| | Smooth Cordgrass Seeds (Photo by John Wozniak) |
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Marc Alan Cohn and James H. Chappell
Establishing smooth cordgrass from seeds is a potentially economical and efficient means for coastal stabilization. While planting seeds is a straightforward procedure for most crops, smooth cordgrass seeds have two special traits that make this operation difficult. First, the seeds are dormant at harvest, similar to many weed seeds. Second, the seeds are recalcitrant, meaning that they die when dried. Research at the LSU AgCenter’s Seed Biology Laboratory is devoted to understanding these two traits and using this information to allow large-scale stand establishment of smooth cordgrass from seeds.
Seed Dormancy
Removing dormancy in smooth cordgrass is easy but time-consuming. Immediately after harvest, the seed heads are stored moist at 40-50 degrees F, or the seeds are submerged in cold water (40 degrees F) for two to six months, depending on the variety. After one of these treatments, seeds germinate readily. It is important to avoid freezing the seeds during moist chilling, however, as this will kill the seeds.
Attempts to shorten or eliminate the need for moist chilling through the use of traditional dormancy-breaking chemicals, such as nitrate and gibberellic acid, have not been successful. This is typical for many plants that require a moist-chilling treatment to break seed dormancy. In the future, new treatments that combine germination stimulants and inhibitors of dormancy-maintaining plant hormones need to be evaluated.
Seed Recalcitrance
In addition to smooth cordgrass, many economically important plants, such as coffee, tea, avocado, mangoes, rubber and many citrus species, have recalcitrant seeds. Recalcitrance not only hampers plant propagation from seeds but also makes the normal conservation of genetic resources more challenging, because unlike most other seeds, recalcitrant seeds cannot be stored dry. As a consequence, seed recalcitrance has been a subject of worldwide research for more than 50 years.
Despite these efforts, no satisfactory solution to the problem has been presented. Several causes are due to elements of experimental design or the properties of the seeds chosen for study. These difficulties exist because, in contrast to smooth cordgrass, most recalcitrant-seed species do not have dormant seeds. Some of these seeds (for example, several species of mangroves) actually germinate on the mother plant and are shed as young seedlings. Therefore, while strong seed dormancy is usually a trait to be avoided in crops, it enhances the ability to study the nature of seed recalcitrance. Furthermore, because most other species with recalcitrant seeds germinate quickly and the seed supply is limited to a short time just after harvest, the physiological characteristics of the seeds as they are dried have not been studied in detail at the molecular level. Since smooth cordgrass seeds are dormant, however, research on recalcitrance can be conducted throughout the year.
Smooth cordgrass seeds have unique characteristics that make them an attractive model in which to study seed recalcitrance. Along with being able to conduct research throughout the year due to dormancy, the storage of the moist seeds at low temperature reduces metabolism. In addition, the seeds are small and easy to handle for experimentation. Because other types of cordgrass, such as Spartina pectinata (prairie cordgrass), produce conventional seeds that remain alive after drying, drying experiments with both smooth cordgrass and prairie cordgrass allow comparison between the normal responses to drying and those that cause seed death.
Because death due to recalcitrance has been studied for some time, our first approach was to confirm if any existing hypotheses for seed death were correct. Past experiments in other laboratories have suggested recalcitrant seeds die because of biochemical or physical causes. We first chose to re-examine the biochemical processes. One of the dominant ideas in the scientific literature is that recalcitrant seed death is caused by lipid oxidation (a mild form of what occurs when butter turns rancid), which destroys the plant cell membranes during drying. Three independent physiological and biochemical procedures, however, show that lipid oxidation is not the cause of seed death. Previous conclusions to this effect were either due to artifacts of the biochemical methods used or because the results were obtained from using nondormant seeds.
Nonetheless, these experiments did show that the chemical antioxidants (substances such as vitamin C) in recalcitrant seeds were severely reduced during drying. This finding prompted the question: Are seed components other than membranes damaged? Preliminary results showed a massive increase in protein oxidation as recalcitrant seeds were dried. Identifying the individual proteins that are damaged in this manner may lead to a way to avoid seed recalcitrance. To this end, we are planning further collaborative research to identify these proteins.
Using the system, with both dormant and non-dormant seeds, and comparing conventional and recalcitrant seeds will permit substantial progress in understanding seed death because of drying. A positive outcome will allow Spartina alterniflora plants to be established from seeds on the large scale necessary for coastal restoration efforts, and the ability to conserve other economically important recalcitrant species as seeds will be possible.
Marc Alan Cohn, Professor, and James H. Chappell, Senior Graduate Research Assistant, Department of Plant Pathology & Crop Physiology, LSU AgCenter, Baton Rouge, La.
(This article was published in the spring 2007 issue of Louisiana Agriculture.) |