Plant Physiology

Plant Physiology (Biology 327) - Dr. Stephen G. Saupe; College of St. Benedict/ St. John's University; Biology Department; Collegeville, MN 56321; (320) 363 - 2782; (320) 363 - 3202, fax; ssaupe@csbsju.edu

Photomorphogenesis: Effect of Light on Seedling Growth & Development

Objectives: Upon completion of this lab you should be able to:

describe the effect of light and dark on plant growth
use the scientific method to test hypotheses
perform an ANOVA statistical test
use the terms etiolation and photomorphogenesis
explain why plants grown indoors often have a spindly or "leggy" appearance

Introduction
Light has a powerful impact on plant growth and development. This phenomenon, termed photomorphogenesis, explains why plants grown in the light, reduced light conditions, or even total darkness, look different than those grown in the light. In this exercise we will observe the effect of light on maize, sunflower and pea.

Pre-Lab

Print a copy of this handout and the Measuring Chlorophyll & Anthocyanin concentrations lab.
Complete the following hypotheses about plant growth under light and dark conditions. Circle your prediction for each of the following and then write the rationale for your response. Email a copy of this table before lab.

	Hypotheses:	Rationale for Prediction
1.	Plants grown in the dark will be (shorter / taller) than those grown in the light.
2.	Plants grown in the dark will have a (thinner / thicker) stem than those grown in the light.
3.	Plants grown in the dark have (less / more ) chlorophyll (green coloration) than those in the light
4.	The leaves of plants grown in the dark will be (smaller / larger) than those grown in the light.
5.	The apex of plants grown in the dark will be (straight / hooked) compared to those grown in the light.

Methods
In each of three pots containing pre-moistened Jiffy Mix plant several seeds each of a cereal (such as maize), bean, sunflower, pea, or amaranth. Moisten the medium, label with your name and date, cover with plastic wrap and then put one pot in the dark, one under high irradiance, and the other under low irradiance light. Once the seedlings appear, remove the plastic wrap. After several days of growth, when the seedlings are approximately 10 cm in height, examine your plants to see whether your data support your hypotheses. Using a ruler to measure the height (in mm) of 10 plants (what exactly will you measure?) and use a caliper to measure the diameter of the stem (or hypocotyl) in mm and complete Table 1. On sheet of paper, lay one representative specimen of each plant grown under each condition and record a digital image with the camera provided (or bring your own). Label on this diagram the following: hypocotyl, epicotyl, epicotyl hook, cotyledon, and coleoptile. Measure the chlorophyll and anthocyanin content of the plants by grinding a tissue sample in solution acetone/Tris buffer (80:20 volume; pH = 7.8) with a mortar and pestle. Transfer the homogenized mixture to a centrifuge tube and spin at the highest setting for 3 minutes. Measure the absorbance of the supernatant at wavelenghts of 470, 537, 647, and 663 nm. Then using the equations of Sims and Gamon (2002) calculate anthocyanin, total chlorophyll and carotenoid concentration.

Results:

Table 1. Comparison of dark and light grown seedlings of maize, sunflower, and pea. Seedlings were grown in four-inch pots in Jiffy-Mix for ______ days at room temperature.
		Dark			Low Light (fluence: )			High Light (fluence: )
		Maize	Sunflower	Bean	Maize	Sunflower	Bean	Maize	Sunflower	Bean
Height (mm)	1
	2
	3
	4
	5
	6
	7
	8
	9
	10
	Mean
Stem thickness (mm)	1
	2
	3
	4
	5
	6
	7
	8
	9
	10
	Mean

Table 2. Pigment data for plants grow under varying light conditions
		Maize	Bean	Sunflower
Wavelength (nm)	470
	537
	647
	663
Calculations	chl a (�mol ml^-1)
	chl b (�mol ml^-1)
	total chlorophyll (�mol ml^-1)
	carotenoids (�mol ml^-1)
	anthocyanins (�mol ml^-1)

Analysis of Hypotheses:

Plant height: Using the data from Table 1, plot a single histogram depicting average plant height vs. treatment for the three treatments (Figure 2). What do you conclude from these data? ANOVA is a statistical test that allows us to determine if there are statistically meaningful differences between our treatments. Run an ANOVA test (Concepts web site: http://www.physics.csbsju.edu/stats/Index.html) and complete the following:

Null hypothesis:
probability value:	p =
Null hypothesis is:	supported rejected

Stem Thickness: Using the data from Table 1, plot a histogram depicting the average plant height vs. treatment for the three treatments (Figure 3). What do you conclude from these data? Run an ANOVA test (Concepts web site: http://www.physics.csbsju.edu/stats/Index.html) and complete the following:

Null hypothesis:
probability value:	p =
Null hypothesis is:	supported rejected

Leaf Size: After examining your plants (and/or digital image), what do you conclude about your hypothesis? How could you make your analysis more quantitative?
Coloration: After examining your plants ((and/or digital image), what do you conclude about your hypothesis? How could you make your analysis more quantitative?
Apex shape: After examining your plants ((and/or digital image), what do you conclude about your hypothesis? How could you make your analysis more quantitative?

Assignment:

Write an abstract of your experiment summarizing the response of the three species to the three conditions (dark, high light, low light). Be sure to discuss/include the terms etiolation and photomorphogenesis in your abstract. Also be sure to cite your p-values in your abstract. Append to your abstract a copy of your digital image of the plants (properly labeled as Figure 1) , Table 1, Table 2, Figure 2 and Figure 3.