Plant water relations
Water potential is an expression of the free energy status of water. It is a measure of the driving force which causes water to move into any system such as plant tissue soil or atmosphere, or from one part of the system to another. Water potential is the determinant of diffusional water movement, and bulk movement also occurs in response to gradients set up by diffusional movement under the control of a water potential gradient. Therefore, four experiments were carried out with the aim of:
i. Understanding the functions of root and stem in water and mineral uptake and transport,
ii. Understanding the concept of water and osmotic potential and the plant cells as osmometers.
iii. Studying the effects of drought stress on plant water potential and
iv. Measurement of stomatal conductance of well watered drought plants.
Leaf porometer is used to measure stomatal conductance of leaves. Stomatal conductance is a function of the density, size and degree of opening of stomata. The leaf porometer measures stomatal conductance by putting the conductance of a leaf in series with two known conductance elements, and comparing the humidity measurements between them.
Materials and methods
I. Mineral and water uptake and transport.
The experiment was carried out to observe water and mineral uptake and transport by Brassica plants with and without roots. Two beakers of water containing red food coloring were prepared. In one beaker, a Brassica plant with roots was allowed to stand in while in the other beaker, the roots of the plant were severed. The roots were severed with a razor blade under water to eliminate air that may have entered into the vascular tissues. The leaves and stem of both plants were observed after two hours. However, it is better to keep the plants in food coloring for 4 to 5 hours. Due to the time constraint, we could only do it for 2 hours.
II. Measurement of osmotic potential in plant tissues
In performing the experiment, fourteen labeled 50ml beakers, each containing 20ml of the test sucrose solution from each of the fourteen sucrose concentrations were prepared. The sucrose solutions were of the following concentrations 0, 0.05, 0.10, 0.15, 0.20, 0.25, 0.30, 0.35, 0.40, 0.45, 0.50, 0.60, 0.70 and 0.80M. Fourteen potato cylinders measuring approximately 1cm in diameter and 4 cm in length were obtained, using a cork borer from 16 large potato tubers and quickly placed between the folds of a moist towel. Using an analytical balance, the weight of each potato cylinder was recorded to the nearest milligram. Each of the potato cylinders was then cut into several uniform slices, approximately 2mm thick after which the slices so obtained from one potato cylinder were placed in one of the test solutions in the beaker for 1.5 hours. Each beaker was taken as a sample and after the 1.5 hours incubation period, the slices in each sample were removed and blotted gently on paper towel and weighed in chronological order in which they were initially placed in the test solutions. The data was then tabulated showing original weight, final weight and change in weight (Table 1).
III. Direct measurement of shoot water potentials of well watered and drought plants [Piper sarmentosum (wild pepper) and Epipremnum acureum (money plant)] by using the pressure chamber.
The effects of drought stress on plant water potential was studied in this experiment where xylem pressure was measured using the pressure chamber (PMS Instrument Co., Corvallia, Oregon USA). The shoot of wild pepper plant (drought stressed) was cut by a sharp razor blade at an angle and the cut end of the stem inserted through the sealing gasket (rubber bung) with the cut end emerging from the smaller end. The gasket with the leaf was then pushed firmly into the lid of the pressure of chamber so that the cut end emerges, after which the lid was placed on the chamber body and turned clockwise to the stop. The pressure was then increased until sap just starts exuding from the cut end. At this point, the control valve was then turned of immediately and the pressure recorded in bars (or Mpa). The control valve was then turned to exhaust and as soon as the chamber pressure reached zero, the chamber cover was removed with the shoot and the process was repeated with a new shoot from a drought stressed money plant. As a control, the experiment was repeated all over again using the same plants, but which were well watered. Three measurements were taken in each case.
IV. Measurement of stomatal conductance of well watered and drought treated plants [flowering Chinese cabbage (Brassica rapa var utilis common name: Chai Sim or Choy Sum) and Chinese broccoli (Brassica albograbra common name: Kai Lan)] by leaf porometer (Decagon Devices).
As a control experiment, well watered Chai Sim and Kai Lan plant leaves were used in comparing the readings on stomatal conductance.
I. In this experiment, the plant with roots intact did not have a change in color but for the plant without roots, it turned red in color. The time for the red dye to travel up to the leaves will depend on the length of the stem and possibly other factors such as humidity in the room, air movement around the leaves and condition of the stem.
II. Table 1 below shows changes in weight of the 14 tissue samples.
Table 1: changes in weight of tissue samples
Sucrose concentration (M)
Initial weight (g)
Final weight (g)
Change in weight (g)
Figure 1: Changes in weight plotted against
III. Table 2 shows results of the experiment.
Table 2: water potentials of well watered and drought plants [Piper sarmentosum (wild pepper) and Epipremnum acureum (money plant)]
Water potential (Bar)
Well watered (control) stem
Drought stress stem
I. Table 3 shows results of the experiment.
Stomatal conductance (mmol H2O m[-2]s[-1])
Well watered (control) leaves
Drought stress stem
Discussion and conclusions
From experiment I, it is evident that roots play a crucial role in determining mineral uptake from the soil and the stem acts as a transport agent of water and minerals within the plant. This is the reason why the plant without roots had the stem stained red as opposed to the one with roots.
In experiment II, sucrose concentration at which there was zero change in weight was 0.3M solution as can be seen from the graph. At this point, the water potential of the potato tissue (Ψw) is equal to the water potential of the sucrose solution (Ψs) given by – miRT at room temperature, where
m = molar concentration of the solution
i = ionization constant (1 for sucrose because it does not ionize in water)
R = universal gas constant (0.083 liter bars/ mole [o]C)
T = absolute temperature ([o]C of solution + 273)
= – (0.3) x 1 x 0.083 x (25+273) = -7.4202 Mpa
This implies that there will be no net flow of water within the potato tissue at the given osmotic potential (Vines and Rees 54) hence a kind of osmotic equilibrium will be achieved.
From experiment III and IV, it is evident that water potential is lower in well watered plant stems as opposed to the ones that are drought stressed and stomatal conductance is higher in well watered plant leaves as opposed to those that are drought treated. All these measures ensure that water movements in the plant occur when required and are controlled to avoid loses. Therefore, the net movement of water depends on the osmotic factors within the plant and the environment within which the plant is placed or grows.
Vines, A E and N Rees. Plant and Animal Biology Volume 2. Toronto: Pitman, 1974.