The amount of water needed daily by plants for the growth and maintenance of tissues is small in comparison to the amount that is lost through the process of transpiration (the evaporation of water from the plant surface). If this water is not replaced, the plant will wilt and may die.
The transport of water up from the roots in the xylem is governed by differences in water potential (the potential energy of water molecules). These differences account for water movement from cell to cell and over long distances in the plant. Gravity, pressure, and solute concentration all contribute to water potential, and water always moves from an area of high water potential to an area of low water potential. The movement itself is facilitated by osmosis, root pressure and adhesion and cohesion of water molecules.
The most significant force moving the water and dissolved minerals in the xylem is an upward pull as a result of transpiration, which creates tension. The “pull” on the water from transpiration results from the cohesion and adhesion of water molecules (although scientists are now also discovering active “pumps” powered by ATP that help the fluids move against gravity).
The Details: Transpiration begins with evaporation of water through the stomata, small openings in the leaf surface which open into air spaces that surround the mesophyll cells of the leaf. As water evaporates from the leaf surface, water is pulled up from the roots. This transpiration pull occurs because of (1) the cohesion of water molecules to one another due to hydrogen bond formation, and (2) by adhesion of water molecules to the walls of the xylem cells.
Many environmental conditions influence the opening and closing of stomata and also affect the rate of transpiration. Temperature, light intensity, air currents, humidity, and leaf size are some of these factors. Different plants also vary in the rate of transpiration.
How Can Transpiration be Measured?
There are several ways to measure water as it moves through plants. For this exercise, we can use a simple method of placing a small seedling in a test tube filled with water. The top of the test tube will be covered in parafilm so that water loss through the plant can be measured. Measure the water loss by marking the water level on the test tube and measuring it over a 24 or 48 hour period.
Your Task - Choose one variable to explore to determine how it affects the rate of transpiration.
Materials Available: Bean seedlings or celery stalks, Light bulbs, Warm/Cold water, Graph paper, Rulers
Experimental Design: Determine which variable you will be testing and design an experiment. Sketch your experimental design below and indicate the dependent variables, independent variables, and a control group if needed.
Data Collection: Gather data and organize into a chart below. You may also want to include any qualitative observations.
ANALYZE DATA: What conclusions can you draw from the data, summarize in a complete statement as a CLAIM and provide an explanation (reasoning) for your results.
Extension: Compare your data to that of other groups, which variable has the MOST effect on the rate of transpiration? Explain.