The Experiment
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How quickly a plant grows depends on many factors, including the amount of sunlight available, the amount and type of minerals in the soil, the pH, or acidity of the soil, salt content of the soil, and water availability. The plant itself produces hormones that can alter growth patterns. Similar to humans, plants often respond to factors in the environment or in their own developmental timetable by producing more or less of some specific hormone. The way a plant responds to drought by closing stomata and slowing growth is thought to be regulated in part by a hormone called abscisic acid, or ABA. The ABA, or more specifically the way in which a leaf responds to ABA is also thought to be affected by the pH of the xylem sap. Salt concentration in soil often increases under drought conditions and this may also affect a plant’s growth. Any of these could be added to sap, or made to vary in the sap fed to a plant for an experiment.

Considering the way plants obtain water, oxygen, nutrients and energy, what are some ways that environments might vary that could in turn affect how well plants grow?

You have already determined the difference in how quickly the plant grows if it was given pure water vs. artificial sap. But, what else might be in the sap that could affect the rate of growth? Will plants grow faster if they are in sunlight 24 hrs/day, or do they benefit from a dark time as well? Is temperature important? How might a hormone like ABA that is produced by the plant affect its growth? Could this be determined by “feeding” ABA to the plant? Does the pH of the xylem sap affect how the plant grows? Does increase salt concentration of the water being fed to plants affect growth? Do plants behave differently if they are under stress, like in drought conditions? Could you simulate drought to test this hypothesis? These are just a few questions that come to mind when I think about this question and I’m sure you will have many more. Discuss possible questions with your group and write some of them in the space below.

Choose one question to investigate. Design an experiment using the leaf growth bioassay to try and answer your question. If you have another idea for an experiment talk it over with your teacher. Be sure to consider all of the variables that might affect your results and how you will control them. In addition to the artificial sap, distilled water, grow lights and the tubes and stoppers, you will also have available to you the following modified solutions of artificial sap:

sap with a 10-6 M concentration of ABA
sap with a 10-5 M concentration of ABA
sap with pH 5.0
sap with pH 5.5
sap with pH 6.0
sap that is 20% solution of NaCl
sap that is 10% solution of NaCl
 

Write an hypothesis and a detailed description of the way in which you plan to test this hypothesis. Due __________It will be returned to you with questions and suggestions on ____________.

You will then perform the experiment and upon completion you will write a formal paper following the format in your “Honors Biology Handbook.” This will then be displayed on a poster that will be presented to the class on ____________.