Section 8

EFFECTS OF INTRA AND INTER SPECIFIC INTERACTIONS ON SEED GERMINATION IN WEEDS


Background

Ecologists study how interactions among organisms and their environments influence the distribution and abundance species. The habitat available to a species depends upon abiotic factors such as climatic factors and interactions with other individuals. Although abiotic factors such as temperature and moisture often place habitat restrictions on species, interactions with other individuals also often influence which habitats are open to a species. We distinguish between intra specific interactions, which involve members of the same species, and inter specific interactions, which involve members of different species.

Most ecological investigations into the effects of intra and inter specific interactions on plant species distributions use adult plants or seedlings. Few studies address whether these interactions operate at the seed phase of the life cycle.

During this lab segment, your group will:

- Pose a question related to the effects of intra and inter specific interactions on seed germination in selected weed species
- Design an experiment that allows you to address that question
- Collect and analyze data from this experiment
- Report your conclusions

BEFORE YOU COME TO LAB

Meet with your lab group and:

- Review the "Potential Research Systems" material
- Discuss which type of system interests you most
- Frame a question that you wish to address using this system
- Discuss your design options for an experiment that will allow you to address your question
- Prepare a brief presentation of your research proposal. You will make an oral presentation of this proposal in lab.


POTENTIAL RESEARCH SYSTEMS

Inter specific Effects of Pokeweed Chemicals on Seed Germination

Pokeweed (Phytolacca americana) tissues contain chemicals that inhibit germination of its own seeds (Edwards et al., 1988). Why would a plant produce compounds that hinder its own reproduction? Pokeweed fruits mature in the fall, and Edwards (1988) suggested that these chemicals might delay germination until the spring when conditions are more suitable for growth and survival. However, several other hypotheses could explain the evolution and maintenance of this situation.

1. The chemicals may limit rodent predation on seeds. In this case, the benefits of rodent avoidance must be greater than any costs associated with inhibition ofseed germination. A group could treat sunflower seeds or baits made of sugar and shortening with pokeweed berry juice and compare predation on this group with a control group.


2. The chemical may limit competition from other species. If the pokeweed chemicals inhibit seed germination of other species or kill the seeds of those species outright, then pokeweed's production of these chemicals may be adaptive. A group could expose seeds of other species such as white campion or rye grass to pokeweed tissue extracts and compare the germination and survival of these seeds with a control group.

References


Intra specific Interactions and Bulbul Germination in Wild Garlic

Wild garlic (Allium vineale ) is the little, chive like plant that you can find growing on just about any lawn in Davidson. Wild garlic can reproduce sexually via seeds or asexually via bulbuls. Most reproduction appears to occur via the bulbuls. All of the bulbuls produced by a wild garlic plant are genetically identical to each other.

Dr. Margaret Ronsheim (1994, 1996) wanted to determine the relative benefits of producing genetically variable vs. genetically identical progeny. Several theories predicted that she would find better germination and growth in arrays of genetically variable individuals than arrays of genetically identical individuals since plants in genetically variable arrays may differ in their resource requirements and susceptibilities to pathogens. Instead, Ronsheim found better germination and growth in genetically identical arrays than in the genetically variable groups in both field and the greenhouse experiments.

Why do genetically identical sets of wild garlic bulbuls perform better than genetically variable arrays? One potential explanation suggests that the bulbuls produce germination enhancing chemicals and that these chemicals are clone specific. In other words individuals surrounded by members of their own clone might receive higher doses of such chemicals than those surrounded by members of other clones. A group could test this hypothesis by creating genetically identical and variable arrays of seeds and comparing the rate of germination and growth of the two groups.

Reference


Effects of Small Population Size on Seed Viability

Most ecologists fear that small, isolated populations will be drawn into an "extinction vortex." The extinction vortex theory (described in Primack, 1995) speculates that as population size becomes small, breeding among close relatives will increases and the fitness of many offspring will be compromised due to a rise in the frequency of individuals homozygous for deleterious alleles. As the average fitness drops, then the population size will become smaller, which will exacerbate inbreeding depression and further lower individual fitness. As such, the population size will spiral down towards extinction.

This extinction vortex scenario constitutes a logical theory, but we have little empirical data available to test the hypotheses associated with this theory. One hypothesis predicts that small, isolated populations should show lower seed viability than large populations. A group could compare seed viability between these types of populations. I have collections of white campion (Silene latifolia) seeds from large populations and small isolated ones. We could also collect morning glory (Ipomea spp.) seeds from large and small populations in the Davidson area. Seed viability can be tested using a simple tetrazolium stain.

References


Effects of Interactions between Varieties of Brassica rapa

Brassica rapa is a plant specially bred for rapid growth and development. Under the correct conditions, this species can go from seed to seed in as little as four weeks. B. rapa var. wild plants reach heights of 13-22 cm tall at maturity while those of B. rapa var. rosette reach heights of only 5-15 cm at maturity. A lack of gibberellic acid production produces dwarfing in the rosette variety, (Wisconsin Fast Plants Program, 1989). Gibberellins are plant hormones which affect stem height and, in many species, promote germination. Although the height differences between the wild and rosette varieties are well documented, we do not know if the lack of gibberellin in the rosette variety also slows the rate of germination. We also do not know if gibberellin from a wild plant can escape into the surrounding environment and influence the germination and growth of the rosette variety.

A group could compare germination rates in wild and rosette varieties of B rapa. The group could also compare germination of isolated rosette seeds and rosette seeds in the presence of wild type seeds.

References


ACKNOWLEDGEMENT: This lab was developed by Dr. Patricia A. Peroni.

© Copyright 2000 Department of Biology, Davidson College, Davidson, NC 28036
Send comments, questions, and suggestions to: macampbell@davidson.edu