Spring.wmf (18300 bytes) 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

Light & Plants

ObjectivesUpon completion of this exercise, the student should be able to:

  1. describe various ways to express light measurements
  2. explain the difference between footcandles and photon fluence
  3. explain why plants grown in an east-facing window appear different than those in a south-facing window.
  4. calculate the electrical cost for lighting
  5. calculate the photosynthetic efficiency of a plant

    Light can be measured in three major ways:  (1) illumination - which refers primarily to the light available for human vision and is measured in lux or footcandles (10.8 lux = 1 ftc); (2) Photon fluence - which refers to the number of photons that are absorbed by a surface per unit area per unit time and is expressed in units of μmol photon m-2 s-1 (or microEinsteins, μE); and (3) energy of light - measured typically in watts m-2 .  Fluence, also called irradiance, is the preferred measure for a plant physiologist because it features the actually number of photons that a plant receives of photosynthetically active radiations (PAR), those with wavelengths from 400 - 700 nm.  

Pre-Lab Study

  1. read this lab and the pertinent material in your text.
  2. complete the hypotheses/predictions


  1. The fluence of sunlight is (the same as / less than / much greater than ) that indoors.
  2. A plant growing in window facing (north / south / east / west ) will be most likely to be light starved.
  3. Fluence (remains the same / decreases slowly/ decreases rapidly / increases ) as you move from a light source.
  4. Given the same illumination (footcandles), the best light source for a plant is (sunlight / cool white fluorescent).
  5. Given the same photon fluence, the best light source for a plant is (sunlight / cool white fluorescent).

Exercise 1:  Comparison of Light Measures
The most readily available (and inexpensive) light meters measure illumination and express values in in footcandles.  To convert footcandles to photon fluence, divide footcandles by the appropriate conversion factor (5 for daylight, 6.9 for cool white fluorescent bulbs, 4.6 for incandescent bulbs).  Now, let's apply this information.

Question 1:  Consider sunlight and cool white fluorescent light that have the same PAR photon fluence rate of 125 μmol photon m-2 s-1 .  Which light source will be better illumination for human vision?  Explain with reference to your calculations.

Question 2:  Now, consider sunlight and cool white fluorescent light that have the same level of illumination of 1000 footcandles. Which will be a better light source for plant growth?  Explain with reference to your calculations.

Exercise 2 - Fluence of different light sources
    Use the LiCor Quantum Radiometer to measure irradiance in the areas listed in Table 1.  Based on these data, is the Greenhouse arranged appropriately with desert plants in the western-most section, and so on?  In which window (east, west, etc) would a plant grow best?  In which window would a plant be least likely to get enough sunlight for growth and show signs of light 'starvation?'   Explain why many houseplants have come from rain forests.  Why are there so few plants that live on the floor of a deciduous forest in late summer?

Table 1.  Irradiance of various light sources
Source Irradiance (μmol photon m-2 s-1)   
Sunlight - Outside  in open area (e.g., parking lot)  
Sunlight - Outside in forest  
Classroom - fluorescent light bank  
Greenhouse - Conservatory  
Greenhouse - Succulent House  
Greenhouse - Project Room  
Window - south facing  
Window - north facing  

Exercise 3.  The Cost of Growing a Plant Indoors

    Consider a bean plant that was growing under our bank of six fluorescent lights for two weeks under continuous illumination.  How much did it cost to run the lights for the duration of the experiment?  Assume that the cost of electricity is $0.10 per kilowatt hour (kWh) and that there are six, 40 watt bulbs in each light bank.  Show your calculations.

Exercise 4.  Photosynthetic Efficiency
    Photosynthetic efficiency (PE) is a measure of how much light energy is converted into dry matter by a plant.  This can be mathematically expressed:  energy (kcal) fixed in plant/total light energy (kcal) x 100.  Let's estimate the PE for a plant. 

    First we need to know the total energy (kcal) fixed in the plant.  To make this calculation we need to know the dry weight of the plant (weigh your plant and record the fresh weight.  Then, put the plant in an oven until dry and reweigh).  From this value we can calculate the crop growth rate (CGR) in units of g plant dry mass produced per meter squared per day (g m-2  d-1).  We also need to know the average plant tissue energy value.  Although this is typically empirically measured with a bomb calorimeter, a reasonable value is 4 kcal g-1 dry weight. 

    Next we need to calculate the total energy that is available to the plant.  To make this calculation we will need to know the irradiance (μmol m-2 s-1) of light during the growth period and the approximate energy available in PAR (use 52 kcal/mol as an approximation).  This is easy if our plant is growing under a fluorescent light bank (simply measure with the quantum radiometer), but is a little more challenging for an outdoor or greenhouse grown crop because light irradiance changes during the course of the day.  As a very simple approximation, If studying a greenhouse-grown plant we need to measure the irradiance in the greenhouse and then assume that this value is constant during the daylight hours.  We also need to know the average day length during the growth period.

    You can use Table 2 to collect the data that you will require to calculate PE. 

    As an aside, from these data you can also calculate the water content of the plant (%)

Table 2.  Data for calculating photosynthetic efficiency

date experiment started

date experiment ended  

Length of experiment (days)

Length of experiment (seconds)  
Fresh weight (g)  
Dry weight ( g)  
Area of plant/container (cm2)  
Area of plant/container (m2)  
Crop growth rate (g m-2  d-1)  
Water content of the plant (%)  
Fluence/Irradiance of light source (measured with radiometer; (μmol m-2 s-1)  
Photosynthetic efficiency (%)  

Exercise 5 - Effect of Distance on Fluence

     Use the LiCor quantum radiometer to measure the irradiance at several distances from the bulbs of the light bank (Table 2). Then, make a graph using Excel plotting irradiance vs. distance (Figure 1).  Find the best curve to fit your data.  What does this tell you about light and distance?  Based on these data, explain why we put out plants so close to the light source?  What implications do these data have for plants grown in your home?

Table 3.  Effect of distance on fluence from a light source

Distance from source (cm)

Fluence (μmol m-2 s-1)

Post-Lab:  Upon completion of this lab:

Write an abstract summarizing the results of this experiment.  Append to your abstract a copy of Tables 1 & 2 and Figure 1 & 2.   In your abstract, you should address the questions indicated in each of the exercises. 


  • Dean, RL (1996) Plotting rates of photosynthesis as a function of light quality. American Biology Teacher 58: 424-5.

  • Hershey, D (1991) Plant light measurement and calculations.  American Biology Teacher 53:  351-3.


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Last updated:  01/07/2009     � Copyright  by SG Saupe