Four O'Clock Plants
The buds and the bloomers

By Sarah Atkinson and Jennifer Guest

Mirabilis jalapa, or more commonly known as Four O'clock Plants and sometimes even the "Marvel of Peru," is a short-day plant, meaning that it needs a short amount of light for it to bloom. The plant actually can determine what time to bloom by the period of darkness and its relativity to the period of light. They belong to the family Nyctaginaceae and are know for their ability to distinguish between seasons by measuring the relative periods of light and darkness. They bloom late in the afternoon, hence the name, because their day lenght requirement is not met until this time.

The Four O’clock plant determines the lenght of darkness and the lenght of day light through a process called photoperiodism. Some plants, such as the Four o’clock plant, have a critical period of darkness. By regulating the periods of lightness and darkness, we hope to control the time it takes for plants to bloom. If this period of darkness is interrupted by a red light (a certain wave length in the magnetic structure), then the plant ceases to bloom. Only if the interruption of red light is followed by a flash of far red light will the plant continue to bloom. (far red light is a light on the spectrum that has nm’s, and is greater in wave length.). Plants are stimulated by the flashes of red light and far red light because of a certain homodimer that is found in the leaves of plants. A homodimer is two identical protein molecules each conjugated to a light absorbing molecule. This specific homodimer is Phytochrome. There are a total of five phytochromes in a plant leaf. They are PhyA, PhyB, PhyC, PhyD, and PhyE. Each consists in two interconvertible forms. These two forms are PR and PFR. The PR absorbs red light, while the PFR absorbs far red light. (the P stands for phytochrome). Once a PR absorbs red light, it is converted into PR (i don’t know what the process is); and vise versa (the PFR converts to PR). During the dark period, all the PR that was converted to PR converts back into its original form, PR. This is because the PR is needed to stimulate flowering. The new PR then triggers florigen.

In more detail: When the PR is exposed to a flash of red light, it is converted into PFR. The PFR then moves through the cytoplasm into the nucleus. Once in the nucleus, it binds to a protein called PIF3 ("phytochrome-interacting factor 3). The PIF3 is a transcription factor, and so its has a structure in the form of a helix-loop-helix. The binding of the two proteins, the PFR and the PIF3, initiates promoters located in the genes. These promoters then encode other transcription factors. These factors, in turn, initiate transcription of a variety of genes which are then expressed when the plant is exposed to light. If the plant is then exposed to far red light, the light causes the PFR to convert back into PR. The PR then dissociates from the PIF3 and retuns to the cytoplasm where it can be triggered again by red light and the process begins again.

Scientists do not know how exactly plants trigger flowering. Some scientists think that PFR is active in promoting plant growth, while PR, which is "inactive" in this process, may be released as a "default" pathway that then induces florigen and eventually flowering.

The Four O'clock Plants have a unique structure to their species. The particular flowers of this plant have no petals, but instead are supported by petal-like bracts that help to stabalize the flower. They have thick leaves, which allow them to survive pollution. They are popular along roads and highways because of this structure and because they are very heat resistant and tolerant of different types of soil. They grow the best under a full sun and well drained soil. They grow from 12 to 36 inches, and bloom in a variety of colors, mainly red, pink, yellow and white.

The Four O'clock Plants are also commonly used in medicinal purposes but are also known to be quite poisonous to animals. The roots and seeds can be mildly toxic when ingested and cause minor skin irritations and dermatitis. Other members of this family include the bougainvillea.


Sarah and I chose this topic whith help from Ms. Fields. (thanks!) The main purpose of this lab was to determine the minimum amount of light under which the flowers would bloom. This topic was also new to both of us and very intriguing. (plus it gave us a chance to get dirty). The purpose of our project was to see waht the minimum amount of light was needed for these plants to sucessfully bloom, whether they could recieve too much light, and if they could, what this maximum amount of light was.


From our background research, we found that these plants follow a bloomage pattern that was directly related to the hours of light they received. In our experiment, we have three different day lengths: 8 hours, 12 hours, and 16 hours. We predict that the plants that recieve the most light, the 16 hour group, will flourish the most. We think that those plants under the second group, the 12 hours, will grow and bloom, but just not as quickly. Finally, we think that the 8-hour plants will not have enough light to bloom at all. Hopefully, the plants will start to show buds a few days after reaching their full hight.


  • 216 Mirabillis jalapa seeds (Four O'Clock Plants)
  • 9 growing pots, each with 10 individual plant spaces.
  • 3 frames (we used boxes; shoe boxes work as well)
  • 3 black peices of clothe
  • 3 lamps
  • 3 light timers
  • potting soil
  • water
  • rular
  • measuring cup
  • string
  • toothpicks
  • paper clips

  • Procedure

    Begin by buying the plants. Because of the convenience, we purchased our seeds online at Swallow Tail Garden Seeds, but they can also be found at Earthly Goods Online. The plants generally require 1 to 2 weeks for germination, and luckily, ours only took 7 days. From our first round of plants, we discovered that plants do better in natural light than under lights. Because of this, we combined a window and a 40-watt bulb to be the light source for the second group of plants.

    After the plants have sprouted and are growing (about two to three weeks), place them under the various day lengths. Place 30 plants under each day lenght and cover the setup with a towel. This will prevent ouside light from intruding on the expirament. Record data each day, noting the number of blooms in each box and the length that each bloom stays open. Note if any of the blooms die, and if they do, when.

    note: everything should be recorded, including the number of hours each seedling was under while growing, the soil type, the amount of water given, the distance of the boxes to the light source, etc.


    First round:
  • It took a long time for those plants that grew to grow, and even these did not survive long.
  • The plants stopped growing after reaching 4 inches

  • Second round:
  • The plants sprouted quickly
  • Once the plants sprouted, they grew quickly
  • The plants continuously took 10 ml of water
  • As the plants grow upwards and become taller, the leaves begin to lose their circular shape and instead become more pointy.

  • Data

    16 Hours

    12 Hours

    8 Hours


    First Round:
    planting: We planted the first round of plants in 108 nine-ounce wax cups. There was only one seed in each cup. Each of the cups had holes in the bottom that alowed water to filter out. The seeds were planted 2 centimeters under the soil surface in "Peter's Professional Potting Soil." (it contained: composted bark, Canadian sphagnum peat moss, horticultural vermiculite and a wetting agent). We began by watering then 50 ml, then 25 ml, then 15ml, but after watering them 15 ml, the soil was still moist and we stopped watering them for a week. The particular type of cup that we used in the first round allowed molding; the constant wet soil suggests that the holes in the bottom of the cups were not big enough, or that there were not enough; the potting soil was not a starter soil and probably wasn't suitable for seeds.

    sprouting: The first round did not grow. This could be contributed to many things. 1) over watering; 2) they were planted too deeply in the soil; 3) the lamps that they were placed under were not strong enough.

    growing: Did not grow.

    Second Round:
    planting: The second round was planted in seed starter pots that were made of cardboard. This prevented molding. We used starting soil this time, which provided the right nutrients (Country Cottage Seeds) and was specifically designed to help seeds grow. The seeds were planted one centimeter below the soil surface this time. This allowed greater access to light. We watered them 10 ml each day using a spray bottle. This prevented water pressure from uprooting the seeds once they began to grow. These seeds were also allowed to grow under natural light. They grew faster than the first round, which suggests that the lamps used in the first round were not strong enough or were not the right light type--those plants which grew and thrived were the ones that were closest to the window.

    sprouting: The plants began to grow after a week and a half. This could be because of stronger light and a more stable watering system of 10 ml a day. They are growing at a slant because the light source is coming from the side (this is also bending the vascular system--use the toothpicks to support them)

    growing: The second round grew faster and had a significantly lower mortality rate. This could be because the plants had natural light and were closer to the light source. The plants were moved to school Monday, May 8. We placed 30 plants under each light source. Each light source had a 14-watt Philips plant bulb. The plants were placed 36 inches under each lamp. The first light source was on 16 hours strait. After two days, it appears that these plants have not yet straitened up. The second group is under 12 hours of solid light. Contrary to our hypothesis, these plants seem to be recieving sufficient light and are thriving and growing upwards. The third group that is under 8 hours has not shown any change. This suggests that the ideal amount of light is greater that 8 hours and less that 16 hours. It also suggests that maybe the plants are two close to the light because in standar spring and summer days, there is more than 12 hours of light.

    Error Analasis

    First Round:
    We decided that initiation time and conclusion time of the light source whould not affect the plants because they were inside and not exposed to any other light sources. By taking away this variable, we were able to reduce the number of plants by one half.

    On the first day that the plants were planted, they were given 50 ml of water over a course of a few hours. The next day they were again given 50 ml of water, but the amount seemed to be too much this time, because some of the water came out through the holes in the bottom of the cup. We then decided to reduce the amount of water for the next day to 25 ml again and see how the plants responded to that. This again was too much, and we went down to 15 ml. This was still too much--the soil remained moist for a week, and so the plants were not watered until one week later. (April 12--this is on the first round) The access water caused mold to form.

    When the plants were placed under a lamp for germination, they were all placed on the floor under the light. However, since there were three boxes, some of the plants may have been more directly under the light than others.

    During the time of germination, outside light may have reached the plants as well. Also during this time, the amount of light the plants recieved each day may have varied by five or ten minutes.

    Second round:
    The walls may have shaded some plants from the light from the window.

    We replanted the seond round of plants of Sarah's desk. These plants grew much faster. The inside light probably was not bright enough to supply them with the right amount of light. They could not grow.

    The second batch of plants grew quickly. Finally, because of lack of time, we decided to change the experiment to just light time, and not include the different types of light sources. This simplified the exprament greatly.

    It took a long time for the seeds to arrive in the first place.

    For the second round, we used starter soil instead of potting soil. (different fertilizers)

    Using cardboard seed starting pots instead of wax paper cups prevented mold growth.

    Pouring water on seeds may have caused some uprooting, so we watered them with a spray bottle until they sprouted and were more stable.


    The conclusion cannot be written yet because of lack of data


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