F510

Activity #4

Leaf Water Potential Activity

Introduction

Today we will be measuring and evaluating the leaf water potential of some different plants.  Water potential is the measure of total free energy of a solution, and the results are presented relative to pure water at a standard temperature and pressure.  In plants, water typically has less free energy than pure water because of the presence of solutes (think simple sugars) and the tension water is under as it is being extracted from the soil.  Therefore (and unfortunately) we are going to be dealing with negative numbers, which are always a bit counter-intuitive.  The lower the free energy of water in the leaf, the more difficult it is for the plants to maintain hydration.  Therefore, the lower the leaf water potential, the more water ‘stressed’ the leaf often is, although there are exceptions to this we will discuss throughout the course.

Now, the leaf water potential is a function of two main processes: 1) the osmotic potential (or osmotic concentration) and 2) the turgor pressure.  Turgor pressure is the pressure that cell membranes exert on the contents of the cell. A water balloon is a good analogy; when you fill up a balloon with water, as the balloon begins to stretch under the pressure of the water inside, the material of the balloon also exerts a force back on the water inside the balloon.  Turgor pressure increases the free energy of the water and has positive values.  The concentration of solutes in cells interact with water molecules and reduce the free energy of the solution; the more solutes the lower the free energy and the lower the osmotic potential – resulting in negative values. The balance of turgor pressure and osmotic potential result in the water potential of a leaf. There is a third force that can play a role in leaf water potential, but we’ll talk about this more in lecture.  But for now, we’ll assume that leaf water potential equals:

Y_leaf = Y_osm + Y_turgor

During lab today we will measure Y_leaf and Y_osm, allowing us to estimate Y_turgor.

Objective

The objective of this activity is to give students hands-on experience measurement leaf water potential and its constituent parts.  In addition, students will learn to interpret the results of leaf water potential as it pertains to water stress in plants.  Finally, we will compare the data we collect in lab to similar data reported in the literature during a class discussion.

Instructions

1. Measure leaf water potential of a species and record its value in the table below. Share your data with other groups so that we can compare among different species and functional groups.  We will go over the methods for estimating leaf water potential during lab.

2. Measure the osmotic potential of a leaf and record its value below. Note: There is a video on my lab’s YouTube channel that you can use for reference.

3. Calculate the turgor pressure of the leaf you measured and record its value below. And then share your data with other groups so that we can look at interspecific and functional group differences in lecture.

Bring your data with you to our next class and we will discuss our results as a group and we will compare that data we collected with the articles you are reading for our next literature discussion.