Plant Defenses Lab Manual and Background (2)

Testing Plant Defensive Compounds

  • Plants face challenges from abiotic stressors (light, desiccation, soil chemistry) and biotic stressors (herbivores, pathogens).

  • Unable to flee from herbivores, plants have evolved different defense mechanisms.

Types of Herbivores and Corresponding Defenses

  • Larger Herbivores (e.g., mammals):

    • Produce structural defenses (e.g., thorns, spines).

    • Example: Cacti have modified leaves as spines for protection in low-precipitation environments.

    • Other examples include acacia tree thorns and rose bushes.

  • Smaller Herbivores (e.g., insects):

    • Structural defenses may be circumvented; instead, plants employ chemical defenses.

    • Allelochemicals: Toxins produced by plants that disrupt the behavior, growth/development, or survival of herbivores.

    • Example: Tannins inhibit digestion by binding to proteins, leading to starvation while feeding.

    • Cardenolides from milkweed can cause cardiac arrest in consumers.

Common Plant Toxins: Alkaloids

  • Alkaloids are compounds derived from amino acids, including caffeine, nicotine, cocaine.

  • Effects vary widely: stimulants, analgesics, depressants, psychedelics.

  • Adaptive value resides primarily in insect deterrence.

Laboratory Investigation

  • Students will measure the effects of plant defensive compounds on small organisms (e.g., heart rate of invertebrates).

  • Formulate a hypothesis for lab report based on knowledge.

Common Plants Tested and Their Effects

Kava-Kava (Piper methysticum)

  • Used in Pacific cultures for its sedative properties.

  • Psychoactive effects attributed to kavalactones in the roots.

  • Preparation involves steeping ground roots in water.

Chili Peppers (Capsicum spp.)

  • Contains capsaicin, responsible for the burning sensation.

  • Deter herbivory in mammals; advantageous for seed dispersal by birds.

  • Integral to various human diets and cultivated strains worldwide.

Tobacco (Nicotiana tabacum)

  • Known for the stimulant nicotine; significant economic impact historically.

  • Dried leaves are consumed through various methods (smoking, chewing).

Belladonna (Atropa belladonna)

  • Extract used to dilate pupils; known as deadly nightshade.

  • Active compound atropine blocks acetylcholine, increases heart rate.

  • Toxic to many animals except for certain livestock, which hints at its ecological role.

Coffee (Coffea sp.)

  • Caffeine is the primary active compound, widely consumed stimulant.

  • Grown in shaded forest understories, major export for Central and South America.

Tea (Camellia sinensis)

  • Source of various teas, differing by processing methods.

  • Contains caffeine, widely consumed globally.

Lavender (Lavandula angustifolia)

  • Used in herbal medicine and culinary dishes; relieves anxiety and aids sleep.

  • Lavender oil utilized in aromatherapy and skincare products.

Laboratory Protocol

  1. Specimen Preparation: Use a pipette to place a brine shrimp (Artemia salina) on a slide and immobilize with a coverslip. Ensure enough water for gill function without swimming.

    • Locate specimen using low magnification.

  2. Heart Rate Measurement:

    • Count heartbeats for 60 seconds, repeat for three trials, ensuring proper disposal of organisms post-experiment.

  3. Adding Extracts for Testing:

    • Perform tests with different extracts (e.g., Extract 1, Extract 2) following the same heart rate measurement protocol.

Data Analysis

  • Calculation of mean and variance for heart rates from different treatments (control vs extracts).

  • Statistical tests used to determine significance:

    • P-value Interpretation: Probability of obtaining observed results under the null hypothesis.

    • Significance determined if P-value <= 0.05.

  • Conduct a Student’s t-test for comparing means across groups:

    • Compare mean heart rates and determine statistical significance based on calculated t-statistics and critical values.

Statistical Calculations

  • Variance calculation involves:

    1. Subtracting individual observations from the mean and squaring the results.

    2. Summing those values and dividing by the number of observations minus one.

  • T-Statistic Calculation:

    • Formula: (t = \frac{|\bar{x}_1 - \bar{x}_2|}{\sqrt{\frac{s_1^2}{n_1} + \frac{s_2^2}{n_2}}})

    • Use degrees of freedom to determine the critical value for decision-making in hypothesis testing.

  • Report findings clearly in lab report, showing significance of results.