The Effect of Auxin Concentration on Apical Dominance in Pea Seedling Growth

Apical dominance is a term used in reference to a circumstance where the central stem of a plant appears to be dominant over other stems. Auxin is vital in the growth of plant since it helps to stimulate growth by elongating the cells at given concentrations (Weinstein, 2020). Thus, auxin is used in apical dominance, where it is produced in apical meristems in the stem tips are transported to the stems through the lateral buds, thus promoting bud growth (Weinstein, 2020). The aim of this experiment is to examine how auxin concentration affects apical dominance, decapitated pea seedlings were treated with different levels of auxin and their amount of growth over time was recorded. It will determine whether lateral shoot growth differs in pea seedlings with different concentrations of auxin. There are three types of auxin concentrations that is 500 ppm and 5000 ppm and control of 0 ppm. The study grouped students where each group comprised of 3-4 students. Each group was assigned pea seedlings and the three auxin concentrations in labeled pots ( Barbier et al., 2017 ). They were required to treat the pea seedings before planning them. The pea seedings were assessed after growth and the auxin concentration measured. The measurements were recorded on a table. The experiment hypothesized that there was a significant high shoot growth in the pea plants treated in a 0 ppm, 500 ppm indicated a medium growth and 5000 ppm had a little growth.

Objective 

To evaluate whether the shoot growth in pea seedlings is significant differs in auxin concentrations. 

Research question

Is there is a significant difference in the pea plant shoot growth treated with the different concentrations of auxin?

Methods

Procedure 

1. Obtain a length of paper towel and the peas treated two weeks ago in a pot. Then tenderly unpot the peas over the bucket provided for vermiculite recycling, then carefully place your peas onto the paper towel. Take caution and carefully handle the pea plants not to break them. 

2. For easy identification of the treatment, stick the label on the paper towel. 

3. Identify each pea plant’s longest sprout and measure its length in mm, between the base and apex. However, do not measure a cut stub. No sprout = 0 mm.

4. The plant that is untreated plant should be labeled "n/a and ignored because it not an experimental group. This s because the treated pea plant germinates. This plant should be recognized by their lack of a cut and treated stub. Any experimental pea should not be discarded. 

5. Write your measurements on the paper towel below each plant, and enter your data in Table and on the whiteboard.

6. Observe everyone's plants, examine outliers, and take relevant notes for your Research Paper.

Materials 

1. Pes seedlings

2. Pot label

3. Goggles 

4. Gloves

5. Sharp razor blade

6. Toothpick

7. Lanolin-plus-auxin

Data Analysis

The data was analyzed using the Wilcoxon test. This a nonparametric test compared two experimental groups. The Wilcoxon Statistic is computed where a calculated Wilcoxon (W) must be equal to or below the critical W in order to be significant.

Results

Table 1. Class Data of Pea Seedling Growth Length (in mm)

Data from Apical Dominance Experiment
	0 ppm (control)	500 ppm	5000 ppm
Sprout Length (mm)	420	469	560
	360	595	132
	570	390	315

Table 2. Class data of pea seedling growth, ranking the control group lengths & the 500 ppm group lengths .

Wilcoxon Rankings for Control and 500 ppm Treatments
Treatment
Control (0ppm)		500ppm
Length (mm)	Rank		Length (mm)	Rank
420	5		469	6
360	3		595	9
570	8		390	4
Average = 450	Sum = 16		Average = 487.6667	Sum = 19
Number of data points for this treatment = 3		Number of data points for this treatment = 3

Table 3. Class data of pea seedling growth, ranking the control group lengths & the 5,000 ppm group lengths

Wilcoxon Rankings for Control and 5000 ppm Treatments
Treatment
Control (0ppm)		5000 ppm
Length (mm)	Rank		Length (mm)	Rank
420	5		560	7
360	3		132	1
570	8		315	2
Average = 450	Sum = 16		Average = 335.6667	Sum = 10
Number of data points for this treatment = 3		Number of data points for this treatment = 3

Table 4. Class data of pea seedling growth, ranking the 500 ppm group lengths & the 5,000 ppm group lengths

Wilcoxon Rankings for 500 ppm and 5000 ppm Treatments
Treatment
500 ppm		5000 ppm
Length (mm)	Rank		Length (mm)	Rank
469	6		560	7
595	9		132	1
390	4		315	2
Average = 484.6667	Sum = 19		Average = 335.6667	Sum = 10
Number of data points for this treatment = 3		Number of data points for this treatment = 3

Table 5. Wilcoxon Statistical Test of Data from Table 3, 4, & 5

	Table 3. Control/500 ppm	Table 4. Control /5000 ppm	Table 5. Control 500/5000 ppm
Your calculated value, W’	24	24	19
Your N1 value	3	3	3
Your N2 value	3	3	3
Your critical (table) value, W	6	6	6

The data from the experimental groups in the Wilcoxon tests indicates that there is a statically significant difference linking the treatments. Table 1 shows that the control group (0 ppm) has the highest growth followed by 500 ppm and 5,000 ppm. Also, it shows the measurements of the treated and untreated (control) pea plants in millimeters. Table 2 compares the shoot growth of the controlled pea plants (0 ppm) and treated with 500 ppm of auxin, then ranks them from smallest to largest. Table 3 compares the soot growth of the controlled plant (0 ppm) with the treated (5,000 ppm) of auxin, then also ranks them from smallest to largest. Table 4 compares treated pea plants with 500 ppm with 5,000 ppm of auxin, then ranks them smallest to largest. Table 5 uses the Wilcoxon statistical test to compare the critical and calculated values gathered from each table.

Discussion

The purpose of the study was to examine whether the lateral shoot growth in decapitated pea seedlings is differ between the two auxin concentrations (500 ppm, 5000 ppm). There were more pea plants treated with 500 ppm, however, those that grew the longest were those treated with lanolin wax, aka the control group. The data in Table 1 indicated that the control group had the highest shoot growth, second was had 500 ppm, then the 5,000 ppm. When compared using the Wilcoxon statistical test, the critical value of Table 3 is lower than the calculated value. 

However, the critical values for Table 4 and 5 and higher than the calculated values, which leads me to accept my hypothesis. The explanation for such results is the natural sensitivity to auxin concentration in a plant’s lateral buds (Koning, n.d.). A plant’s lateral buds react to the slightest exposure of auxin in an inhibitory manner, so when high concentrations are introduced, the less growth the lateral buds experience. In a similar experiment, it was witnessed that replacing auxin in temperate woody species to observe apical dominance held the same effect if auxin were applied in some non-woody species (Cline, 2000). Some factors that may have biased our results include hindsight-bias and human error in plant measurement. It is believed hindsight-bias, a phenomenon in which a person overestimates their ability to predict the outcome of something even though the outcome is impossible to predict, plays a key factor in our results. This is because, as a group, we had recently learned what auxin is and how it effects the growth of plants. However, we could not have known how higher concentrations of auxin might have affected pea seedling growth. For future studies, I recommend that the gestation period for the plants should be increased after treatment with the sample size in each group. This will help in providing more sights on the effect of auxin concentration of plant growth. 

References

Barbier, F. F., Dun, E. A., & Beveridge, C. A. (2017). Apical dominance.  Current Biology ,  27 (17), R864-R865.

Cline, M. G. (2000). Execution of the auxin replacement apical dominance experiment in temperate woody species [Abstract]. American Journal of Botany, 87 (2). doi:10.2307/2656904

Koning, R. (n.d.). Apical Dominance. http://plantphys.info/apical/apical.html  

Weinstein, S. (2020). BSC 121 Principles of biology for majors laboratory manual (18th) . Cincinnati, Ohio: Val-Griner Learning.