Pressure bomb

     Author: Burkhard Schulz

This video is a visual 10 step instruction manual how to use a pressure bomb for measuring water pressure in the xylem of transpiring leaves. It is an entry for the www.ChloroFilms.org video competition. Water in the xylem is under negative pressure. This tension can be measured by a “pressure bomb”. Tension pressure in a non-transpiring leaf is “equivalent” to water potential of cells surrounding the xylem (mesophyll cells), because the xylem’s osmotic potential is negligible. A single leaf is sealed in the pressure chamber with the cut surface protruding through a rubber stopper. Pressure is applied to the shoot from a tank of compressed air (or nitrogen) until xylem sap appears at the cut ends of the xylem. The hydrostatic pressure inside the xylem elements is then identical to the applied external pressure. The amount of pressure that must be applied to force water out of the leaf cells into the xylem is regarded as equal to the xylem tension when the leaf was cut from the plant.

This video was created to serve as visual instruction in a Plant Physiology course (HORT301) at Purdue University. Videos are powerfull tools to present experimental approaches, research methodology, and theory that the students have to deal with during their course work. This is especially true for experimental components of plant biology courses where students have to work hands-on with material and equipment which is often new to them. This way, additional background material that explains necessary equipment, concepts and essential steps of an experimental procedure can be made accessible as a video reference on course websites. This will engage students in critical discussion with contemporary research and permit its presentation to peers in the course and the public. The idea is for students to translate basic scientific information into a more popular visual language, that engages the students. The objective is not to let the students do the work of generating teaching material, but involve them in a process of active and action learning which enhances the study success and connects the students with the course and the teaching content more intensely. This video was produced with the help of graduate students who were involved as teaching assistents in this course. Video camera equipment was supplied by Purdue University’s Digital Learning Collaboratory at ITaP (http://dlc.purdue.edu/cs4.cfm) and through an Instructional Innovation grant by the College of Agriculture. Editing of the material and production of musical score has been done by Diana Nucera (Video artist, Detroit, MI). More background information on our video productions can be found at http://www.hort.purdue.edu/hort/academics/HortMedia/hortmedia.html.