**4. Results**

#### **4.1. Leaf morphology**

Leaves of *Rhizophora* showed large differences between sites. They were thin and green at Ricoa, but showed a leathery texture and a yellowish color at the two hypersaline sites. At the Tacuato sites, leaves were smaller, had their edges bent downward, and showed an angle well above 45° from the horizontal. In *Avicennia*, differences in leaf morphology were not that obvious, but leaf inclination was also more pronounced in the hypersaline sites. In *Avicennia,* crystals of secreted salt could be observed on the leaf surface at both Tacuato sites.

Leaf length (L) in both species was reduced in the hypersaline sites, while leaf width (W) was reduced only in *Rhizophora* (**Table 1**). As a result, *Avicennia* leaves from hypersaline sites tended to be rounder than those of the low salinity site (smaller L/W ratio).

**4.2. Osmotic adaptation to salinity of the soil solution**

**Table 2.** Area/mass relationships in adult leaves collected at the different sites.

by tides.

*Rhizophora mangle*

*Avicennia germinans*

Statistical notations as in **Table 1**.

Units: mmol kg−1

Statistical notations as in **Table 1**

between sites were all significant.

**Site n Osmolality soil** 

**solution**

Salinity of interstitial water differed by more than one order of magnitude between the low and high salinity sites (**Table 3**). Within the high salinity site, the Tacuato-lagoon showed always larger osmolalities than the Tacuato-fringe, because the former was not always in contact with the bay water, so that concentration through evaporation could not be compensated

**Species and Sites n Area Dry mass Fresh mass Dry mass/Area Fresh/Dry mass**

Ricoa 11 50.9 a,0 0.92 1,0 2.89 a,0 180 a,0 3.16 a,0 Tacuato-fringe 21 32.1 b,0 0.77 b,0 2.11 b,0 239 b,0 2.73 b,0 Tacuato-lagoon 15 21.1 c,0 0.50 c,0 1.28 c,0 237 b,0 2.58 c,0

Ricoa 9 14.8 A,1 0.28 A,1 0.75 A,1 187 A,0 2.70 A,1 Tacuato-fringe 20 10.9 B,1 0.27 A,1 0.68 AB,1 250 B,0 2.41 B,1 Tacuato-lagoon 14 9.7 B,1 0.26 A,1 0.63 B,1 267 B,1 2.38 B,1

**cm2 g g g m−2 g g−1**

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Mangroves in Contrasting Osmotic Environments: Photosynthetic Costs of High Salinity Tolerance

Leaf sap osmolality was also higher for both species in the high salinity sites, particularly at Tacuato-lagoon, but absolute values were only 1.5–1.8 times higher than at Ricoa. Average data of **Table 3** show that osmolality was well above 100 mmol kg−1 higher in *Avicennia* than in *Rhizophora* for all values of soil salinity. The leaf sap-soil osmolality difference in *Rhizophora* decreased from nearly 900 mmol kg−1 in Ricoa to negative values approaching 100 mmol kg−1 in Tacuato. In *Avicennia*, the reduction of Δ leaf sap-soil was also very strong, but average values were always positive. The sap-soil differences were larger in *Avicennia*, and the differences

> **n Osmolality of leaf sap**

Ricoa 3 127a 11 1037 a,0 914 a,0 9 1226A,1 1103A,1 Tacuato-fringe 6 1666b 21 1631 b,0 −63 b,0 20 1859B,1 180B,1 Tacuato-lagoon 5 1862c 15 1893c,0 −92 b,0 14 2027B,1 54C,0

**Table 3.** Average values of osmolality of soil solution and leaf sap of *R. mangle* and *A. germinans* at the different sites.

*Rhizophora mangle Avicennia germinans*

**∆ sap-soil n Osmolality of** 

**leaf sap**

**∆ sap-soil**

For both species, average area of a single fully expanded leaf was greater at the low salinity site (**Table 2**). Leaves from the two hypersaline sites differed in size only for *Rhizophora*. Reduction in leaf area from low to high salinity site was more pronounced for *Rhizophora* (37–59%) than for *Avicennia* (26–34%). Leaf dry mass decreased significantly in hypersaline sites only in the case of *Rhizophora*, but fresh mass decreased in both species. The fresh mass/ dry mass ratio was higher for *Rhizophora* at all sites. Leaf dry mass/area ratios were significantly lower for both species at the Ricoa site, and the differences between species within sites were only significant in the case of Tacuato-lagoon (**Table 2**).


In columns, different superscript letters denote significant differences (P < 0.05) between sites and leaves of one species in different sites; different superscript numbers denote significant differences (P < 0.05) between species at the same site

**Table 1.** Average values of leaf dimensions from adult leaves collected at the different sites.

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**Table 2.** Area/mass relationships in adult leaves collected at the different sites.

#### **4.2. Osmotic adaptation to salinity of the soil solution**

significant when P ≤ 0.05. Differences between means of measured parameters in the two species at the same site were tested with students t-test at the level of P = 0.05. All statistical

74 Photosynthesis - From Its Evolution to Future Improvements in Photosynthetic Efficiency Using Nanomaterials

Leaves of *Rhizophora* showed large differences between sites. They were thin and green at Ricoa, but showed a leathery texture and a yellowish color at the two hypersaline sites. At the Tacuato sites, leaves were smaller, had their edges bent downward, and showed an angle well above 45° from the horizontal. In *Avicennia*, differences in leaf morphology were not that obvious, but leaf inclination was also more pronounced in the hypersaline sites. In *Avicennia,*

Leaf length (L) in both species was reduced in the hypersaline sites, while leaf width (W) was reduced only in *Rhizophora* (**Table 1**). As a result, *Avicennia* leaves from hypersaline sites

For both species, average area of a single fully expanded leaf was greater at the low salinity site (**Table 2**). Leaves from the two hypersaline sites differed in size only for *Rhizophora*. Reduction in leaf area from low to high salinity site was more pronounced for *Rhizophora* (37–59%) than for *Avicennia* (26–34%). Leaf dry mass decreased significantly in hypersaline sites only in the case of *Rhizophora*, but fresh mass decreased in both species. The fresh mass/ dry mass ratio was higher for *Rhizophora* at all sites. Leaf dry mass/area ratios were significantly lower for both species at the Ricoa site, and the differences between species within sites

crystals of secreted salt could be observed on the leaf surface at both Tacuato sites.

**Site n Length (cm) Width (cm) L/W**

Ricoa 21 12.1 a,0 5.6 a,0 2.2 a,0 Tacuato-fringe 78 9.8 b,0 4.3 b,0 2.3 a,0 Tacuato-lagoon 41 8.6 c,0 3.3 c,0 2.6 a,0

Ricoa 27 7.9 A,1 2.6 A,1 3.1 A,1 Tacuato-fringe 85 5.4 B,1 2.6 A,1 2.1 B,1 Tacuato-lagoon 87 5.1 B,1 2.5 A,1 2.1 B,1

**Table 1.** Average values of leaf dimensions from adult leaves collected at the different sites.

In columns, different superscript letters denote significant differences (P < 0.05) between sites and leaves of one species in different sites; different superscript numbers denote significant differences (P < 0.05) between species at the same site

tended to be rounder than those of the low salinity site (smaller L/W ratio).

were only significant in the case of Tacuato-lagoon (**Table 2**).

analyses were done using Statgraphics 5.0.

**4. Results**

*Rhizophora mangle*

*Avicennia germinans*

**4.1. Leaf morphology**

Salinity of interstitial water differed by more than one order of magnitude between the low and high salinity sites (**Table 3**). Within the high salinity site, the Tacuato-lagoon showed always larger osmolalities than the Tacuato-fringe, because the former was not always in contact with the bay water, so that concentration through evaporation could not be compensated by tides.

Leaf sap osmolality was also higher for both species in the high salinity sites, particularly at Tacuato-lagoon, but absolute values were only 1.5–1.8 times higher than at Ricoa. Average data of **Table 3** show that osmolality was well above 100 mmol kg−1 higher in *Avicennia* than in *Rhizophora* for all values of soil salinity. The leaf sap-soil osmolality difference in *Rhizophora* decreased from nearly 900 mmol kg−1 in Ricoa to negative values approaching 100 mmol kg−1 in Tacuato. In *Avicennia*, the reduction of Δ leaf sap-soil was also very strong, but average values were always positive. The sap-soil differences were larger in *Avicennia*, and the differences between sites were all significant.


**Table 3.** Average values of osmolality of soil solution and leaf sap of *R. mangle* and *A. germinans* at the different sites.

### **4.3. Concentration of compatible solutes**

In *Rhizophora*, the main compatible solute is the cyclitol 1D-1-O-methyl-*muco*-inositol (OMMI) [29]. Its concentration in the leaf sap increased with osmolality from nearly 80 mmol L−1 in Ricoa to about 160 mmol L−1 at Tacuato (**Table 4**). The other cyclitols present in *Rhizophora* (L-quebrachitol, L-*chiro*-inositol, and D-pinitol) were present only as minor components.

The only compatible solute in *Avicennia* is the quaternary ammonium compound glycinebetaine [14]. It reached concentrations of about 120 mmol L−1 at Ricoa to 180 mmol L−1 at Tacuato. Glycinebetaine contained between 15 and 21% of total leaf N with higher values found at the hypersaline sites (calculated with values from **Tables 4** and **5**). At all sites, concentrations of glycinebetaine in the leaf sap of *Avicennia* were higher than those of cyclitols in *Rhizophora*.

#### **4.4. Total phosphorus, nitrogen, and chlorophyll concentrations**

Both total P and N concentrations per unit leaf mass were higher in *Avicennia* than in *Rhizophora* (**Table 5**). In both species, no differences in total P concentration between sites were detected. However, concentrations of P and N per unit leaf area increased with salinity in both species because of the higher leaf mass/area ratios. The N to P molar ratios varied between 23 and 29 in both species, suggesting that P was not a limiting nutrient in these soils.

Concentration of Chltot per leaf area at a given site was always higher in *Avicennia* than in *Rhizophora*, but there was not a clear pattern relating chlorophyll concentration with salinity (**Table 5**). The ratio Chlortot/N was higher in *Rhizophora*, but the differences disappear if the amount of N invested in glycinebetaine is subtracted, suggesting similar N allocation to photosynthetic structures. In both species, this ratio decreases significantly in hypersaline sites.

**4.5. Photosynthetic response to light intensities and internal CO2**

freshwater site, while compensation values were nearly unchanged.

measured (4) does not allow a definitive conclusion.

linear part of the curve to the plateau of CO2

*Rhizophora mangle*

*Avicennia germinans*

Number of samples for P.

Statistical notation as in **Table 1**.

\*\*Number of samples for the rest of the columns.

\*

**4.6. Average values of gas exchange parameters**

lagoon site, Asat was even lower, especially in *Rhizophora*.

Photosynthetic response to increasing intercellular CO<sup>2</sup>

Light response curves for both species showed a clear reduction in light saturated photosynthesis in hypersaline sites, 36% in *Rhizophora* and 21% in *Avicennia* (**Figure 2**). *Avicennia* had higher rates of Asat than *Rhizophora* at low and high salinity sites. The reductions in photosynthetic light use efficiency (Φ) reached 45% in *Rhizophora* and nearly 37% in *Avicennia*. Light compensation points were not so much affected by salinity, although the number of curves

**Table 5.** Average nitrogen, phosphorus, and chlorophyll concentrations of leaf samples taken from the different sites.

**SITE P N N N/P Chlortot Chlor/N GB**

RICOA (4) 50 A,1 (9) 1365 A,1 258 A,1 27 467 A,1 1.82A,1 2.1 TACUATO-fringe (5) 52 A,1 (20) 1208 B,1 304 B,1 23 359 B,1 1.19B,1 1.5 TACUATO-lagoon (5) 50 A,1 (14) 1204 B,1 324 B,1 24 411 c,0 1.29B,1 1.6

RICOA (5) \* 31 a,0 (11)\*\* 910 ab,0 161 a,0 29 328 a,0 2.04a,0 TACUATO-fringe (5) 33 a,0 (21) 817 a,0 195 b,0 25 325 a,0 1.67b,0 TACUATO-lagoon (5) 31 a,0 (15) 1017 b,0 240 b,0 33 383 b,0 1.61b,0

μmol g−1 μmol g−1 mmol m−2 molar μmol m−2 mmol mol−1 mmol mol−1

Mangroves in Contrasting Osmotic Environments: Photosynthetic Costs of High Salinity Tolerance

near that of ambient air (≈350 ppm) and below (**Figure 3**). Hence, the transition from the

the curve, representing carboxylation efficiency, was steeper in *Avicennia*, and in this species compensation values were lower than in *Rhizophora*. The carboxylation efficiency was reduced at the hypersaline site by 39% in *Rhizophora* and 26% in *Avicennia*, compared to that at the

Average light intensities and temperature recorded during the measurement of Asat under natural conditions were similar for both species at each site, but the temperature was higher at the hypersaline sites (**Figure 4**). At any given site, Asat was higher in *Avicennia* than in *Rhizophora*. Differences between sites were significant for both species. At Tacuato-fringe, the values of Asat in both species were only about 70% of those measured at Ricoa. At the Tacuato-

 **concentration**

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concentrations

was measured at CO2

saturation was not reached. The initial slope of


OMMI = ortho-methyl-muco-inositol (other cyclitols include quebrachitol, chiroinositol and pinitol). Statistical notations as in **Table 1**

**Table 4.** Concentration of compatible solutes in *Rhizophora mangle* (cyclitols) and *Avicennia germinans* (glycinebetain) in adult leaves collected at the different sites.

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\* Number of samples for P.

**4.3. Concentration of compatible solutes**

in *Rhizophora*.

*Rhizophora mangle*

Statistical notations as in **Table 1**

adult leaves collected at the different sites.

In *Rhizophora*, the main compatible solute is the cyclitol 1D-1-O-methyl-*muco*-inositol (OMMI) [29]. Its concentration in the leaf sap increased with osmolality from nearly 80 mmol L−1 in Ricoa to about 160 mmol L−1 at Tacuato (**Table 4**). The other cyclitols present in *Rhizophora* (L-quebrachitol, L-*chiro*-inositol, and D-pinitol) were present only as minor components.

76 Photosynthesis - From Its Evolution to Future Improvements in Photosynthetic Efficiency Using Nanomaterials

The only compatible solute in *Avicennia* is the quaternary ammonium compound glycinebetaine [14]. It reached concentrations of about 120 mmol L−1 at Ricoa to 180 mmol L−1 at Tacuato. Glycinebetaine contained between 15 and 21% of total leaf N with higher values found at the hypersaline sites (calculated with values from **Tables 4** and **5**). At all sites, concentrations of glycinebetaine in the leaf sap of *Avicennia* were higher than those of cyclitols

Both total P and N concentrations per unit leaf mass were higher in *Avicennia* than in *Rhizophora* (**Table 5**). In both species, no differences in total P concentration between sites were detected. However, concentrations of P and N per unit leaf area increased with salinity in both species because of the higher leaf mass/area ratios. The N to P molar ratios varied between 23 and

Concentration of Chltot per leaf area at a given site was always higher in *Avicennia* than in *Rhizophora*, but there was not a clear pattern relating chlorophyll concentration with salinity (**Table 5**). The ratio Chlortot/N was higher in *Rhizophora*, but the differences disappear if the amount of N invested in glycinebetaine is subtracted, suggesting similar N allocation to photosynthetic structures. In both species, this ratio decreases significantly in hypersaline sites.

**mmol L−1**

**4.4. Total phosphorus, nitrogen, and chlorophyll concentrations**

29 in both species, suggesting that P was not a limiting nutrient in these soils.

**Site n OMMI ∑cyclitols**

Ricoa 11 77.2 a 88.1 a Tacuato-fringe 19 125.4 b 141.3 b Tacuato-lagoon 15 159.4 <sup>c</sup> 172.6 c

Ricoa 9 120.1 A 14.8 A Tacuato-fringe 20 165.8 <sup>B</sup> 20.1 <sup>B</sup> Tacuato-lagoon 14 178.1 <sup>B</sup> 21.1 <sup>B</sup> OMMI = ortho-methyl-muco-inositol (other cyclitols include quebrachitol, chiroinositol and pinitol).

*Avicennia germinans* Glycinebetaine mmol L−1 GB N/Total N %

**Table 4.** Concentration of compatible solutes in *Rhizophora mangle* (cyclitols) and *Avicennia germinans* (glycinebetain) in

\*\*Number of samples for the rest of the columns.

Statistical notation as in **Table 1**.

**Table 5.** Average nitrogen, phosphorus, and chlorophyll concentrations of leaf samples taken from the different sites.

#### **4.5. Photosynthetic response to light intensities and internal CO2 concentration**

Light response curves for both species showed a clear reduction in light saturated photosynthesis in hypersaline sites, 36% in *Rhizophora* and 21% in *Avicennia* (**Figure 2**). *Avicennia* had higher rates of Asat than *Rhizophora* at low and high salinity sites. The reductions in photosynthetic light use efficiency (Φ) reached 45% in *Rhizophora* and nearly 37% in *Avicennia*. Light compensation points were not so much affected by salinity, although the number of curves measured (4) does not allow a definitive conclusion.

Photosynthetic response to increasing intercellular CO<sup>2</sup> was measured at CO2 concentrations near that of ambient air (≈350 ppm) and below (**Figure 3**). Hence, the transition from the linear part of the curve to the plateau of CO2 saturation was not reached. The initial slope of the curve, representing carboxylation efficiency, was steeper in *Avicennia*, and in this species compensation values were lower than in *Rhizophora*. The carboxylation efficiency was reduced at the hypersaline site by 39% in *Rhizophora* and 26% in *Avicennia*, compared to that at the freshwater site, while compensation values were nearly unchanged.

#### **4.6. Average values of gas exchange parameters**

Average light intensities and temperature recorded during the measurement of Asat under natural conditions were similar for both species at each site, but the temperature was higher at the hypersaline sites (**Figure 4**). At any given site, Asat was higher in *Avicennia* than in *Rhizophora*. Differences between sites were significant for both species. At Tacuato-fringe, the values of Asat in both species were only about 70% of those measured at Ricoa. At the Tacuatolagoon site, Asat was even lower, especially in *Rhizophora*.

**Figure 3.** Dependence of photosynthetic rate on intercellular CO2

**Figure 4.** Average light intensity, leaf temperature, and gas exchange characteristics of the investigated species. In the upper left panel, the numbers within the columns indicate the number of leaves measured, while in the right-hand panel, they indicate the difference in temperature between the leaf and the surrounding air. On top of the columns, letters indicate significance of differences between sites for a given species. Numbers indicate differences between species at

species at each site.

the same site.

concentration measured in four leaves from each

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Mangroves in Contrasting Osmotic Environments: Photosynthetic Costs of High Salinity Tolerance

**Figure 2.** Photosynthetic rate (μmol CO<sup>2</sup> m−2 s−1) versus light intensity (μmol m−2 s−1) measured in four leaves per species at each site. φ: Apparent quantum yield; Asat: light saturated photosynthetic rate.

Leaf conductance to water vapor (gl ) showed a similar pattern to that of Asat; however, relative differences between the low salinity site and the hypersaline sites were more pronounced in the former. Differences between the two hypersaline sites were significant in *Rhizophora* only (**Figure 4**). Intrinsic water use efficiency (A/g<sup>l</sup> ) was higher in both species at the hypersaline sites, maximum values corresponding to *Rhizophora* at Tacuato.

#### **4.7. Carbon isotope discrimination**

As expected, values fo δ13C followed a pattern opposite to A/gl (**Figure 4**). 13C discrimination (Δ) was calculated from leaf δ<sup>13</sup>C values using formulation of Farquhar *et al.* [51] (δ<sup>13</sup>C air = −8‰). Discrimination values were lower at the hypersaline sites for both species, whereas they showed no significant differences at the hypersaline site, between Tacuato-fringe and Tacuato-lagoon. Differences between species were only significant at the Ricoa site, with *Rhizophora* having higher ∆ values. This pattern in Ricoa was expected as *Rhizophora* showed distinctly higher leaf conductance.

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**Figure 3.** Dependence of photosynthetic rate on intercellular CO2 concentration measured in four leaves from each species at each site.

Leaf conductance to water vapor (gl

**Figure 2.** Photosynthetic rate (μmol CO<sup>2</sup>

**4.7. Carbon isotope discrimination**

(**Figure 4**). Intrinsic water use efficiency (A/g<sup>l</sup>

sites, maximum values corresponding to *Rhizophora* at Tacuato.

at each site. φ: Apparent quantum yield; Asat: light saturated photosynthetic rate.

*Rhizophora* showed distinctly higher leaf conductance.

) showed a similar pattern to that of Asat; however, relative

m−2 s−1) versus light intensity (μmol m−2 s−1) measured in four leaves per species

) was higher in both species at the hypersaline

differences between the low salinity site and the hypersaline sites were more pronounced in the former. Differences between the two hypersaline sites were significant in *Rhizophora* only

78 Photosynthesis - From Its Evolution to Future Improvements in Photosynthetic Efficiency Using Nanomaterials

As expected, values fo δ13C followed a pattern opposite to A/gl (**Figure 4**). 13C discrimination (Δ) was calculated from leaf δ<sup>13</sup>C values using formulation of Farquhar *et al.* [51] (δ<sup>13</sup>C air = −8‰). Discrimination values were lower at the hypersaline sites for both species, whereas they showed no significant differences at the hypersaline site, between Tacuato-fringe and Tacuato-lagoon. Differences between species were only significant at the Ricoa site, with *Rhizophora* having higher ∆ values. This pattern in Ricoa was expected as

**Figure 4.** Average light intensity, leaf temperature, and gas exchange characteristics of the investigated species. In the upper left panel, the numbers within the columns indicate the number of leaves measured, while in the right-hand panel, they indicate the difference in temperature between the leaf and the surrounding air. On top of the columns, letters indicate significance of differences between sites for a given species. Numbers indicate differences between species at the same site.

#### **4.8. Efficiency of photosynthetic resource use**

Mass-based assimilation rate can be used as a measure of the biomass use efficiency in photosynthesis. As previously shown for Asat per unit area, at any given site, *Avicennia* also showed higher assimilation rates per unit leaf dry mass than *Rhizophora* (**Table 6**). Comparing sites, values were higher at the low salinity site and decreased with salinity.

Water use efficiency was estimated as the ratio of A to transpiration E (calculated using leaf conductance, ambient relative humidity, and temperature). The ratio A/E showed the same pattern of intrinsic water use efficiency for short-term water use, that is, similar for both species at Ricoa, but higher for *Rhizophora* in the hypersaline sites.

Nitrogen use efficiency (Asat/N) was lower in *Avicennia* at all sites, when using total N concentration as basis for calculation. When the amount of N bound in glycinebetaine from total N in *Avicennia* is subtracted, differences between the species disappeared at Ricoa and Tacuatofringe, but at Tacuato-lagoon, *Rhizophora* still showed a lower A/N index.

**5. Discussion**

Statistical significance:\*\*(P = 0.05);

μmol CO<sup>2</sup>

\*\*\*(P = 0.01).

**5.1. Leaf morphology, leaf size, and leaf water content**

leaves convectional cooling is more effective.

conductance to water vapor and nitrogen concentration.

rounder than those of low salinity sites.

**5.2. Osmotic adaptation**

importance in controlling leaf temperature [31].

A conspicuous visual feature observed in the field was that leaves had a high degree of inclination at the hypersaline sites. Ball *et al.* [31] showed that the high degree of leaf inclination found in mangrove species in nature effectively reduces the intensity of radiant heat loading. Furthermore, in both species, a marked reduction in leaf area in hypersaline sites was observed. This may also improve the energy balance in saline and dry sites, as in smaller

**Table 7.** Correlation coefficients between saturated photosynthesis under field conditions and specific leaf area, and leaf

**Photosynthesis** *Rhizophora mangle Avicennia germinans*

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kg−1 dry mass. s−1 vs (n = 47) (n = 43)

Leaf conductance (mol m−2 s−1) 0.91\*\*\* 0.90\*\*\* Specific leaf area (m−2 g−1 dry mass) 0.79\*\*\* 0.88\*\*\* Osmolality (mmol kg−1) −0.79\*\*\* −0.81\*\*\* Nitrogen (μmol g−1 dry mass) 0.78\*\* (n = 11) 0.67\*\*\* N-glycinebetaine (μmol g−1 dry mass) — 0.79\*\*\*

Lin and Sternberg [32] found a reduction in leaf size in dwarf scrub mangroves in contrast to tall growing fringe mangroves in Florida. Salinity, as well as nutrient level, may cause reductions in leaf area while neither sulfide nor original growth form had an influence [33].

In the present study, reductions in leaf area and dry mass associated with salinity were, respectively, 59 and 46% in *Rhizophora* compared to the much lower reductions of 34 and 7% in *Avicennia*, suggesting a higher salt sensitivity in the former species. In *Avicennia*, not only leaf size but also leaf shape was affected by high salinity, leaves of high salinity sites being

Larger leaf dry mass/area ratios at the hypersaline sites have been reported before and attributed to increased succulence [34, 35, 36], although there was also evidence for scleromorphy [34]. We observed in both species a significant decrease in the fresh mass/dry mass ratio that together with the increase in leaf mass/area ratio are rather symptoms of scleromorphy than of succulence. Generally, higher leaf mass/area ratios increase heat capacity and may be of

Increases of leaf sap osmolality with soil salinity in both species counteracted the lower soil water potential at the hypersaline sites. In *Rhizophora*, differences between leaf sap and soil

#### **4.9. Relationships between leaf gas exchange and specific leaf area, and osmotic properties**

We calculated correlations for a complete subset of data including Asat on a dry mass basis, leaf conductance, specific leaf area, osmolality, and N. For these calculations, average values of photosynthesis of the leaves pooled for chemical analyses were used. In all cases, higher correlations were found when using dry mass as a reference basis.

In both species, leaf conductance, specific leaf area, and N concentration were positively, whereas osmolality was negatively, and significantly correlated with photosynthetic rate (**Table 7**). The N-photosynthesis correlation in *Avicennia* increased from 0.67 to 0.79 when the glycinebetaine-N was subtracted from total N.


A: Maximum photosynthetic rate; E: Transpiration rate, N: Nitrogen concentration; N-GlBet. N: Nitrogen concentration minus nitrogen bound in glycine betaine; Chlor: Total chlorophyll concentration. Statistical notations as in **Table 1**.

**Table 6.** Efficiency of resource use in photosynthesis.


**Table 7.** Correlation coefficients between saturated photosynthesis under field conditions and specific leaf area, and leaf conductance to water vapor and nitrogen concentration.
