**3. Materials and methods**

#### **3.1. Interstitial water**

Interstitial water was sampled by digging 10–20 cm into the mud with a perforated plastic tube. Water salinity was determined *in situ* with a refractometer (ATAGO) calibrated with distilled water just before the measurement. Osmolality of the sampled water was calculated from salinity (in ppt) as in [22].

#### **3.2. Gas exchange measurements**

Gas exchange measurements were carried out with an open IRGA system of the type LCA 3 (ADC3, Analytical Development Co.) combined with a Parkinson leaf chamber of 6.25 cm<sup>2</sup> . A photometer and a thermocouple attached to the chamber allowed the measurement of incoming light intensity and leaf temperature. Photosynthetic rates used for correlations with leaf conductance (gl ), and the concentrations of N and chlorophyll, were measured under natural conditions at saturating intensities of photosynthetic active radiation (PAR) ≥ 1000 μmol m−2 s−1 (Asat). Leaves were oriented at 90° to the incoming radiation during measurements. To obtain a range of quantum fluxes, leaves were shaded in the field by a set of fine wire nets. The wire nets covered the photosynthesis chamber until gl stabilized (2–3 min). A light response curve was a composite of measurements conducted on four leaves. Curves were fitted to the data using Sigmaplot 2.01 (Jandel Corporation 1994) and the following equation [23, 24]:

$$\mathbf{A} = \left( (\boldsymbol{\Phi} \, \mathbf{Q} + \mathbf{A}\_{\text{sat}}) - \text{sqrt} \left( (\boldsymbol{\Phi} \, \mathbf{Q} + \mathbf{A}\_{\text{sat}})^2 - 4 \, \boldsymbol{\Phi} \, \boldsymbol{\Phi} \, \mathbf{Q} \, \mathbf{A}\_{\text{sat}} \right) / 2 \boldsymbol{\Theta} \right) - \mathbf{R}\_{\text{d}} \tag{1}$$

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 apparent quantum yield (φ).

To obtain different leaf internal CO<sup>2</sup> concentrations (ci ), the concentration of CO2 in the air 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 measured during late morning and early afternoon (10–15 hours).
