Low soil temperature inhibits the effect of high nutrient supply onphotosynthetic response to elevated carbon dioxide concentration inwhite birch seedlings
TITUS F. AMBEBE,1 QING-LAI DANG1,2 and JUNLIN LI1
1 Faculty of Forestry and the Forest Environment, Lakehead University, 955 Oliver Road, Thunder Bay, ON P7B 5E1, Canada2 Corresponding author ([email protected])
Received August 25, 2009; accepted November 1, 2009; published online December 8, 2009
Summary To investigate the interactive effects of soil tem-perature (Tsoil) and nutrient availability on the response ofphotosynthesis to elevated atmospheric carbon dioxide con-centration ([CO2]), white birch (Betula papyrifera Marsh.)seedlings were exposed to ambient (360 μmol mol−1) or ele-vated (720 μmol mol−1) [CO2], three Tsoil (5, 15 and 25 °Cinitially, increased to 7, 17 and 27 °C, respectively, 1 monthlater) and three nutrient regimes (4/1.8/3.3, 80/35/66 and 160/70/132 mg l−1 N/P/K) for 3 months in environment-controlledgreenhouses. Elevated [CO2] increased net photosynthetic rate(An), instantaneous water-use efficiency (IWUE), internal toambient carbon dioxide concentration ratio (Ci/Ca), triosephosphate utilization (TPU) and photosynthetic linear electrontransport to carboxylation (Jc), and it decreased actual photo-chemical efficiency of photosystem II (ΔF/Fm′), the fractionof total linear electron transport partitioned to oxygenation(Jo/JT) and leaf N concentration. The low Tsoil suppressedAn, transpiration rate (E), TPU,ΔF/Fm′ and Jc, but it increasedJo/JT. The low nutrient treatment reduced An, IWUE, maxi-mum carboxylation rate of Rubisco, light-saturated electrontransport rate, TPU, ΔF/Fm′, Jc and leaf N concentration,but increased Ci/Ca. There were two-factor interactions forCi/Ca, TPU and leaf N concentration, and a significant effectof CO2 × Tsoil × nutrient regime on An, IWUE and Jc. The sti-mulations of An and IWUE by elevated [CO2] were limited toseedlings grown under the intermediate and high nutrient re-gimes at the intermediate and high Tsoil. For Jc, the [CO2] effectwas significant only at intermediate Tsoil + high nutrient avail-ability. No significant [CO2] effects were observed under thelow Tsoil at any nutrient level. Our results support this study’shypothesis that low Tsoil would reduce the positive effect ofhigh nutrient supply on the response of An to elevated [CO2].
Keywords: Betula papyrifera Marsh., boreal forest, CO2
enrichment, CO2–Tsoil–nutrient interaction, gas exchange,global environmental change.
Introduction
The photosynthetic and growth responses of C3 plants to ele-vated carbon dioxide concentration ([CO2]) show consider-able diversity, ranging from highly positive to neutral and,in rare cases, even negative (Poorter 1993, Gunderson andWullschleger 1994, Miglietta et al. 1996, Zhang and Dang2007). Such variability in response complicates the predictionof ecosystem changes as CO2 continues to accumulate in theearth’s atmosphere. Plant responses to elevated [CO2] aremodified by growing conditions (Miglietta et al. 1996, Midg-ley et al. 1999, Olszyk et al. 2003, Zhang and Dang 2006,Zhang et al. 2006, Cao et al. 2007, Zhang and Dang 2007).For instance, elevated [CO2] increases photosynthesis (Daveyet al. 1999, Eguchi et al. 2004) and growth (Baxter et al.1997, Oren et al. 2001) in nutrient-rich but not in nutrient-poor soils. Other environmental factors that are known to in-fluence the responses of C3 plants to elevated [CO2] includesoil moisture (Mishra et al. 1999, Robredo et al. 2007), light(Zebian and Reekie 1998, Marfo and Dang 2009) and airtemperature (Allen et al. 1990, Pessarakli 2005). However,multiple factors often interact in natural ecosystems to affectplants, and the interactive effects may be of greater value thanthe main effects in predicting plant responses to elevated at-mospheric [CO2].Soil temperature (Tsoil) is an important environmental factor
controlling the growth of northern forests (Bonan and Shugart1989, Bonan 1992). There is great heterogeneity in Tsoil amongdifferent sites within the boreal forest, ranging from nearzero over permafrost to 35 °C on south-facing slopes andnewly burnt sites (Bonan and Shugart 1989, Zasada et al.1997). Low Tsoil reduces root growth and nutrient uptake(Chapin 1974, Tachibana 1982, Pastor et al. 1987, Pritchardet al. 1990, Paré et al. 1993, Peng and Dang 2003). Plantsgrowing in cold soils may experience feedback inhibitionand photoinhibition of photosynthesis because of reducedsink strength (Bagnall et al. 1988, Lambers et al. 2008).Furthermore, low shoot water potentials associated with in-creased soil water viscosity and decreased root permeability
Tree Physiology 30, 234–243doi:10.1093/treephys/tpp109
at low Tsoil have been implicated in stomatal closure andstomatal limitation of carbon assimilation (Benzioni andDunstone 1988, Dang and Cheng 2004). In spite of its neg-ative effects on the physiology and overall growth of C3
plants, low Tsoil has been surprisingly neglected in moststudies examining the impact of rising atmospheric [CO2]on boreal forest trees.In this study, we examined the interactive effects of Tsoil
and nutrient availability on the response of net photosynthe-sis (An) in white birch (Betula papyrifera Marsh.) to elevated[CO2]. White birch is a pioneer boreal tree species with arapid rate of initial growth and a high nutrient demand(Burns and Honkala 1990, Zhang and Dang 2006). Thereare strong relationships between leaf nutrient concentrationsand photosynthetic performance (Grassi et al. 2002, Bownet al. 2007). Nitrogen and phosphorus supply to leaves affectRubisco activity (Fredeen et al. 1990, Jacob and Lawlor 1991,Warren and Adams 2004). Furthermore, Brooks (1986) hasdemonstrated significant reductions in the regeneration ofthe carboxylation substrate, RuBP, with declining leaf phos-phorus levels. Plants that are provided with favorable nutrientconditions in cold soils may suffer from low nutrient stressbecause of the decrease in root uptake capacity (Setter andGreenway 1988). Thus, we hypothesized that low Tsoil wouldreduce the positive effect of high nutrient supply on the re-sponse of An to elevated [CO2].
Materials and methods
Plant materials
Seeds of white birch were sown in germination trays in agreenhouse at Lakehead University. The growing mediumwas a 1:1 (v/v) mixture of peat moss/vermiculite. The green-house was maintained at 26/16 °C (day/night temperature),and the natural photoperiod was extended to 15 h by high-pressure sodium lamps. The growing medium was wateredtwice a day with normal tap water using a spray bottle. After8 weeks, seedlings were selected for uniformity and trans-ferred to plastic pots (13.5 cm tall and 11/9.5 cm top/bottomdiameter) containing the same composition of growing medi-um described above. The pots were a built-in component of theTsoil control system that is described in the following section.
Experimental design
The treatments consisted of two [CO2] (360 and 720 μmolmol−1), three Tsoil (5, 15 and 25 °C initially, increased to 7,17 and 27 °C, respectively, 1 month later) and three nutrientregimes (4/1.8/3.3, 80/35/66 and 160/70/132 mg l−1 N/P/K).The CO2 treatments correspond to approximately ‘present’and ‘year 2100’ atmospheric CO2 levels (Watson et al.1990, Long et al. 2004, Zhou and Shangguan 2009). The Tsoiltreatments are in line with Tsoil conditions at different siteswithin the ecological range of white birch in the boreal forest.The leaf nutrient concentrations for the intermediate and high
nutrient treatments are comparable to those in white birchtrees that are naturally growing on nutrient-rich sites whilethe values for the low nutrient treatment are lower than thelowest levels in the field (Kopinga and van den Burg 1995,Zhang et al. 2006).The experiment was a split-split plot design in which the
[CO2] treatments were the main plots, Tsoil were the sub-plotsand nutrient regimes were the sub-sub-plots. Two separate en-vironment-controlled greenhouses were maintained at 360μmol mol−1 and two at 720 μmol mol−1, representing two re-plications per CO2 treatment. The elevated [CO2] was sup-plied by Argus CO2 generators (Argus, Vancouver, BC,Canada). Three Tsoil control boxes (one per Tsoil treatment)were placed on separate benches in each greenhouse. The tar-get Tsoil was achieved by circulating temperature-controlledwater between the pots fixed to the bottom of the Tsoil controlbox. Each pot had a drainage hole drilled through the bottomof the box. For a detailed description of the Tsoil control sys-tem, see Cheng et al. (2000). There were 10 randomly as-signed seedlings in each of the three nutrient regimes withineach Tsoil control box. The nutrient treatments were appliedonce a week. Treatments started on 1 January 2008.During the experiment, the four greenhouses were subjected
to 26/16 °C (day/night air temperature) and a 16-h photoperiod(the natural light was supplementedwith high-pressure sodiumlamps on cloudy days, early mornings and late evenings). Allthe environmental conditions were monitored and controlledwith an Argus environmental control system. The seedlingswere watered regularly to keep the growing medium moist.
Simultaneous measurements of in situ gas exchange andchlorophyll fluorescence
Two seedlings were randomly chosen from each greenhouseand Tsoil × nutrient treatment for gas exchange measurementsafter 3 months of treatments. The measurements were made onthe fifth youngest fully developed leaf with a PP-SystemsCIRAS-1 open gas exchange system (Hitchin, Hertfordshire,UK). The response of photosynthesis (A) to intercellular [CO2](Ci) was measured over a range of eight external CO2 partialpressures (Ca) from ~50 to 1100 μmol mol−1. The environ-mental conditions in the leaf chamber were 26 °C air temper-ature, 800 μmol m−2 s−1 photosynthetic photon flux densityand 50% relative humidity. The A/Ci curves were fitted usingPhotosyn Assistant software (Dundee Scientific, Scotland,UK) and analyzed with a biochemically based model (Farqu-har et al. 1980, Harley et al. 1992) to determine the maximumcarboxylation rate (Vcmax), light-saturated electron transportrate (Jmax) and triose phosphate utilization (TPU) using the ki-netic parameters of Wullschleger (1993). An, stomatal conduc-tance (gs) and transpiration rate (E) were obtained from A/Ci
curves where Ca equaled 360 μmol mol−1 for ambient and 720μmol mol−1 for CO2-enriched leaves. All gas exchange para-meters were calculated according to Farquhar et al. (1980).Chlorophyll fluorescence was measured with a portable
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[CO2], SOIL TEMPERATURE, NUTRITION AND WHITE BIRCH 235
Instruments, Norfolk, UK). Maximum (Fm′) and steady-state(Fs) fluorescence of light-adapted leaves were measured si-multaneously with each gas exchange measurement withthe chlorophyll fluorescence probe integrated into the leafchamber of the CIRAS-1, whereas maximum (Fm) and min-imum (Fo) fluorescence yields of dark-adapted leaves wereindependently sampled from the gas exchange measurementsafter dark adapting the leaves for 1 h in leaf clips. Thesemeasured variables were used to determine actual (ΔF/Fm′ =(Fm′ − Fs)/Fm′) and potential (Fv/Fm = (Fm − Fo)/Fm) photo-chemical efficiency of photosystem II (PSII).The rate of total electron transport through PSII (JT) and
the partitioning of electrons between carboxylation (Jc) andoxygenation (Jo) were calculated according to Farquhar etal. (1980), Genty (1989) and Epron et al. (1995).
Leaf nitrogen assay
Following the gas exchange and chlorophyll fluorescencemeasurements, leaves were harvested and dried to constantmass at 70 °C. Total leaf nitrogen (N) concentration was de-termined by the dry combustion method using a LECO CNS-2000 analyzer (LECO Corporation, St. Joseph, MI, USA).
Statistical analysis
Data were analyzed with Data Desk 6.01 (Data Description1996). All the data were examined graphically for normalityand homogeneity of variance using probability plots and scat-ter plots, respectively. The above tests showed that all the datasatisfied the assumptions for analysis of variance (ANOVA).The effects of [CO2], Tsoil, nutrient regime and their inter-actions were then tested using a three-factor, split-split plotANOVA. Differences were considered marginally significantat P ≤ 0.10 and significant at P ≤ 0.05. When the effect of aninteraction or a treatment involving more than two levels was
significant (P ≤ 0.10) for a given parameter, Scheffe’s F testfor post hoc pair-wise comparisons was conducted.
Results
In situ gas exchange
There was a significant main effect of Tsoil and nutrient supplyon An. In addition, the effects of [CO2] and [CO2] × Tsoil ×nutrient supply on this parameter were marginally significant(Table 1). An significantly increased from the low to the highnutrient regime but only at the intermediate and high, not atthe low Tsoil where no significant nutrient effects were ob-served (Figure 1A). Furthermore, there were no significantdifferences between the intermediate and high nutrient re-gimes at intermediate and high Tsoil (Figure 1A). The low Tsoilsignificantly reduced An only at the intermediate and high butnot at the low nutrient regime (Figure 1A). The differences inAn between the intermediate and high Tsoil were not statistical-ly significant (Figure 1A). The [CO2] elevation significantlyincreased An under the intermediate and high Tsoil at the inter-mediate and high but not the low nutrient level (Figure 1A).However, An was unaffected by [CO2] at low Tsoil (Figure 1A).No significant effects of [CO2] or nutrient supply alone or
in combination were observed on gs (data not shown) and E(Table 1). In contrast, there was a significant main effect ofTsoil on both parameters, but no Tsoil-related interaction (Table1). gs and E differed among all the three Tsoil treatments andthe responses were lowest in the low and highest in the inter-mediate Tsoil (Figure 1B).Only nutrient regime, but not [CO2] or Tsoil, had a signif-
icant main effect on IWUE (Table 1). However, the effect of[CO2] × Tsoil × nutrient regime was significant (Table 1).While IWUE was significantly higher in the high thanlow nutrient regime only at the low and intermediate Tsoil
Table 1. P-values of ANOVA for the effects of [CO2], soil temperature (Tsoil), nutrient regime (N) and their interactions on net photosynthesis(An), transpiration rate (E), instantaneous water-use efficiency (IWUE), internal to ambient CO2 concentration ratio (Ci/Ca), maximum carbox-ylation rate (Vcmax), light-saturated electron transport rate (Jmax), triose phosphate utilization (TPU), potential photochemical efficiency ofphotosystem II (Fv/Fm), actual photochemical efficiency of photosystem II (ΔF/Fm′), photosynthetic linear electron transport to carboxylation(Jc), the fraction of total photosynthetic linear electron transport partitioned to oxygenation (Jo/JT) and mass-based leaf nitrogen concentration([N]mass) of white birch. Seedlings were subjected to two [CO2] (360 and 720 μmol mol−1), three Tsoil (5, 15 and 25 °C initially, increased to 7,17 and 27 °C, respectively, 1 month later) and three N (4/1.8/3.3, 80/35/66 and 160/70/132 mg l−1 N/P/K) regimes for 3 months.
Source CO2 Tsoil N CO2 × Tsoil CO2 × N Tsoil × N CO2 × Tsoil × N
in ambient [CO2], the nutrient effect in elevated [CO2] wassignificant only at the two higher but not at the low Tsoil(Figure 1C). Furthermore, there were no significant differ-ences between nutrient treatments at the high Tsoil in ambi-ent [CO2] (Figure 1C). The ranking of nutrient treatmentsfor IWUE differed between [CO2] treatments: in ambient
[CO2], the intermediate nutrient regime was not signifi-cantly different from either the low or high nutrient levelsat low Tsoil, whereas there were no significant differencesbetween the low and intermediate nutrient treatments at theintermediate Tsoil; in elevated [CO2], no significant differ-ences were observed between the intermediate and high treat-ments at both the intermediate and high Tsoil (Figure 1C).IWUE was not significantly affected by Tsoil at any nutrientregime in ambient [CO2] (Figure 1C). In elevated [CO2],however, the low Tsoil significantly suppressed IWUE onlyat the intermediate and high but not at the low nutrient regimewhere the effect of Tsoil was insignificant (Figure 1C). In ad-dition, no significant differences were detected between the in-termediate and high Tsoil in elevated [CO2] (Figure 1C).Elevated [CO2] significantly increased IWUE under the inter-mediate Tsoil at the intermediate nutrient regime and underthe high Tsoil at the intermediate and high nutrient levels(Figure 1C). However, the effect of [CO2] on IWUE wasgenerally insignificant under the low Tsoil (Figure 1C).There was no effect of Tsoil alone or in combination on in-
ternal to ambient CO2 concentration ratio (Ci/Ca; Table 1). Incontrast, the effect of [CO2] × nutrient regime was marginallysignificant (Table 1). Values of Ci/Ca were lowest in high nu-trient regime + ambient [CO2] and highest in low nutrient re-gime + elevated [CO2]; however, the differences between theambient and elevated [CO2] at the low nutrient regime werenot statistically significant (Figure 1D). Ci/Ca increased fromambient to elevated [CO2] at the intermediate and high nutri-ent regimes (Figure 1D).
In vivo Rubisco activity
No significant individual or interactive effects of [CO2] orTsoil on Vcmax or Jmax were found (Table 1). Nevertheless,the effect of nutrient was significant for Jmax and marginallysignificant for Vcmax (Table 1). Vcmax and Jmax were signifi-cantly higher at high than low nutrient regime, whereas therewere no differences between the low and intermediate or theintermediate and high nutrient treatments (Figure 2A and B).There was a significant effect of nutrient regime, and a
marginal effect of Tsoil and [CO2] × nutrient regime, onTPU (Table 1). TPU generally increased from the low to the
Figure 1. Effects of [CO2], soil temperature (Tsoil) and nutrient regime(N) on net photosynthesis (An), transpiration rate (E), instantaneouswater-use efficiency (IWUE) and internal to ambient CO2 concentra-tion ratio (Ci/Ca; mean ± SE, n = 2) in white birch. Seedlings weregrown under two [CO2] (360 and 720 μmol mol−1), three Tsoil (5, 15and 25 °C initially, increased to 7, 17 and 27 °C, respectively, 1 monthlater) and three N (4/1.8/3.3, 80/35/66 and 160/70/132 mg l−1 N/P/K)regimes for 3 months. The significance levels (**P≤ 0.01, *P≤ 0.05,+P ≤ 0.1) are based on ANOVA. In Figure 1A and C, the lowercaseletters indicate CO2 × Tsoil × N interactions. In Figure 1D, the lettersindicate CO2 × N interaction. The uppercase letters indicate Tsoil ef-fect. Means with different letters are significantly different from eachother, according to Scheffe’s F test. Note: when there was no CO2-related interaction for a given parameter, only the bars on the sideof the ambient [CO2] were labeled. L, I and H represent the low,intermediate and high Tsoil, respectively.
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[CO2], SOIL TEMPERATURE, NUTRITION AND WHITE BIRCH 237
high nutrient regime in both [CO2] treatments (Figure 2C).However, the differences between the low and intermediatenutrient regimes in ambient [CO2] were not statistically sig-nificant (Figure 2C). Furthermore, no significant differenceswere observed between the intermediate and high nutrientregimes in elevated [CO2] (Figure 2C). The [CO2] elevationsignificantly increased TPU only at the intermediate but notat the low and high nutrient regimes (Figure 2C). Also, [CO2]and Tsoil had a significant interactive effect on TPU (Table 1).TPU was lowest in low Tsoil + elevated and highest in inter-mediate Tsoil + elevated [CO2], but the differences betweenlow Tsoil + elevated [CO2] and low Tsoil + ambient [CO2] were
statistically insignificant (Figure 2C). TPU was significantlyhigher at the high than intermediate Tsoil in ambient [CO2](Figure 2C). The [CO2] elevation significantly increasedTPU only at the intermediate but not at the low and high Tsoil(Figure 2C).
Photochemical efficiency of PSII
No significant effects of [CO2], Tsoil or nutrient regimewere observed on Fv/Fm (Table 1, Figure 3A). However,ΔF/Fm′ significantly responded to all three environmentalfactors, but not to their interactions (Table 1). ΔF/Fm′ de-clined with increased [CO2] (Figure 3B). Both low Tsoil andlow nutrient regime significantly decreased ΔF/Fm′, whereasthere were no significant differences between the intermediateand high Tsoil or the intermediate and high nutrient regimes(Figure 3B).Jc was significantly affected by Tsoil, nutrient regime,
[CO2] × nutrient regime and [CO2] × Tsoil × nutrient regime(Table 1). However, the effect of the three-factor interactionwas marginal. The low nutrient regime significantly reducedJc at the intermediate and high but not at the low Tsoil wherethe effect of nutrient regime was insignificant (Figure 3C).However, there were no significant differences between theintermediate and high nutrient regimes at the intermediateand high Tsoil (Figure 3C). The low Tsoil significantly sup-pressed Jc only at the high nutrient regime in ambient[CO2] and at the intermediate and high nutrient regimes inelevated [CO2], but no significant differences were noted be-tween the intermediate and high Tsoil in either ambient or el-evated [CO2] (Figure 3C). The effect of Tsoil on Jc was notsignificant at the intermediate nutrient regime in ambient[CO2] and at the low nutrient regime in both ambient and el-evated [CO2] (Figure 3C). The [CO2] elevation significantlyincreased Jc only in intermediate Tsoil + high nutrient regime(Figure 3C). No significant effect of [CO2] on Jc was detectedin any other treatment.There was no significant effect of nutrient alone or in com-
bination on Jo/JT (Table 1). In contrast, there was a significantmain effect of both [CO2] and Tsoil: Jo/JT significantly de-creased from ambient to elevated [CO2] and low to high Tsoil(Table 1, Figure 3D). However, the differences in Jo/JT be-tween the intermediate Tsoil and either the low or high Tsoilwere not statistically significant (Figure 3D).
Total leaf N concentration
There was a marginally significant main effect of [CO2] andnutrient regime and also a significant effect of [CO2] × Tsoilon leaf N concentration (Table 1). Leaf N concentration wasthe highest in ambient [CO2] + low Tsoil and the lowest inelevated [CO2] + low Tsoil and elevated [CO2] + high Tsoil(Figure 4). There were no significant differences betweenthe intermediate and high Tsoil in ambient [CO2] (Figure 4).Elevated [CO2] significantly decreased leaf N concentrationat all Tsoil (Figure 4). Leaf N concentration was significantlyhigher at the high than low nutrient regime, whereas the dif-
Figure 2. Effects of [CO2], soil temperature and nutrient regime onmaximum carboxylation rate (Vcmax), light-saturated electron trans-port rate (Jmax) and triose phosphate utilization (TPU; mean ± SE, n= 2). In Figure 2A and B, the letters indicate N effect. In Figure 2C, thelower- and uppercase letters indicate CO2 × N and CO2 × Tsoil inter-actions, respectively. See caption of Figure 1 for other explanations.
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ferences between the intermediate nutrient treatment and ei-ther the low or high nutrient levels were not statistically sig-nificant (Figure 4).
Discussion
Elevated [CO2] significantly increased An under the interme-diate and high Tsoil but only at the intermediate and high nu-trient regimes, not at the low nutrient level. Similar responsesof An to elevated [CO2] and nutrient availability have beenobtained with seedlings of loblolly pine (Pinus taeda L., Tis-sue et al. 1993), perennial ryegrass (Lolium perenne L., Daveyet al. 1999) and Japanese larch (Larix kaempferi Carr., Eguchiet al. 2004) grown under favorable Tsoil conditions. In a pre-vious study with white birch seedlings (Zhang and Dang2006), the increase in An from ambient to elevated [CO2] athigh nutrient regime was associated with an increase in Vcmax.The important role of Vcmax in the photosynthetic response ofboth coniferous and deciduous species to elevated [CO2] hasbeen demonstrated (Tissue et al. 1997, Murray et al. 2000,Ainsworth et al. 2002). In the present study, however, elevat-ed [CO2] did not increase Vcmax at any Tsoil or nutrient re-gime, suggesting that the observed increases in An cannotbe explained by higher Vcmax. The lack of positive responseof Vcmax to elevated [CO2] could be possibly attributed to thedecline in leaf N concentration under elevated [CO2] (Griffinand Seemann 1996, Midgley et al. 1999). Ellsworth et al.(2004) have reported strong correlations between the re-sponses of leaf N concentration and Vcmax to [CO2] eleva-tion. For 4-month-old potted white birch seedlings, Zhangand Dang (2006) have demonstrated that supplying optimalnutrient levels at least twice a week is crucial for maintaininghigher leaf N concentrations and, consequently, Vcmax in el-evated than ambient [CO2]. The high [CO2]-related decreasein leaf N concentration at the intermediate and high nutrientlevels under the warmer Tsoil conditions in this study wasprobably reflective of the low frequency of fertilizer applica-
Figure 4. Effects of [CO2], soil temperature and nutrient regime ontotal leaf nitrogen concentration (mean ± SE, n = 2). See captions ofFigures 1 and 2 for other explanations.
Figure 3. Effects of [CO2], soil temperature and nutrient regime onpotential photochemical efficiency of photosystem II (Fv/Fm), actualphotochemical efficiency of photosystem II (ΔF/Fm′), photosynthet-ic linear electron transport to carboxylation (Jc) and the fraction oftotal photosynthetic linear electron transport partitioned to oxygena-tion (Jo/JT; mean ± SE, n = 2). The absence of labels indicates nosignificant effects (P > 0.1). See captions of Figures 1 and 2 for otherexplanations.
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[CO2], SOIL TEMPERATURE, NUTRITION AND WHITE BIRCH 239
tion. On the other hand, reduced plant sink strength at thelow Tsoil might have induced an accumulation of excess car-bohydrates in the leaf under elevated [CO2], diluting N con-centration (DeLucia 1986, Bagnall et al. 1988).According to Farquhar et al. (1980) and Hymus et al.
(2001), high atmospheric [CO2] would stimulate electronflow to the photosynthetic carbon reduction cycle and com-petitively suppress electron allocation to the photorespira-tory carbon oxidation pathway in plants growing undernon-limiting nutrient conditions, leading to increased An.In accord with this hypothesis, the [CO2] elevation signifi-cantly increased Jc but decreased Jo/JT under intermediateTsoil + high nutrient supply. However, no significant effectsof elevated [CO2] on Jc were found in any other treatments,suggesting that other factors were more important than Jc forthe observed increases in An under elevated [CO2]. Ci sig-nificantly increased from ambient to elevated [CO2] at eachTsoil and nutrient level (data not shown). Our data supportthe finding of other researchers (e.g. Agrawal 1999, Midgleyet al. 1999) that Ci is the decisive factor for the higher An inelevated [CO2].The lack of positive response of An to elevated [CO2] at the
low nutrient regime reflects photosynthetic down-regulation(Evans 1989, Murray et al. 2000, Eguchi et al. 2004, Caoet al. 2007). To support this claim, we found a 39% declinein An from ambient to elevated [CO2] at the low nutrient re-gime, when An of ambient and CO2-enriched leaves werecompared at a common Ca of 360 μmol mol−1 (data notshown). An generally increases in response to elevated[CO2] (Drake et al. 1997, Saxe et al. 1998, Zhang and Dang2006, Huang et al. 2007). However, if the increased produc-tion of carbohydrate cannot be utilized due to nutrient limita-tion to growth, carbohydrates will accumulate in the leaf.Two mechanisms have been proposed whereby the accumu-lation of starch can directly inhibit An in elevated [CO2].First, high levels of carbohydrates may hinder CO2 diffusionfrom the intercellular airspaces to the stroma in chloroplasts(Makino 1994, Eguchi et al. 2004). Second, carbohydrate ac-cumulation may induce a feedback inhibition of carbohydratesynthesis, with the result that An is inhibited because Pi is notregenerated rapidly enough (Lambers et al. 2008). Stitt(1991) has proposed another feedback mechanism in whichcarbohydrate loading indirectly causes a decrease in the le-vels of proteins and other components of the photosyntheticapparatus. Because elevated [CO2] did not significantly re-duce Vcmax, Jmax and TPU at any nutrient level in this study,high diffusion resistance in chloroplasts may be the maincause for the down-regulation of An in elevated [CO2] +low nutrient supply.The low Tsoil suppressed An at the intermediate and high
nutrient regimes, consistent with the works of King et al.(1999), Dang and Cheng (2004) and Zhang and Dang(2005). Furthermore, there were no significant differencesin An between the ambient and elevated [CO2] at this Tsoil.The fact that low Tsoil significantly decreased gs and E buthad no significant effects on Vcmax and Jmax suggests that
the reduction in An was primarily caused by the decline ings. Zhang and Dang (2005) have found that gs is the mainlimiting factor for An in white birch and jack pine seedlingsgrowing under low Tsoil conditions in ambient and elevated[CO2]. The reduction in gs may be ascribed to a decline in leafwater potential (Benzioni and Dunstone 1988, Dang andCheng 2004). Low Tsoil may reduce water supply to the shootby increasing soil water viscosity and decreasing root perme-ability (Kaufmann 1977, Gurdarshan and Reynolds 1996, Ri-chardson 2000, Öpik and Rolfe 2005). Alternatively, thedecrease in gs could be related to non-hydraulic signals thatroots sense in cold soils (Day et al. 1991). There were nosignificant effects of Tsoil on An at the low nutrient regime,indicating a stronger nutrient than Tsoil effect on An at thisnutrient level.Lambers et al. (2008) have suggested that a decrease in An
under low Tsoil could be potentially associated with photoin-hibition. However, this study’s finding that low Tsoil did notsignificantly affect Fv/Fm and all the values of Fv/Fm werewithin the normal range (0.75–0.85, Ball et al. 1994) fornon-stressed plants points to the absence of photoinhibition.In other words, there was no loss in the yield of PSII photo-chemistry due to the low Tsoil. Similar results have been ob-tained with jack pine (Pinus banksiana Lamb., Zhang andDang 2005), trembling aspen (Populus tremuloides Michx.,Landhäusser and Lieffers 1998) and Scots pine (Pinus sylves-tris L., Domisch et al. 2001). The significant decrease in ΔF/Fm′ and increase in Jo/JT under low Tsoil were possibly relat-ed to photoprotective mechanisms. Photorespiration couldprevent the photosynthetic apparatus from photodamage byconsuming excessive assimilatory power, a prerequisite ofwhich should be an increase in photorespiratory activity or,at least, no decrease (Zhang and Dang 2005, Zhang andDang 2006, Lambers et al. 2008).In conclusion, the [CO2] elevation significantly increased
An under the intermediate and high Tsoil at the intermediateand high nutrient regimes but not at low nutrient availability.In contrast, no significant differences in An were observedbetween ambient and elevated [CO2] at the low Tsoil. Thesefindings support our hypothesis that low Tsoil reduces thepositive effect of high nutrient availability on the responseof An to elevated [CO2]. Like atmospheric [CO2], soil fertil-ity is predicted to increase in the future due to an increase inanthropogenic nitrogen deposition (Galloway et al. 2004, Le-Bauer and Treseder 2008). Although [CO2] and nutrientavailability are known to synergistically affect An of whitebirch growing under favorable Tsoil conditions (Zhang andDang 2006), the results of the present study reveal that ele-vated [CO2] + high soil fertility may not be expected to in-crease An in trees growing in cold soils. In other words, theenhancement of photosynthesis by [CO2] elevation dependson both nutrient availability and Tsoil. The differences in re-sponse can have important implications on the productivityof different sites within the boreal forest, given the great var-iation in Tsoil across the boreal landscape. However, becausethis short-term study with seedlings was conducted in a con-
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trolled environment, the findings may not accurately reflectlong-term acclimation of mature trees to field conditions(Curtis and Wang 1998, Pritchard et al. 1999, Bond 2000,Wigley and Schimel 2000). Therefore, the results shouldnot be applied directly to trees growing in forests withoutvalidation. They, however, highlight a necessity to take Tsoilinto account when investigating the effect of nutrient avail-ability on the response of An to high atmospheric [CO2]. Thiscontrolled-environment study should pave the way for fur-ther research to determine whether the different responsesof An to [CO2] resulting from the various combinations ofTsoil and nutrient availability are exhibited in the field.
Acknowledgments
This research was supported by a grant from NSERC to Q.L.D. andscholarships from Lakehead University and the Ontario LegacyForest Trust to T.F.A.
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