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

Water potential of stem

The water potential of the stem is made up of 3 components, the pressure differential due to gravity (negligible in the case of a shrub), the osmotic or solute potential resulting from the concentration of salts in the sap (say 40 mol/m3), which is small and the sap pressure potential within the plant is estimated using the Scholander bomb.

Students working with Scholander Bomb

(Bill and Mark looking on at a safe distance!)



Close up of bomb
A suitable sized twig is cut cleanly from the mangrove shrub, wrapped in a plastic bag and taken immediately to the bomb where it is inserted through the lid with the cut end uppermost. The twig, still wrapped in plastic is then sealed in the bomb and pressure slowly applied. At first the cut end of the twig appears dry then, quite suddenly, begins to look wet and shining and droplets of water are forced from the xylem exposed at the cut.

Safe operational practices and eye protection are necessary



The pressure required to produce the first wet appearance is recorded. It is assumed that this is equal but opposite to the negative tension which existed inside the twig before it was cut.
Record pressure required for a number of plants.

If the cut end of the twig was rough, would it be a good idea to re-cut it so that you could see clearly when the 'wetness' first appeared?