Long-Term Research Into Recalcitrant Seeds Begins to Bear Fruit

Saving seeds is common practice for farmers and has been since the dawn of agriculture approximately 10,000 years ago. According to USDA’s Agricultural Research Service, the best way to store seeds is to dry them to about 20 percent relative humidity and store in vapor proof containers in a cold place like a home freezer. When properly dried, most undamaged seeds can survive in conventional storage for about 100 years. The exception is a variety of crops with recalcitrant seeds, seeds that die when dried.

Citrus fruits, mangoes, lychee, coffee, tea, avocados and cacao (aka chocolate) possess recalcitrant seeds. Although it is possible to store some of these seeds moist for up to a year, the majority cannot be stored for more than a few weeks. Because these seeds require moisture in order to survive, the only way to preserve their germplasm long term is to grow the plants vegetatively in nurseries, an expensive proposition. For the home gardener, propagating some of them is less expensive. For example, one can suspend an avocado seed in a cup of water until roots and a stem sprout from the underside. However, you must begin this process almost immediately after removing the seed from the fruit. This is not a sustainable practice for commercial growers. Farmers might opt to purchase trees from nurseries, or graft their own.

Meanwhile, many important hardwoods with recalcitrant seeds experience overharvesting, particularly in Africa, so researchers are striving to learn how to transport recalcitrant seeds.

Louisiana State University AgCenter professor of seed biology and crop physiology, Marc Alan Cohn, leads the university’s research project to determine why most recalcitrant seeds die upon desiccation instead of going dormant. His goal is to resolve the commercial and conservation issues related to this type of seed. Some recalcitrant seeds, such as Cohn’s experimental model Spartina alterniflora (smooth cordgrass), will go dormant for a short period if they are moist or submerged in cold water. Because recalcitrant Spartina seeds have antimicrobial properties, and resist fungal growth, storing them moist isn’t a problem as it would be for most other seeds.

Research into recalcitrant seeds has been ongoing all over the world for over 40 years, but Cohn says most scientists are not focusing on this area of research. Researchers who attempt to work with or conserve these seeds know it’s difficult in the long term. So far, research into recalcitrant seed biology has produced a few ideas about why the seeds die:

1.Recalcitrant seeds lack proteins and sugars that can protect other dry seeds.

2.Recalcitrant seeds suffer physical damage more easily than orthodox seeds.

3.Recalcitrant seeds suffer oxidative stress during desiccation; in other words, the seed’s metabolism becomes abnormal during the drying process.

According to Cohn, he and his students at LSU AgCenter were the first to consider Spartina alterniflora as a model for studying recalcitrant seed death. The LSU researchers found that Spartina seeds have several important properties that make them good to work with:

1.It is atypical for recalcitrant seeds to have dormancy, but Spartina alterniflora seeds are dormant when they are ripe for harvest. This dormant period allows researchers to store the seeds and study them for up to a year. Most recalcitrant seeds must be worked on within weeks of harvest, so unless the researcher has a steady supply of them, they can’t do any more work.

2. Spartina alterniflora seeds are cold-tolerant. Most recalcitrant seeds are sensitive to cold, so if drying doesn’t kill them, cooling will. Cohn and his colleagues can work with smooth cordgrass seeds year-round, because they can store them in water in the refrigerator. However, after 12 months, the seeds actually germinate in the cold water and require replacing.

3. Spartina alterniflora seeds are small. The LSU researchers can do the variety of treatments necessary on these seeds and have reasonable sample sizes for the design of the study.

4.Cohn says the most important feature of smooth cordgrass seeds is that within the genus Spartina, there are about two dozen species, and some of these species have orthodox (normal) seeds that can survive drying.

The type of Spartina that Cohn works with is recalcitrant, but the existence of orthodox Spartina species provides Cohn and his team with a control for their experiments. The classic scientific method is to have a controlled experiment. The approach for most recalcitrant seed research is to take the seeds, dry them under controlled conditions and measure whatever the scientist intends to study about why they die, but for most recalcitrant seeds, there is no comparable control mechanism.

A researcher can take the orthodox Spartina, the species that does not die when dried, and dry that in the same way as one would dry the recalcitrant S. alterniflora seeds. By examining seeds that die when they dry and comparing them to seeds in the same genus that dry and survive, Cohn and doctoral student James Chappell (now at Harvard) were able to study the oxidative stress levels in each species to determine whether such stress creates a metabolism abnormal enough to kill the recalcitrant seeds. “What we found was evidence of this kind of stress metabolism in recalcitrant Spartina, but we also found the same thing in the orthodox Spartina,” he reports.

In ongoing research, Cohn and his current student, Yi Wang, have found about two dozen proteins present in orthodox Spartina that are missing from the recalcitrant seeds. It is possible that one or a group of these proteins is responsible for keeping the dried orthodox seeds alive. If that’s the case, Cohn and Wang will have some answers.

Will the answers to Cohn’s questions about Spartina help growers of fruits borne from recalcitrant seeds, such as avocado and mango? “Our hope always is that what we find will be generally applicable, not only for growers of specialty crops, but also for generation of hardwood species that are being depleted all along the continent of Africa. Many important crops possess recalcitrant seeds. Our hope is if what we’re seeing in Spartina is a general phenomenon associated with recalcitrant seeds, what we have will be applicable for the conservation of seed resources for all recalcitrant plants,” says Cohn.

Don’t look for the application any time soon, however. Although the research has been ongoing for a decade, it’s still in its infancy.

The author is a freelance writer based in Massachusetts and a monthly contributor to Growing.