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