# Control of salt content

## Measurement of salt secretion by Avicennia marina

 In A. marina, salt glands are located on the lower surface of the leaf, shown highlighted in the RH image.
 Student washing a leaf Selected leaves on a number of Avicennia marina trees were washed with deionised water during the morning and the time at which they were washed recorded. Later in the afternoon, these leaves were carefully washed again with deionised water and the washings collected in a clean container. The time at which each leaf was washed again was noted and the elapsed time calculated. The collected washings from each leaf were made up to a constant volume and the electrical conductivity of the resulting solution measured.

Student measuring conductivity of leaf washings
In each case the washings are 'made up' to the same volume (100 ml ). Here is typical set of results for a single leaf 19.13 sq cm in area.
 Initial wash : 11.00 am Final wash : 2.58 pm Elapsed time : 14220 sec.
 Volume of washings : 100 ml *Conductivity : 78.8 microS/cm Quantity of salt secreted = 78.8 micromols
*NaCl Concentration.
This is an approximate value given by the following relation:
(conductivity of washings - conductivity of purified water)/100 = [NaCl] mol/m-3

After washing, the leaves were removed from the tree and the area of each leaf was recorded.

### The rate of salt excretion for this leaf was 2.9 micromol salt per square metre per second

#### Calculations

A small mangrove shrub might have a total leaf area of 1 sq. metre Assuming that the rate of salt secretion remains constant, about how much salt will it secrete in one day? If the water round the roots of the plant is 500 mM with respect to sodium chloride and the plant excludes 95% of this salt from the water it absorbs, about how much water is transpired by the plant during a day?

## Water relations of mangroves

For water to flow through the mangrove plant there must be a gradient of water potential from the water around the roots of the plant (sea water), through the stem and leaves and out to the air. In the sea water the total water potential is made up almost entirely of the solute potential while in the stem it is represented largely by the (negative) pressure potential. In the air the water potential is dependent on the temperature and relative humidity. All of these values can be estimated.
In this exercise we will assume a value for sea water of Y = -2.5 MPa