**1. Introduction**

Mangrove species in the neotropics are found along large latitudinal ranges including dry and wet coastal environments [1, 2]. Their distribution along steep salinity gradients provide

© 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2018 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

an opportunity to test *in situ* the impact of soil salinity on osmolyte accumulation in plant tissues, and analyze under similar light, temperature, and air humidity conditions, on their leaf development and photosynthetic performance.

The structural development and complexity of mangrove communities including height, leaf area index, leaf size, stem diameter, branching, litter production, and productivity are inversely related to interstitial soil water salinity. In neotropical mangroves, these properties have been shown to be strongly correlated [3–8].

Photosynthesis decreases significantly with salinity of interstitial water in several mangrove species [7–11]. Some species appear to be more sensitive to soil salinity than others, a characteristic that may be associated with specific metabolic and structural properties such as synthesis of compatible solutes, root permeability, salt excretion, and compartmentation of excess ions [12–17].

We studied the differentiation in leaf morphology, accumulation of osmotically active solutes and the photosynthetic response of two mangrove species, *Rhizophora mangle* L. and *Avicennia germinans* (L.) Stearn, reportedly differing in their salt tolerance [18–20] (**Figure 1**). These species coexist in neotropical mangroves and differ in their mechanisms of salt tolerance. *Rhizophora mangle* is considered as salt excluder, dominant in fringe mangroves throughout the neotropics, whereas *A. germinans* possesses numerous salt secreting glands in their leaves and typically dominates basin mangrove vegetation [20, 21]. Measurements were conducted under field conditions, in contrasting environments regarding fresh water availability and salt concentration of the soil interstitial water. Our objective was to assess quantitatively the impact of high salinity environments on photosynthetic performance and leaf expansion in association with inorganic and organic osmolyte accumulation.

In this chapter, we present results relating leaf sap osmolality and concentration of compatible solutes (cyclitols and glycinebetaine) to leaf morphology and patterns of gas exchange. The compatible solutes are presumably accumulated in the cytoplasm and counteract the osmotic effect of inorganic ions predominantly accumulated in the vacuole [14]. Accumulation of these compounds requires energy and carbohydrates from photosynthesis, and in the case of glycinebetaine, it needs additional amounts of N. The latter probably affects photosynthesis through the reduction of N availability for synthesis of photosynthetic enzymes.

Our hypotheses for this study were: (1) the reduction of photosynthesis resulting from salt accumulation in leaf cell sap is stronger in the species assumed to have lower salt tolerance, *R. mangle*; (2) salinity affects photosynthesis through diminished nutrient uptake, such as N, affecting protein synthesis, and phosphorus (P), possibly affecting N use efficiency; and (3) water use efficiency is higher in the high salinity (drier) environment, as a result of the combined effect of increased cell sap and interstitial soil water osmolalities on leaf conductance, leading to a proportionally larger reduction of transpiration compared to photosynthesis.

**2. Study sites**

Field work was carried out at two locations in the Caribbean coast of northern Venezuela, both in the State of Falcón. The site, further on called Tacuato, is a low stature mangrove

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

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

71

**Figure 1.** Flowering twig of the species studied. Notice the differences in leaf size.

Mangroves in Contrasting Osmotic Environments: Photosynthetic Costs of High Salinity Tolerance http://dx.doi.org/10.5772/intechopen.74750 71

**Figure 1.** Flowering twig of the species studied. Notice the differences in leaf size.
