onions

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Inside of a Seed: Two Monocots

“Seeds are travelers in space and time – small packages of DNA, protein, and starch that can move over long distances and remain viable for hundreds of years. These packages have everything they need not only to survive, but also to grow into a plant when they encounter the right conditions.”      — The Book of Seeds by Paul Smith

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As illustrated in last week’s post, the mature seeds of dicots – depending on the species – can be either with or without endosperm (a starchy food packet that feeds a growing seedling upon germination). Seeds without endosperm store these essential sugars in their cotyledons. Monocotyledons (or monocots, for short) are a group of flowering plants (i.e. angiosperms) whose seedlings are composed of a single cotyledon. With the exception of orchids, the seeds of monocots always contain endosperm.

The first of two examples of monocot seeds is the common onion (Allium cepa). The embryo in this seed sits curled up, surrounded by endosperm inside of a durable seed coat.

If you have ever sown onion seeds, you have watched as the single, grass-like cotyledon emerges from the soil. The seed coat often remains attached to the tip of the cotyledon like a little helmet as it stretches out towards the sky. Soon the first true leaf appears, pushing out from the base of the cotyledon. The source of this first leaf is the plumule hidden within the cotyledon.

The fruit of plants in the grass family – including cereal grains like wheat, oats, barley, rice, and corn – is called a caryopsis. In this type of fruit, the fruit wall (or pericarp) is fused to the seed coat, making the fruit indistinguishable from the seed. The embryos in these seeds are highly developed, with a few more discernible parts. A simplified diagram of a corn seed (Zea mays) is shown below. Each kernel of corn on a cob is a caryopsis. These relatively large seeds are great for demonstrating the anatomy of seeds in the grass family.

In these seeds there is an additional layer of endosperm called aleurone, which is rich in protein and composed of living cells. The cells of the adjacent endosperm are not alive and are composed of starch. The embryo consists of several parts, including the cotyledon (which, in the grass family, is also called a scuttelum), coleoptile, plumule, radicle, and coleorhiza. The coleoptile is a sheath that protects the emerging shoot as it pushes up through the soil. The plumule is the growing point for the first shoots and leaves, and the radicle is the beginning of the root system. The emerging root is protected by a root cap called a calyptra and a sheath called a coleorhiza.

Germination begins with the coleorhiza pushing through the pericarp. It is quickly followed by the radicle growing through the coleorhiza. As the embryo emerges, a signal is sent to the endosperm to start feeding the growing baby corn plant, giving it a head start until it can make its own food via photosynthesis.

corn seeds (Zea mays)

Up Next: We’ll take an inside look at the seeds of gymnosperms.

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White Rot and the Quarantine Zone, revisited

This is a revised version of a post I wrote in July 2013 during the inaugural year of Awkward Botany.

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It’s garlic planting season in the northern hemisphere. A few years ago, while helping out with the garlic harvest at a local farm, I had the chance to learn about some of the challenges involved in growing garlic in southern Idaho. Apart from the fact that it is a very labor intensive crop to grow, one of the major challenges stems from a disease called white rot – easily one of the worst diseases that garlic and onion growers face.

White rot is caused by a fungus (Sclerotium cepivorum), and it affects all plants in the Allium genus, including garlic, onions, chives, and ornamentals. The disease causes the leaves of alliums to die back, their bulbs to decay, and their roots to rot, ultimately turning the plants to mush. Sclerotia, the dormant stage of the fungus, are small (about the size of a poppy seed), black, spherical structures that can survive in soil for more than 20 years. They remain dormant until the exudates of allium plants awaken them, at which point they begin to grow, unleashing their destruction. Sclerotia can be moved around by farm equipment, floods, irrigation water, wind, and by attaching themselves to plant material. Once this fungus has established itself in a field, it is extremely difficult to eradicate, making the field virtually unfit for allium crops.

The threat of white rot and the monetary damage that it can cause led to the establishment of a quarantine zone in southern Idaho in order to protect its $55 million dollar a year onion industry. Due to the quarantine zone (which encompasses 21 counties), all garlic that is grown for seed within the zone must be inspected and certified. [“Seed” in this case refers to the garlic cloves themselves; onions, on the other hand, are grown from actual seeds and are not subject to the same protocol.] Any seed garlic that is brought into the zone must go through a rigorous testing process in order to ensure that it is free of the white rot pathogen before it can be planted. Garlic is a specific threat because the cloves can readily carry sclerotia, compared to onion seeds, which are not likely to harbor them.

This process significantly limits the amount and variety of garlic that can be grown in the quarantine zone. While the quarantine is essential for warding off the threat of this particular pathogen, it stifles the garlic growing industry and makes it difficult for new garlic growers to establish themselves.

Garlic farming is already incredibly demanding due to the amount of time and physical labor that goes into planting, harvesting, drying, grading, etc. The quarantine, while understandable, is an added challenge. Learn more about this issue by listening to this story on PRX.

See Also:

Garlic emerging in the spring.

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Year of Pollination: Pollinator Walk at Earthly Delights Farm

Last week I had the privilege of attending a pollinator walk with a local entomologist at Earthly Delights Farm, a small, urban farm in Boise, Idaho. The entomologist was Dr. Karen Strickler, an adjunct instructor at College of Western Idaho and the owner of Pollinator Paradise. A small group of us spent a couple of hours wandering through the farm looking for pollinators and discussing whatever pollinator or non-pollinator related topic that arose. Earthly Delights Farm, along with growing and selling produce using a subscription-based model, is a seed producing farm (and part of a larger seed growing operation called Snake River Seed Cooperative), so there were several crops flowering on the farm that would typically be removed at other farms before reaching that stage, such as lettuce and carrots. The farm also shares property with Draggin’ Wing High Desert Nursery, a nursery specializing in water efficient plants for the Intermountain West, which has a large demonstration area full of flowering plants. Thus, pollinators were present in abundance.

A series of isolation tents over various crops to help prevent cross pollination between varieties.

A series of isolation tents placed over various crops to help prevent cross pollination between varieties – an important component of seed saving.

While many groups of pollinators were discussed, including leafcutter bees, bumblebees, honeybees, sweat bees, hummingbirds, and beetles, much of our conversation and search was focused on syrphid flies. Flies are an often underappreciated and overlooked group of pollinators. While not all of the 120,000 species of flies in the world are pollinators, many of them are. The book Attracting Native Pollinators by the Xerces Society has this to say about flies: “With their reputation as generalist foragers, no nests to provision, and sometimes sparsely haired bodies, flies don’t get much credit as significant pollinators. Despite this reputation, they are often important pollinators in natural ecosystems for specific plants, and occasionally for human food plants.” They are especially important pollinators in the Arctic and in alpine regions, because unlike bees, they do not maintain nests, which means they use less energy and require less nectar, making them more fit for colder climates.

One food crop that flies are particularly efficient at pollinating is carrots. According the Xerces Society, carrot flowers are “not a favorite of managed honeybees.” Most flies do not have long tubular, sucking mouthparts, so they search for nectar in small, shallow flowers that appear in clusters, such as plants in the mint, carrot, and brassica families. Flower-visiting flies come in search of nectar and sometimes pollen for energy and reproduction. While acquiring these meals they can at times inadvertently collect pollen on their bodies and transfer it to adjacent flowers. They are generally not as efficient at moving pollen as other pollinators are, but they can get the job done.

Blister beetle on carrot flowers (a preferred food source of flies). Beetles can be important pollinators, even despite chewing on the flowers as they proceed.

Blister beetle on carrot flowers (a preferred food source of flies). Beetles can be effective pollinators as well, even despite chewing on the flowers as they proceed.

During the pollinator walk, we were specifically observing flies in the family Syrphidae, which are commonly known as flower flies, hoverflies, or syrphid flies. Many flies in this family mimic the coloring of bees and wasps, and thus are easily confused as such. Appearing as a bee or wasp is a form of protection from predators, who typically steer clear from these insects to avoid being stung. The larvae of syrphid flies often feed on insects, a trait that can be an added benefit for farmers and gardeners, particularly when their prey includes pest insects like aphids. Other families of flies that are important pollinators include Bombyliidae (bee flies), Acroceridae (small-headed flies), Muscidae (house flies), and Tachinidae (tachinid flies).

Common banded hoverfly (Syrphus ribesii) - one species of hundreds in the syrphid fly family, a common and diverse family of flower visiting flies (photo credit: www.eol.org)

Common banded hoverfly (Syrphus ribesii) – one species of thousands in the syrphid fly family, a common and diverse family of flower-visiting flies (photo credit: www.eol.org)

Because many species of flies visit flowers and because those flies commonly mimic the appearance of bees and wasps, it can be difficult to tell these insects apart. Observing the following features will help you determine what you are looking at.

  • Wings – flies have two; bees have four (look closely though because the forewings and hindwings of bees are attached with a series of hooks called hamuli making them appear as one)
  • Hairs – flies are generally less hairy than bees
  • Eyes – the eyes of flies are usually quite large and in the front of their heads; the eyes of bees are more towards the sides of their heads
  • Antennae – flies have shorter, stubbier antennae compared to bees; the antennae of flies also have bristles at the tips
  • Bees, unlike flies, have features on their legs and abdomens designed for collecting pollen; however, some flies have mimics of these features
Bumblebee on Echinacea sp.

Bumblebee visiting Echinacea sp.

Another interesting topic that Dr. Strickler addressed was the growing popularity of insect hotels – structures big and small that are fashioned out of a variety of natural materials and intended to house a variety of insects including pollinators. There is a concern that many insect hotels, while functioning nicely as a piece of garden artwork, often offer little in the way of habitat for beneficial insects and instead house pest insects such as earwigs. Also, insect hotels that are inhabited by bees and other pollinators may actually become breeding grounds for pests and diseases that harm these insects. It is advised that these houses be cleaned or replaced regularly to avoid the build up of such issues. Learn more about the proper construction and maintenance of insect hotels in this article from Pacific Horticulture.

A row of onions setting seed at Earthly Delights Farm. Onions are another crop that is commonly pollinated by flies.

A row of onions setting seed at Earthly Delights Farm. Onions are another crop that is commonly pollinated by flies.

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White Rot and the Quarantine Zone

It’s garlic harvesting season in the northern hemisphere, so recently while helping out with the harvest at a local farm, I had the chance to learn about a challenge involving growing garlic in southern Idaho. The challenge stems from a disease called white rot. It’s caused by a fungus (Sclerotium cepivorum), and it affects all alliums, including garlic, onions, chives, and ornamental alliums. This disease causes the leaves of alliums to die back, their bulbs to decay, and their roots to rot, ultimately turning the plants to mush. Sclerotia, the dormant stage of the fungus, are small (about the size of a poppy seed), black, spherical structures that can survive in soil for more than 20 years. They remain dormant until the exudates of allium plants awaken them, at which point they begin to grow, unleashing their destruction. Sclerotia can be moved around by farm equipment, flood and irrigation water, wind, and by attaching themselves to plant material. Once this fungus has established itself in a field, it is extremely difficult to eradicate, making the field virtually unfit for allium crops.

The threat of white rot and the monetary damage that it can cause led to the establishment of a quarantine zone in southern Idaho in order to protect its $55 million dollar a year onion industry. Due to the quarantine zone, all garlic that is grown for seed within the zone must be inspected and certified, and any garlic seed (i.e. garlic cloves) that are brought into the zone must go through a rigorous testing process in order to be certain it is free of the white rot pathogen before it can be planted. Garlic is a specific threat, because while onions in the zone are typically grown from seed and so are largely free from harboring sclerotia, garlic is grown from cloves, which can readily carry sclerotia. This process significantly limits the amount and the variety of garlic that can be grown in the quarantine zone. While the quarantine is essential for warding off the threat of this particular pathogen, it stifles the garlic growing industry and makes it difficult for new garlic growers to establish themselves.

Growing garlic is already an incredibly challenging pursuit due to the amount of time and physical labor that goes into planting, harvesting, drying, grading, etc. The quarantine, while understandable, is yet another added challenge. Learn more about this issue by reading this article found at Northwest Food News.

garlic

photo credit: wikimedia commons

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Onion Seed Viability, etc.

Seeds don’t remain viable forever. However, each species is different – the seeds of some species can remain viable for many years (decades even), while some species have seeds that will no longer be viable after a single year. This, of course, is something to keep in mind when planting seeds.

Recently I planted some onion seeds. I was curious to see if they would germinate because they were a few years old –  collected in 2007. My experience with onion seeds is that they germinate fairly quickly, within a week or so. However, three weeks have passed and my seeds have not yet germinated, despite being kept in moist potting soil in a sunny, warm corner of the house. This experience has led to me to think about seed viability.

Like I said, seeds of different species remain viable for different lengths of time. For vegetable gardeners, there are a variety of places to go to learn more about seed viability. Iowa State University Extension has a great chart which shows the number of years that the seeds of popular vegetable crops should remain viable. It is interesting to note that onion seeds only remain viable for one year. As it turns out, my seeds were far past their prime.

Seed storage can make a huge difference, though. Ideally, seeds should be stored in a cool, dry location. If they are exposed to too much heat or moisture, their metabolism will increase and their viability will decrease. This is because seeds are living organisms, despite appearing dead or dormant. Their metabolic processes are proceeding at an extremely slow rate,  but they are still proceeding. If metabolism increases (due to excessive heat or moisture, for example), the embryonic resources of seeds can become depleted, and viability (or germination potential) decreases.

Some sources recommend that you keep your seeds in the refrigerator, provided that they are sealed in plastic to keep them dry. Regardless, the ideal conditions for seed storage are cool and dry. I have always kept my seeds in a shoebox at room temperature (which isn’t always that cool because in the summer I refrain from using the air conditioner as much as possible). Thus, the viability of my seeds may in fact be reduced simply due to the conditions in which they are being stored.

There is a way to determine the viability of your seeds if you are curious. Just place some seeds on a moist paper towel, roll up the paper towel, and place it inside of a plastic bag. Wait a few days and then remove the paper towel from the plastic bag. Count the number of germinated seeds and divide that number by the total number of seeds originally placed on the paper towel. This will give you the germination percentage and will help you determine how many seeds to place in each hole or pot when you are planting them.

onion seed packet

This is the seed packet for the onion seeds that I planted. 2/15/2013 is the date that I planted them. After three weeks they had not germinated. I guess I’ll have to try some newer seeds.