Introduction

A need confronting current educational practices is the incorporation of meaningful investigatory classroom practices that emulate the scientific method (AAAS, 2011; National Research Council, 2012). We offer two activities involving the response of a plant (maize) to exposure to different concentrations of household chemicals, using as an example three alcohols in a concentration gradient. The following questions could be used as prompts for student discussion and planning: (a) Would the household chemicals have an effect on plant growth (i.e., either enhancement or reduction of growth, or no effect)? (b) Would there be a difference between a full-strength chemical solution and a diluted solution? (c) If a chemical were toxic, how could the toxic concentration be determined? These experiments relate to AP Biology Science Practice 3 “engage in scientific questioning to extend thinking or to guide investigation,” Practice 4 “plan and implement data collection strategies,” and Practice 5 “perform data analysis and evaluation of evidence” (College Board, 2015). Of the Next Generation Science Standards (NGSS), these activities address two disciplinary core ideas at the middle school level (LS1.B and LS4.C) and two at the high school level (LS1.A and LS4.C) (NRC, 2012). Two elements of the NOS matrix of the NGSS are addressed: “Scientific investigations use a variety of methods,” and “Scientific knowledge is based on empirical evidence” (NGSS Lead States, 2013). These activities are relevant to two Core Competencies and Disciplinary Practices as outlined by the AAAS (2011): the ability to (1) apply the process of science, and (2) use quantitative reasoning.

Materials

Possible household solutions for these experiments are listed in Table 1, providing students with choices for experimentation. Students can also test additional household liquids of their choice. We recommend that ammonia, bleach, and nail polish remover solutions be handled by the teacher in a well-ventilated area with protective clothing (gloves and a lab coat). The other solutions can be handled by students with proper supervision.

In setting up the experiments, discuss with students the proposed treatments with undiluted (full-strength) and diluted solutions. A dilution gradient can be prepared in defined increments (e.g., increases of 2.5, or 5, or 10 percent, with three to five concentrations in a series for a single chemical) to determine the effects of different concentrations on shoot growth and to detect possible lethal concentrations. Dilutions are calculated with the equation V₁C₁ = V₂C₂, where V₁ and V₂ are the starting concentrated solution and final diluted solution volumes, respectively, and C₁ and C₂ are the stock solution and final diluted solution concentrations, respectively (Adams, 2003). As an example, we demonstrate the results of dilution calculations with an alcohol series of 2.5, 5, 7.5, and 10 percent (Table 2).

Procedure

As an example we will discuss two experiments that use the three alcohols and four dilutions (Table 2).

The seed germination experiment can be completed within three weeks (21 days) and requires the following materials:

• 13 100-mm plastic petri plates

• Several sheets of white construction paper

• 117 maize seeds

With a permanent (indelible ink) marker, label the outside bottom of a plate with the alcohol and concentration to be assayed. Label one plate as a water control. Place a 3 × 3 inch square of white construction paper into the bottom of each plate. Label 9 positions on the paper square (1–9) and the experimental treatment or control. Add 10 ml of the chemical solution (or water control) to be tested. Place nine maize seeds in each plate, one on each numbered position (Figure 1). We determine the onset of germination when approximately 5 mm of the radicle (initial root) emerges from the seed (Figure 2). Germination typically occurs in the water control within 3–5 days, later for the alcohol treatments.

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Figure 1.

A petri plate containing a moistened paper square and four germinating maize seeds. Seed numbers 1, 5, 7, and 8 are all exhibiting germination.

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Figure 2.

Effects of alcohols on maize seedling growth after 28 days. The 7.5% and 10% alcohol concentrations were lethal to the plant.

The shoot growth experiment typically requires 28 days. The materials needed are:

• 39 8-oz. clear plastic drinking cups

• approximately 2 cubic feet of Perlite (available from a garden supply store)

• 351 maize seeds

The afternoon before the experiment, put the seeds in tap water to soak until the next day. Fill each of 39 8-oz. drinking cups to approximately 7/8 capacity with Perlite, and place four maize seeds on top. Three cups will be the water controls. For each alcohol (three) and each dilution (four), use three cups (4 seeds/cup, total of 12 seeds per treatment) for replication. Hydrate the Perlite with either water (control) or an alcohol solution. Cover the seeds with a thin layer (1/4 inch) of Perlite. Place the cups near a light source, and keep the surface of the Perlite moist. After 28 days, measure the length of the shoots from the base to the tip. Record the data in a Microsoft Excel spreadsheet.

Statistical Approaches

We determine the means and standard deviations for the control shoot heights and each chemical treatment set of shoot heights to address the question: Do the differences among the different chemicals and their concentrations affect seedling germination and shoot growth? We first determine the average (mean) height of the shoots for each specific treatment at the end of the experiment (day 28). The standard deviation is then calculated to determine the variation in the data for each chemical treatment and for the control. For this portion of the activity students can be trained using either the online Microsoft Excel tutorial (https://support.office.com/en-us/article/Excel-training-9bc05390-e94c-46af-a5b3-d7c22f6990bb) or any of several You Tube tutorials. They will calculate the mean, median, and standard deviation (https://www.youtube.com/watch?v=2rEhWFhSqnI) and graph the results in Excel (https://www.youtube.com/watch?v=NhUFj_yg8zI). A composite bar graph containing results for all the treatments and control allows for a comparison. Students are asked: Do their predictions match the actual results?

As an example, we present an analysis of shoot growth results from the alcohol experiment. The alcohol with the greatest inhibitory effect was ethanol (Figure 2). The 7.5% and 10% ethanol treatments were lethal (that is, no shoot growth was seen). The mean shoot height of plants exposed to the low concentration (2.5%) of isopropanol was similar to that of the water control, with increasing concentrations of the alcohols resulting in increased inhibition (reduction) of shoot growth.

Students could be asked to develop other quantitative measures of plant growth (e.g., root length, shoot or root biomass, total biomass). They could measure and analyze weekly germination or growth results to determine if the effects of chemical exposure are either constant over the duration of the experiment or vary as the experiment progresses.