**5. Discussion**

**4.8. Efficiency of photosynthetic resource use**

**properties**

was subtracted from total N.

*Rhizophora mangle*

*Avicennia germinans*

Statistical notations as in **Table 1**.

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

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,

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

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 spe-

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 Tacuato-

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

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

Ricoa 11 51.6 a,0 1.5 a,0 56.7 a,0 — 27.7 a,0 Tacuato-fringe 21 28.3 b,0 2.5 b,0 34.6 b,0 — 20.9 B,0 Tacuato-lagoon 15 18.6 c,0 2.8 c,0 18.4 c,0 — 11.7 C,0

Ricoa 9 68.5A,1 1.7 A,0 49.7 A,0 58.8 A,0 27.2 A,0 Tacuato-fringe 20 35.0B,1 2.3 B,1 28.7 B,0 34.6 B,0 24.1 A,1 Tacuato-lagoon 14 29.4B,1 2.2 B,1 24.3 B,1 28.1 B,1 18.8 B,1

A: Maximum photosynthetic rate; E: Transpiration rate, N: Nitrogen concentration; N-GlBet. N: Nitrogen concentration

**n Asat A/E A/N A/N-GlBet.N A/Chlor**

 **g−1 s−1 mmol mol−1 μmol CO2**

 **mol−1 s−1**

values were higher at the low salinity site and decreased with salinity.

fringe, but at Tacuato-lagoon, *Rhizophora* still showed a lower A/N index.

correlations were found when using dry mass as a reference basis.

**μmol CO2**

minus nitrogen bound in glycine betaine; Chlor: Total chlorophyll concentration.

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

cies at Ricoa, but higher for *Rhizophora* in the hypersaline sites.

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

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 leaves convectional cooling is more effective.

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 rounder than those of low salinity sites.

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 importance in controlling leaf temperature [31].

#### **5.2. Osmotic adaptation**

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 solution osmolalities were small or negative at the hypersaline sites, whereas in *Avicennia* differences were always positive. Scholander *et al.* [21], and Walter and Steiner [37] found that the osmotic potential in mangrove leaves exceeded that of the seawater surrounding them. However, in a field study in Venezuela, Rada *et al.* [38] found that turgor loss occurred at midday in leaves of *Conocarpus erectus* and *Rhizophora mangle* during drought periods. We did not measure water potential in the investigated plants, but both conductance and photosynthetic rates measured did not indicate turgor loss even at the most stressful sites.

present study. Values of Chltot/N at a given site did not differ significantly between species, when the amount of N bound in glycinebetaine in *Avicennia* was subtracted from total N. It seems that under similar salinity stress, both species invest a similar N fraction into the

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

The fractional investment of leaf N into chloroplast protein-pigment complexes can be calculated using the N to chlorophyll ratio in thylakoids estimated by Evans [44] (50 mol thylakoid N/mol chlorophyll). Both species had values ranging from 9% of leaf N in low salinity sites to 5–6% in high salinity sites. *Rhizophora* had always slightly higher values than *Avicennia*. Those values are about half of the average reported for lowland trees in humid tropical forest (107 species, 23.7 ± 0.8% of leaf N) [45]. The large difference underscores the photosynthetic cost

*Avicennia* showed consistently higher assimilation rates than *Rhizophora*, in accordance with previous reports on other species of the same genera [11, 46, 47]. Our results showed that both species had lower Asat at the hypersaline sites. However, the depression of Asat related to high

Light saturated photosynthetic capacity reflects the maximum possible benefits from a given investment in photosynthetic machinery [48]. Zotz and Winter [49] showed a linear relation-

canopy species. This would explain the low growth rates and the shrubby stature of the plants at the hypersaline sites. In addition, as constructing and maintaining photosynthetic machinery is energetically expensive, photosynthetic capacity should be tuned to the constraints of the environment [48]. The most prominent factor in mangrove habitats is salinity. In *Aegiceras corniculatum* and *Avicennia marina*, photosynthetic capacity was found to decrease with increasing salinity [50, 51], and Asat was negatively related to salinity in a range of mangrove species under field conditions [11]. Other environmental factors such as low nutrient availability [33] and temporal variation in salinity [52] are also known to depress maximum assimilation rate of mangroves. Extreme low values of Asat in *Rhizophora* leaves at Tacuato-lagoon may be related

Values of light saturated A calculated from light response curves confirmed that the photosynthetic capacity was generally higher in *Avicennia* compared to *Rhizophora* and was reduced

Quantum yield (φ) on an incident light basis was depressed at the hypersaline sites in both species. Björkman *et al.* [46] found that quantum yield in mangrove leaves decreased due to the combination of low leaf water potentials with high irradiance; whereas salinity, and the resulting leaf water deficit, had no negative effect on the quantum yield of mangrove leaves

ear, with a slope proportional to the *in vivo* activity of Rubisco (carboxylation efficiency, CE [53]). The lower CE of both species at the hypersaline site indicates that decreases of Asat with salinity were not only due to stomatal limitation, but also due to the result of changes in the

uptake in a range of rainforest

http://dx.doi.org/10.5772/intechopen.74750

83

concentrations equal or below ambient were lin-

construction of photosynthetic structures.

salinity was more pronounced in *Rhizophora*.

in both species at the hypersaline sites.


biochemical properties of photosynthesis.

protected from direct sunlight.

The Ci

ship between diurnal carbon gain and maximum rate of CO2

to a combination of salinity with one or more of the latter mentioned factors.

of high salinity tolerance.

**5.4. Photosynthetic capacity**

At the low salinity site for both species, osmolalities of the leaf sap were found to be about 10 times higher than that of the soil solution, indicating their halophytic (salt accumulating behavior) character [15, 17]. The generally larger values of leaf sap osmolality in *Avicennia* are consistent with the higher salinity tolerance of species of this genus [13, 14, 19, 20, 37].

Concentrations of compatible solutes were clearly correlated to soil salinity and were within the range reported in other field studies [14]. Glycinebetaine concentrations were higher than those of cyclitols at each site, and in both cases their concentration increased with leaf sap osmolality. This is in accordance with their postulated role in keeping osmotic equilibrium between cytoplasm and vacuole. However, the cyclitol/osmolality ratio was 1.25 in *Rhizophora,* while the glycinebetaine/osmolality ratio in *Avicennia* was only 0.7, suggesting a higher osmoprotective efficiency of this compound.

#### **5.3. Phosphorus, nitrogen, and chlorophyll**

Studies of nutrient availability in soils under several mangrove stands showed that P availability may be limiting growth, especially under oxidized conditions of well drained soils [39]. Growth limitation by P was confirmed by fertilization studies on dwarf red mangrove in Belize [40], where an increase in growth was brought about only by application of NPK or P. In the present study, leaf P concentration did not differ between sites but was always higher in *Avicennia* compared to *Rhizophora*. Besides, N to P molar ratios were below 35 suggesting that P supply was not limiting mangrove growth in the study sites.

Leaf N concentrations of *Avicennia* were significantly higher than those of *Rhizophora* as has been found earlier [20, 41] and reported for Australian species of these genera by Popp *et al*. [14]. Part of the difference can be explained by the amount of glycinebetaine in *Avicennia,* representing 15–21% of total leaf N. Differences in N concentration between species disappear when this fraction is subtracted from total N.

Differences in N concentrations between sites were mainly related to differences in leaf mass/ area ratio in both species. While N concentration decreased with salinity when calculated on a dry mass basis, they increased based on leaf area. *Rhizophora* leaves at Tacuato-lagoon, however, showed a higher concentration of N per leaf area as well as per g dry mass, indicating a strong reduction in nitrogen use efficiency.

Chlorophyll concentrations per leaf area were similar or lower than those reported earlier for the same species in dry and wet habitats [20], or for mangrove species in Australia and India growing on a range soil salinities [42, 43]. The average Chltot/N ratios decrease markedly in hypersaline sites, pointing to a reduction in N investment in photosynthetic structures due to salt stress. Besides, these ratios were low compared with earlier reports on these species [20], a fact perhaps related to the much lower leaf N/P ratios found in the present study. Values of Chltot/N at a given site did not differ significantly between species, when the amount of N bound in glycinebetaine in *Avicennia* was subtracted from total N. It seems that under similar salinity stress, both species invest a similar N fraction into the construction of photosynthetic structures.

The fractional investment of leaf N into chloroplast protein-pigment complexes can be calculated using the N to chlorophyll ratio in thylakoids estimated by Evans [44] (50 mol thylakoid N/mol chlorophyll). Both species had values ranging from 9% of leaf N in low salinity sites to 5–6% in high salinity sites. *Rhizophora* had always slightly higher values than *Avicennia*. Those values are about half of the average reported for lowland trees in humid tropical forest (107 species, 23.7 ± 0.8% of leaf N) [45]. The large difference underscores the photosynthetic cost of high salinity tolerance.

## **5.4. Photosynthetic capacity**

solution osmolalities were small or negative at the hypersaline sites, whereas in *Avicennia* differences were always positive. Scholander *et al.* [21], and Walter and Steiner [37] found that the osmotic potential in mangrove leaves exceeded that of the seawater surrounding them. However, in a field study in Venezuela, Rada *et al.* [38] found that turgor loss occurred at midday in leaves of *Conocarpus erectus* and *Rhizophora mangle* during drought periods. We did not measure water potential in the investigated plants, but both conductance and photosynthetic

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

At the low salinity site for both species, osmolalities of the leaf sap were found to be about 10 times higher than that of the soil solution, indicating their halophytic (salt accumulating behavior) character [15, 17]. The generally larger values of leaf sap osmolality in *Avicennia* are

Concentrations of compatible solutes were clearly correlated to soil salinity and were within the range reported in other field studies [14]. Glycinebetaine concentrations were higher than those of cyclitols at each site, and in both cases their concentration increased with leaf sap osmolality. This is in accordance with their postulated role in keeping osmotic equilibrium between cytoplasm and vacuole. However, the cyclitol/osmolality ratio was 1.25 in *Rhizophora,* while the glycinebetaine/osmolality ratio in *Avicennia* was only 0.7, suggesting a higher osmo-

Studies of nutrient availability in soils under several mangrove stands showed that P availability may be limiting growth, especially under oxidized conditions of well drained soils [39]. Growth limitation by P was confirmed by fertilization studies on dwarf red mangrove in Belize [40], where an increase in growth was brought about only by application of NPK or P. In the present study, leaf P concentration did not differ between sites but was always higher in *Avicennia* compared to *Rhizophora*. Besides, N to P molar ratios were below 35 sug-

Leaf N concentrations of *Avicennia* were significantly higher than those of *Rhizophora* as has been found earlier [20, 41] and reported for Australian species of these genera by Popp *et al*. [14]. Part of the difference can be explained by the amount of glycinebetaine in *Avicennia,* representing 15–21% of total leaf N. Differences in N concentration between species disappear

Differences in N concentrations between sites were mainly related to differences in leaf mass/ area ratio in both species. While N concentration decreased with salinity when calculated on a dry mass basis, they increased based on leaf area. *Rhizophora* leaves at Tacuato-lagoon, however, showed a higher concentration of N per leaf area as well as per g dry mass, indicating a

Chlorophyll concentrations per leaf area were similar or lower than those reported earlier for the same species in dry and wet habitats [20], or for mangrove species in Australia and India growing on a range soil salinities [42, 43]. The average Chltot/N ratios decrease markedly in hypersaline sites, pointing to a reduction in N investment in photosynthetic structures due to salt stress. Besides, these ratios were low compared with earlier reports on these species [20], a fact perhaps related to the much lower leaf N/P ratios found in the

gesting that P supply was not limiting mangrove growth in the study sites.

consistent with the higher salinity tolerance of species of this genus [13, 14, 19, 20, 37].

rates measured did not indicate turgor loss even at the most stressful sites.

protective efficiency of this compound.

**5.3. Phosphorus, nitrogen, and chlorophyll**

when this fraction is subtracted from total N.

strong reduction in nitrogen use efficiency.

*Avicennia* showed consistently higher assimilation rates than *Rhizophora*, in accordance with previous reports on other species of the same genera [11, 46, 47]. Our results showed that both species had lower Asat at the hypersaline sites. However, the depression of Asat related to high salinity was more pronounced in *Rhizophora*.

Light saturated photosynthetic capacity reflects the maximum possible benefits from a given investment in photosynthetic machinery [48]. Zotz and Winter [49] showed a linear relationship between diurnal carbon gain and maximum rate of CO2 uptake in a range of rainforest canopy species. This would explain the low growth rates and the shrubby stature of the plants at the hypersaline sites. In addition, as constructing and maintaining photosynthetic machinery is energetically expensive, photosynthetic capacity should be tuned to the constraints of the environment [48]. The most prominent factor in mangrove habitats is salinity. In *Aegiceras corniculatum* and *Avicennia marina*, photosynthetic capacity was found to decrease with increasing salinity [50, 51], and Asat was negatively related to salinity in a range of mangrove species under field conditions [11]. Other environmental factors such as low nutrient availability [33] and temporal variation in salinity [52] are also known to depress maximum assimilation rate of mangroves. Extreme low values of Asat in *Rhizophora* leaves at Tacuato-lagoon may be related to a combination of salinity with one or more of the latter mentioned factors.

Values of light saturated A calculated from light response curves confirmed that the photosynthetic capacity was generally higher in *Avicennia* compared to *Rhizophora* and was reduced in both species at the hypersaline sites.

Quantum yield (φ) on an incident light basis was depressed at the hypersaline sites in both species. Björkman *et al.* [46] found that quantum yield in mangrove leaves decreased due to the combination of low leaf water potentials with high irradiance; whereas salinity, and the resulting leaf water deficit, had no negative effect on the quantum yield of mangrove leaves protected from direct sunlight.

The Ci -Asat relationships obtained for CO2 concentrations equal or below ambient were linear, with a slope proportional to the *in vivo* activity of Rubisco (carboxylation efficiency, CE [53]). The lower CE of both species at the hypersaline site indicates that decreases of Asat with salinity were not only due to stomatal limitation, but also due to the result of changes in the biochemical properties of photosynthesis.

CO2 compensation points were higher in *Rhizophora* than in *Avicennia*, suggesting higher photorespiration rates in the former species. However, species characteristic compensation values were similar at Tacuato and Ricoa. Similar results were obtained by Ball and Farquhar [12] with mangrove species grown at different salinities.

range reported for mangroves in the literature [9, 20, 32, 47, 58]. These results confirmed for

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

http://dx.doi.org/10.5772/intechopen.74750

85

The relationships between the set of physiological properties associated with high salinity stress in both mangrove species studied here can be depicted along two sequences of events operating simultaneously (**Figure 5**). Increases in interstitial water salinity affect

**Figure 5.** Scheme depicting the assumed sequence of events affecting photosynthesis and resource use efficiency caused by exposure to high salinity conditions. The driving forces for environmental salinity are encapsulated as tides (sea water supply), rainfall (dilution and washing-out effects), atmospheric evaporative demand (air water saturation deficit), and soil properties. Thick red arrows indicate the direction of change resulting from long-term exposure to high salinity. Thin black arrows indicate the plant-environment interface. Green arrows depict the hypothesized dependence of biological processes triggered by increases in cell sap osmolality and leaf nutrient status. The connections between the boxes are not necessarily linear, and processes affected may show differential sensitivity toward interstitial soil water salinity. Generally, mangrove-environment interactions under high salinity conditions lead to higher water use and lower N use efficiencies.

the long-term, the patterns found for short-term water use efficiency discussed above.

**6. Conclusions**

#### **5.5. Water use and N use efficiency in photosynthesis**

At the low salinity site, gl was significantly higher for both species. The generally higher values of gl of *Avicennia* compared to those in *Rhizophora* are correlated with their assimilation rates. At the hypersaline sites, gl was reduced to a greater proportion than Asat in both species, but in *Rhizophora*, the reduction of these parameters was more pronounced indicating the lower salinity tolerance of this species.

Intrinsic water use efficiency (A/g<sup>l</sup> ) evaluates the role of biological components in determining water-carbon exchange relationships [50, 51]. In both species, A/gl was higher at the hypersaline sites, because of the relatively larger reduction of gl compared to Asat. Values of A/gl were similar to those calculated from data from Smith *et al.* [54] for *Avicennia germinans* and *Conocarpus erectus* at a coastal site in northern Venezuela and to those calculated from data from Lin and Sternberg [32] for *Rhizophora mangle* at a site in coastal Florida. In an extensive field study, Clough and Sim [11] found higher water use efficiency in mangroves with increasing salinity and decreasing air humidity. As air humidity in our study did not differ much between sites, changes in A/gl with salinity were less drastic than in the study mentioned above.

Water use efficiency was higher in *Avicennia* at the low salinity site, but it was higher in *Rhizophora* in the high salinity sites. More conservative water use in the latter species at the hypersaline sites is probably related to its non-salt secreting character. Water loss is minimized as salt exclusion mechanisms at the roots impose a large resistance to water flow [55]. The higher water use efficiency in *Avicennia* at the low salinity site may be related to the association of this species with the more saline soils [15]. However, as in *Avicennia*, leaf-to-soil osmolality difference was at least 50 mmol kg−1 at the hypersaline sites, and as accumulation of NaCl can be counteracted by salt secretion, and in this species restriction of water loss from leaves under hypersaline conditions was lower than in *Rhizophora*.

Nitrogen use efficiency in photosynthesis based on total leaf N was higher in *Rhizophora* at all sites. Similar results were reported by Alongi *et al.* [5] in Australian mangrove forests of *R. stylosa* and *A. marina*. However, those differences disappear if the amount of N invested in glycinebetain is subtracted from the total amount of N. In both species, the NUE decreases in hypersaline sites. In an experimental study, Cardona-Olarte *et al.* [16] did not find differences in WUE based on gas exchange of seedling grown in nutrient solutions with salinities between 10 and 40 ppt; however, PNUE decreased from about 85 μmol/mol N at 10 ppt to nearly 60 at 40 ppt.

#### **5.6. Carbon isotope discrimination**

The carbon isotope ratio δ13C is related to a long-term average of ci and can be taken as an indicator of water use efficiency [56, 57]. Values of δ<sup>13</sup>C ranged between −24.3‰ and −29.4‰ (Δ = 16.7 to 22.1‰), with a larger variation found in *Rhizophora*. They were well within the range reported for mangroves in the literature [9, 20, 32, 47, 58]. These results confirmed for the long-term, the patterns found for short-term water use efficiency discussed above.
