et al. 2000, and biochemical component factors (Geiger and Servaites
2008), suggesting adaptive constraints to physiological 1994) and as a result are likely to fluctuate based on
capacity. However, arctic sedges show reduced photosyn- diurnal differences in carbon sequestration. While prior
thesis and stomatal conductance in response to reduced studies have examined seasonal changes in metabolic
root temperatures, implying robust controls on gas- carbon pools in diverse arctic tundra growth forms (Cha-
exchange rates by soil temperatures (Starr et al. 2004). pin et al. 1986; Olsrud and Christensen 2004), to our
Furthermore, long-term experimental manipulations of knowledge only one study has examined diurnal patterns
nutrients (nitrogen and phosphorus), air temperature, of total foliar carbohydrates in Oxyria digyna (Warren
and light availability reveal that at the species level, pho- Wilson 1954). This study found a diurnal pattern in total
tosynthesis responds most to increased light intensities foliar carbohydrate concentrations during continuous
(Chapin and Shaver 1996). “daylight”. Leaf total nonstructural carbohydrate (TNC)
To date, only two prior studies have examined diurnal concentrations decreased or remained stable in deciduous
patterns from the Alaskan arctic tundra, but these have shrubs, and increased in forbs, graminoids and evergreens
been limited a) in the number of species examined and b) as the growing season progresses toward fall (Chapin
to the peak of the growing season. Tieszen and Johnson et al. 1986) implying an important role of TNCs as winter
(1975) reported on the diurnal pattern of photosynthetic storage resources. High concentrations of TNCs found in
uptake in the grass Dupontia fischeri, as part of a study stems and roots of deciduous species in the fall indicates
on seasonal patterns of light-saturated photosynthesis in that these plant components are the main storage organs
this species and found a peak in photosynthesis around for carbohydrate resource and are used for leaf growth in
midday, with the lowest uptake at night, corresponding the spring. Thus, the varied growth forms found in the
with drops in temperature and reduced light intensity, Arctic show distinct strategies in their TNC concentration
(however, no formal methods were used to statistically toward survival and growth. However, when total sugars
test this relationship). Similar midday photosynthetic are considered in isolation, all growth forms show a
peaks were observed in two sedges (Eriophorum vagina- consistent, uniform pattern with high foliar concentra-
tum and E. angustifolium) in moist and wet sites in the tions in the spring, significant drops in the summer and
Alaskan arctic tundra, respectively, when measured under elevated concentrations in the fall (Chapin and Shaver
ambient light conditions (Gebauer et al. 1998). However, 1988). However, the dynamics of diurnal gas-exchange
these midday peaks were observed on overcast days and and metabolite concentration patterns as the growing
peak photosynthetic rates in both species shifted to earlier season progresses remain unknown.
in the day (between 7–8 am) with high light quantities Here, we examine diurnal patterns of leaf gas-exchange
and greater associated air temperatures. Both of these (photosynthesis and dark respiration), chlorophyll fluo-
studies were confined to grasses and sedges, thereby leav- rescence, and metabolite concentrations in seven arctic
ing out other commonly occurring growth forms, such as tundra species during three distinct time periods (spring
deciduous and evergreen shrubs. melt out, peak summer, and fall) of the short growing
376 ª2013TheAuthors.PublishedbyBlackwellPublishingLtd.
R.Patankaretal. DiurnalPhotosynthesisinArcticPlants
season. Furthermore, we determine which among a suite at a constant reference CO concentration of
2
of environmental factors drive these patterns among the 400 lmol m(cid:1)2 s(cid:1)1 using an infrared gas analyser
species and, finally we compare diurnally averaged pat- (Li-6400XT portable photosynthesis system; LI-COR,
terns in leaf gas-exchange and metabolite concentration Lincoln, Nebraska). A measurements were made at a
max
across the three time periods. On the basis of prior stud- saturating light intensity of 1500 lmol m(cid:1)2 s(cid:1)1 (Obe-
ies that found departures from circadian rhythms under rbauer and Oechel 1989; Starr et al. 2008; 1200 in the
constant high light conditions (Hennessey and Field spring and fall), and following this, the chamber light
1991), we hypothesized that given continuous high-qual- source was switched off. R was measured after a period
d
ity light, tundra plants would move away from circadian of ~ 5–7 min in darkness, when dark respiratory rates
rhythms and photosynthesize at or close to, their (net) had stabilized. Leaves were collected and brought back to
maximum capacity. Changes in photosynthetic assimilate the lab where leaf areas (Li-3100 area meter; LI-COR,
concentrations (from neighboring plants) on the other Lincoln, Nebraska) were measured (to correct for area-
hand were predicted to not change diurnally (Yang et al. based gas-exchange measurements) following the 24-h
2002; Sun et al. 2009), but instead are influenced/gov- sampling period. Due to rain events and dew formation
erned by changing internal and external stressors such as at several time intervals during the study period, leaves
temperature, water vapor deficits, or herbivory. were often wet and despite best efforts, this lead to inac-
curate/unreliable estimates of stomatal conductance.
Hence diurnal patterns of field g measurements are not
Methods and Materials s
presented here. However, diurnally integrated values of
g were used to examine correlations between A and
Study site and experimental design s max
g (Table 1).
s
The study was conducted in a moist acidic tussock tundra
site at the Toolik Lake field station, Alaska (68°37′39″N,
Microclimate variables
149°35′51″W). Seven species representing four growth
forms and comprising ~80% of the vascular plant ground Air (T ) and leaf temperature (T ), ambient pressure
air leaf
cover (Walker et al. 1994) were selected for the study. (P) and leaf to air vapor pressure deficit (D) were
These included: Vaccinium vitis-idaea L., Cassiope tetrag- recorded along with gas-exchange parameters using the
ona, Ledum palustra L. (evergreen shrubs), Betula nana infrared gas analyser (Li-6400XT portable photosynthesis
L., Salix pulchra Cham.(deciduous shrubs), Eriophorum system; LI-COR, Lincoln Nebraska).
vaginatum L.(wintergreen sedge), and Carex bigelowii
Torr.(sedge). A detailed description of the community
Diurnal metabolite measurements
can be found in Bliss and Matveyeva (1992). To examine
seasonal variations in diurnal ecophysiology, measure- To examine diurnal differences in the partitioning of
ments were made: (1) immediately after melt out in the recently acquired carbon, foliar tissue from adjacent
spring (30 May–7 June 2011), B) peak summer (8–20 stems, as not to affect our physiological measurements,
July) and C) fall (7–17 September). Five of the seven were collected separately at each time interval (n = 6 per
species were measured immediately after melt out in May
–June, S. pulchra and C. bigelowii were included from the
July sampling period onward (as neither had fully devel- Table1. Correlation coefficients between Amax and gs in seven
oped foliage in the spring). We measured six individuals arctictundraspeciesfromthreetimeperiodsduringthearcticgrowing
season.
of each species at 4-h intervals (00:00, 04:00, 08:00, 12:00,
16:00, 20:00 hours) per 24-h sampling period. Meltout Peak Fall
Betulanana 0.011 0.146* 0.013
Diurnal physiological measurements Salixpulchra – 0.514*** 0.023
Carexbigelowi – 0.474*** –
At each sampling interval, plants (n = 6 per species) were Eriophorumvaginatum 0.276** 0.745*** 0.034
dark-adapted for a minimum of 10 min and Fv/Fm (rep- Cassiopetetragona 0.122* 0.006 0.067
resenting potential quantum efficiency of photosystem II Vacciniumvitis-idea 0.009 0.837*** 0.156*
[PSII]) was measured using an OS5-FL modulated- Ledumpalustre 0.017 0.399*** 0.178*
fluorometer (Opti-Sciences, Tyngsboro, Massachusetts).
Levelofsignificance(P)isrepresentedbythe“*”symbolasfollows:
Immediately thereafter, Amax (maximum photosynthetic *P<0.05.
capacity at saturating light), stomatal conductance (gs) **P<0.01.
and net foliar dark respiration Rd were measured in situ ***P<0.001.
ª2013TheAuthors.PublishedbyBlackwellPublishingLtd. 377
DiurnalPhotosynthesisinArcticPlants R.Patankaretal.
time interval) and immediately stored in the field at (cid:1)40°C 12:00 time interval in four species, but was highest at
and transferred within 30 min to a (cid:1)80°C freezer. 16:00 in Vaccinium (Fig. 1 h–n). Diurnal differences in
Samples were then freeze-dried for 24 h at (cid:1)80°C (Lab- F /F were only significant in Cassiope and this temporal
v m
conco Co., Kansas City, Missouri), ground and extracted variability was driven by the lower midnight value when
with 80% ethanol, and analyzed enzymatically for soluble comparedtotheothertimeintervals(Fig. 1o–u).
sugars (glucose, fructose, sucrose) and starch concentra- Clear diurnal patterns were observed during peak sum-
tions according to Boehringer (1984). mer (July), but unlike earlier in the season, peak time
intervals in A differed between species. A in both
max max
grasses species (Eriophorum and Carex) peaked at 08:00
Statistical analyses
(Fig. 1 c, d) and at 12:00 in both deciduous species (Betu-
Data were examined for deviations from normality and in la and Salix) (Fig. 1 f, g). Among the evergreens, A
max
cases where this occurred, the data were normalized using peaked at 12:00 in Vaccinium, at 16:00 in Ledum and
appropriate transformations (Zar 1984). For each species, reached similar levels at 08:00 and 12:00 in Cassiope. R
d
a one-way analysis of variance (ANOVA) was performed was highest at 12:00 for Vaccinium, Ledum, and Betula,
to determine differences in gas-exchange parameters and was highest at 08:00 for Eriophorum, 16:00 for Salix and
metabolite concentrations between the six diurnal time peaked at 08:00, and 20:00 for Cassiope (Fig. 1 h–n).
intervals. To test for differences in physiological parame- F /F showed significant diurnal changes in only two of
v m
ters between seasons and species (and the interaction the seven species examined during the peak summer per-
between them) diurnal data for the individual response iod (Table 2). In both species, these differences were dri-
variables were pooled as there was no significant variation ven by low values from a single time interval (16:00 in
in diurnal ranks between seasons and species (i.e., diur- both Betula and Vaccinium) in comparison to other time
nals patterns were similar across seasons and species), and intervals (Fig. 1 o, t).
a two-way ANOVA was used with the above two variables Carex leaves had senesced before measurements com-
as fixed factors (a student’s t-test was used for Salix pul- menced in the fall and were thus excluded from this time
chra to determine seasonal differences between summer period.A inEriophorumpeakedat08:00,andwassignif-
max
and fall; C. bigelowi was only measured during peak sum- icantlylowerat12:00throughmidnight(Fig . 1c).Among
mer). To determine which environmental variables con- theevergreens,VacciniumandLedumbothhadA peaks
max
tributed to variations in physiological parameters, at08:00and12:00,whereasinCassiope,rateswerecompa-
response (gas-exchange and metabolite) data from the rablyhighfrom04:00to16:00(Fig. 1a,b,c).A inboth
max
different diurnal time intervals, and growing periods were Betula and Salix peaked at 12:00, and were significantly
pooled separately for each species, and a multiple linear higher than at midnight (Fig. 1 f, g, Table 2). R rates
d
regression model was run using three environmental vari- during the fall showed diurnal peaks in five of six species
ables – air temperature (°C), pressure (kPa), and leaf to examined, but differences between the time intervals were
air vapor pressure deficit D (kPa) – as predictors. Leaf only significant in Vaccinium, Ledum and Salix (Table 2).
L
temperature was highly correlated with air temperature, R peakedat12:00inVacciniumandLedumandat16:00in
d
and was thus excluded from the analysis to avoid collin- Salix (Fig. 1 h, i, n). Diurnal differences in F /F were
v m
earity. significant in Betula and the evergreens Cassiope and
Ledum (Table 2), and were driven by lower midnight val-
ues (Cassiope, Ledum) and 04:00 (Betula) compared to the
Results
otherdiurnaltimeintervals(Fig. 1i,l,n).
Diurnal patterns of gas-exchange and
chlorophyll fluorescence Diurnal patterns of metabolite pools
All five species examined following melt outshowed peaks Significant differences in diurnal patterns of metabolite
in A at midday (12:00 time interval) that were signifi- concentration during the melt out period were seen in
max
cantly higher than A measured at midnight (Fig. 1 a–g, Betula, Cassiope, Vaccinium, and Ledum (Table 3). This
max
Table 2). Leaf dark respiration similarly peaked at the was due to decreased glucose-fructose concentrations at
Figure1. Diurnalpatternsofgasexchange(light-saturatedphotosynthesisA [a–g],respiration[h–n])andchlorophyllfluorescence(F F [o–u])
max V M
fromsevenarctictundravascularplantsfromToolikLakeFieldStation,Alaskameasuredatthreetimeperiods(meltout,peak,fall)in2010.Filled
circles = melt out, filled squares = peak, and filled diamonds = fall period of the growing season. All symbols are means of n=6 plants (+- 1
SEM);x-axis:diurnaltimeinterval.
378 ª2013TheAuthors.PublishedbyBlackwellPublishingLtd.
R.Patankaretal. DiurnalPhotosynthesisinArcticPlants
A R F/F
max d v m
(a) ((hh)) (o)
Vacinium Vacinium Vacinium
–2–1 (CO μmol m s)max211205050 MPFaeelalltkout –2–1 (CO μmol m s)Rd2–––32101 MPFaeelalltkout F/FVM000...789 MPFaeelalltkout
A –4 0.6
0 4 8 12 16 20 0 4 8 12 16 20 0 4 8 12 16 20
Timeofday Timeofday Timeofday
(b) ((ii)) (p)
Ledum Ledum
–2–1A (CO μmol m s)max211205050 0 4 8 12 16 20 MPFaeelalltkout –2–1R (CO μmol m s)d2––––864202 0Ledum4 8 12 16 20 MPFaeelalltkout F/FVM0000....6789 0 4 8 12 16 20 MPFaeelalltkout
Timeofday Timeofday Timeofday
(c) ((jj)) (q)
Eriophorum Eriophorum Eriophorum
–2–1A (CO μmol m s)max21122050505 MPFaeelalltkout –2–1R (CO μmol m s)d2–––––5432101 MPFaeelalltkout F/FVM0000....6789 MPFaeelalltkout
0 4 8 12 16 20 0 4 8 12 16 20 0 4 8 12 16 20
Timeofday Timeofday Timeofday
(d) ((kk)) (r)
Carex Carex Carex
–2–1A (CO μmol m s)max2110505 Peak –2–1R (CO μmol m s)d2––––43210 Peak F/FVM0000....6789 Peak
0 4 8 12 16 20 0 4 8 12 16 20 0 4 8 12 16 20
Timeofday Timeofday Timeofday
(e) Cassiope ((ll)) Cassiope (s) Cassiope
–2–1O μmol m s)2150 MPFaeelalltkout –2–1O μmol m s)2––2101 MPFaeelalltkout F/FVM00000.....56789 MPFaeelalltkout
A (Cmax 0 R (Cd––43 00..34
0 4 8 12 16 20 0 4 8 12 16 20 0 4 8 12 16 20
Timeofday Timeofday Timeofday
(f) ((mm)) (t)
Betula Betula Betula
–2–1 (CO μmol m s)max211205050 MPFaeelalltkout –2–1R (CO μmol m s)d2––––––65432101 MPFaeelalltkout F/FVM000000......456789 MPFaeelalltkout
A –7 0.3
0 4 8 12 16 20 0 4 8 12 16 20 0 4 8 12 16 20
Timeofday Timeofday Timeofday
(g) Salix ((nn)) 1 Salix (u)0.9 Salix
–2–1m s)1105 PFaelalk –2–1m s) 0 PFaelalk 00..78 PFaelalk
O μmol 2 5 O μmol 2––21 F/FVM000...456
A (Cmax 0 R (Cd––43 00..23
0 4 8 12 16 20 0 4 8 12 16 20 0 4 8 12 16 20
Timeofday Timeofday Timeofday
ª2013TheAuthors.PublishedbyBlackwellPublishingLtd. 379
DiurnalPhotosynthesisinArcticPlants R.Patankaretal.
Table2. FandPvaluesofanalysisofvariance(ANOVA)sexaminingdiurnalpatternsofgasexchange(photosynthesisandrespiration)andchloro-
phyllfluorescenceinsevenarctictundravascularspecies.
A Respiration F/F
max v m
Meltout Peak Fall Meltout Peak Fall Meltout Peak Fall
B.nana 3.73** 11.49*** 4.22** 13.35*** 7.41*** 1.2 2.09 6.71*** 3.36*
S.pulchra – 10.39*** 3.17* – 3.43** 3.12* – 1.68 1.13
C.bigelowi – 6.26*** – – 1.49 – – 1.92 –
E.vaginatum 7.49*** 7.99*** 6.12*** 31.75*** 1.37 2.07 1.00 1.47 1.6
C.tetragona 4.87*** 6.52*** 9.16*** 27.09*** 8.27*** 1.96 3.35** 1.81 4.37**
V.vitis-idea 8.68*** 10.71*** 22.57*** 19.28*** 8.35*** 6.38*** 2.29 2.81* 1.73
L.palustre 8.77*** 5.47*** 47.68*** 14.41*** 6.45*** 11.2*** 1.23 1.63 2.88*
Levelofsignificance(P)isrepresentedbythe“*”symbolasfollows:
*P<0.05.
**P<0.01.
***P<0.001. B.nana = Betula nana, S.pulchra = Salix pulchra, C.bigelowi = Carex bigelowi, E.vaginatum = Eriophorum vaginatum,
C.tetragona=Cassiopetetragona,V.vitis-idea=Vacciniumvitis-idea,L.palustre=Ledumpalustre.
Table3. FandPvaluesofanalysisofvariance(ANOVA)sexaminingdiurnalpatternsofmetaboliteconcentrationinsevenarctictundravascular
species.
Glucose-Fructose Sucrose Starch
Meltout Peak Fall Meltout Peak Fall Meltout Peak Fall
B.nana 2.90* 2.70* 1.65 5.79*** 4.79** 3.06* 2.94* 1.62 0.63
S.pulchra – 5.45** 2.10 – 3.26* 2.45 – 9.27*** 8.92***
C.bigelowi – 1.39 – – 12.8*** – – 1.18 –
E.vaginatum 2.27 3.67* 1.29 1.17 0.74 4.29** 1.18 1.63 1.82
C.tetragona 13.83*** 4.89*** 1.15 0.47 11.4*** 6.69*** 7.37*** 12.1*** 3.06*
V.vitis-idea 4.34** 0.54 3.66* 7.70*** 3.99** 3.85** 0.59 4.17** 0.79
L.palustre 2.05 1.19 8.49*** 5.22** 0.94 30.30*** 1.06 1.10 2.92*
Levelofsignificance(P)isrepresentedbythe“*”symbolasfollows:
*P<0.05,
**P<0.01,
***P<0.001. B.nana = Betula nana, S.pulchra = Salix pulchra, C.bigelowi = Carex bigelowi, E.vaginatum = Eriophorum vaginatum,
C.tetragona=Cassiopetetragona,V.vitis-idea=Vacciniumvitis-idea,L.palustre=Ledumpalustre.
time interval 0:400 in Betula and Vaccinium, and incre- least one of the three metabolites measured, Ledum being
ased glucose-fructose concentrations at 12:00 in Cassiope the exception. Salix and Cassiope showed diurnal differ-
(Fig. 2 a, e, f). Sucrose likewise varied in Betula, Vaccini- ences in all three types of metabolites, whereas Betula and
um, and Ledum (Table 3), with concentrations signifi- Vaccinium showed diurnal differences in two of the three
cantly lower at 04:00 in Betula, at 08:00 in Ledum, and at measured metabolites (Table 3). The graminoids Eriopho-
20:00 in Vaccinium compared to other times (Fig. 2 a, b, rum and Carex showed differences in glucose-fructose
f). Starch concentration was significantly different and sucrose, respectively (Table 3). Significant differences
between time intervals in Betula and Cassiope, with the in Salix were driven by low glucose-fructose concentra-
main difference seen in low starch concentrations at 04:00 tions at midnight and at 08:00, low sucrose concentra-
in Betula and significantly higher concentrations at 16:00 tions at 04:00, and low starch concentrations at 04:00 and
in Cassiope (Fig. 2 e, f). 08:00 (Fig. 2 g, n, u). Cassiope exhibited the opposite
Six of the seven species examined during peak summer pattern, with significantly higher glucose-fructose, sucrose,
showed significant differences between time intervals in at and starch concentrations at midnight and 04:00
Figure2. Diurnalpatternsofconcentrationofmetabolites(glucose-fructose[a–g],sucrose[h–n],starch[o–u])fromsevenarctictundravascular
plantsfromToolikLakeFieldStation,Alaskameasuredatthreetimeperiods(meltout,peak,fall)in2010.Filledcircles=meltout,filledsquares=
peak, and filled diamonds = fall period of the growing season. All symbols are means of n=6 plants (+- 1 SEM); x-axis: diurnal time interval,
y-axis=concentrations(gramsper100gramsofdrytissuemass).
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R.Patankaretal. DiurnalPhotosynthesisinArcticPlants
(a) (h) (o)
(b) (i) (p)
(c) (j) (q)
(d) (k) (r)
(e) (l) (s)
(f) (m) (t)
(g) (n) (u)
ª2013TheAuthors.PublishedbyBlackwellPublishingLtd. 381
DiurnalPhotosynthesisinArcticPlants R.Patankaretal.
compared to other time intervals (Fig. 2e, l, s). Diurnal Betula and Salix exhibiting the largest drops (Fig. 3c).
differences in Betula carbohydrate concentrations were Fluorescence levels were similar in the five species exam-
driven by higher concentrations of glucose-fructose and ined during melt out (Fig. 3c). Glucose-fructose levels
sucrose at midnight and 16:00, respectively (Fig. 2 f, m), were highest in the fall across all species (F = 43.87,
whereas in Vaccinium, differences were driven by high P < 0.001) except in the grass Eriophorum, which showed
and low concentrations of sucrose and starch, respec- no seasonal differences. Glucose-fructose concentrations
tively, at midnight (Fig. 2 a, o). Glucose-fructose concen- were consistently lowest during melt out except in Betula,
tration in Eriophorum was lowest at the 12:00 time where summer concentrations were marginally lower
interval (Fig. 2 c), whereas in Carex sucrose concentra- (F = 43.87, P < 0.001). The deciduous species had the
tions were lowest at midnight and at 04:00 (Fig. 2 d). highest fall concentrations of glucose-fructose, whereas
All six species measured in the fall showed diurnal dif- the grasses Carex (summer) and Eriophorum (all periods)
ferences in at least one of the three metabolites types. had the lowest concentrations among the seven species
During this time period, Ledum had diurnal differences in examined (Fig. 3d). Sucrose concentrations were highest
the concentrations of all three metabolites (unlike in the in the peak summer among the deciduous (Betula, Salix)
summer where no diurnal differences were found in this and graminoid (Carex, Eriophorum) species, but were
species). Glucose-fructose concentrations in Ledum were either lower (Cassiope) or did not change (Ledum, Vacci-
significantly lower at 12:00 and 16:00, whereas sucrose nium) in the evergreens (F = 16.94, P < 0.001, Fig. 3e).
and starch concentrations were significantly higher during Starch concentrations were consistently lowest in the fall
these time intervals (Fig. 2 b, i, p). The other two in six of the seven species (Carex was excluded during
evergreens, Vaccinium and Cassiope showed diurnal differ- this period) (F = 133.71, P < 0.001; Fig. 3f). Similarly,
ences in glucose-fructose/sucrose and sucrose/starch con- starch concentrations were highest in peak summer (sig-
centrations, respectively. Vaccinium glucose-fructose nificantly so in Betula, Eriophorum, and Ledum) (Fig. 3 f).
concentrations were highest at midnight, whereas sucrose
was highest at 16:00 (Fig. 2 a, h); Cassiope sucrose and
Physiological and metabolic response to
starch concentrations were highest at noon (Fig. 2 l, s).
environmental variables
Sucrose concentrations in Eriophorum and Betula were
both significantly lower at midnight compared to other The linear multiple regression model revealed that (spe-
time intervals, whereas starch concentration in Salix was cies-pooled) A and R were significantly correlated
max d
significantly higher at this time interval (Fig. 2 j, m, n). with (air) temperature (positive) and D (negative), but
not with ambient pressure (A : r2 = 0.373, F = 120.89,
max
P < 0.001; R : r2 = 0.391, F = 130.37, P < 0.001). F /F
Seasonal differences in physiology and d v m
was also positively correlated with temperature and
metabolite concentration
negatively with D, although these correlations were
Diurnally averaged A was significantly higher during weaker (r2 = 0.137, F = 32.19, P < 0.001). All three
max
the peak summer and lowest in the fall in all five species metabolite concentrations exhibited weak, but significant
that were examined across these three time periods (Salix correlations with at least one, the environmental predic-
was not measured during melt out and Carex was only tor. Glucose-fructose was weakly (negatively) correlated
measured in the summer) (F = 41.94, P < 0.001, Fig. 3 a). with temperature (r2 = 0.07, F = 15.39, P < 0.001),
Cassiope had the lowest melt out and summer A rates, sucrose was correlated with temperature (positive) and
max
whereas Betula and Eriophorum had the highest melt out pressure (negative) (r2 = 0.153, F = 36.73, P < 0.001),
and peak summer A values, respectively (Fig. 3a). Fall and starch was positively correlated with temperature and
max
A rates did not vary significantly between species. R pressure (r2 = 0.162, F = 37.17, P < 0.001).
max d
showed similar seasonal patterns as A except in the
max
evergreen Vaccinium where R was lowest during melt
d Discussion
out rather than fall (F = 15.55, P < 0.001; Fig. 3b). Betula
had the highest spring and peak summer R rates,
d Diurnal and seasonal patterns in leaf gas
whereas Vaccinium and Carex had the lowest spring and
exchange
summer rates, respectively (Fig. 3b). Similar to A , fall
max
R rates did not differ significantly between species. Chlo- Contrary to our expectation, all seven species of arctic
d
rophyll fluorescence also peaked in the summer and were tundra vascular plants showed clear diurnal patterns in
similarly high across all species (F = 38.52, P < 0.001; leaf gas-exchange with peak rates of photosynthesis
Fig. 3c). All species except Eriophorum showed significant and dark respiration around midday, and lulls at night.
declines in F /F in the fall, with the deciduous species Arctic tundra plants thus appear to exhibit similar diurnal
v m
382 ª2013TheAuthors.PublishedbyBlackwellPublishingLtd.
R.Patankaretal. DiurnalPhotosynthesisinArcticPlants
(a) (d)
(b) (e)
(c) (f)
Figure3. Seasonal differences in foliar gas-exchange, chlorophyll fluorescence and metabolite concentration in seven arctic tundra plants from
ToolikLakeFieldStation,Alaskain2011.Foreachspecies,barsnotconnectedbythesamelettersdiffersignificantlyatP<0.05,andrepresent
diurnally averaged means (+1 s.e.m). Black bars = melt out period (May–June), white bars = peak growing period (July), gray bars = fall
(September).
gas-exchange rhythms regardless of growth form (ever- (Table 1), suggesting strong controls of water loss on leaf
greens, graminoids or deciduous). The diurnal declines in carbon uptake. Our hypothesis was based on prior lab
carbon uptake during midnight and early morning hours and greenhouse studies on common species that showed
persisted even under saturating light conditions during departures from photosynthetic circadian rhythms under
times when photoperiod was 24 h, suggesting that these constant, uniform light conditions. For example, Hennes-
species do not take advantage of light availability, but sey and Field (1991) examined red kidney bean (Phaseolus
instead are constrained by a) stomatal water loss and b) vulgaris L.) photosynthetic circadian rhythms under
internal physiological adaptations governed by circadian tightly controlled greenhouse conditions and found that
clocks (Geiger and Servaites 1994; Dodd et al. 2005). clear circadian rhythms were maintained at constant air
While diurnal patterns of stomatal conductance were not temperatures and ambient CO concentrations under a
2
consistent across species (data not shown), daily averaged fixed light-dark photoperiod, but did not persist under a
conductance values were strongly correlated with daily constant low light regime (200 lmol m(cid:1)2 s(cid:1)1). There are
photosynthesis values, at least during the peak summer compelling alternative explanations for the observed
ª2013TheAuthors.PublishedbyBlackwellPublishingLtd. 383
DiurnalPhotosynthesisinArcticPlants R.Patankaretal.
diurnal patterns. First, in our study, while plants were thetic peaks were observed in Eriophorum vaginatum and
measured at a constant PPFD (just above saturation E. augustifolium measured at ambient light conditions on
point) and a fixed CO concentration, the plants were a sunny day in arctic tussock tundra (Gebauer et al.
2
not continuously sampled and thus no individual leaf or 1998), but shifted toward midday under overcast condi-
tiller was subjected to long periods of constant light. tions. This shift was attributed to photosynthetic
Hence it is probable that extended periods of high light responses of tillers to water stress, and more specifically
exposure (and not “spot” measurements) on individual external (leaf to air vapor deficit) rather than internal
leaves might alter the observed diurnal patterns. Alterna- (water potential) water stress. Given the lack of a signifi-
tively, plants in the field experience far from “ideal” envi- cant association between A (and also respiration) and
max
ronments, and are thus subject to a number of stresses D in both graminoids (data not shown), our findings
L
(temperature, light, water status, nutrient availability, suggest that irrespective of season, peak diurnal A rates
max
herbivory) that act in a mutually nonexclusive manner, in the graminoids are reached relatively early during the
and are thus likely to maintain circadian patterns to day thereby suggesting a requirement to accelerate carbon
optimize the costs and benefits of carbon uptake (Dodd acquisition as early as possible. Diurnally averaged A
max
et al. 2005). Additionally, the plants in this study showed a consistent seasonal pattern, with the highest
experienced high levels of light intensities during peak rates in July (peak) and the lowest rates in the September
summer (peak summer midday PAR was frequently above (fall). Senescing leaves, largely devoid/depleted of chlo-
1000 lmol m(cid:1)2 s(cid:1)1) and were measured at PAR levels rophyll, likely resulted in the low rates of A
max
well above saturation. Hence, we suggest, based on this (~ 0–1 lmol m(cid:1)2 s(cid:1)1) seen in the deciduous species Be-
and prior studies measuring in situ diurnal patterns in tula and Salix in the fall. Midday respiration rates were
the arctic (Tieszen 1973; Gebauer et al. 1998), that arctic highest during the peak summer, except in Betula, where
plants are constrained in their ability to fully exploit spring rates were noticeably higher. As mentioned previ-
available light, and this is likely due to a combination of ously, diurnal peaks in respiration lagged behind peak
adaptive constraints, such as stomatal water loss, and A rates in Vaccinium, Cassiope and Salix, suggesting a
max
diurnal circadian clocks (which might in fact confer sig- recovery phase after potentially significant water loss in
nificant advantages to photosynthesis, likely leading to these species. Diurnally averaged seasonal trends here
greater growth and survival; see Dodd et al. 2005 for a resembled those of A , with high midsummer rates and
max
recent example). This is further reinforced by the fact that the lowest respiration rates in the fall, (except in V. vitis-
while prior studies examining arctic plants (Tieszen 1973; idea where spring rates were lowest). Dark-adapted PS II
Gebauer et al. 1998) have shown a close matching quantum efficiency rates generally remained relatively
between diurnal photosynthesis and photoperiod (imply- constant during a 24-hr period, suggesting that PS II
ing external regulation/influence), we show that depar- quantum efficiency does not fluctuate diurnally, but
tures from normal diurnal light regimes (i.e., artificially rather changes as the growing season progresses, as seen
provisioned saturating light levels) do not result in modi- by the peak values in July and declines in fall (Fig. 3).
fications to diurnal patterns (implying internal control). Observed seasonal trends in all three physiological traits
Any measured differences in PS II efficiency, on the other are consistent with prior studies that have examined sea-
hand, had more to do with large changes between any sonal changes in leaf physiology in arctic tundra plants,
two times intervals, as opposed to diurnal changes consis- with midsummer peaks in A , R , and F /F (see Defo-
max d v m
tent with gas-exchange parameters implying, for the most liart et al. 1988; Starr et al. 2008). The significant
part, minimal diurnal variation in the photochemical decreases in fall gas-exchange traits compared to the other
pathway of host leaves. two seasons is likely due to declines in leaf functioning in
While marked diurnal patterns in gas exchange were response to decreases in diurnal photoperiods from a
observed, the timing of peaks and lulls differed between summer peak of 24 h to about ~14 h in the fall.
species. Photosynthetic capacity in both graminoids (Erio-
phorum and Carex) peaked relatively soon after lulls in
Diurnal and seasonal patterns in leaf
midnight and early morning intervals, as seen by the
metabolites
peaks at ~08:00 during summer in both species, whereas
peaks were closer to noon or even 16:00 in the evergreens Unlike with leaf physiological traits, diurnal patterns in
and deciduous species. Corresponding respiration rates in leaf metabolic pools were less consistent, with patterns
the two graminoids peaked at similar times, as opposed changing across seasons and growth forms (Fig. 2). In
to the lags in peak respiration seen in three of the five many instances, no clear diurnal patterns were detected,
other species (diurnal respiration rates in Carex, however, as we had predicted based on earlier findings (Yang et al.
were not significant). Similar early morning photosyn- 2002; Sun et al. 2009). Glucose-fructose concentrations
384 ª2013TheAuthors.PublishedbyBlackwellPublishingLtd.