Sarah Rodell

Lindsey Calvi

Field Research Project:

Plant Population

Abstract

We hypothesized plant density would increase growth productivity. We used WI Fast Plant: Rapid-cycling Brassica rapa seeds and placed a different amount of seeds in each pot. We then measured number of stems per pot. The pots that contained larger numbers of WI Fast Plant seeds grew more efficiently than the pots with a less number of seeds with a correlation of 0.9854.

Keywords: seed density, plant growth

Introduction

We hypothesized that increased seed density in a given area will lead to plants that have a lower growth productivity. Competition among plants can affect any stage of a plant’s life cycle sporadically (Vila & Sardans, 1999). Increasing the number of plants per area could potentially lead to an increase in competition. Large populations of plants could result in fewer nutrients in the soil for that particular plant. In order for a plant to consume the nutrients in the soil, a plant should place its roots in an area where there aren’t any other roots from other plants (Maina, Brown, & Gersani, 2002). High plant density could also lead to a decreased ability to reach sunlight and have less water to soak up from the soil. Therefore, a lack in these vital resources would lead to less growth in plants that are more crowded together.

Materials & methods

We tested whether the seed density in a given area has an effect on the growth productivity of the WI Fast Plant. We used 102 WI Fast Plant: Rapid-cycling Brassica rapa seeds. First we gathered 12-7.62 cm x 7.62 cm planting pots, and one 54 cm x 28 cm potting tray. We used a black Sharpie marker and blue masking tape to label each of the pots. We labeled the pots according to the number of seeds being placed in the pot. To lesson our error in our experiment we planted the same number of seeds in a second row of pots, we deemed that row; Row B. See figure 1.

Figure 1: Pictorial description of how pots were labeled and organized.

After creating the labels we cut 12-17 cm strips of wick. We then thread the wick through the bottom of each pot, so that half of the wick was inside of the planting pot and the other half was sitting in a 54 cm x 28 cm potting tray. Next, we moistened 2844 mL Pro-Mix Soil. We used a large planting pot to help moisten the soil. We filled a large planter with soil, added water slowly, and mixed the soil with our hands until the soil was damp. Since the soil was so dry we had to do this twice in order to get all of the necessary soil moistened. We then filled each pot with approximately 237 mL of soil. After adding the soil we placed the pots (in order; pot A in front of pot B) in the potting tray. We gently placed the correct number of seeds on top of the soil in the corresponding pots. We tried to keep the seeds as close to one another as much as possible, because this would truly show if seeds have problems competing for space at the beginning stages of growth. Because the soil was moistened already we used a 400 mL beaker and added 600 mL of water to the bottom of the potting tray. Lastly, we placed the potting tray under the 24 hour lighting. WI Fast Plants need a lot of lighting when they are first beginning to grow. After one and a half weeks we moved the planting tray to 12 hour on-12 hour off lighting.

Results

Our results from our correlation test, 0.9854 indicated that we had a strong correlation between the number of seeds in each pot and the average number of stems in each pot. Within four days of planting, the first seedlings began to emerge. The first seedlings sprouted in pots 5a/b and 6a/b. There was new growth over the next few days in pots 3b, 4a/b, 5a/b, and 6a/b. There was no growth in pots 1a/b or 2a/b during the entire duration of the experiment. Pot 3a did not have any growth until the last week of the experiment. Pot 3a only grew one stem out of the four seeds that were planted. Seed growth was quick in the beginning, and then the rate slowed down after a few days. Seeds sprouted after the first week, but at a very slow pace.

Most of the seedlings sprouted to the height of about 1cm. However, none of the plants increased in height much after they sprouted. Pots 1a/b and 2a/b had no growth. Pots 3a/b had an average of 1.2 stems and pot 4a/b had an average of 2.8 stems. Pot 5a/b had an average of 3.2 stems and pot 6a/b had an average of 6.3 stems (See figure 1). As the number of seeds in each pot was increased the average number of stems also increased.

Figure 2: Correlation between average number of stems per pot and the number of seeds per pot.

Number of seeds per pot	Average number of stems per pot
2	0
4	0
8	1.17
16	2.83
24	3.16
48	6.3

Table 1: Number of seeds per pot versus the average number of stems.

Discussion

Our results did not support our hypothesis; higher seed density did affect the growth of the WI Fast Plant: Rapid-cycling Brassica rapa seeds. However, we found it odd that there was absolutely no sign of growth in the four pots containing seed densities of 1 seed and 2 seeds.

Some of the contributions that we believe could have affected the growth of any of our planting pots, but mainly pots 1a, 1b and pots 2a, 2b are; the amount of light, the amount of water, not adding any type of fertilizer to speed up the growing process, and finally the type of soil.

After one and a half weeks we decided to take the planting tray out of the 24 hour on lighting because we believed the lighting was too intense. The lighting could have been causing the soil to dry out too quickly and prohibiting the seeds to sprout. Due to such intense lighting (at first) we had to add 600mL of water to the soil almost every other day because the water was evaporating too fast. WI Fast Plant: Rapid-cycling Brassica rapa seeds grow best under cool fluorescent lighting, and complete their growing process in 35-40 days after being planted (Hafner, 1990). If we were to do this experiment again we would place our plants in the 12 hour on and 12 hour off lighting conditions. This way the lighting would not be a contributing factor in a follow-up investigation.

A second contributing factor could have been the amount of water we were adding. We did not directly add water to the soil, because with WI Fast Plants you are not supposed to add water directly to the plant unless you are using a pipette, or a spray bottle. WI Fast Plant seeds do not tolerate drying out (Lee, 2003, pp. 137-138). In order to control the amount of water and the soil moisture levels of each pot we added a wick to each planting pot. The purpose of the wick is to keep the soil moist, however we believe that somewhere along the line too much water was added and this produced mold growth in our planting pots. Because the planting pots and the planting tray were dry we added water to the potting tray every 3 days, but we should not have. If we were to do this experiment again we would prefer using a pipette, or a spray bottle to measure the amount of water being put into each pot. We believe that doing this would eliminate the confusion of when to add water and when not to add water. We would also prevent the soil from becoming too moist and growing mold.

A third factor we believed was that we should have added a fertilizer. We believed that because WI Fast Plants grow so rapidly that we did not have to add a fertilizer. If we would have added a fertilizer we could have possibly had some growth in pots 1a, 1b and pots 2a, 2b. If we were to do this again we would use a fertilizer. We would have to do some additional research to see what type of fertilizer works best for WI Fast Plants.

Our last contributing factor could very well be the type of soil we used. In the WI Fast Plant Growing Instructions guide it specifically says to use a potting soil, and in the laboratory equipment room we used Pro-Mix Soil. We thought that this soil would have the same effect as potting soil. However, overall there was very little growth in all of our planting pots. We realized that potting soil would have worked much better. In the future, we would definitely use potting soil for growing WI Fast Plants.

Bibliography

Hafner, R. (Jan. 1990). Fast Plant: Rapid-cycling Brassicas. The American Biology Teacher, Vol. 52, No. 1 (p.40-46). Retrieved November 2009 from JSTOR.

Lee, C. A. (2003). A Learning Cycle Inquiry into Plant Nutrition . The American Biology Teacher, Vol. 65, No. 2 (p.136-141). Retrieved November 2009 from JSTOR.

Maina, G. G., Brown, J. S., & Gersani, M. (2002). Intra-Plant versus Inter-Plant Root Competition in Beans: Avoidance, Resource Matching or Tragedy of the Commons . Plant Ecology, Vol. 160, No. 2 (p.235-247). Retrieved November 2009 from JSTOR.

Vila, M., & Sardans, J. (1999). Plant Competition in Mediterranean-Type Vegetation . Journal of Vegetation Science, Vol. 10, No. 2 (p.281-294). Retrieved November 2009 from JSTOR.