**6. Conclusions**

CO2

ues of gl

changes in A/gl

nearly 60 at 40 ppt.

**5.6. Carbon isotope discrimination**

At the low salinity site, gl

rates. At the hypersaline sites, gl

Intrinsic water use efficiency (A/g<sup>l</sup>

lower salinity tolerance of this species.

 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]

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

of *Avicennia* compared to those in *Rhizophora* are correlated with their assimilation

but in *Rhizophora*, the reduction of these parameters was more pronounced indicating the

lar 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,

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

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

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

was significantly higher for both species. The generally higher val-

was reduced to a greater proportion than Asat in both species,

) evaluates the role of biological components in determining

compared to Asat. Values of A/gl

was higher at the hypersaline

and can be taken as an

were simi-

with mangrove species grown at different salinities.

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

water-carbon exchange relationships [50, 51]. In both species, A/gl

leaves under hypersaline conditions was lower than in *Rhizophora*.

The carbon isotope ratio δ13C is related to a long-term average of ci

sites, because of the relatively larger reduction of gl

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.

essential nutrients uptake and salt accumulation, determining increases of tissue sap osmolality. Both processes lead to a nutritional limitation of photosynthesis resulting in a strong reduction of nitrogen use efficiency for growth. In addition, the differential soil–plant osmotic potential decreases reducing the amount of water available for transpiration, and inducing an accumulation of compatible solutes that protect cytoplasmic organelles from dehydration and toxic ionic effects. As a result, leaf conductance is reduced to a larger proportion than photosynthesis, thereby increasing leaf temperature and water use efficiency. The connections between the boxes of **Figure 5** are not necessarily linear, and processes affected may show differential sensitivity toward interstitial soil salinity (as shown in the ratio of conductance to photosynthesis). In the processes documented in the present chapter, *Rhizophora* appears to be more sensitive than *Avicennia*, and we speculate that the ultimate cause of this difference may reside in the higher efficiency of glycinebetaine as an osmoprotectant compared to cyclitols.

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