Module 3: Harvesting and processing

Introduction

Welcome to the third module of this online course. In module 2, you learned about some key factors influencing longevity and viability of seeds while in genebank storage. However, the influences on a seed’s ability to germinate do not start at the moment it goes into storage.

In this module, we will take you on a journey through the early stages of a seed’s life. You will discover natural processes, such as seed development, climate, irrigation, and the plant’s own seed dispersal mechanisms. All of these can affect the longevity of seeds in your care, and you can exploit or mitigate some of these natural processes. By knowing how much to irrigate, when to harvest, and how to manage the processing that goes on in the lab between harvesting and storage, you can make a difference to your genebank’s success in conserving biodiversity.

By the end of this module, you will be in a good position to apply what you have learned to your own work. We will give you the opportunity to read about the science of this in more depth in our ‘Useful publications’ section.

Overview of genebank processes

Figure 1: key genebank processes

Figure 1 (above) highlights the stages of a genebank’s operations you will consider in module 3. Here, you will shift your focus to the science underlying regeneration, characterization and processing. Parts of this module are also relevant to the collection of brand new accessions, being placed in a genebank for the first time.

During seed regeneration (which in some contexts is referred to as ‘rejuvenation’ or ‘multiplication’), there is a period when the crop is growing in the field. On the parent plant, seeds develop from tiny globules into mature seeds, capable of germinating into new plants. They must be harvested at exactly the right level of maturity. After harvesting comes threshing, cleaning and processing after the material enters the genebank. All of these can affect a seed’s longevity when it enters storage.

In this module, we will take a deep dive into the biological mechanisms that occur while seeds mature, and that continue during the human-imposed processes that are conducted in a genebank, before the seeds go into storage.

Some definitions

Fertilization, which happens after pollination, is the fusion of the male reproductive cell (pollen) with the female reproductive cell (ovum) to form a zygote.

The male and female reproductive cells are usually haploid – they each have one copy of each chromosome. Once they combine, the resulting zygote is usually diploid - it has two copies of each chromosome.

The process of seed development is ultimately regulated by genetic controls, which can switch developmental processes on and off.

As a seed matures, seed filling is the process of transferring carbohydrates, lipids, proteins, and other components into the developing seed.

The stage of mass maturity is when maximum seed dry matter has accumulated, at the end of the seed filling phase.

During seed dispersal, seeds are spread or transported away from the parent plant, giving them the chance to germinate and grow into new plants.

The embryo of a seed is the tiny, furled up living baby plant, which in future may become a full adult plant.

Attached to the embryo are the cotyledons, stores of nutrient that protect and nourish the growing embryo, and become the new plant’s first leaves.

Seeds of some species, such as cereals, have most of their food storage reserves in tissue called the endosperm. Other species, such as legumes, have food storage in their cotyledons, while yet other species have a combination of both these food storage systems.

Development of a soybean seed

Seeds are essential to plants’ survival: they protect and nourish the developing embryo that will eventually become the next-generation plant. In legumes, the seed has two cotyledons, which become more defined as the seed develops.

Figure 2 (below) shows you the developmental process of soybean, a typical legume. You might like to view and download this image at high quality. To do this, first click the 'Maximise' link below the image: to download, right click or control click once the image has opened.

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Figure 2: soybean seed development

Our story starts with a flower on the parent soybean plant, dusted with pollen grains. Making its way to the female reproductive cells or ‘gametes’ inside the flower, a tube develops from each pollen grain, allowing male gametes to travel to the base of the flower. This is where fertilization occurs.

After fertilization, the first diploid cells to appear are a tiny globular mass of dividing cells, which develop into a heart-shaped embryo. It takes some days for the cotyledons to become distinguishable, but once they do, they grow larger and larger, eventually becoming bigger than the embryo. Inside the cotyledons, proteins, carbohydrates and lipids are being built up in a process called ‘seed filling’. Once the seed has matured, water is expelled. As the seed continues to lose water, it shrinks a little, so that by the time it is ready for harvest, the cotyledons are more compact than they had been earlier in the process.

In the rest of this section, you will discover more about what happens during seed development, and why this matters so much to genebanks.

Pre-storage factors

Everything that happens to seeds before they go into storage can affect their longevity. One reason for seeds failing to germinate when they come out of storage could be that they were immature when they first arrived at the genebank. Figure 3 (below) shows the changes that normally take place when seeds mature on the parent plant, while they are still in the field.

Figure 3: seed development

A very young seed is effectively a tiny, globular mass of cells in a bag of water. Figure 3 shows the changes that follow, as it develops into a mature seed. The brown curve shows how dry weight increases, as the seed fills with carbohydrates, lipids, proteins and so on. The blue curve shows how moisture content decreases over the same period.

The large molecules build up until the point we call ‘mass maturity’, where the brown curve flattens. At this point, there ceases to be a vascular connection between the parent plant and the developing seed. No more large molecules can enter the seed. Whereas the blue curve shows how moisture content continues to decline steeply, even after mass maturity.

It should come as no great surprise that if you harvest immature seeds, you’ll have a hard job convincing them to germinate. However, as a genebank scientist, the germination of freshly harvested seeds is not the only thing you need to think about. You also need to be sure your seeds will germinate after being dried and stored for many years at low temperature!

Figure 4: effect of seed development on germination

Figure 4 (above) superimposes the results of germination tests over the development graph shown in Figure 3. The green curve shows how germination improves for fresh seeds the longer after flowering they are harvested. These fresh seeds are already close to their maximum capacity for germination when they reach mass maturity.

By contrast, the orange curve shows what happens when the seeds are air-dried before attempting to germinate them. For these seeds, harvesting at mass maturity does not yield optimal germination rates – their longevity will improve if you wait and harvest later than mass maturity.

The purple plot shows longevity when seeds are both dried and kept in storage: these seeds are even less likely to germinate if harvested at mass maturity. They only come into their own very late in the maturing process, at around the time when the wild plant naturally disperses its seeds. This is less common in domesticated crops, where shattering resistance is a common trait, meaning that the seeds remain on the plant until they are harvested.

Activity 1

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Seed dispersal mechanisms

In the wild, plants have developed various seed dispersal strategies to ensure the survival of their seeds. They come into play after seeds have matured, and can give the seeds a competitive advantage. In such plants, seed longevity continues to improve as the seeds dry down to equilibrium with ambient conditions. Viability does not peak until very close to seed dispersal. Let’s look at a couple of examples.

Case study: bursting with viability

In wild relatives of crop species such as peas, the natural seed dispersal mechanism is what biologists call ‘explosive’. After maturing, the pod splits open suddenly so that the seeds burst out, and are propelled far enough to land some distance away from the parent plant. This helps young seedlings to survive in situ, by reducing competition with the parent plant.

Crop species descended from plants with this dispersal mechanism retain the tendency for longevity to improve until very late in seed development. While the weight of the seeds stabilizes, they are still drying out in preparation for dispersal. This lower moisture content has a beneficial effect on longevity in storage, which keeps improving until very close to the time of dispersal. This is why, in many crops, it is a good idea to harvest as close to the point when the plant naturally disperses its seeds as possible.

Explosiveness can be a challenge in a wide variety of crops. For instance, some wild varieties of rice are highly shattering. They may release seeds before the seeds are fully mature, while their moisture content is still high. By harvesting too soon, the seeds will be immature, and not ready for storage. But if you wait too long before harvesting, the seeds you have carefully cultivated could disperse and be lost to your genebank. Gathering seeds from the ground is not good practice. CGIAR genebanks have developed a range of strategies to overcome these challenges, which you will learn about next.

Deciding when to harvest

CGIAR scientists at the International Rice Research Institute (IRRI) conducted a series of experiments to investigate the impact of seed maturity at harvest on longevity in storage. In one set of experiments, twenty Oryza sativa accessions were harvested at 24, 31, 38 and 45 days after flowering. They were then dried in the genebank’s drying room, grown-out, and their longevity (p₅₀) calculated. In many cases, the greatest longevity was in seeds that had been harvested at 45 days, although some peaked at 38 days and declined after that. Figure 5 shows what a difference a delay can make:

Figure 5: effect of harvest time on longevity

By allowing seeds to mature for as long as possible on the parent plant, it is possible to improve their longevity in storage. But the seed’s maturation process is not the only pre-storage factor that can have an effect on longevity. The weather can make a difference too.

Conditions in the field

Imagine you had one lot of a rice cultivar growing in a cool climate (20-28°C), and another lot of the same cultivar growing in a warmer climate (24-32°C). What difference would the different climates and temperatures make to the seeds’ longevity in storage? A research team from the University of Reading did some experiments to find out.

Figure 6 (below) shows the behavior of the seeds of Japonica rice, a type of rice normally grown in temperate environments. The square readings are for seeds grown in a warmer climate, the round ones are for seeds grown in cooler conditions. You can see that at both temperatures, the moisture content drops and the mass initially increases, then stabilizes at mass maturity, as you’d expect from the generalized patterns you saw in Figure 3 on the page entitled ‘Pre-storage factors’.

Figure 6: effect of temperature during development

However, something else must be going on: the researchers discovered differences in longevity associated with the two growing conditions after those seeds have been harvested, dried and stored. The seeds grown in the cooler climate achieved higher longevity than those grown under warmer conditions. In other words, for Japonica rice, the quality of seeds grown under certain climatic conditions is better than the quality of seeds grown under other conditions.

If temperature during development can affect how seeds behave in storage, you might wonder if the presence or absence of rain (or indeed irrigation) could be another factor. Another team from Reading set out to investigate, this time using Brassica seeds with different irrigation regimes, to simulate the effect of drought.

Figure 7: effect of irrigation during seed development

Figure 7 (above) shows three samples of Brassica. The red triangles at the base show the time of mass maturity. The graph with round points shows what happens when the parent plant was continuously irrigated. For the graph with triangular points, irrigation was stopped 16 days after pollination, and for the graph with square points, irrigation was stopped 24 days after pollination, imitating what might happen during a severe drought.

Activity 2

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Genebankers’ perspectives

Let’s revisit the two genebanks, IITA and IRRI. The next documentary explores what goes on while seeds develop in the field, and how seed scientists and genebank managers decide on the best time to harvest. The video tells the story of a seed’s early life, from flowering to threshing. By tracking this development carefully, the scientists can make a difference to the physiological quality of seeds when they enter the genebank.

As you watch, think about how the genebanks harness seeds’ developmental processes to maximize their longevity in storage.

Download this video clip.Video player: Video 1: from field to genebank

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Transcript: Video 1: from field to genebank

Narration:

What happens to crops growing in the field will affect the longevity of the seeds once they enter the genebank. Let’s explore the seed’s life story a bit more. It starts with a flower, and a chance of fertilization.

Sola Owoborode:

This is a soybean plant at flowering stage. The color ranges from purple to white color, and soybeans are self-fertilizing crops, there is no cross-pollination.

Narration:

After fertilization, a series of transformations take place.

Fiona Hay:

So when we talk about seed development, we talk about three phases. So after fertilization, you get the first phase, which is where there’s cell differentiation, and embryogenesis. So the structures of the seed are forming.

Olaniyi Oyotomi:

The initial part of the seed development is that the embryo is formed, and then it’s attached to the walls of the uterus, as it may be for the type of crop or species.

Fiona Hay:

In the second stage we get cell expansion and dry matter accumulation. So dry matter is moving from the mother plant into the developing seeds. That’s when we see big increases in the dry weight of the seed.

Olaniyi Oyotomi:

Full mass maturity of the seeds signifies the end of the development of the seeds.

Narration:

At ‘mass maturity’, the dry weight of the seed reaches a plateau. It no longer needs nutrients from the parent plant. The connection between the seed and its parent plant becomes blocked.

Fiona Hay:

Because the vascular connection is now stopped, it’s blocked. What we also see then is that's when the seeds start to dry down to ambient conditions, so they start to equilibrate with the ambient humidity and the ambient temperature. And we call that last phase, at this point we call this the ‘maturation-drying phase’. And this maturation-drying phase is very important for the physiological quality of the seeds. So it's very important that we try and let the seeds on the plants go through this maturation-drying phase to get the maximum quality longevity of the seeds for the harvest.

Narration:

Changes taking place inside the seeds are reflected in changes to their external appearance.

Sola Owoborode:

This is a young soybean plant, with immature pods, they are still very fresh with immature pods and not yet ready for harvest. While over here, we have a matured soybean plant, with mature dry pods and the detachment of the leaves from the mother plant.

Olaniyi Oyotomi:

And the seeds start to signify with various signs of maturity like, the seed coat is formed and it becomes thickened. The color of the seed cover begins to change color to brown in cowpea, and then that signifies the end of maturity of the seeds, and at that time the seeds are ready for harvest.

Narration:

The signs to look out for depend on the species. This is rice.

Andres Godwin Sajise:

The best criteria to check when the materials are ready to harvest is your data on flowering. This will tell you what time the seeds have fertilized, and the process of maturity will start around thirty or even more number of days. Second is the ripeness, or the color of the seeds, should be uniform from the tip to the panicle base.

Narration:

Once the seeds have reached this level of maturity, the harvesters can move in. In legumes, too, there are physical changes to show seeds are ready for harvest.

Oladepo Adebowale:

These are cowpea plants. We have pods that are at pod filling stage, pods that are mature but not ready for harvesting, and we have mature pods, gone pale brown; and when you press, you’re going to hear it crack.

Narration:

These maize plants give different clues that they’re ready to harvest.

Olubiyi Adeyemi:

One of the determinants that we look at before harvesting maize, is we look at the color of the plant itself. Then we also look at the color of the cob, when the cob is brown. Then we also look out for when the silk detaches from the cob, which means the seeds must have dried.

Narration:

IRRI has twenty-three species of wild rice in its collection. These bring particular challenges for harvesting. The net bags are added very early, to get over the plants’ natural behavior.

Arma Kristal Pabro:

We bag the panicles after flowering because most of the wild rice species, they have highly shattering panicles. So we do the harvesting at around thirty days after bagging. We collect the seeds, using the net bags, because at maturity, the seeds would be falling off the panicles.

Narration:

These are cowpeas from IITA. They must be threshed soon after harvest while they’re still fresh. The seed covers are blown away, leaving behind the cleaned seeds, ready for genebank storage. At IRRI, the newly harvested rice must be threshed in ways that don’t damage the precious seeds.

Andres Godwin Sajise:

We have two ways of threshing the samples after harvest. First is the machine threshing for samples that are easy or intermediate to thresh. But we do hand threshing for the samples that have difficult thresh-ability. In this way we are getting good seed recovery and high quality seeds for our conservation.

End transcript: Video 1: from field to genebank

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Video 1: from field to genebank

Conditions in the genebank

Until a consignment of seeds is packed for storage in a genebank’s active and base collections (or Svalbard, or another safety duplicate location), time is of the essence. Delays between the seeds’ arrival at the genebank and their going into storage can make a big difference to longevity and viability. So can differences in humidity and temperature while they wait to be processed.

Although conditions in the field and at harvest are not always under the control of a genebank, there is tantalizing evidence that, by making small changes to the conditions and timescale before they are processed, it may be possible for a genebank to compensate for less-than ideal growing or harvesting conditions. Let’s start with humidity and temperature.

Humidity and temperature

You have already learned that seeds arriving at a genebank may be immature, or may contain more moisture than is optimal for longevity in storage. This may happen if weather conditions in the field are poor, or if the seed’s natural dispersal mechanisms make it expedient to harvest before the seeds shatter and become spread over the ground, therefore lost to science. In this situation, once the immature seeds arrive at the genebank, they can mature under safe conditions in the drying rooms.

Many genebanks dry seeds according to the Genebank Standards: at temperatures between 10 and 25°C, and humidity between 10 and 15%. This contrasts with traditional practices of farmers in countries with hot, dry climates, who leave their crops out to dry in the sun, where temperatures can be around 40°C. The traditional method is low cost and tried and tested, although from a genebank’s point of view, it has limitations: prolonged exposure to the sun can damage seeds and cause a reduction in viability, and drying outside can be impractical during the rainy season.

So which of these two approaches would you expect to be more effective – drying at high temperature, like traditional farmers, or at temperatures that are prescribed in the Genebank Standards? Scientists at IRRI set out to find out.

In the case of rice, it seems the prescription in the Genebank Standards may not be optimal for producing seeds with high viability and longevity. The IRRI researchers tested the viability of samples of Oryza sativa that had been dried according to the Genebank Standards, and compared their survival with the survival of other samples, initially dried for two days in a batch-dryer at 40–45°C, before being transferred to the drying room for the rest of the drying period. Intriguingly, initial drying at high temperatures, perhaps more akin to ideal conditions when the seeds are maturing in the field, or traditional roadside drying, had a lot to offer.

Figure 8 (below) shows the improvements in longevity that resulted from initial drying at a higher temperature. There were no negative effects, and in many cases, a significant increase in longevity (p₅₀).

Figure 8: improvements in longevity due to initial drying at high temperature

Activity 3

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Dried to perfection

The benefit from initial drying at high temperature is not unique to cultivated varieties of Oryza. Figure 9 (below) shows the results for two wild relatives. The two wild species remained viable for longest when they had initially been dried at 45°C (the blue curve) before a second stage of drying at temperatures of around 15°C. They remained viable for less time when they had only been dried at 15°C (the black curve) throughout the drying process. Even seeds that were initially dried at a sweltering 60°C (the pink curve) performed slightly better in germination tests than the seeds grown according to Genebank Standards (the black curve).

Figure 9: benefits of drying at a higher temperature

These findings suggest that for some species, longevity in storage can be improved by adapting the conditions under which the seeds are dried. These results were compelling enough to convince IRRI to change their procedures. Instead of drying rice according to the Genebank Standards, IRRI now carry out two-stage drying, where seeds are initially dried at 30°C for a couple of days, then finished under conditions the Genebank Standards recommend: 15% humidity and 15°C. This has allowed IRRI to make demonstrable improvements to seed viability.

There is evidence that other species may benefit from drying at different temperatures too. Scientists at the Institute for Tropical Agriculture (IITA) in Nigeria found similar improvements in soya beans and Bambara groundnuts. Although there is a lot we still don’t know, it is likely that if seeds have had to be harvested early, for instance due to poor weather, drying them under conditions that simulate natural conditions in the field during a good year can compensate for losses in viability as a result of being harvested prematurely.

Time between harvest and storage

So far, you have learned the danger of waiting too long to harvest - we don’t want to allow seeds to disperse and be lost. You have learned that seeds should not go into storage when they are immature, but it is also important that they also should not go into storage when they are too old. Watch the animation, which is based on data about the fate of seed lots regenerated by a genebank in 1990. As you watch, think about what was happening to the longevity of these seeds as time went by.

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Transcript: Video 2: processing time and viability

In 1990, a fresh accession of seeds was checked in to a genebank.

Some of these seeds were processed in less than 6 months.

Some were processed in less than a year.

Others were left between one and two years before they were dried, cleaned, packed and placed into storage.

What difference did this delay make to their long-term viability once they had gone into storage?

Let’s start with the seeds that were processed within six months.

After 3 years, the results of germination tests ranged from fifty to 100%. The lower results are probably because some of the seeds were dormant.

After 12 years, this situation has improved: most of the dormancy has worn off.

After 16 years, the results are satisfactory - about 85 to 98%.

Even after 20 years, viability has not declined a lot.

What about the seed lots which went into storage after 6 months to one year?

Once again, the initial results could perhaps be explained by dormancy.

But by 10 years, dormancy can’t explain why the lowest viability has dropped to a measly 30%.

And after 16 years, when you’d expect dormancy to have passed, there is still a wide spread of viability.

At 20 years, although some seeds are highly viable, the results are more patchy than for the seeds that were fresh when they first went into storage.

Much the same is apparent for seeds that waited between one and two years before they went into storage.

There’s a noticeable tendency for these seeds to perform less consistently in germination tests compared to the viabilities of the seed lots that had been placed into storage in less than 6 months.

These results suggest that when freshly harvested seeds arrive at your genebank, there’s no time to waste.

End transcript: Video 2: processing time and viability

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Video 2: processing time and viability

My genebank and me

For your next blog entry, think about what happens in your genebank between harvesting and storage.

Are there delays? If so, can you think of any ways you could reduce those delays, and therefore safeguard viability? What could you do in your personal practice, to make sure your newly regenerated seeds remain as viable as possible once they are in storage? If you would like to share your ideas with other participants and the moderators of the course, write a post in the Forum.

Follow this link to the Blog

Beating the clock

This module should have convinced you how important it is to protect seeds while they are being processed between harvest and storage. Genebanks can control the temperature and humidity of the areas where seeds are processed: this does a lot to reduce aging reactions. But there are a lot of processes to get through, so perhaps the biggest challenge is controlling the amount of time it takes to process the seeds.

Watch this video, which shows the number of processes a genebank must carry out before seeds are ready for storage. Scientists at IRRI and IITA demonstrate these processes, while Fiona Hay and Olaniyi Oyatomi discuss some steps genebanks can take to ensure all processes are carried out efficiently. As you watch, think about how the scientists ensure the seeds’ passage through the genebank is as swift as it can be.

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Transcript: Video 3: beating the clock

Narration:

A new batch of freshly threshed rice. These seeds are as viable as they ever will be. Now they must be processed before they lose their ability to survive years in storage.

Fiona Hay:

When we harvest the seeds, we first take them off the plants, what we can say is that the, the clock is ticking. And if we keep the seeds too long in high humidity or high temperature, then that clock is going round faster and the seeds will be aging already.

Narration:

These maize seeds are starting their journey to be stored in the IITA genebank.

Olaniyi Oyotomi:

The life in the seeds are embryos, and then these embryos also are living things. So each time and all the time when seed are being processed for storage, the life of the embryo is the target to be protected.

Narration:

That protection starts in the drying room, so it’s important to get seeds in here as soon as possible. Inside the seeds, the biochemical reactions that promote ageing slow down, as moisture diffuses out into the cool, dry air. At IRRI, rice goes through two stages of drying. Three days at a higher temperature, then the process continuing in a cooler drying room.

Marionette Alana:

Every three days, we check again for its moisture. We want to if know once the seeds already reach less than 30% ERH, then that’s the time when we’re going to transfer the seeds in the net bags into paper bags.

Narration:

These drawers contain the original samples of every type of seed in the genebank’s collection. It’s important that new accessions match the old ones exactly.

Marionette Alana:

We need to authenticate first the harvest, because we have to know if the harvest they give us is the correct one. We compare it using our reference material, on what we call the seed file.

Narration:

These checks are essential for seed quality - whatever the crop.

Kofoworola Olatunbosun:

It is important to do the conformity check with a reference sample, to ensure we have seed purity in our genebank conservation, and to eliminate the debris, the broken seed and the off-type.

Narration:

Seeds that have come straight from the field may have been contaminated during planting, harvesting or threshing.

Kofoworola Olatunbosun:

you can see, the seed shape and the eye pattern are the same with this, while these are quite different, so that is why we do the conformity and the cleaning, to eliminate the bad seed from the good seed.

Narration:

Other seeds may be rejected because they are immature. Only the ripest and most well formed seeds will remain viable during storage. At IRRI, the unfilled seeds and chaff are literally blown away. This is cultivated rice. The genebank’s collections of wild rice require special treatment.

Arma Kristal Pabro:

Unlike the cultivated rice species, where we use the seed blower, for the wild rice species we use manual seed cleaning, where we take out the plant debris and unfilled grains in the harvest.

Narration:

Cleaning rice requires an eye for detail. For instance, the green colour of this seed tells the technician it is immature, and must be discarded. This makes manual cleaning a time-consuming process. But for some types of rice, technology can help. This is IRRI’s automatic seed-sorting device. First, the machine is trained on hand-selected rice of the ideal color, size and shape for the particular accession. Then, when a new seed lot is introduced, the machine will identify the very best seeds. This automated process reduces the time it takes to get through the sorting. But the seeds are still not ready for storage.

Marionette Alana:

After we have finished sorting the seeds we’re now going to give it to our quality personnel. She’s going to check again those selected materials and then prepare the seeds for viability testing and also the materials that will be sent to the seed health unit.

Narration:

At IITA, one of the seed health procedures is fumigation. It’s a chemical pest control treatment, to ensure the seeds are free of insects by the time they go into storage. If the seeds pass the phytosanitary checks, they’re ready to be counted.

Opeyemi Oyatomi:

What I am doing right now is to know the number of seeds. The first thing I do is to count one hundred seeds, to know the hundred seeds’ weight. Then I do the total weight of the seed. The system will do the calculation, and determine the number of seed we are having for conservation.

Narration:

At last, the seeds are ready for packing. They are placed in foil packets, and the air is removed to avoid oxidation. Cool dry and sealed, they’re now prepared for their long state of suspended animation. Throughout all these processes, the precious embryos have been kept alive.

Fiona Hay:

We always say that it's very important to get the seeds into a controlled environment as soon as possible. And of course, the best-controlled environment is the genebank storage rooms. The longer it takes before getting the seeds into genebank storage, the more the seeds will be aging and losing viability, and then their longevity will be compromised when they are actually in the genebank as well.

End transcript: Video 3: beating the clock

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Video 3: beating the clock

Genetic basis of longevity

Within any species, there is variation in seed longevity, even within closely-related germplasm. This can provide vital information for seed genebanks about the molecular basis of this trait. Consider the development of a soybean seed, which you learned about at the beginning of this module. Every protein stored in the developing cotyledons is encoded by a gene; every process is switched on and off by genetic mechanisms, such as DNA methylation. These genetic controls continue as long as the seed survives. By deepening our understanding of these processes, there is potential to make further improvements in seed longevity.

A team at IRRI set out to determine the relative seed longevity of various accessions of Indica rice (Oryza sativa Indica), harvested at intervals between four and five weeks after flowering. The accessions were equilibrated at 10.9% relative humidity and 45°C. Samples were taken at regular intervals, and germination tests were carried out, to reveal the loss of viability during experimental storage. As you might expect, longevity tended to be higher in samples with lower harvest moisture content, and less in samples with higher harvest moisture content.

The study also gave glimpses of a possible genetic basis for longevity. In a genome-wide analysis, eight major loci were identified as potentially having some control over seed longevity. Located on chromosomes 1, 3, 4, 9 and 11, they were shown to enhance p₅₀ by ratios of 0.22–1.86. Comparing these findings with existing knowledge about the rice genome suggested that the mechanisms conferring high seed longevity might be related to DNA repair and transcription, sugar metabolism, reactive oxygen species scavenging and embryonic and/or root development.

Investigating variations in longevity

Genebanks rely on the ability of seeds to go into an induced latent state when they are dried and cooled. Therefore in genebank storage, seeds must be stored at low temperatures and low moisture content to prevent them from aging.

By contrast, scientists investigating seed longevity deliberately place seeds under conditions that will accelerate the aging process. This allows the scientists to explore processes that normally take decades or even centuries, within a realistic timeframe for a scientific experiment. They subject seeds to conditions of high temperature and humidity, which are described variously as ‘experimental storage’ or ‘accelerated aging’.

Figure 10 (below) shows results from the IRRI experiment described on the previous page. The box covers the 25th to 75th percentiles. The green line across the box is the median; the red line is the mean longevity. The whiskers show the 5th and 95th percentile.

Figure 10: variation in longevity in Indica rice

These data show that there is a lot of variation in longevity, even within one subspecies, Oryza sativa Indica. The mean and median p₅₀ are less than half the maximum longevity shown by a few exceptional individuals, and the box for the 25th to 75th percentiles is somewhat skewed towards the minimum longevity, suggesting there might be scope for improvement.

By working with just one variant of rice, there should be fewer alleles to consider, rather than having to worry about the entire genome of the whole species. However, the variation even within this single variety group of rice underlines how much there is to investigate about the genetic basis of seed longevity.

The Seed Information Database

The photograph below places 20 varieties of rice in rows reflecting the landscapes where they are grown - from lowland paddies to sloping uplands. It shows some of the diversity within this crop that is visible just by looking at the seeds. In fact, Asian cultivated rice is divided into different ‘variety groups’ based on their genetic relatedness. Indica rice (at the top) is one of those variety groups, and the previous study focused on accessions from within this variety group. Let’s use the Seed Information Database to model the behaviour of Indica rice in storage.

The FAO’s Genebank Standards

The conditions for acquisition currently laid out in the FAO’s Genebank Standards recognize the need to avoid conservation of immature seeds, seeds that have been exposed to the elements for too long, and to ensure that seeds are handled carefully after collection and before they are transferred to controlled conditions.

• 4.1.2 Seed collecting should be made as close as possible to the time of maturation and prior to natural seed dispersal, avoiding potential genetic contamination, to ensure maximum seed quality.
• 4.1.3 To maximize seed quality, the period between seed collecting and transfer to a controlled drying environment should be within three to five days, or as short as possible, bearing in mind that seeds should not be exposed to high temperatures and intense light, and that some species may have immature seeds that require time after harvest to achieve embryo maturation.
• 4.1.5 The minimum number of plants from which seeds should be collected is between 30 to 60 plants, depending on the breeding system of the target species.

Discussion point: how well are genebanks doing?

This is the third discussion point of the course.

Participation in this and the other discussions within the course is essential for you to gain your end-of-module badge and completion certificate. The course organizers will visit the discussion spaces now and then, and provide feedback and guidance, so please do go back to the discussion space to check the organizers’ feedback.

You should use these discussions to:

• Explore common dilemmas in seed quality management
• Listen to the opinions of other learners and the facilitator
• Come to a collective view on how to proceed when faced with this type of scenario in your own work

Discussion space: driving progress

In module 3, you have learned about improvements some genebanks were able to make to the longevity of certain crop species, as a result of scientific investigations. It is important to emphasize that for most crop species, genebanks are doing a good-enough job in conserving diversity. What, then, are the challenges genebanks face in their attempts to improve their processes still further? How might they go about designing incremental improvements?

What measures can genebanks take to improve on their record?

driving progress 

 

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Key words and concepts

Throughout this course, we offer you quizzes to help you keep track of how much you have learned. We hope you find them enjoyable. The quizzes within modules are not graded, and your results will not be shared with colleagues, but engaging with these quizzes is crucial. It is important to check your answers and read the feedback we have written: this feedback is often the best way to learn. It will help you to develop your own understanding.

Working through the quizzes in every module will build your confidence until by the end of the course, you will be ready to tackle the end-of-course quiz. Unlike the quizzes within modules, this final quiz will count towards your badge and statement of participation.

Have a go at this quiz, testing your understanding of key ideas in module 3.

  Question 1

  Question 2

  Question 3

  Question 4

  Question 5

  Question 6

The Forum

Throughout this course, the Forum gives you the opportunity to meet course organizers, ask them questions, and discuss what you have learned so far. The Forum is also the place where you can see other students’ questions and comments, and the experts’ responses to them, so it is a good idea to get into the habit of going to the Forum regularly, using the menu on the left hand side.

A live event will accompany this and the next module. Look out for emails from the course organizers about when this will happen. It is important to make sure that you get through the online material for both modules (modules 3 and 4) before attending the live event.

Follow this link to the Forum

Summary

You have reached the end of module 3. In this module you have studied how seeds develop in the field, and how this development can be influenced by external factors such as temperature, climate, humidity and the passage of time.

Once seeds have entered a genebank, you have discovered how drying at different temperatures can help improve their longevity, and how important it is to make sure delays in processing are kept to a minimum. You have glimpsed some of the experiments that are starting to shed light on the genetic basis of seed longevity.

In the next online learning section, module 4, you will shift your attention to water activity in seeds, and discover what happens inside seeds when they are dried.

Useful publications

If you were interested in some of the issues in this module, you might like to download and read these articles, which we have selected for you.

Click here to move on to the next module