Plant Physiology

Plant Physiology (Biology 327) - Dr. Stephen G. Saupe; College of St. Benedict/ St. John's University; Biology Department; Collegeville, MN 56321; (320) 363 - 2782; (320) 363 - 3202, fax; ssaupe@csbsju.edu

Objectives: The purpose of this lab experience is to provide an opportunity to:

Introduction:
A seed is essentially a baby in a suitcase carrying its lunch. "Baby" refers to the embryo, or immature plant, that will grow and develop into the seedling and ultimately the mature plant. The dicot embryo consists of: (1) radicle (embryonic root) - develops into the root system; (2) epicotyl (embryonic shoot) - develops into the stems bearing leaves; (3) cotyledons (seed leaves) - serve to store nutrients for germination and/or as an organ to transfer nutrients from the endosperm into the embryo; and (4) hypocotyl - embryonic stem that connects the epicotyl and radicle. The embryos of grasses like maize and oats differ slightly (see any botany text). The "suitcase" is the seed coat (or testa) that surrounds the seeds and "lunch" refers to the nutritive source for the germinating seedling. The food for the germinating seedling may be stored in part of the embryo itself, such as the fleshy cotyledons of a bean seed, or it may take other forms including endosperm, which is a special starch-rich storage tissue that surrounds the embryo.

     A seed is officially considered to have germinated when the radicle emerges from the seed coat. To germinate, a seed requires three things � water, oxygen, and a suitable temperature. Water uptake, also called imbibition, is the first stage of seed germination. During this process the dry seed, which typically has a water content of less than 10%, absorbs water and swells. This process serves to hydrate the dry components of the seed and active the metabolic machinery necessary for germination. Among the early metabolic activities occurring in the seed is the breakdown of starches stored in the seed into simple sugars that can be used for energy and building blocks for necessary cellular structures.

    Except for the first half-hour or so of germination when little oxygen is present, seed germination and subsequent seedling growth requires oxygen. It is required, in large part, for use in the cellular structures, called mitochondria, to produce ATP (energy). A suitable temperature is necessary to optimize the metabolic reactions required for germination. The seeds of every species have an optimal temperature for germination; some species, such as the gourds and squashes prefer warm temperatures while other species such as radish can tolerate cooler temperatures for germination.

    A seed that has not germinated because it is lacking one or more of the necessary requirements for germination is termed quiescent. These seeds are simply "resting", waiting for the appropriate conditions for germination. Given water, oxygen and/or a suitable temperature, a quiescent seed will germinate. However, even if given the proper conditions, a seed may not germinate. These seeds may fail to germinate because the seed is either dormant or "dead".

    Dormant seeds have the potential to germinate but are prevented from doing so by some mechanism. Thus, even though all the proper growth conditions are present, they don't germinate unless they are "primed." Or in other words, their dormancy mechanism has been overcome. There are many dormancy mechanisms in seeds. For example, when some seeds, like hemp, are shed they have immature embryos that will not germinate until they undergo a period of development (called after-ripening). Other seeds, like apple, require a cold treatment, called stratification, for germination. Many of our native plant seeds must be stratified. Some seeds have a hard seed coat that needs to be nicked (called scarification) for germination. This usually occurs as the result of natural freeze-thaw cycles. Still other seeds require a period of heat in order to germinate. Many of these species are winter annuals that germinate in the late summer/early fall.

    Ultimately, the function of these varied dormancy mechanisms is to enable the seed time to disperse from the parent plant and to avoid germinating during unfavorable weather. Humans have attempted to breed dormancy mechanisms from our crop plants. Although an advantage for a wild plant, dormancy is a problem if a farmer who want the crop to germinate uniformly and immediately upon planting.

    It�s not easy to tell if a seed is �dead�. Only if it fails to germinate when provided the proper conditions and any dormancy mechanisms are broken can we consider a seed �dead�.

    Seed companies typically test the germination of seeds before sale. The results of these tests, the germination percentage, are typically provided on a seed packet. Most crop seeds lose viability rapidly after a few years. However, a few long-lived seeds are known. For example, mustard seeds show good germination after even 50 years.

Exercises: In today's lab we will do a variety of exercises concerning seed germination. We will attempt to answer the following questions:

Pre-Lab: Bring to lab, a copy of the exercises listed at the end of each question above.

Post-lab: Write an abstract summarizing the results of these investigations. The abstract should address each of the questions above and appended data (tables/graphs/statistical analyses) to support your conclusions.