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  Circadian Rhythms  
in Russian Giant Sunflowers

A Science Fair Research Project - Botany

Winner of a first place at the PJAS local science fair, a first place at the PJAS state science fair, and a first honorable mention at the Montgomery County Science Research Competition.

This project contains:
           Abstract (a summary of the project)
           Background Information
           References Sited


         In my project I tried to identify a circadian rhythm in Russian Giant Sunflowers. A circadian rhythm is a 24-hour cycle of metabolic activity that continues independent of environmental cues. I grew my plants in a completely constant environment. After my plants had grown for about 4 weeks, I severed their stems just below the cotyledons. I then laid the plants on their sides on the strip chart recorder. The recorder rolled a graduated strip of paper along at a slow (but constant) pace. The cut stems dripped on the moving paper and made a distinct mark. For each plant I calculated the times when the exuded sap dripped on the paper. The stems were cut at 10:55 PM. During the early morning hours of the next day there were very few drops; however, at about 7:00 AM the drops became more frequent. The hourly number of drops peaked at 4:00 PM and then gradually subsided (all drops ended before 8:00 PM). This data indicates that the plants transported more sap during the day than at night – a sign of increased metabolic activity. I found the data to be consistent with my hypothesis; a daytime oriented rhythm does exist in the sunflower. Though the plants could only sustain exudation for one day, I am fairly confident that the rhythm would repeat daily.

  Background Information 

         Chronobiology is the study of biological rhythms. Most chronobiologists study circadian rhythms, cycles of behavioral or biological activity centered around a period of about 24 hours. These rhythms continue in the absence of environmental cues (endogenous). The human sleep-wake cycle is an example of a Circadian rhythm. Circadian rhythms are generated by an internal clock that is synchronized to the day and night of the environment and other daily cues. This clock, which is encoded into the DNA, is expressed through the production of certain hormones (these hormones differ with each organism).
         In 1920, a landmark paper was written by W.W. Garner and H.A. Allard in which they showed that tobacco plants would flower only if exposed to a certain number of hours of light. The term "photoperiodism" was used to designate the response of the organisms to relative lengths of day and night. Garner and Allard showed that plants could tell time. This experiment opened up the possibility that other organisms, besides animals, functioned on circadian rhythms. The changing times of dawn and dusk contain seasonal information as well as time of day information so that the organisms have, in effect, an internal clock and calendar. The ability to sense day length and time of year is important for all organisms, especially plants, to grow, reproduce, and develop to their fullest potential.


         Does the Russian Giant Sunflower have a regular circadian rhythm? In other words…Does the sunflower operate on a 24-hour cycle independent of all environmental cues?


         If the rate of sap exudation is measured in Russian Giant sunflowers for a period of 2 days and all environmental factors (light, temperature, water, and mineral nutrition) are kept constant, then a predictable 24 hour cycle will emerge that will have the highest activity level at midday.


Growing the Plants

  1. 5 identical plastic pots (3 inches deep) are labeled numerically from 1 to 5
  2. The pots are filled with potting soil
  3. Five seeds of Russian Giant sunflower (Helianthus annus) are sown in each pot ½ inch deep, indoors
  4. Growing Guidelines
    • Water as needed (probably every 3 to 4 days) by placing the pots in a pan of water
    • Maintain temperature at 60 to 65 degrees
    • Keep the plants under fluorescent lighting constantly: the lights should rest 2 to 3 inches above the plants

Measuring the Sap Exudation
  1. After growing for 4 to 5 weeks, cut through the stem just below the cotyledons
  2. Pull a plastic sandwich bag with a hole in the bottom over each pot. The severed stem should fit through the hole
  3. Color the strip chart recorder paper with water based markers, so that any drops will make a clear mark on the paper
  4. Place the pots on their sides on the strip chart recorder with their stems bent downwards so that the drops will fall freely on to the chart
  5. Space the stem-ends along a starting line on the paper
  6. Record the initial time next to the starting line
  7. Start the recorder at 100 millimeters per hour
  8. Keep the temperature constant and do not water the plants
  9. Mark the time on the paper three times each day
  10. After two days have passed, remove the paper and mark it in hourly increments
  11. Present results

  • Air temperature
  • Light (24 hours per day)
  • Mineral nutrition
  • Watering times

  Data - Click for the enlarged graph  

Click to enlarge

Click to enlarge

  Pictures - Click to enlarge; hold cursor over picture for a description 

Plants under the lighting system (24 hr/day coverage) Plants just prior to the cutting
Plants with their stems severed The strip chart recorder
Plants arranged on the strip chart recorder The red circle surrounds a spot where the plant fluid dripped on the paper


         After cutting the plants at 12:00 AM, there were very few drops until about 7:00 AM. Between 9:00 AM and 7:00 PM were the highest concentrations of sap drops, while there were very few sap drops at any other time. Unfortunately, the plants could only sustain exudation for one day.


         I found that a cycle does exist in the Russian Giant Sunflower. I cannot be certain that the cycle repeats every 24 hours because exudation ended after one day; however, the evidence that I gathered points to a daily cycle. If no circadian rhythm was present, the plants would exude a high amount of drops right after the cutting and gradually decrease. The data shows that this was not the case: the drops did not come in full force until around 9:00 AM, even though they were cut at 12:00 AM. The high concentration of drops around midday is indicative of a daily cycle in the plants, oriented towards the middle of the day. Basically, the plants were more active at day than at night. This is particularly fascinating because the cycle was independent of environmental cues: I kept the light on my plants 24 hours a day, the temperature constant, and utilized an irregular watering cycle. The plants were deprived of all environmental stimuli. Next time, I could grow the plants for a longer period. As a result, they might be able to sustain exudation for more than one day.

  References Cited 

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