There is a large degree of variation in the general leaf anatomy of Spartina species (Fig. 1). In all species studied, the abaxial side of the leaf is flat; the adaxial side is characterized by ridges. The size and shape of leaf ridges was quite variable across Spartina species. In all species, irrespective of their anatomy, the height of ridges is maximal in the central part of the leaf blade and decreases towards the lateral margins.
Figure 1. Light micrographs showing cross-sections of Spartina leaves. (a, b) low marsh species, (c–g) high marsh species, and (h–j) freshwater Spartina species. M, mesophyll; BS, bundle sheath; BSE, bundle sheath extension; PC, parenchyma cells. Bars, 200 µm.
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The low marsh species Spartina alterniflora and Spartina anglica have moderately thick leaves, up to approx. 340 µm (Table 2). The ridges are uniform (no distinction between ridges over major and minor leaf veins) with flat tops (Fig. 1). Leaf thickness was not significantly different for both species (Table 2).
Table 2. Thickness of the leaf blades between veins, from the abaxial side to the bottom of the furrows (minimal), and to the top of the largest ridges (maximal)
|Spartina species||Natural habitat||Leaf thickness (µm)||Distance between veins(µm)|
|S. alterniflora||Low marsh||343.0 ± 33.9 (d)||146.3 ± 16.9 (ab)||37.4 ± 4.1 (ab)|
|S. anglica||Low marsh||342.3 ± 13.1 (d)||118.5 ± 13.1 (bc)||45.1 ± 10.2 (a)|
|S. argentinensis||High marsh||672.3 ± 41.0 (a)||168.0 ± 18.6 (a)||35.4 ± 4.2 (ab)|
|S. bakeri||High marsh||455.0 ± 45.4 (c)||90.3 ± 12.1 (cd)||26.4 ± 4.6 (bc)|
|S. densiflora||High marsh||594.3 ± 24.9 (ab)||135.3 ± 14.1 (ab)||37.6 ± 4.5 (ab)|
|S. patens||High marsh||524.8 ± 76.7 (bc)||74.9 ± 8.1 (d)||16.9 ± 1.2 (c)|
|S. spartinae||High marsh||578.5 ± 30.3 (ab)||153.8 ± 9.5 (ab)||24.6 ± 3.8 (bc)|
|S. cynosuroides||Freshwater||207.0 ± 8.6 (e)||140.7 ± 8.6 (ab)||30.0 ± 7.8 (abc)|
|S. gracilis||Freshwater||264.0 ± 21.2 (de)||113.9 ± 4.3 (bc)||19.8 ± 2.1 (c)|
|S. pectinata||Freshwater||232.3 ± 20.9 (e)||113.7 ± 14.6 (bc)||35.3 ± 4.5 (ab)|
The second group of species occurs mostly in high marsh zones, and includes S. argentinensis, S. bakeri, S. densiflora, S. patens, and S. spartinae. These species have much thicker leaves than low marsh species, with the highest maximal and minimal leaf thickness in S. argentinensis, S. densiflora, and S. spartinae (maximal is up to 672 µm and minimal is up to 168 µm; Table 2). High marsh species have much larger leaf ridges over major vascular bundles compared with low marsh species. Large leaf ridges alternate with ridges of small and medium size in different ways in different species. Distal balloon-like ends of major leaf ridges on adaxial leaf surfaces are filled with large colorless parenchyma cells that have direct contact with xylem between extensions of chlorenchyma. Vascular bundles located near the abaxial epidermis are separated from it by groups of mechanical cells. But in S. argentinensis and S. spartinae, all vascular bundles are separated from the abaxial epidermis by one or two layers of colorless parenchyma cells (Fig. 1). Thus, in all high marsh species, vascular bundles have direct contact with colorless parenchyma on their xylem, and often also with strands of parenchyma cells on their phloem. Minimal leaf thickness in high marsh species is from 14 to 25% of the total leaf thickness.
The thinnest leaves were from the three freshwater species, S. gracilis, S. cynosuroides, and S. pectinata (maximal thickness is up to 264 µm, Table 2). In addition, these three species have the least undulated adaxial leaf surfaces. Groups of bulliform cells located at the bottom of furrows are well-developed in these three species (and in the high marsh S. bakeri), while in all other species they are not very prominent. In S. cynosuroides, a layer of flattened, large parenchyma cells underlie groups of bulliform cells. Minimal leaf thickness in freshwater species ranges from c. 50% (S. bakeri and S. pectinata) to 75% (S. cynosuroides) of the total leaf thickness.
All the Spartina species studied have Kranz type leaf anatomy. However, only S. gracilis has layers of mesophyll and bundle sheath chlorenchyma arranged in the classical way, by surrounding vascular bundles (Fig. 1i), whereas others have bundle sheath extensions. The most impressive forms of bundle sheath extensions are in S. alterniflora and S. anglica, the low marsh species (Fig. 1a,b), where specialized bundle sheath cells surround vascular bundles located near the abaxial epidermis and extend, usually in two rows, up to the adaxial epidermis in each ridge. In the other species, the chlorenchymatous bundle sheath creates different forms of extensions, especially in major veins, while some of the minor veins are the classical Kranz type. In all high marsh species having large ridges over the major veins, bundle sheath cells also surround vascular bundles on the phloem side and form different shapes of extensions above their xylem towards the colorless parenchyma. In all species, vascular bundles have an internal mestome sheath and outer Kranz sheath with randomly or centrifugally arranged chloroplasts, except for S. densiflora which has peripheral, or sometimes centripetal, chloroplast positioning in bundle sheath cells.
The difference between vein density in the species studied (the minimal distance between the closest bundle sheath cells of adjoining vascular bundles) varies from 16.9 µm in high marsh S. patens to 45.1 µm in low marsh S. anglica (Table 2); there was no significant relationship to different habitats.
Species also differed in the distribution of sclerenchyma in leaves. The low marsh species, S. anglica and S. alterniflora, have groups of mechanical fibers under the phloem of veins while the flat tops of ridges are fully lined by one or two layers of mechanical cells (Fig. 1a,b). In high marsh species, the tops of major ridges and some smaller veins are also lined with one layer of mechanical cells, and on the abaxial side mechanical fibers are distributed in groups under the phloem or, as in S. argentinensis and S. spartinae (Fig. 1c,f), one layer of mechanical cells underlie the abaxial epidermis with rare gaps. In the freshwater species S. cynosuroides, S. gracilis, and S. pectinata (Fig. 1h–j), small groups of mechanical fibers are located under the phloem of most veins near the abaxial epidermis. On the adaxial side, fibers underlie the epidermis on top of the ridges in S. gracilis and S. pectinata and are distributed only in small groups above xylem in large veins of S. cynosuroides.
Cuticle thicknesses are approx. 1.7- to 4-fold higher on the abaxial side than on the adaxial side of the leaf (Fig. 2, Table 3). The highest mean values for cuticle thickness on the abaxial side was 0.63 µm for epidermal cells over mechanical tissue and 0.51 µm for cells between veins in high marsh species, while on the adaxial side the mean values were similar (0.13–0.22 µm) in all species. Thickness of the outer epidermal cell wall is rather uniform across the adaxial side of leaf blades (measured on top of ridges and close to the bottom of furrows) and across the abaxial side (over mechanical tissue and between veins). However, mean thickness of the outer epidermal cell wall was much higher (1.4- to 2.7-fold) on the abaxial than on the adaxial side of the leaf.
Figure 2. The ultrastructure of the leaf cuticle in representative species. On the abaxial side, in all species except for Spartina cynosuroides, the cuticle of the epidermal cell is over the mechanical tissue. On the adaxial side of the leaf, the cuticle of the epidermal cell is on the top of the ridges (over the mechanical tissue) in (b), (f), and (h), and epidermal cells of the lateral side of the furrow closer to the bottom on (d), (j), and (l). CP, cuticle proper; CW, cell wall; RL, reticulate layer; RLL, reticulate-lamellate layer. Bars, 0.5 µm.
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Table 3. Thickness of the epidermal cell wall and cuticle complex
|Spartina species||Abaxial side||Adaxial side|
|Over mechanical tissue (over veins)||Over parenchyma tissue (between veins)||Over mechanical tissue (at the top of the ridge)||Over parenchyma tissue (near bottom of furrows)|
|C||CW(µm)||C + CW||C||CW(µm)||C + CW||C||CW(µm)||C + CW||C||CW(µm)||C + CW|
|S. alterniflora||0.53 ± 0.01||1.61 ± 0.07||2.14 ± 0.08||0.36 ± 0.02||1.98 ± 0.06||2.33 ± 0.07||0.23 ± 0.01||0.83 ± 0.03||1.06 ± 0.03||0.13 ± 0.02||1.35 ± 0.04||1.48 ± 0.05|
|(cd) (A)||(d) (B)||(g) (A)||(c) (B)||(d) (A)||(e) (A)||(bc) (C)||(e) (D)||(e) (C)||(bd) (D)||(ef) (C)||(ef) (B)|
|S. anglica||0.46 ± 0.04||2.23 ± 0.16||2.70 ± 0.20||0.33 ± 0.01||2.30 ± 0.09||2.63 ± 0.10||0.20 ± 0.01||0.86 ± 0.11||1.06 ± 0.13||0.13 ± 0.01||1.02 ± 0.05||1.16 ± 0.06|
|(cd) (A)||(c) (A)||(ef) (A)||(c) (B)||(cd) (A)||(ce) (A)||(bcd) (C)||(de) (B)||(e) (B)||(bd) (C)||(gh) (B)||(gh) (B)|
|S. argentinensis||0.53 ± 0.01||3.07 ± 0.05||3.62 ± 0.05||–|| || ||0.15 ± 0.01||1.67 ± 0.13||1.80 ± 0.13||0.13 ± 0.003||1.62 ± 0.16||1.75 ± 0.16|
|(c) (A)||(b) (A)||(bd) (A)|| || || ||(de) (B)||(c) (B)||(c) (B)||(cd) (B)||(de) (B)||(de) (B)|
|S. bakeri||0.22 ± 0.01||2.41 ± 0.09||2.64 ± 0.10||0.16 ± 0.01||2.46 ± 0.09||2.62 ± 0.09||0.10 ± 0.01||1.19 ± 0.04||1.29 ± 0.04||0.07 ± 0.003||0.97 ± 0.02 ||1.04 ± 0.01|
|(e) (A)||(c) (A)||(f) (A)||(d) (B)||(cd) (A)||(de) (A)||(f) (C)||(d) (B)||(de) (B)||(e) (D)||(h) (C)||(h) (C)|
|S. densiflora||0.91 ± 0.06||2.40 ± 0.11||3.32 ± 0.10||0.75 ± 0.06||2.51 ± 0.02||3.26 ± 0.06||0.25 ± 0.01||1.56 ± 0.06||1.81 ± 0.06||0.14 ± 0.01||1.81 ± 0.06||1.95 ± 0.07|
|(a) (A)||(c) (A)||(cd) (A)||(a) (B)||(cd) (A)||(be) (A)||(b) (C)||(c) (B)||(c) (B)||(bd) (C)||(cd) (B)||(cd) (B)|
|S. patens||0.84 ± 0.03||4.35 ± 0.08||5.18 ± 0.11||0.59 ± 0.03||4.68 ± 0.06||5.24 ± 0.08||0.22 ± 0.01||1.55 ± 0.04||1.66 ± 0.04||0.15 ± 0.003||1.26 ± 0.09||1.41 ± 0.09|
|(a) (A)||(a) (B)||(a) (A)||(b) (B)||(a) (A)||(a) (A)||(bc) (C)||(c) (C)||(c) (B)||(abc) (C)||(fg) (D)||(fg) (B)|
|S. spartinae||0.67 ± 0.02||3.08 ± 0.10||3.75 ± 0.10||0.55 ± 0.02||3.34 ± 0.05||3.89 ± 0.06||0.29 ± 0.01||1.27 ± 0.20||1.58 ± 0.22||0.15 ± 0.01||0.76 ± 0.02||0.92 ± 0.02|
|(b) (A)||(b) (A)||(b) (A)||(b) (B)||(bc) (A)||(bcd) (A)||(a) (C)||(cd) (B)||(cd) (B)||(ab) (D)||(h) (C)||(h) (C)|
|S. cynosuroides||0.19 ± 0.01||3.36 ± 0.09||3.55 ± 0.09||0.16 ± 0.01||3.73 ± 0.72||3.89 ± 0.74||0.14 ± 0.01||3.29 ± 0.06||3.42 ± 0.06||0.11 ± 0.01||3.54 ± 0.12||3.65 ± 0.12|
|(e) (A)||(b) (A)||(bd) (A)||(d) (B)||(b) (A)||(b) (A)||(ef) (BC)||(a) (A)||(a) (A)||(d) (C)||(a) (A)||(a) (A)|
|S. gracilis||0.42 ± 0.02||2.63 ± 0.08||3.06 ± 0.08||0.38 ± 0.02||3.20 ± 0.05||3.58 ± 0.06||0.21 ± 0.01||2.24 ± 0.04||2.45 ± 0.04||0.17 ± 0.01||2.53 ± 0.03||2.70 ± 0.03|
|(d) (A)||(c) (B)||(ce) (B)||(c) (B)||(bc) (A)||(bc) (A)||(c) (C)||(b) (C)||(b) (D)||(a) (D)||(b) (B)||(b) (C)|
|S. pectinata||0.21 ± 0.01||4.73 ± 0.06||4.94 ± 0.06||0.20 ± 0.01||4.68 ± 0.11||4.89 ± 0.11||0.15 ± 0.01||2.30 ± 0.08||2.45 ± 0.08||0.12 ± 0.002||1.97 ± 0.07||2.09 ± 0.07|
|(e) (A)||(a) (A)||(a) (A)||(d) (A)||(a) (A)||(a) (A)||(de) (B)||(b) (B)||(b) (B)||(d) (C)||(c) (C)||(c) (C)|
The freshwater species S. cynosuroides and S. pectinata, and the high marsh species S. bakeri, had the lowest cuticle thickness (c. 0.21 µm) on the abaxial side of the leaf. Otherwise, high marsh species had significantly thicker cuticles, ranging from 0.53 to 0.91 µm (ANOVA, P < 0.05). In the freshwater species S. gracilis the value was approx. 0.4 µm, an intermediate value that was comparable with the low marsh species S. alterniflora and S. anglica, and the high marsh species S. argentinensis. Also, cuticle thickness on the abaxial side of leaves is usually higher in cells located above mechanical tissue compared with cells adjoining parenchyma cells, and this difference is very pronounced in all high and low marsh species. By contrast, cuticle thickness is similar across the leaf in freshwater species. On the adaxial side, the thickness of the cuticle is less near the bottom of furrows compared with the top of ridges (from 1.2 times in S. gracilis up to 1.9 times in S. spartinae).
Total thickness of the cell wall + cuticle complex for salt marsh species is, at a minimum, twofold higher on the abaxial compared with the adaxial side of the leaf. This trend also exists in one freshwater species, S. pectinata, while in the freshwater species S. cynosuroides and S. gracilis total thickness is similar on both sides of the leaf. In freshwater species, cuticle + cell wall thickness on adaxial leaf surfaces is greater than in salt water species, both at tops of ridges and in furrows between ridges (ANOVA, P < 0.05; Table 3).
Analyses by TEM show that the main cuticle structural types could be subdivided in two groups. Species of the first group have a homogenous-reticulate type cuticle on both sides of leaves (S. anglica, S. alterniflora, S. patens, and S. cynosuroides; Fig. 2a,b,i–l). In this type, the outermost part consists of a homogeneous cuticle proper while the inner layer is reticulate; the net of microfibrils is mostly uniformly distributed across the reticulate layer but their thickness and density vary between species (e.g. Fig. 2a,k). Species of the second group (the remainder) have different cuticle structures on adaxial and abaxial sides of the leaf (Fig. 2c–h). The abaxial side of the leaf is covered by a homogeneous-lamellate–reticulate type cuticle (Fig. 2c,e,g) with a homogeneous cuticle proper and a polylamellated external part of the inner reticulated layer. Density and positioning of lamellae vary between species: some species have primarily periclinal lamellation (which is dense in S. argentinensis and S. spartinae in comparison with more loosely arranged lamellae in S. densiflora), or they have more anticlinally or chaotically orientated rare lamellae (S. bakeri, S. pectinata, and S. gracilis). These six species have mostly a homogeneous-reticulate type of cuticle on the adaxial side, but sometimes cells located above mechanical tissue have a cuticle with very rare single lamellae in the reticulate layer (Fig. 2d,f).
External leaf morphology
Different leaf ridge morphologies seen in Fig. 1 lead to different dimensions between ridge and furrow when viewed by SEM from the adaxial side (Fig. 3). Terminology for characters follows those used for grasses and other species of Spartina (Ellis, 1979, 1986; Watson & Dallwitz, 1992; Koyro & Huchzermeyer, 2004). Presence/absence of specific micromorphological features on the adaxial and abaxial epidermis of species of Spartina, as well as descriptions of prickles, silica cells, salt glands, papillae, and stomata, are summarized in Table 4.
Figure 3. Scanning electron micrographs showing adaxial surfaces of Spartina leaves. Bars, 150 µm. Micrograph of Spartina pectinata by Jessica L. Casey. P, papillae; Pr, prickle.
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Table 4. Characteristics of the adaxial and abaxial epidermis of Spartina species examined with SEM for this study
|Spartina species||Prickles||Silica cells||Salt glands||Papillae||Stomata||Other features/remarks|
|S. alterniflora||0||0||r||r||+||+||s||0||+||+||Papillae overarching stomata|
|S. anglica||0||0||r||s||+||+||s||0||+||1||Papillae overarching stomata|
|S. argentinensis||+||0||r||r||0||0||s, l||0||+||0||Abundant lobed papillae|
|S. bakeri||+||0||0||r||0||0||s||0||+||0||Abundant large prickles|
|S. densiflora||+||0||0||0||0||+||s||0||+||1|| |
|S. patens ||+||0||r||s, r||0||0||s||0||+||0||Abundant large prickles|
|S. spartinae ||+||0||c, r||c, s||0||0||s||0||+||+|| |
|S. cynosuroides||+||0||r, s||r, s||0||+||s||0||+||+||Adaxial subsidiary cells with papillae|
|S. pectinata||+||0||0||c, s||+||+||s, l||0||+||+|| |
|S. gracilis||+||0||s||s, r||+||+||s||0||+||+||Adaxial subsidiary cells with papillae|
Morphological descriptions of the epidermis in grasses are commonly organized into costal (over veins) and intercostal (between veins) areas. All species of Spartina examined in this study had ridges on adaxial costal areas that alternate with furrows. In species with uniformly-sized ridges, such as S. alterniflora (Figs 1a, 3a) and S. anglica (Figs 1b, 3b), each furrow includes a distinguishable intercostal area, but in species with both major and minor ridges, such as S. gracilis (Figs 1i, 3i), adaxial intercostal areas are obscure.
Ridges were absent from the abaxial surface in all species, and distinctions between costal and intercostal areas were often not clear (Fig. 4). In some species, such as S. gracilis and S. pectinata, rows of stomata and salt glands could be used to determine margins of veins (Fig. 4i,j).
Figure 4. Scanning electron micrographs showing abaxial surfaces of Spartina leaves. Micrographs by Claudia M. Dasilva-Carvalho (Spartina patens) and Jessica L. Casey (Spartina pectinata). LC, long cell; S, stomata; SC, short cell; SG, salt gland. Bars, 150 µm.
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Several cell types and micromorphological features were observed on adaxial and abaxial leaf surfaces (Figs 3, 4). Long cells with sinuous edges were the most common type of cell in the epidermis of both surfaces, but were often obscured on the abaxial surface by the cuticle or by an abundance of papillae on the adaxial surface. Papillae were only found on adaxial surfaces as outgrowths from long cells and subsidiary cells (Figs 3, 5a–d) and were seen commonly overarching stomata and salt glands (Fig. 5b–d). Papillae were generally simple, but in S. argentinensis were consistently bilobed (Fig. 3c). Unicellar prickles with dilated bases and sharp apices were found on all taxa except S. alterniflora and S. anglica. The largest prickles were often at margins of major ridges (Fig. 3d,g,h,i,j) or along tops of minor ridges as in S. gracilis (Fig. 3i). Salt glands were observed on adaxial surfaces of S. alterniflora (Fig. 5c,d), S. anglica, S. gracilis, and S. pectinata, and abaxial surfaces of S. alterniflora, S. anglica, S. cynosuroides, S. densiflora, S. gracilis (Figs 4i, 5f), and S. pectinata (Fig. 4j). Adaxial salt glands were located in rows above the rows of stomata on walls of ridges. On abaxial surfaces salt glands were also found in rows, often alternating with stomata, as in S. gracilis and S. pectinata (Fig. 4i,j). Stomata with dome-shaped subsidiary cells were observed in rows near the base of ridges (i.e., close to the bottom of furrows; Fig. 5a,b) on adaxial surfaces on all taxa, but on the abaxial surface were only seen at margins of veins on S. cynosuroides (Fig. 4h), S. gracilis (Fig. 4i), and S. pectinata (Fig. 4j). Except for S. bakeri (abaxial only), S. densiflora (neither surface), and S. pectinata (abaxial only), crecentric, rounded, or saddle-shaped silica cells were seen on both leaf surfaces of the remaining species. On both leaf surfaces silica cells generally alternated with long cells over the veins. Densities of cork cells were less than 8 mm−2. Cork cells were crescent shaped; they are apparently restricted to abaxial leaf surfaces in Spartina and were only observed on S. alterniflora, S. anglica, S. argentinensis, S. spartinae, S. pectinata, and S. gracilis.
Figure 5. Illustration of some structural features of leaf ridges, leaf curling, stomata, and salt glands in Spartina species. (a) Spartina spartinae. Stomata are located near the bottoms of leaf furrows between ridges on the adaxial leaf surface (arrow). (b) Enlargement of box region in (a). A cross section of a stomate can be seen. (c) Spartina alterniflora. Frequently papillae are densely congregated near stomata and salt glands. (d) Spartina alterniflora. Higher magnification of an adaxial salt gland surrounded by papillae. (e) Spartina densiflora. Illustration of leaf ridges fitting together tightly as the leaf is rolled. (f) Spartina gracilis. Enlargement of abaxial salt gland. Micrographs by Jessica J. Bitner (S. spartinae) and Jerad L. Gorney (S. densiflora). P, papillae; S, stomata; SG, salt gland. Bar, (a,c) 150 µm, (b) 25 µm, (d,f) 10 µm, (e) 1000 µm.
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Leaf stomatal densities on abaxial surfaces ranged from 0 to 103 mm−2 across species in the study (Table 5). On the abaxial side of leaves, freshwater species had high stomatal densities, while salt marsh species had significantly lower stomatal densities (ANOVA, P < 0.0001). In all species there were higher numbers of stomata on adaxial surfaces, with values ranging from 114 to 209 mm−2 (Table 5). However, there were no reliable differences in adaxial stomatal densities between species (ANOVA, P = 0.204).
Table 5. Densities of stomata on adaxial and abaxial leaf surfaces in Spartina grasses
|Spartina species||Natural habitat||Adaxial stomata (mm−2)||Abaxial stomata (mm−2)|
|S. alterniflora||Low marsh||191.0 ± 21.4 (8); a||0.58 ± 0.58 (5); c|
|S. anglica||Low marsh||168.4 ± 9.4 (5); a||0.72 ± 0.61 (9); c|
|S. argentinensis||High marsh||129.9 ± 17.9 (4); a||0.0 ± 0.0 (4); c|
|S. bakeri||High marsh||136.1 ± 6.7 (3); a||0.0 ± 0.0 (6); c|
|S. densiflora||High marsh||176.7 ± 23.7 (3); a||2.4 ± 1.7 (7); c|
|S. patens||High marsh||194.5 ± 59.2 (3); a||0.0 ± 0.0 (3); c|
|S. spartinae||High marsh||151.4 ± 30.6 (3); a||2.8 ± 1.6 (4); c|
|S. cynosuroides||Freshwater||114.1 ± 11.8 (6); a||99.7 ± 16.0 (5); ab|
|S. pectinata||Freshwater||209.2 ± 47.3 (5); a||74.6 ± 19.1 (6); b|
|S. gracilis||Freshwater||135.9 ± 6.4 (3); a||103.1 ± 5.0 (4); a|
Gas exchange and carbon isotope composition
Photosynthesis rates and leaf conductance (gv) were measured on adaxial and abaxial leaf surfaces in S. alterniflora, S. anglica, and S. cynosuroides. Leaves of S. cynosuroides had similar gv and rates of CO2 uptake from both surfaces, while in S. alterniflora and S. anglica CO2 uptake by leaves only occurred on the adaxial side, which correlated with gv (Figs 6, 7). The CO2 compensation points for S. alterniflora, S. anglica, and S. cynosuroides were 4.7, 5.0, and 4.1 µmol mol−1, respectively.
Figure 6. Relationship of leaf photosynthesis rates (A) to intercellular CO2 concentrations (Ci) in Spartina alterniflora, Spartina anglica, and Spartina cynosuroides. Closed symbols, measurements on adaxial leaf surfaces; open symbols, measurements on abaxial leaf surfaces. The results represent the average values for two experiments.
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Figure 7. Relationship between leaf conductance (gv) and intercellular CO2 concentrations (Ci) in Spartina alterniflora, Spartina anglica, and Spartina cynosuroides. Closed symbols, measurements on adaxial leaf surfaces; open symbols, measurements on abaxial leaf surfaces. The results represent the average values for two experiments.
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Leaf δ13C ranged from −14.5 to −13.5‰ across species (Table 6). Although there were significant differences in δ13C between species (ANOVA, P = 0.034), there were no relationships to habitat type (ANOVA, P = 0.328).
Table 6. Leaf carbon isotope composition (δ13C) of the Spartina species in this study
|Spartina species||Natural habitat||Leaf δ13C (‰)|
|S. alterniflora||Low marsh||−14.50 ± 0.19 (a)|
|S. anglica||Low marsh||−13.70 ± 0.15 (bc)|
|S. argentinensis||High marsh||−13.53 ± 0.14 (c)|
|S. bakeri||High marsh||−13.70 ± 0.22 (bc)|
|S. densiflora||High marsh||−14.31 ± 0.05 (a)|
|S. patens||High marsh||−14.10 ± 0.33 (abc)|
|S. spartinae||High marsh||−14.22 ± 0.06 (ab)|
|S. cynosuroides||Freshwater||−13.61 ± 0.09 (c)|
|S. pectinata||Freshwater||−13.91 ± 0.17 (abc)|
|S. gracilis||Freshwater||−13.68 ± 0.32 (bc)|
Leaf porometer measurements demonstrate a greater leaf surface conductance on adaxial surfaces compared with abaxial surfaces across species (Fig. 8). The ratio gvad : gvab ranged from 2.5 in freshwater S. gracilis to 8.2 in low marsh S. anglica. There were significant differences between species (ANOVA, P < 0.0001), with salt marsh species S. alterniflora, S. anglica, S. argentinensis, S. bakeri, and S. densiflora having the highest gvad : gvab ratios, and freshwater species S. gracilis, S. cynosuroides, and S. pectinata having the lowest ratios. In addition, salt marsh species S. patens and S. spartinae had similarly low values for gvad : gvab (Fig. 8).
Figure 8. Ratio of leaf conductance on adaxial surfaces (gvad) to abaxial surfaces (gvab) of Spartina species. Bars indicate ± SE (n = 6–22). Letters indicate significant differences at α = 0.05.
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