Synergic effect of salinity and CO2 enrichment on growth and photosynthetic responses of the invasive cordgrass Spartina densiflora.

Spartina densiflora is a C(4) halophytic species that has proved to have a high invasive potential which derives from its clonal growth and its physiological plasticity to environmental factors, such as salinity. A greenhouse experiment was designed to investigate the synergic effect of 380 and 700 ppm CO(2) at 0, 171, and 510 mM NaCl on the growth and the photosynthetic apparatus of S. densiflora by measuring chlorophyll fluorescence parameters, gas exchange and photosynthetic pigment concentrations. PEPC activity and total ash, sodium, potassium, calcium, magnesium, and zinc concentrations were determined, as well as the C/N ratio. Elevated CO(2) stimulated growth of S. densiflora at 0 and 171 mM NaCl external salinity after 90 d of treatment. This growth enhancement was associated with a greater leaf area and improved leaf water relations rather than with variations in net photosynthetic rate (A). Despite the fact that stomatal conductance decreased in response to 700 ppm CO(2) after 30 d of treatment, A was not affected. This response of A to elevated CO(2) concentration might be explained by an enhanced PEPC carboxylation capacity. On the whole, plant nutrient concentrations declined under elevated CO(2), which can be ascribed to the dilution effect caused by an increase in biomass and the higher water content found at 700 ppm CO(2). Finally, CO(2) and salinity had a marked overall effect on the photochemical (PSII) apparatus and the synthesis of photosynthetic pigments.

Introduction

Global enviropan class="Chemical">nmental changes such as climatic change and biological invasions are common conservation problems affecting ecosystems worldwide (Occhipinti-Ambrogi, 2007). Climate change is likely to alter patterns of alien plant invasions through its effect on three general aspects: the invasibility of the ecosystem, climate impacts on indigenous species, and the invasive potential of the alien species (Dukes and Mooney, 1999).

n class="Species">Spartina densiflorapan> is a C4 halophytic species with a South American originpan> that is inpan>vadinpan>g salt marshes as far apart as southern Europe (Tutin, 1980; Mateos-Naranjo ), North Africa (Fennane and Mathez, 1988) and North America (Kittelson and Boyd, 1997). In Spain, S. densiflora has proved to have a high invasive potential which derives from its prolific seed production and from its clonal growth (Figueroa and Castellanos, 1988; Nieva ). In addition, its physiological and morphological versatility apparently allows S. densiflora to tolerate a wide range of salinity, tidal submergence, and drainage (Castillo ; Mateos-Naranjo ). This is consistent with S. densiflora having colonized high, middle, and low marshes, with their different characteristic assemblages of native species (Nieva ). Many ecological and physiological aspects of S. densiflora have hitherto been analysed (Castillo ; Mateos-Naranjo , 2008a, b). Nevertheless, so far no studies have assessed the influence of climatic change and of rising atmospheric n class="Chemical">CO2 concentrations on the invasive potential of S. densiflora.

Predictiopan class="Chemical">ns regarding climate change indicate that the CO2 concentration inpan> the atmosphere is expected to have unpan>dergone a 2-fold inpan>crease by the end of the century with a value of c. 760 ppm by 2100 (IPCC, 2001). There is a general consensus on the direct physiological impact of inpan>creasinpan>g CO2 concentration on plant photosynthesis and metabolism, stimulating growth and development in hundreds of plants species (Ghannoum ). However, recent evidence from free-air CO2 enrichment experiments suggested that elevated CO2 concentration did not directly stimulate C4 photosynthesis. Nonetheless, drought stress can be ameliorated at elevated CO2 concentration as a result of lower stomatal conductance and greater intercellular CO2 concentration (Leakey, 2009). Furthermore, the effects of CO2 enrichment on plants can be modified by other environmental factors, such as salinity (Lenssen , 1995; Rozema, 1993) and temperature.

The aims of this study were to investigate (i) whether papan class="Chemical">n class="Chemical">CO2 enpan>richment stimulates the growth of the inpan>vasive species S. densiflora, and whether this stimulation is mediated by an improvement of photosynthetic activity, and/or (ii) whether salinity stress can be ameliorated at high CO2 concentration. The specific objectives were to: (i) to analyse the growth of plants in experimental salinity concentrations from 0 to 510 mM NaCl at ambient and elevated n class="Chemical">CO2 concentrations (380 and 700 ppm, respectively); (ii) determine the extent of the effects on the photosynthetic apparatus (PSII chemistry), gas exchange characteristics, phosphoenolpyruvate carboxylase activity (PEPC), and photosynthetic pigments; and (iii) examine the possible role of concentrations of mineral matter (ash), calcium, potassium, sodium, and zinc accumulated and C/N ratio in response to increasing external salinity at both CO2 levels.

Materials and methods

Plant material

Seeds of n class="Species">S. densiflorapan> were collected in December 2006 from Odiel Marshes (37°15’ N, 6°58’ W; SW Spainpan>), and subsequently stored at 4 °C (inpan> darkness) for three months. After the storage period, seeds were placed inpan> a germinpan>ator (ASL Aparatos Científicos M-92004, Madrid, Spainpan>), and subjected to an alternatinpan>g diurnal regime of 16 h of light (photon flux rate, 400–700 nm, 35 μmol m−2 s−1) at 25 °C and 8 h of darkness at 12 °C, for a month. Seedlinpan>gs were planted inpan> inpan>dividual plastic pots (9 cm and 11 cm of height and diameter, respectively) filled with perlite and placed inpan> a greenhouse (durinpan>g sprinpan>g 2007) with minpan>imum-maximum temperatures of 21–25 °C, 40–60% relative humidity and natural daylight (minpan>imum and maximum light flux: 200 and 1000 μmol m−2 s−1, respectively). Pots were carefully irrigated with 20% Hoagland's solution (Hoagland and Arnon, 1938) as necessary.

Growth conditions

In April 2007, after a mopan class="Chemical">nth of seedling cultures, the pots were allocated to three NaCl treatments inpan> shallow trays: 0, 171, and 510 mM inpan> n class="Chemical">Hoagland's solution. Afterwards, they were exposed to ambient (380 ppm) or elevated n class="Chemical">CO2 concentration (700 ppm), in a controlled-environment chamber, in the same greenhouse and supplied with Hoagland's solution with or without NaCl (ten pots per tray, with one tray per NaCl and CO2 treatments) for a further three months. The CO2 concentration was within 10% of the target concentration for 85% of the time, on the basis of 1 min averages. NaCl concentrations were chosen to cover variations recorded by Mateos-Naranjo in the salt marshes of the Odiel River where S. densiflora occurs.

The n class="Chemical">CO2pan> concentration inpan> the greenhouse with ambient CO2 was not controlled, but it was measured with a CO2 analyser (Testo 535, Germany). The controlled-environment chamber consists of transparent chamber tops, 3.3×1.4×1.1 m (length×width×height) made with 0.005 thick acrylic glass, and aluminium angular frame elements. The CO2 level in the enriched chamber was maintained by supplying pure CO2 from a compressed gas cylinder (Air liquide, B50 35K) into the chamber. The CO2 concentration in the chamber was continuously recorded by a CO2 sensor (Vaisala CARBOCAP GMT220, Finland), the signal being received by a computer (ASCON M3, Italy) that activated, if necessary, CO2 injection into the enriched chamber so as to reach the desired 700 ppm.

At the beginpan class="Chemical">ning of the experiment, 3.0 l of the appropriate solution were placed in each of the trays down to a depth of 1 cm. During the experiment, the levels in the trays were monitored and they were topped up to the marked level with 20% Hoagland's solution as a way to limit the change of NaCl concentration due to water evaporation of the nutritive solution.

Growth analysis

At the beginpan class="Chemical">ning and at the end of the experiment, three and seven entire plants (roots and leaves) from each treatment, respectively, were dried at 80 °C for 48 h and then weighed. Dried, ground samples were ignited in lidded, ceramic crucibles and ash weights were recorded; the furnace temperature was raised slowly over 6 h to 550 °C and this temperature was maintained for a further 8 h. Also, the number of tillers was measured.

A classical growth analysis (Evapan class="Chemical">ns, 1972) was carried out with ash-free dry masses. The relative growth rate in whole plant dry mass (RGR) was calculated and partitioned into its three components, unit leaf rate (ULR), specific leaf area (SLA), and leaf mass fraction (LMF), using the software tool of Hunt :

where t is time, W is total dry mass per plant, LA is total leaf area per plapan class="Chemical">nt, and LW is total leaf dry mass per plant. Leaf area was calculated by superimposing the surface of each leaf over a mm-square paper.

Leaf elongatiopan class="Chemical">n rate (LER) was measured in random leaves (n=14, per treatment; two measurements per plant) at 90 d of treatment by placing a marker of inert sealant at the base of the youngest accessible leaf. The distance between the marker and the leaf base was measured after 24 h (Mateos-Naranjo ).

Gas exchange

Gas exchange measuremepan class="Chemical">nts were taken on random, fully expanded penultimate leaves (Fig. 1; n=10, one measurement per plant and three extra taken randomly) using an infrared gas analyser in an open system (Li-6400, Li-Cor Inc., Nebraska, USA) after 7, 30, and 90 d of treatment. Net photosynthetic rate (A), intercellular CO2 concentration (Ci), and stomatal conductance to CO2 (Gs) were determined at ambient CO2 concentration of 380 and 700 ppm CO2, temperature of 20 °C, 50±5% relative humidity, and a photon flux density of 1000 μmol m−2 s−1. A, Ci, and Gs were calculated using standard formulae of Von Caemmerer and Farquhar (1981). Photosynthetic area was approximated as the area of a trapezium. The water use efficiency (WUE) was calculated as the ratio between A and transpiration rate (mmol CO2 assimilated mol−1 H2O transpired).

Fig. 1.

Pot of Spartina densiflora with fully expanded penultimate leaves marked in red. (This figure is available in colour at JXB online.)

Pot of papan class="Chemical">n class="Species">Spartina densiflora with fully expanpan>ded penultimate leaves marked inpan> red. (This figure is available inpan> colour at JXB onlinpan>e.)

Leaf water content

Leaf n class="Chemical">waterpan> content (WC) was calculated after 90 d of treatment as:where FW is the fresh mass of the leaves, and DW is the dry mass after oven-dryinpan>g at 80 °C for 48 h.

Chlorophyll fluorescence

n class="Chemical">Chlorophyllpan> fluorescence was measured inpan> random, fully developed penultimate leaves (n=10, one measurement per plant and three extra taken randomly) usinpan>g a portable modulated fluorimeter (FMS-2, Hansatech Instrument Ltd., England) after 7, 30, and 90 d of treatment. Light- and dark-adapted fluorescence parameters were measured at dawn (stable, 50 μmol m−2 s−1 ambient light) and at midday (1600 μmol m−2 s−1) to inpan>vestigate whether NaCl and CO2 concentration affected the sensitivity of plants to photoinhibition.

Plapan class="Chemical">nts were dark-adapted for 30 min, using leaf-clips exclusively designed for this purpose. The minimal fluorescence level in the dark-adapted state (F0) was measured using a modulated pulse (<0.05 μmol m−2 s−1 for 1.8 μs) which was too small to induce significant physiological changes in the plant. The data stored were an average taken over a 1.6 s period. Maximal fluorescence in this state (Fm) was measured after applying a saturating actinic light pulse of 15 000 μmol m−2 s−1 for 0.7 s. The value of Fm was recorded as the highest average of two consecutive points. Values of the variable fluorescence (Fv=Fm–F0) and maximum quantum efficiency of PSII photochemistry (Fv/Fm) were calculated from F0 and Fm. This ratio of variable to maximal fluorescence correlates with the number of functional PSII reaction centres, and dark-adapted values of Fv/Fm can be used to quantify photoinhibition (Krivosheeva ).

The same leaf section of each plapan class="Chemical">nt was used to measure light-adapted parameters. Steady-state fluorescence yield (Fs) was recorded after adapting plants to ambient light conditions for 30 min. A saturating actinic light pulse of 15 000 μmol m−2 s−1 for 0.7 s was then used to produce the maximum fluorescence yield () by temporarily inhibiting PSII photochemistry.

Using fluorescence parameters determined in both light- and dark-adapted states, the following were calculated: quantum efficiency of PSII and non-photochemical quenching ; Redondo-Gómez ).

Photosynthetic pigments

At the end of the experimepan class="Chemical">nt period, photosynthetic pigments in fully expanded penultimate leaves (n=5) were extracted using 0.05 g of fresh material in 10 ml of 80% aqueous acetone. After filterinpan>g, 1 ml of the suspension was diluted with a further 2 ml of acetone and chlorophyll a (Chl a), chlorophyll b (Chl b) and carotenoid (Cx+c) contents were determined with a spectrophotometer (Hitachi U-2001, Hitachi Ltd, Japan), using three wavelengths (663.2, 646.8, and 470.0 nm). Concentrations of pigments (μg g−1 FW) were obtained by calculation, using the method of Lichtenthaler (1987).

Determination of sodium, potassium, calcium, magnesium, zinc, and nitrogen

In accordapan class="Chemical">nce with protocols of Redondo-Gómez , at the end of the experiment leaf and root samples were dried at 80 °C for 48 h and ground. Leaves and roots were carefully washed with distilled water before any further analysis. Then 0.5 g samples, taken from a mixture of the leaves or the roots belonginpan>g to the seven plants used for each treatmenpan>t, were tripicately digested with 6 ml n class="Chemical">HNO3, 0.5 ml n class="Chemical">HF, and 1 ml H2O2. Ca2+, K+, Mg2+, Na+, P, and Zn were measured by inductively coupled plasma (ICP) spectroscopy (ARL-Fison 3410, USA). Total N and C concentrations were determined for undigested dry samples with an elemental analyser (Leco CHNS-932, Spain).

Preparation of desalted protein extracts

Plapan class="Chemical">nts were exposed to 2 h of direct sunlight at midday. To extract PEPC (n=5), 0.2 g of leaf tissue were taken and immediately ground in a chilled mortar with 1 ml of extraction buffer A containing 100 mM TRIS-HCl pH 7.5, 20% (v/v) glycerol, 1 mM EDTA, 10 mM MgCl2, 14 mM β-mercaptoethanol, 1 mM phenylmethysulphonyfluoride (PMSF), 10 μg ml−1 chymostatin, 10 μg ml−1 leupeptin, and 10 mM potassium fluoride. The homogenate was centrifuged at 15 000 g for 2 min and the supernatant was filtered through Sephadex G-25 equilibrated with buffer A without β-mercaptoethanol. The desalted extract was used rapidly to determine the activity and sensitivity of PEPC to L-malate, as described below.

Assay of PEPC activity and its inhibition by L-malate

PEpan class="Chemical">PC activity was measured spectrophotometrically at the optimal and suboptimal pH values of 8 and 7.3, respectively, using the NAD-malate dehydrogenase-coupled assay at 2.5 mM phosphoenolpyruvate (PEP) described by Echevarria et al. (1994). Assays were initiated by the addition of an aliquot of crude extract (n=5). An enzyme unit is defined as the amount of PEPC that catalyses the carboxylation of 1 μmol of phosphoenolpyruvate min−1 at pH 8 and 30 °C. Malate sensitivity was determined at suboptimal pH 7.3 in the presence or absence of various concentrations of L-malate (IC50, 50% inhibition of initial PEPC activity by L-malate; Echevarría ). A high IC50 is related to a high degree of PEPC phosphorylation.

Protein quantification

Proteipan class="Chemical">n amounts were determined by the method of Bradford (1976), using n class="Species">bovine serum albuminpan> (BSA) as standard.

Statistical analysis

Statistical analysis was carried out usipan class="Chemical">ng Statistica v. 6.0 (Statsoft Inc.). Pearson coefficients were calculated to assess correlation between different variables. Data were analysed using one-, two-, and three-way analysis of variance (F-test). Data were first tested for normality with the Kolmogorov–Smirnov test and for homogeneity of variance with the Brown–Forsythe test. Significant test results were followed by Tukey tests for identification of important contrasts. The comparison between measurements of fluorescence at dawn and midday and between the means of ambient and elevated n class="Chemical">CO2 concentration treatments inpan> all parameters were made by usinpan>g the Student test (t test).

Results

Total dry mass showed a broad optimum at 171 mM n class="Chemical">pan class="Chemical">NaCl externpan>al salinity for both 380 and 700 ppm CO2 (Fig. 2A); dry mass was substantially reduced at the highest salinity (510 mM NaCl) for both CO2 concentrations. On the other hand, total dry mass was higher at 700 ppm n class="Chemical">CO2 and 0 and 171 mM NaCl than at 380 ppm CO2 for the same salinity treatments (two-way ANOVA: CO2×salinity, F2,32=4.1, P <0.05; Fig. 2A). The same trends were evident in mean relative growth rate (RGR; Fig. 2B), and the effects of salinity and CO2 concentration on RGR were highly significant after 90 d of treatment (two-way ANOVA: CO2×salinity, F2,34=3.6, P <0.05). The peak at 171 mM NaCl was associated with higher values of total leaf area (Fig. 2C) and specific leaf area (Fig. 2D), whereas the lower values of RGR at 510 mM were linked to lower values of unit leaf rate (Fig. 2E). Leaf mass fraction did not show any relationship either with salinity or CO2 treatments, showing values c. 0.7 g g−1 in all cases (data not presented).

Fig. 2.

Growth analysis of Spartina densiflora in response to treatment with a range of salinity concentrations at ambient and elevated CO2 concentration over 90 d. Total dry mass (A), relative growth rate (B), total leaf area (C), specific leaf area (D), unit leaf rate (E), leaf elongation rate (F), and number of tillers (G). Values represent mean ±SE, n=7 (n=14 for total leaf area). The analysis was carried out on an ash-free dry mass basis. Different letters indicate means that are significantly different from each other (two-way ANOVA, CO2×salinity; Tukey test, P <0.05).

Growth analysis of papan class="Chemical">n class="Species">Spartina densiflora inpan> response to treatment with a range of salinpan>ity concentrations at ambient and elevated CO2 concentration over 90 d. Total dry mass (A), relative growth rate (B), total leaf area (C), specific leaf area (D), unit leaf rate (E), leaf elongation rate (F), and number of tillers (G). Values represent mean ±SE, n=7 (n=14 for total leaf area). The analysis was carried out on an ash-free dry mass basis. Different letters indicate means that are significantly different from each other (two-way ANOVA, CO2×salinity; Tukey test, P <0.05).

Finally, RGR was directly correlated with leaf elopan class="Chemical">ngation rate (LER; r=0.84, P <0.0001) and number of tillers (r=0.81, P <0.0001) at 700 ppm n class="Chemical">CO2. There were not significant correlations between these parameters at 380 ppm n class="Chemical">CO2, although they showed similar trends (Fig. 2F, G).

Overall net photosypan class="Chemical">nthetic rate (A) data were similar for all salinity and CO2 treatments at each of the three measurement times; except at the 30 d treatment, where planpan>ts grown at 700 ppm CO2 with 0 and 171 mM NaCl showed higher A values than those at ambient n class="Chemical">CO2 (t test, P <0.01; Fig. 3A–C). Stomatal conductance (Gs) showed a trend that was extremely similar to that of A (Fig. 3D–F). Nonetheless, Gs values were lower at 700 ppm CO2 after 30 d and 90 d of treatment (t test, P <0.05). Intercellular CO2 concentration (Ci) at 700 ppm CO2 responded differently to salinity at the earlier stages of the experiment than at the later stage: salinity had no effect on Ci after 7 d and 30 d but Ci reached a peak at 171 mM NaCl after 90 d (Fig. 3G–I). Ci values were higher at 700 ppm CO2 at each of the three measurement times for all salinity treatments (t test, P <0.05).

Fig. 3.

Net photosynthetic rate, A (A–C), stomatal conductance, Gs (D–F), and intercellular CO2 concentration, Ci (G–I) in randomly selected, fully expanded penultimate leaves of Spartina densiflora in response to treatment with a range of NaCl concentrations at ambient and elevated CO2 concentration after 7 d (A, D, G), 30 d (B, E, H), and 90 d (C, F, I). Values represent mean ±SE, n=10.

n class="Chemical">Npan>et photosynthetic rate, A (A–C), stomatal conductance, Gs (D–F), and inpan>tercellular CO2 concentration, Ci (G–I) in randomly selected, fully expanded penultimate leaves of Spartina densiflora in response to treatment with a range of NaCl concentrations at ambient and elevated CO2 concentration after 7 d (A, D, G), 30 d (B, E, H), and 90 d (C, F, I). Values represent mean ±SE, n=10.

Plapan class="Chemical">nts grown at 700 ppm CO2 showed consistently higher npan> class="Chemical">water use efficiency (WUE), although significant differences were only recorded at 0 mM NaCl (t test, P >0.05). Leaf water content (WC) was higher at 700 ppm CO2 for all salinity treatments after 90 d of treatment (t test, P >0.0001; Fig. 4). Furthermore, WC was higher at 171 mM NaCl at 700 ppm CO2 concentration (one-way ANOVA: F2,17=23.4, P <0.0001).

Fig. 4.

Water use efficiency, WUE (A) and leaf water content (B) in randomly selected, fully expanded penultimate leaves of Spartina densiflora in response to treatment with a range of NaCl concentrations at ambient and elevated CO2 concentration over 90 d. Values represent mean ±SE, n=10 and n=7 for WUE and water content, respectively.

n class="Chemical">Waterpan> use efficiency, WUE (A) and leaf water content (B) in randomly selected, fully expanded penultimate leaves of Spartina densiflora in response to treatment with a range of NaCl concentrations at ambient and elevated n class="Chemical">CO2 concentration over 90 d. Values represent mean ±SE, n=10 and n=7 for WUE and water content, respectively.

Values of Fv/Fm and quapan class="Chemical">ntum efficiency of PSII (ΦPSII) at dawn were high at both 380 ppm and 700 ppm CO2 at all external NaCl concentrations after 7, 30, and 90 d of treatment, varying between 0.81 and 0.84 for Fv/Fm, and between 0.80 and 0.83 for ΦPSII. Fv/Fm and ΦPSII, respectively, were always lower at midday and the reductions resulted mainly from lower values of Fm and qP (data not presented), respectively, at midday than at dawn (t test, P <0.05). On the other hand, the midday Fv/Fm values at both CO2 concentrations decreased during the course of the experiment, especially in plants grown with 700 ppm CO2; again as a consequence of lower values of Fm (Fig. 5A–C). The same trends were evident in ΦPSII at midday (Fig. 5D–F).

Fig. 5.

Maximum quantum efficiency of PSII photochemistry, Fv/Fm (A–C) and quantum efficiency of PSII, ΦPSII (C–E) at midday in randomly selected, fully expanded penultimate leaves of Spartina densiflora in response to treatment with a range of salinity concentrations at ambient and elevated CO2 concentration after 7 d (A, C), 30 d (B, D), and 90 d (C, E). Values represent mean ±SE, n=10. Different letters indicate means that are significantly different from each other (two-way ANOVA, CO2×salinity; Tukey test, P <0.05).

Maximum quantum efficiency of PSII photochemistry, Fv/Fm (A–C) and quantum efficiency of PSII, ΦPSII (C–E) at midday in randomly selected, fully expanded penultimate leaves of Spartina densiflora inpan> response to treatment with a range of salinpan>ity concentrations at ambient and elevated CO2 concentration after 7 d (A, C), 30 d (B, D), and 90 d (C, E). Values represent mean ±SE, n=10. Different letters indicate means that are significantly different from each other (two-way ANOVA, CO2×salinity; Tukey test, P <0.05).

The effects of salinity apan class="Chemical">nd of CO2 concenpan>tration on Fv/Fm at midday were highly significant after 90 d of treatment (two-way ANOVA: CO2×salinity, F2,41=5.0, P <0.05; Fig. 5C). Moreover, ΦPSII values at midday increased with external salinity in plants grown at 380 ppm CO2 after 90 d of treatment (one-way ANOVA; F2,46=4.0, P <0.05; Fig. 5F); and ΦPSII values were lower at 700 ppm CO2 in the presence of NaCl than at 380 ppm CO2 (t-test, P <0.05).

Finally, plapan class="Chemical">nts treated with both n class="Chemical">CO2 concenpan>trations mainpan>tainpan>ed nearly constant NPQ at each of the three measurement times, irrespective of the salinpan>ity treatment (c. 1.0; P >0.05).

The effects of salinity apan class="Chemical">nd CO2 concenpan>tration on pigment concentrations (Chl a, Chl b, and Cx+c, all in μg g−1 FW; Fig. 6A–C) were highly significant after 90 d of treatment (two-way ANOVA, CO2×salinity: Chl a, F2,23=17.7, P <0.0001; Chl b, F2,22=4.6, P <0.05; Cx+c, F2,23=5.0, P <0.05). Pigment concentrations increased with increasing salinity treatment at both CO2 concentrations (one-way ANOVA, P <0.05). Contrary to that, leaf nitrogen content diminished with increasing salinity (Fig. 6D). Furthermore, plants treated with n class="Chemical">NaCl showed higher Chl a and b concentrations at 380 ppm CO2 than under 700 ppm CO2 concentration (t test, P <0.0001). In the case of carotenoids, there were not any differences between CO2 treatments (P >0.05).

Fig. 6.

Chlorophyll a, Chl a (A), chlorophyll b, Chl b (B), carotenoid, Cx+c (C), and leaf nitrogen (D) concentrations in randomly selected, fully expanded penultimate leaves of Spartina densiflora in response to treatment with a range of salinity concentrations at ambient and elevated CO2 concentration over 90 d. Values represent mean ±SE, n=5. Different letters indicate means that are significantly different from each other (two-way ANOVA, CO2×salinity; Tukey test, P <0.05).

n class="Chemical">Chlorophyll apan>, n class="Chemical">Chl a (A), n class="Chemical">chlorophyll b, Chl b (B), carotenoid, Cx+c (C), and leaf nitrogen (D) concentrations in randomly selected, fully expanded penultimate leaves of Spartina densiflora in response to treatment with a range of salinity concentrations at ambient and elevated CO2 concentration over 90 d. Values represent mean ±SE, n=5. Different letters indicate means that are significantly different from each other (two-way ANOVA, CO2×salinity; Tukey test, P <0.05).

Overall, the mineral (ash) copan class="Chemical">ntents of both leaves and roots were higher at 380 ppm CO2, and inpan>creased with inpan>creasinpan>g external NaCl concentration. Likewise, ash content was greater in roots than in leaves (three-way ANOVA, CO2×salinity×tissue: F2,23=3.7, P <0.05; Table 1).

Table 1.

Ash, total sodium, potassium, calcium, magnesium and zinc, and C/N ratio for leaves and roots of Spartina densiflora in response to a treatment with a range of salinity concentrations at ambient and elevated [CO2] over 90 d

[CO2] (ppm)	380
Tissue	Leaves			Roots
Salinity (mM)	0	171	510	0	171	510
Ash (%)	13.4 (0.03) a	15.8 (0.13) b	15.4 (0.02) b	19.0 (0.22) c	19.4 (0.39) c	23.8 (0.43) d
Na (mg g−1)	8.5 (0.10) a	31.9 (0.10) b	47.4 (0.27) c	5.5 (0.04) de	23.8 (0.05) f	38.2 (0.22) g
K (mg g−1)	53.8 (1.24) a	24.8 (0.62) b	14.7 (0.12) c	41.6 (0.17) e	37.7 (0.30) ef	50.1 (0.58) a
Ca (mg g−1)	7.9 (0.05) a	6.2 (0.02) b	3.8 (0.00) c	3.8 (0.02) c	2.1 (0.00) d	1.7 (0.00) e
Mg (mg g−1)	3.6 (0.02) a	4.1 (0.01) b	2.0 (0.01) c	6.1 (0.03) d	5.2 (0.01) e	2.2 (0.00) f
Zn (mg kg−1)	40.7 (0.93) a	30.9 (0.56) b	41.8 (0.23) ac	44.2 (0.56) c	40.0 (0.26) a	87.7 (1.24) d
C/N	15.3 (0.05) a	17.6 (0.00) b	17.1 (0.10) b	10.0 (0.02) c	14.1 (0.23) d	16.4 (0.30) e
[CO2] (ppm)	700
Tissue	Leaves			Roots
Salinity (mM)	0	171	510	0	171	510
Ash (%)	11.3 (0.05) e	15.4 (0.15) b	16.9 (0.27) b	11.3 (0.36) e	12.9 (0.42) f	21.2 (0.48) g
Na (mg g−1)	6.3 (0.02) d	36.3 (0.29) h	47.8 (0.64) c	5.4 (0.05) de	23.6 (0.42) f	44.5 (0.40) i
K (mg g−1)	32.8 (0.62) fg	17.4 (0.16) cd	10.6 (0.15) c	30.3 (0.87) g	19.6 (0.36) d	25.4 (0.23) b
Ca (mg g−1)	6.7 (0.00) f	5.0 (0.00) g	4.2 (0.00) h	2.6 (0.00) i	1.5 (0.00) j	1.2 (0.00) k
Mg (mg g−1)	2.7 (0.03) g	3.7 (0.04) a	2.5 (0.02) h	3.7 (0.05) a	3.9 (0.03) b	2.9 (0.02) i
Zn (mg kg−1)	20.2 (0.04) e	21.6 (0.07) e	28.4 (0.14) bf	16.9 (0.23) g	17.0 (0.07) g	28.9 (0.16) bf
C/N	16.0 (0.05) e	16.3 (0.12) e	16.4 (0.05) e	12.9 (0.08) f	16.2 (0.00) e	14.2 (0.11) d

Values represent mean ±SE, n=3. Different letters indicate means that are significantly different from each other (three-way ANOVA, CO2×salinity×tissue; Tukey test, P <0.05).

Ash, total n class="Chemical">sodiumpan>, n class="Chemical">potassium, n class="Chemical">calcium, magnesium and zinc, and C/N ratio for leaves and roots of Spartina densiflora in response to a treatment with a range of salinity concentrations at ambient and elevated [CO2] over 90 d

Values represent meapan class="Chemical">n ±SE, n=3. Different letters indicate means that are significantly different from each other (three-way ANOVA, n class="Chemical">CO2×salinpan>ity×tissue; Tukey test, P <0.05).

By the end of the experimepan class="Chemical">nt, tissue Na concentrations were greater in leaves than in roots, and increased markedly with external NaCl concentration (t test, P <0.001). By contrast, leaf K, Ca, and Mg concentrations decreased with increasing salinity at both CO2 treatments. In addition, leaf and root K, Ca, and Mg concentrations were higher at 380 ppm CO2 (t test, P <0.05).

On the other hand, tissue zinc concentration was greater at 380 ppm CO2 (t test, P <0.0001) and Zn content was higher in the roots than in the leaves at ambient CO2 concentration (Table 1).

Finally, C/pan class="Chemical">N ratio was considerably higher in leaves than in roots for all salinity and CO2 treatments (three-way ANOVA, P <0.0001). Tissue C/N ratio inpan>creased with external NaCl concentration at 380 ppm CO2. Furthermore, C/N ratio was greater at 380 ppm CO2 for leaves and at 700 ppm CO2 for roots.

PEPC activity

The effects of salinity apan class="Chemical">nd of CO2 concenpan>tration on PEPC activity were significant after 90 d of treatment (two-way ANOVA: CO2×salinity, F2,18=3.8, P <0.05). The lowest value of PEPC activity was recorded at 380 ppm CO2 and 171 mM NaCl, and the highest value at the same salinity and elevated CO2 concentration (Fig. 7A). Contrary to that, IC50 for malate was higher at 380 ppm CO2 and 171 mM n class="Chemical">NaCl, and lower at 700 ppm CO2 and the same salinity (Fig. 7B).

Fig. 7.

PEPC activity (A) and IC50 values for L-malate (B) in crude extracts of illuminated leaves of Spartina densiflora in response to treatment with a range of salinity concentrations at ambient and elevated CO2 concentration over 90 d. Values represent mean ±SE, n=5. Different letters indicate means that are significantly different from each other (two-way ANOVA, CO2×salinity; Tukey test, P <0.05).

PEpan class="Chemical">PC activity (A) and IC50 values for L-malate (B) inpan> crude extracts of illuminpan>ated leaves of npan> class="Species">Spartina densiflora in response to treatment with a range of salinity concentrations at ambient and elevated CO2 concentration over 90 d. Values represent mean ±SE, n=5. Different letters indicate means that are significantly different from each other (two-way ANOVA, CO2×salinity; Tukey test, P <0.05).

Discussion

Significapan class="Chemical">nt long-term (i.e. months) effects of CO2 concentration onpan> the growth of the C4 n class="Species">Spartina densiflora were observed, with plants grown at elevated n class="Chemical">CO2 concentration producing 35% and 20% more biomass, at 0 and 171 mM NaCl, respectively, than their ambient CO2-grown counterparts; although this effect was counterbalanced by high salinity (510 mM NaCl), recording similar total dry mass and RGR at ambient and at elevated CO2 concentrations. This response was apparent as the RGR of ash-free dry mass, total leaf area, leaf elongation rate and, by inference, the number of tillers produced. Castillo found the highest rate of leaf elongation of S. densiflora in distilled water. Nevertheless, in our experiment, the growth at 171 mM NaCl was slightly higher than in the absence of salt under both CO2 concentrations. In this and other studies, the absence of salt has been proved to affect neither the photosynthetic function of S. densiflora nor its growth (Mateos-Naranjo ).

Enhapan class="Chemical">nced growth at elevated CO2 concenpan>tration disagrees with results reported by Lenssen , who found that elevated CO2 concentration reduced plant weight of Spartina anglica by 20%, this reduction being associated with decreased SLA. In the current study, the stimulation of growth in S. densiflora at elevated CO2 concentration and 171 mM NaCl was linked to higher SLA, while leaf mass fraction was not affected by either salinity or n class="Chemical">CO2 concentration. The increase of SLA with CO2 concentration and salinity could be mediated by an improvement in water relations and/or an induction of cell expansion. Rozema found that a higher turgor pressure might stimulate leaf expansion. In this way, higher leaf water content was observed in S. densiflora at elevated CO2 concentration in the presence of salt. On the other hand, De Souza et al. (2008) found that elevated CO2 concentration in sugarcane induced cell expansion through the action of XTH (xyloglucan endotransglycosylase/hydrolase) on leaf cell walls. A similar effect was observed by Ferris for Populus species.

Enhapan class="Chemical">nced growth of S. densiflora was higher than that reported by Rogers for C4 inpan>vasive plants of Cyperus rotundus and C. esculentus (10–15%), even higher than the growth stimulation described in the literature for other C4 plants in response to a doubling of the current ambient n class="Chemical">CO2 concentration under non-saline conditions (22–33%; Ghannoum ). On the other hand, Rozema noted that Spartina patens showed higher RGR values at elevated CO2 concentration when plants were treated with low salinity (10 mM NaCl), while RGR of those treated with 250 mM NaCl decreased at 580 ppm CO2 (–48.3%, under aerated conditions in the culture solution). In the case of S. densiflora, the decrease of RGR at an elevated CO2 concentration was recorded in plants treated with 510 mM NaCl. Reduced RGR at high salinity can be attributed to lower unit leaf rate. This component of RGR was the most sensitive to salinity and CO2 concentration, underlining the primary importance of the rate of assimilation per unit leaf area.

Little effect of salinity apan class="Chemical">nd of CO2 concenpan>tration was detected on the net photosynthetic rate of S. densiflora. In this regard, starch accumulation has been described as a possible cause of the reduction of a CO2 stimulation of photosynthesis during long-term elevated CO2 concentration levels (DeLucia ). However, our results showed lower values of foliar C/N ratio at 700 ppm CO2. Yelle found in n class="Species">Lycopersicon esculentum that acclimation to elevated CO2 concentration was not a result of starch accumulation but was instead related to decreased activity of ribulose-1,5-biphosphate carboxylase/oxygenase (Rubisco).

Contrastipan class="Chemical">ngly, De Souza et al. (2008) found that A of sugarcane decreased because of root growth limitation after 22 weeks at 720 ppm CO2, since it is known that root growth limitation can result in lower photosynthetic rates (Arp, 1991). Thus, roots of S. densiflora occupied 5% less of the volume of the pot for all treatments after three months, so pot size did not limit root growth.

On the other hapan class="Chemical">nd, different works have shown that one of the most consistent responses of C4 plant species to elevated atmospheric CO2 concentration is a decrease inpan> Gs (Ainpan>sworth and Rogers, 2007). In the case of S. densiflora this decrease was only recorded after 30 d and 90 d of treatment. Robredo explained that increased Ci originated by the elevated CO2 concentration could promote partial stomatal closure, although the mechanism whereby stomata respond to the CO2 signal remains unknown. Nevertheless, higher Ci values of S. densiflora at elevated CO2 were also recorded after 7 d of treatment without there being a lower Gs, which could be explained by the lower A values.

By contrast, the levels of pan class="Chemical">PEPC protein of S. densiflora leaves were high at elevated CO2 concentration. Intermediate salinity and CO2 concentration enhanced PEPC activity, which could also contribute to the acclimation of A to CO2 enrichment. On the other hand, the lower PEPC activity recorded at 171 mM NaCl and ambient CO2 concentration was counterbalanced by a higher activation of PEPC (see IC50 for malate). This response has previously been reported by Echevarría and García-Mauriño , who found that PEPC-kinase activity of Sorghum increased in salt-treated plants. Contrary to this, Sankhla and Huber (1974) found that high concentrations of Na within the plants might limit the activity of PEPC. Nevertheless, PEPC activity of S. densiflora was not limited by the progressive accumulation of Na+ in root and shoots, with increasing external salinity, at either CO2 concentration treatment.

Accumulation of pan class="Chemical">Na+ with increasing external salinity concentration was accompanied by the decrease of leaf K, Ca, and Mg levels. Similar results have been recorded for other halophytes (Khan ; Redondo-Gómez ). The higher minpan>eral (ash) contents reported at ambient CO2 concentration could be accounted for by the higher leaf and root K, Ca, Mg, and Zn concentrations. Rogers reported that plant nutrient concentrations often decline under elevated CO2 concentration, and this decline can be ascribed to the dilution effect caused by increases in biomass. In the case of S. densiflora, this decline could be also ascribed, to a great extent, to the dilution effect caused by the higher water content found at elevated CO2. Ainsworth and Rogers (2007) explained this decline by a reduction of leaf transpiration rate under elevated CO2 concentration, which may cause a lower flux of nutrient through the soil to the root surface, thereby reducing nutrient uptake. All these possibilities could explain the lower nutrient content recorded in S. densiflora under elevated CO2 concentration.

On the other hand, Randall and Bouma (1973) found that carbonic anhydrase (inpan>volved inpan> photosynthesis, facilitatinpan>g the diffusion of CO2 through the liquid phase of the cell to the chloroplast) demonstrated a lower efficiency at concentrations approaching CO2 saturation with n class="Disease">Zn deficiency, suggesting that zinc deficiency impairs the biochemical capacity of the plant to fix CO2. Nonetheless, net photosynthesis of S. densiflora was not affected, instead a lower leaf Zn concentration was recorded at 700 ppm CO2.

There was evidence that elevated salipan class="Chemical">nity and CO2 concentrationpan> affected the integrity or function of the photochemical apparatus inpan> the long term, and there was an impact on chlorophyll concentrations in the leaves. Chlorophyll content was enhanced by salinity; although at an elevated CO2 concentration this enhancement was only recorded at 510 mM NaCl. According to Geissler , plants seemed to use the additional energy supply under an elevated CO2 concentration for increasing the investment in salinity tolerance mechanisms; for example, for reducing oxidative stress and water loss. The NaCl-induced increase in the chlorophyll level has been previously reported in S. densiflora (Castillo ). Furthermore, as in our experiment, Chen found that the n class="Disease">impairment of the photosynthetic pigments was greater in crop plants at an elevated CO2 concentration. These findings seem to point to CO2 enrichment as an accelerator of pigment degradation and of leaf senescence (Munns, 1993). However, leaf N concentration was not lower at elevated CO2, so there was no link between leaf nitrogen and pigment concentration, as would be expected due to the fact that N is mostly contained in chlorophyll molecules (Torres Netto ). This may be explained by the previously referred dilution effect, caused by the higher water content found at elevated CO2. In fact, the response of leaf N concentration and of water content to salinity and CO2 concentration were quite similar.

On the other hand, the response of A to salinity and to CO2 concentration did not track that of ΦPSII, sinpan>ce the quantum efficiency of PSII data suggest a long-term negative effect of high CO2 in the presence of salt. This disparity could have been caused by changes in the relative rates of CO2 fixation, photorespiration, nitrogen metabolism, and electron donation to oxygen (the Mehler reaction; Fryer ). Redondo-Gómez suggested that photorespiration and cyclic electron transport could be mechanisms to protect n class="Species">Arthrocnemum macrostachyum against an excess of radiation under high salinities. Both pathways can lead to the additional consumption of reducing equivalents and can thus function as sinks for excessive excitation energy (Asada, 1996). These two physiological processes could be relevant mechanisms to protect S. densiflora against an excess of radiation under high salinities, since, as in the case of A. macrostachyum, the relatively stable NPQ across the salinity range could indicate that salt does not cause an increase in thermal dissipation in the PSII antennae. By contrast, Chen reported enhanced ΦPSII yield at 700 ppm CO2 without additional NaCl in the nutrient solution.

The maximum quantum efficiepan class="Chemical">ncy of PSII photochemistry (Fv/Fm) did show a significant reduction at midday compared to dawn values. This midday depression of Fv/Fm was greater inpan> the long term. It was also dependent on salinpan>ity treatmenpan>t, in the same way as ΦPSII. The decreased Fv/Fm values at midday with the duration of treatment and the NaCl and CO2 concentration could have been caused by a lower proportion of open reaction centres (lower values of Fm), which could be attributed to a decrease in chlorophyll a content. The fact that photoinhibition was not more severe in salt-adapted plants, even when exposed to high light, suggests that they have mechanisms by which excess energy is dissipated safely (Qiu ).

In summary, the comparisopan class="Chemical">n of growth and photosynthetic responses of S. densiflora has provided a new inpan>sight inpan>to the response to climatic chanpan>ge and risinpan>g atmospheric CO2 concentration in a competitive coastal invader, which experiences salinity levels as high as those present in seawater. Differences in growth rate over this range of salinity can be accounted for by the ability to develop and maintain an assimilatory surface area combined with an improvement of water relations at elevated n class="Chemical">CO2 concentration, rather than by variations in net photosynthetic rate. Salinity and CO2 concentration have a marked effect on the photochemical (PSII) apparatus in the long term, but not on photosynthesis. However, the absence of a CO2-enrichment effect on net photosynthesis might be explained by enhanced PEPC carboxylation capacity. By contrast, photosynthetic pigments seem to be adversely affected by elevated CO2 concentration in the presence of NaCl. Finally, our results suggest that the productivity of this invader might increase in a scenario of future increase in atmospheric CO2 concentration for plants growing at salinities ranging between 0 mM and 171 mM NaCl. Nevertheless, the salt stress that may be experienced by plants growing at 510 mM NaCl would offset this fertilizer effect.