**2. Study sites**

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

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

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

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

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

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 photosyn-

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

development and photosynthetic performance.

have been shown to be strongly correlated [3–8].

excess ions [12–17].

osmolyte accumulation.

thetic enzymes.

photosynthesis.

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 stand (<5 m tall) of the species *Rhizophora mangle* and *Avicennia germinans* growing in a hypersaline lagoon (salinity >1000 mmol kg−1 ≈ 35 ppt) located south of the village Tacuato on the Paraguaná peninsula (11°41′40″N, 69°49′52″W). The climate is dry (<400 mm rainfall) with one rainy season from September to December. The lagoon has access to the gulf of Tacuato with an average salinity of 45 ppt (1600 mmol kg−1). The water depth of the lagoon in the sampling area varied between 0 and 20 cm, depending on rainfall events and tides. Diurnal air humidity was about 70–80%, whereas day air temperatures ranged from 27 to 37°C. The tallest trees of both species in the middle part of the lagoon reached a height of 5 m. Trees used for measurements were smaller, but mature, as they were flowering and fruiting. The site was divided into two sub-sites: the fringe-region and an inner site nearly 20 m apart from the fringe, differing in their average osmolality of interstitial soil water (1600 and 1800 mmol kg−1, respectively).

nets covered the photosynthesis chamber until gl

apparent quantum yield (φ).

described elsewhere [30].

**3.4. Statistical analysis of data**

To obtain different leaf internal CO<sup>2</sup>

**3.3. Chemical analyses of the samples**

was a composite of measurements conducted on four leaves. Curves were fitted to the data

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

A = ((Φ Q + Asat) − sqrt ((Φ Q + Asat)2 − 4 θ Φ Q Asat)/2θ) − Rd (1)

where Q is the measured quantum flux and A is the rate of photosynthesis. By this procedure, we obtained the maximum photosynthetic capacity at saturating light intensity (Asat) and the

concentrations (ci

entering the leaf chamber was reduced stepwise below ambient by passing a part of the air flow over soda lime. Photosynthetic rates were found to be higher in the second and third leaves below the branch apex, and these leaves were used for all measurements. Photosynthesis was

After gas exchange measurements, leaves were detached and gently cleaned with a wet tissue to remove salt from their surfaces. About 7–10 leaves were used to obtain one sample. Petioles and midribs were removed. Every leaf was cut into halves of which one was put into a plastic syringe (for leaf sap extraction) and the other was put into a plastic bag (for the determination of chlorophyll, N, and P). The samples prepared in that way were immediately frozen on dry

Fresh mass was determined in the field by a battery powered balance (precision ±0.01 g). Samples were dried at approximately 70°C in a ventilated oven until constant weight. Total chlorophyll (a + b) (Chlortot) concentration of leaf disks was measured by spectrophotometry of acetone extracts [25]. Syringes containing the samples were thawed, and leaf sap was squeezed out with a pressure device [20]. Osmolality of the leaf sap was determined with a dew point osmometer (WESCOR 5500). Total P concentration was measured in acid digested dry leaf material following the procedure of Murphy and Riley [26]. Nitrogen concentration was measured using a standard microKjeldahl procedure [27]. These measurements were contrasted with the parallel analysis of calibrated leaf material (peach leaf or citrus leaf, National Institute of Standards and Technology, USA). Sample preparation for organic compounds analyses and the chromatographic determinations of cyclitols in *Rhizophora mangle* and glycinebetain in *Avicennia germinans* have been described in detail elsewhere [14, 28, 29]. Measurement of carbon isotope ratios (δ13C values) of leaf material was performed at the Institute of Botany and Microbiology, University of Munich following standard procedures

Significant differences between means of species and sites were tested with a one-way analysis of variance (ANOVA) and a multiple range test after Scheffé. Differences were considered

measured during late morning and early afternoon (10–15 hours).

ice. Upon returning to the laboratory, they were stored in a freezer at −5°C.

using Sigmaplot 2.01 (Jandel Corporation 1994) and the following equation [23, 24]:

stabilized (2–3 min). A light response curve

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

), the concentration of CO2

in the air

73

The second study site, further on called Ricoa, is located at the fringes of the estuary of the Ricoa River west to the village of Tocópero (11°30′21″N, 69°12′19″W). Annual precipitation is about twice that of Tacuato (970 mm in average) with peaks in May–July and November– December. The soil water salinity averaged 127 mol kg−1 (2–3 ppt), the diurnal air humidity was about 70–80%, like that at Tacuato; day air temperatures were in general lower with highest values around 33°C. Reduced soil salinity was a consequence of higher rainfall and the contribution of the river water run-off. At this site, the trees used for measurements were located at the estuary flood plain, and had approximately the same height as the plants used in Tacuato. Measurements and sample collection of *R. mangle* and *A. germinans* (from now on designated by their genus names) were carried out during seven field trips distributed over 9 months (from October to June), thus including dry and rainy seasons at both sites.
