As I was dissecting various geophyte species in my Floriculture Crop Production class to learn about their anatomy, I realized none of them were making me tear up like the common onion bulb does. Although I was wearing latex gloves to avoid skin dermatitis, none of these flowering bulbs were exhibiting the same defense mechanism as the onion. I became intrigued. Why do we cry and can we do anything about it?
It is one of life’s trivial annoyances that just seems unavoidable, and if you ever stepped foot in a kitchen, you probably shed a tear over an onion. The familiar teary and stinging eyes that make us look like the onion said terrible things to us, are something we just put up with while prepping meals.
Despite the consequences of cutting the common onion, they are cultivated and enjoyed in kitchens around the world. The common white onion is traditionally used in Mexican cuisine, exhibiting a golden color and hint of sweetness when sautéed. Yellow onions are your typical everyday onion, turning a dark brown when caramelized, while the purple onions are perfect in salads and fresh dishes.
All of these cultivars belong to the Allium genus. The garden onion in its binomial nomenclature is known as Allium cepa. A biennial vegetable, it completes its life cycle in two seasons, but in cultivation grows as an annual. Farmers harvest the onion bulbs in late summer or cut the onion foliage in the late spring to produce “spring” onions.
Let us delve into the many applications of this versatile vegetable and learn about its anatomy while exploring how all those plant structures work together to provide us with that deliciousness. Maybe we’ll even find out how to mitigate those aggravating effects of cutting onions!
PHYSIOLOGY & ANATOMY
The onion we are most familiar with is a specialized underground organ consisting of nutrient reserves that plant has stored throughout the season. Allium cepa is a true bulb consisting of a tunic to protect the fleshy scales from mechanical injury and drying, as well as a basal plate where adventitious roots, bulb scales and a vertical stem axis that encloses the flower arise.
Unlike scaly or imbricate bulbs, onions have bulb scales arranged in continuous concentric sheathing leaf bases; whereas imbricate bulbs like garlic have separate non-sheathing scales. Other true tunicate bulbs include daffodils, hyacinths and grape hyacinth.
In the center of the bulb, the more leaf like scales protect either a vegetative meristem or an unexpanded flowering shoot. Throughout the rest of the fleshy, nutrient reserve filled scales you can also see growth points which will eventually become an offset bulblet.
This ability to propagate through division in addition to seed dispersal increases the bulbs chances of survival and is quite convenient for gardeners.
Flowering for many of the spring flowering bulbs as well as our garden onion requires certain environmental ques. Temperature, light intensity, and photoperiod are all stimuli that Allium cepa must perceive in a certain sequence in order to flower.
Onions are a cold-season crop hardy to USDA zone 3 that can be straight sown into a garden bed in early spring. Success is increased when sets of onions are planted. These sets can be harvested as spring onions or scallions in just a few weeks, or you can let the bulb develop until environmental ques like day length and temperature tell it to pack it in for the winter.
This period of cold is known as vernalization, and is required by many temperate region plants to initiate flowering or vegetative growth. If left unharvested, the bulb would overwinter in the ground and flower the next growing season.
"Vernalization". Encyclopædia Britannica Online. Retrieved 2011-01-24. "vernalization, the artificial exposure of plants (or seeds) to low temperatures in order to stimulate flowering or to enhance seed production. By satisfying the cold requirement of many temperate-zone plants, flowering can be induced to occur earlier than normal or in warm climates lacking the requisite seasonal chilling."
Since flowering of vegetables reduces the vigor of vegetative growth and in our case the size of the bulb, understanding what makes our onions flower will help us become better gardeners.
Temperature is the regulating factor for onion production and is what sends the onions into dormancy. Since onions are biennials, they have a tendency to bolt or send up a flower shoot when temperature fluctuations deceive it into thinking it has experienced two growth cycles.
We want to harvest onions in late summer when the vegetative growth begins to brown and bend and before the cool temperatures start to set in. After some curing and drying time, the onions are ready to eat and we are faced with contending with their defense mechanisms.
Onions are equipping themselves to ward off predators well before we slice into them. During their growth, they amass Sulphur from the earth and combine it with amino acids in their cells to produce amino acid sulfoxides.
When onions are sliced or cells walls that are broken from attack of fungi and insects, enzymes called alliinases breakdown into amino acid sulphoxides generating sulphenic acids. One of these acids is further rearranged by combining with a second enzyme called the lachrymatory factor synthase or LFS. The byproduct is a volatile gas called the lachrymatory factor gas.
This gas is diffused through the air, where it reaches the eyes and its sensory neurons, producing a stinging sensation. Tears follow to wash the eyes from irritants and we are left in a blubbering mess.
So the question is, do onions want our help to reproduce? Certain plants have qualities which make them desirable for reproduction, but the onion must not care. They must know that despite their defenses humans will not be able to resist their unique taste. But this is a conversation that is beautifully articulated in Michael Pollan’s book, The Botany of Desire
Since the onions won’t cooperate, we can take certain measures to mitigate their effects on our eyes. Keeping your onions refrigerated reduces the metabolic processes of cells which molecules need to produce the amino acids responsible for lachrymatory factor release.
Another good method for reducing the gas from reaching your eyes is cutting the onions under water or supplying ample water to where the onion is being sliced. Slicing the onion with a sharp blade will also reduce the amount of cell walls being broken, resulting in reduced numbers of enzymes combing to produce the irritating gas.
You can even use a fan to blow air away from your face or goggles if your eyes become especially irritated. Now that we know how to breach the onions defenses, let us examine how we can use the onion besides the obvious culinary applications.
Like many vegetables, onions possess an array of health benefits. Some may be unsubstantiated and some hold some merit, but overall an increased onion diet has shown to have health benefits. Chemical compounds in the onions may have anti-inflammatory, anticholesterol and antioxidant properties that in some studies have shown to reduce head and neck cancers.2
In undeveloped parts of the world onions are used to treat blisters and boils as well as to reduce swelling from bee stings, while companies in the United States use onion extract in topical scar treatment creams.
Onions have also been found to be beneficial to women as they age due to their increased risk of osteoporosis during menopause.2 Enzymes from the onion break down osteoclasts, mitigating the symptoms of osteoporosis.
Some other alternative applications include; using onion juice as an insect repellant, boiling onions for a sore throat remedy, polishing metal, cleaning your grill, and you can even use it to improve your skin’s complexion or stimulate hair growth. For a more thorough list of onion use, refer to this article. http://www.stylecraze.com/articles/amazing-health-benefits-of-onions/#gref
“Despite their benefits to humans, onions are toxic to cattle, cats, and dogs, and, to a lesser extent, sheep and goats (Cope 2011, Merck 2011). Consumption by these animals of large amounts of onion may lead to anemia and impaired oxygen transport.”2
Now that we know all the wonderful benefits of onions and how to avoid the stinging of our eyes, I am definitely more inclined to plant some in my garden.
The first step to a successful onion harvest is to know which variety you have. Different species require different photoperiod times to start initiating a bulb.
European varieties are considered “long-day” onions and will only produce bulbs once 15 plus hours of daylight has been reached. On the other hand, “short-day” varieties only require 10 hours of daylight to begin bulb formation and can be planted in the fall whereas long-day varieties need to be planted in spring.
You can either sow seed straight into your garden or plant onion sets which have a greater chance of survival.
For short-day varieties, sowing seed in the fall directly into the garden will require thinning next February. This practice will ensure your bulbs are not competing with each other. The thinned plants can be used as spring onions or transplanted to an area with more space.
Due to the high nutrient requirements, onions need a good amount of nitrogen to thrive. An application of 21-0-0 Ammonium sulfate or Ammonium nitrate should be applied once a month. In addition, adding phosphorous to the soil prior to planting will provide the young seedlings with a needed boost.
Prepare the soil by creating raised beds that are at least 4 inches high and 20 inches wide. The phosphorous can be added by creating a trench within this raised bed. Plants should be set out at least 4 inches on center and 1 inch deep.
A well-draining soil with plenty of air circulation and sun are the perfect conditions for onions. They are susceptible to fungal diseases and onion thrip if conditions are favorable for those pathogens. Making sure your onions are happy will go a long way in preventing these issues from manifesting.
Harvest your onions in late summer when the foliage begins to yellow and bend. If you wait too long and temperatures suddenly drop, your onions will bolt and compromise the bulb. Once harvested, dry the onions and clip the roots and foliage.
If you keep your onions separated in a cool and dry place, they can have a shelf life of up to a year. Store them preferably in a mesh bag to increase circulation and you can enjoy them until next harvest. As a general rule, sweet onions and their high sugar content have a shorter shelf-life than more pungent cultivars.
When harvesting, ponder this;
Onion’s skin very thin, Mild winter coming in; Onion’s skin thick and tough, Coming winter cold and rough.
Thank you for taking the time to read! Perhaps you can share some of your favorite onion recipes in the comments below? I will be updating this post later this growing season, but for now happy planting!