Ecologically Meaningful Germination Studies

An understanding of seed germination ecology is enhanced by information on kind of dormancy in the seeds, life cycle of the species and seasonal changes in environmental conditions such as temperature and precipitation (and soil salinity for halophytes) in the habitat from the time of seed dispersal to germination. Data from studies done in the laboratory, greenhouse and field can be used to help explain the timing and control of seed germination in nature, and this chapter deals with some of the many problems related to collecting data in such a manner that they have ecological meaning.

Keywords

alternating and constant temperatures; ecological life cycle; embryoless seeds; germination percentages and rates; germination substrate; light vs. dark for germination; seed germination ecology; seed maturity; timing of germination in the field; viability testing

Purpose

Numerous germination studies have been done with the goal of achieving a better understanding of how germination is controlled in nature. Frequently, however, the research was performed in such a way that the results cannot be extrapolated to the field situation, or extrapolation must be done with great caution. The purpose of this chapter is to show how data from seed germination studies in the laboratory and greenhouse can be used to help explain timing and control of seed germination of species in nature. Of course, this means the germination data need to be collected in such a way that the results have ecological meaning.

An understanding of seed germination ecology is enhanced by knowledge of the (1) physiological (germination responses), morphological (development of embryo) and physical (permeability of coats) states of seeds at the time they are matured, (2) changes in physiological, morphological and physical states of seeds that must precede germination, (3) environmental conditions required for these changes to take place and (4) environmental conditions occurring in the habitat between the time of maturation and germination. Thus, information is needed on the life cycle of the species, especially the seed dispersal and germination phases, in relation to seasonal changes in environmental conditions such as temperatures and precipitation and, for halophytes, soil salinity. Further, germination experiments conducted during the course of the natural dormancy-breaking period will provide data on the rate of dormancy break. The questions are: what kinds of experiments are needed, and how should they be done?

By pooling data from many authors and using our own experience, some guidelines for conducting studies on seed germination ecology have been developed. First, seed dormancy will be defined, and then we will consider things that need to be kept in mind when laboratory experiments are conducted to determine the dormancy-breaking and germination requirements of seeds. Finally, we will discuss how laboratory studies can be done to supplement those done in the field, slathouse, nonheated greenhouse or transplant garden.

Definition of Seed Dormancy

To many people, seed dormancy simply means that a seed has not germinated, but we will soon see that this definition is inadequate. Unfavorable environmental conditions are one reason for lack of seed germination. That is, seeds could be in a paper bag on the laboratory shelf (i.e., lack of water), buried in mud at the bottom of a lake (i.e., insufficient oxygen and/or light) or exposed to temperatures that are above or below those suitable for plant growth. These obviously unfavorable conditions for germination are examples of how the environment rather than some factor associated with the seed per se prevents germination.

A second reason why seeds may not germinate is that some property of the seed (or dispersal unit) prevents it. Thus, the lack of germination is a seed rather than an environmental problem (Eira and Caldas, 2000). Dormancy which results from some characteristic of the seed is called organic (versus imposed) dormancy (Nikolaeva, 1969, 1977), and this kind of dormancy usually is of most interest to seed biologists and ecologists. In fact, throughout this book, we will be concerned with organic seed dormancy.

There are many definitions of seed dormancy, but often the term means the failure of seeds to germinate although environmental conditions including water, temperature, light and gases are favorable for germination (Koornneef and Karssen, 1994; Vleeshouwers et al., 1995; Bewley, 1997a; Eira and Caldas, 2000; Geneve, 2005). The ecological consequence of seed dormancy is that germination is prevented (although conditions are favorable for germination) at a time of the year when the environment does not remain favorable long enough for seedlings to become established and thus survive (Vleeshouwers et al., 1995; Eira and Caldas, 2000; Cmelik and Perica, 2007). Therefore, seed dormancy plays an important role in regulating the timing of germination so that environmental conditions are favorable for seedling survival and eventually maturation of the plant (Geneve, 2003; Finch-Savage and Leubner-Metzger, 2006). Not surprisingly, then, an “interesting feature of seed dormancy is that plants have evolved different mechanisms for inducing dormancy” (Penfield and King, 2009), and also, as we will explore in this book, the different ways of breaking dormancy that have evolved in plants. It should be noted, however, that dormancy also is found in Monera, Protista, fungi and animals (see Footitt and Cohn, 2001).

As will become apparent in subsequent chapters of this book, seeds are not only dormant during the season(s) unfavorable for seedling survival, but the conditions of this season may be required to break dormancy. Thus, the dormant-seed stage in the life cycle of many plant species should be visualized as a period of time when things are happening in the seed, e.g., physiological/biochemical and morphological/anatomical changes, growth of the embryo, mobilization of food reserves and certainly activation and deactivation of genes. Since dormancy break occurs during the unfavorable season for seedling growth (or immediately after the environment becomes favorable for seedling growth in the case of many seeds with water-impermeable seed/fruit coats), seeds potentially can germinate at the beginning of the favorable period. Thus, seedlings can grow during all of the favorable season.

Also discussed in this book is the fact that, after dormancy has been broken, another set of environmental conditions may be required to stimulate germination. In other words, for seeds of many, but not all species, different conditions are required for dormancy break and germination. As explained by Vleeshouwers et al. (1995), dormancy break and germination are two different processes and may have different requirements. Thus, the seed biologist must pay attention to both dormancy breaking and germination requirements to successfully germinate the seeds of many species (Vleeshouwers et al., 1995; Srivastava, 2001; Thompson and Ooi, 2010).

However, as we will discuss in Chapter 8, molecular biologists have determined that many genes are activated and/or suppressed during dormancy break and germination, e.g., in buried seeds of Arabidopsis thaliana exposed to temperate-zone annual seasonal temperature (and moisture) conditions (Footitt et al., 2011). After a dormancy-breaking period, the last thing required for seeds of A. thaliana to germinate, assuming that moisture and temperatures are within the range of those favorable for germination, is exposure to light. With regard to exposure to light (and other factors such as ethylene, nitrate and smoke), there is a real difference of opinion about what these factors are doing. Are these factors, and in particular light, breaking the last part of dormancy or promoting the first part of germination? To molecular biologists, light is breaking the last part of dormancy, i.e., suppresses the last genes inhibiting germination (e.g., Finch-Savage and Leubner-Metzger, 2006; Footitt et al., 2011; Finch-Savage and Footitt, 2012). To seed ecologists, light is one of the environmental signals that can promote germination after dormancy is broken (e.g., Thompson and Ooi, 2010).

Both groups of seed scientists recognize, however, that factors such as light usually do not promote radicle emergence until after seeds have been given treatments such as warm and/or cold stratification. That is, the process of going from a freshly matured nongerminating seed to one that has germinated is a series of steps, and it is the last step (just prior to radicle emergence) that has caused differences of opinion. Seed ecologists think about seeds in this “last step” as being nondormant but requiring a signal about the favorability of the environment such as presence of light (or various other factors) before the radicle will emerge. Seed molecular biologists know that seeds have to reach this “last step” before some environmental factor such as light will promote germination; however, as yet they have not given a name to this “last step.” Although Footitt et al. (2011) referred to dormancy in seeds of A. thaliana as being “deep” and “shallow,” this is not a good solution. The terms nondeep, intermediate and deep have already been used (Nikolaeva,...