PLANT PHYSIOLOGY: TRANSPIRATION
When you arrive in the lab, do not engage in unstructured exploration of the computer and electronic apparatus. Leave the computer cover closed and wait for instructions from the TA. As you work with the computer, do not change desktop and sound settings. We need to have the computers look and operate exactly as they did when we set them up. Thank you.
Flowering plants (angiosperms) transport water in the form of a continuous column of liquid inward from the root surface to the xylem and upward in the stem from root to shoot and out into the leaves. In the leaves and other aerial organs possessing stomata (flower petals and sepals, some fruits, stems) liquid water turns to vapor and diffuses out into the atmosphere by the process of transpiration. A continuous supply of water is needed to replace water lost from aerial parts of the plant by transpiration. This replacement water moves upward in plant tissue, xylem, that is adapted for rapid, long distance transport.
The rate of transpiration, and hence of upward movement of water in the plant, depends on several factors. Read about this topic in your text and be prepared to suggest in a lab discussion several factors and how their relative contribution to water movement may be assessed.
The rate of water movement up a cut stem of a shoot (the stem is part of a shoot bearing leaves) caused by transpiration can be quantitatively estimated by a potometer. The potometer is a device designed for measuring rates of water uptake by a stem. The cut end of a stem with leaves is inserted into a chamber containing water. The chamber is a sealed, flexible, uncalibrated tube connected to a pressure sensor. Uptake of water by the plant from the chamber causes a decrease in pressure in the water column (a negative pressure) and this decrease is detected and quantified by the pressure sensor. The advantage of using an electronic pressure...