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How are there multiple varieties of the potato?

How are there multiple varieties of the potato?


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The potato appears to propagate by growing an 'eye'/'bud' which eventually grows into a new plant. So there would probably be single representative of the potato species in the world with all others being genetically identical.

Yet wikipedia indicates there are several closely related species of the potato; apparently as a result of selective breeding. To my understanding (which is probably wrong) there is only a single potato that grows the eyes without the need for a second parent - so there should probably only be the same genes repeated in each potato/plant.

What am I missing here? Why are there multiple varieties of the potato?


You're missing that potatoes also reproduce sexually.

They're flowering plants, and they produce seeds that are not genetically identical to their parent plants.


Well, uvesten is correct in saying that potatoes are flowering plants and as such they can reproduce sexually. However, as everyone mentions potatoes can, like many plants, reproduce asexually by putting out clones.

Since clones are (by definition) genetically identical to the parent plant, this would seem to rule out the possibility of producing different varieties (or even species) from clonal propagation.

But! this very interesting study by Jiang et al. looked at Arabidopsis lineages (Brassica family). They found:

"in vitro regeneration of Arabidopsis plants results in a high frequency of heritable phenotypic variation "

That is, regenerant Arabidopsis plant lineages displayed extensive phenotypic somaclonal variation - the cloned "offspring" were not genetically identical. They attributed most of this genetic variation to an increased base substitution frequency in the regenerant offspring but there may also be unknown epigentic factors as well.

Jiang et al. summarise:

… somatic mutation rates are characteristically higher than germline rates in multicellular organisms [26] and has important particular potential consequences for the evolution of plants, given that they frequently adopt life cycle strategies that involve regeneration from somatic tissues.

So, perhaps some of the variation we see in plant species which commonly propagate asexually actually arose during this process and not via sexual reproduction… ? This would be good news for houseplants which are nearly always propagated asexually.

However, I am not sure whether this extends to potatoes.

Jiang et al. 2011, Current Biology, 21, 1385, Regenerant Arabidopsis Lineages Display a Distinct Genome-Wide Spectrum of Mutations Conferring Variant Phenotypes


What Are The Differences Between Determinate And Indeterminate Potatoes

Determinate and indeterminate potatoes are defined by growth patterns. Several different varieties of potato fall into each category, so there are plenty from which to choose. Select between determinate and indeterminate varieties depending on factors like yield, garden space, and amount of labor.


Potato

There are over 4,000 edible varieties of potato, mostly found in the Andes of South America. Potato is the third most important food crop in the world after rice and wheat in terms of human consumption. More than a billion people worldwide eat potato, and global total crop production exceeds 300 million metric tons. Potato is a critical crop in terms of food security in the face of population growth and increased hunger rates. For example, China, the world’s biggest consumer of potatoes, expects that fully 50% of the increased food production it will need to meet demand in the next 20 years will come from potatoes.

For more information, see the World Potato Atlas

POTATO FACTS AND FIGURES

The potato (Solanum tuberosum) belongs to the solanaceae family of flowering plants. It originated and was first domesticated in the Andes mountains of South America.The potato is the third most important food crop in the world after rice and wheat in terms of human consumption. More than a billion people worldwide eat potato, and global total crop Production exceeds 374 million metric tons. There are more than 4,500 varieties of native potatoes, mostly found in the Andes. They come in many sizes and shapes. There are also between 100-180 wild potato species. Though they are too bitter to eat, their important biodiversity includes natural resistances to pests, diseases, and climatic conditions.

HOW POTATO GROWS

The potato (Solanum tuberosum) is an herbaceous annual that grows up to 100 cm (40 inches) tall. As the potato plant grows, its compound leaves manufacture starch that is transferred to the ends of its underground stems (or stolons). The stems thicken to form a few or as many as 20 tubers close to the soil surface. The number of tubers that actually reach maturity depends on available moisture and soil nutrients. Tubers may vary in shape and size, and normally weigh up to 300 g (10.5 oz) each. At the end of the growing season, the plant’s leaves and stems die down to the soil level and its new tubers detach from their stolons. The tubers then serve as a nutrient store that allows the plant to survive the cold and later regrow and reproduce. Each tuber has from two to as many as 10 buds (or “eyes”), arranged in a spiral pattern around its surface. The buds generate shoots that grow into new plants when conditions are again favorable.

POTATO NUTRITION

Potatoes are an excellent, low-fat source of carbohydrates, with one-fourth the calories of bread. Boiled, they have more protein than maize and nearly twice the calcium. An average serving of potatoes with the skin on provides about 10 percent of the recommended daily intake of fiber.

NATIVE POTATO VARIETIES

Over 4,000 varieties of native potatoes grow in the Andean highlands of Peru, Boliva, and Ecuador. Selected over centuries for their taste, texture, shape and color, these potato varieties are very well adapted to the harsh conditions that prevail in the high Andes, at altitudes ranging from 3,500 to 4,200 meters. Farmers generally produce these native varieties with minimal or no use of agrochemicals. Diversity is conserved on farms and in communities for subsistence use and as a highly valued heritage. Most of these varieties never see a market they are traded among highland and lowland communities and given as gifts for weddings and other occasions. The varieties differ from community to community.

WILD POTATO SPECIES

There are between 100-180 known species of wild potato. These inedible species are the original ancestors of today’s cultivated potato. Wild species are found from southwestern United States to southern Chile, with most species concentrated in Peru and Bolivia. They grow in diverse soils and climates, from the dry desert along the Peruvian coast, to the inter-Andean valleys, up to altitudes of 4,200 meters above sea level.

POTATO PROCESSING AND USES

Potatoes are used for a variety of purposes, and not only as a vegetable for cooking at home. In fact, it is likely that less than 50 percent of potatoes grown worldwide are consumed fresh. The rest are processed into potato food products and food ingredients fed to cattle, pigs, and chickens processed into starch for industry and re-used as seed tubers for growing the next season’s potato crop.

POTATO PESTS AND DISEASES

Increasing potato production while protecting producers, consumers, and the environment requires an integrated approach encompassing a range of strategies: encouraging natural pest predators, breeding varieties with pest/disease resistance, planting clean seed, rotating with other crops, and organic composting to improve soil quality.

Click here for more information on potato diversity held at the CIP Genebank

CIP Headquarters Avenida La Molina 1895, La Molina Apartado 1558, Lima 12, Peru | Tel: 511-3496017


Scientists have bred and released colorful new varieties of potato

High-nutrient, great-tasting Purple Fiesta potato variety, a fingerling midseason specialty potato with smooth purple skin and dark purple flesh. Credit: Stephen Ausmus.

America's favorite vegetable is almost fat free and a good source of potassium, iron, and vitamin C. It has 3 grams each of protein and fiber, low sodium, and no cholesterol, and costs a quarter per serving at the produce aisle.

If you haven't guessed which vegetable yet, it's only going to get harder. Agricultural Research Service scientists have bred and released colorful new varieties. Some have relatively high levels of beneficial red-to-purple pigments called "anthocyanins," and others have high levels of yellow-pigmented carotenoids.

We're talking about the popular potato. ARS researchers have developed three new varieties of potatoes with red and purple flesh and skin, which are now available to consumers. If you can get the new varieties soon, you'll have colorful potatoes gracing your holiday dinner table. See below for information on availability.

All potatoes contain an assortment of nutrients and other health-promoting compounds. The colored-flesh potatoes have anthocyanins and carotenoids. The amount and type depend on the variety of the potato. Breeding efforts by ARS researchers in Beltsville, Maryland, led to the release of yellow-pigmented potatoes with up to three times more carotenoids than a yellow-fleshed imported variety.

As the most-eaten U.S. vegetable, phytonutrient-rich potatoes can have a strong impact on health, according to plant geneticist Charles Brown, who is with ARS in Prosser, Washington. In a study, Brown and his colleagues analyzed and compared concentrations of phytochemicals in yellow- and purple-pigmented potatoes and in white potatoes. The team reported that yellow potatoes had a 45-fold greater concentration of carotenoids than white potatoes, and purple potatoes had a 20-fold greater concentration of anthocyanins than yellow potatoes. No detectable amounts of anthocyanins were found in white potatoes.

During the same study, the team also compared sensory evaluations of pigmented potatoes to those of white potatoes. When yellow, purple, and white potatoes were ranked by a consumer panel, no significant differences in flavor or overall acceptance were observed. The study was published in Food and Nutrition Sciences in 2013.

High-nutrient, great-tasting AmaRosa potato variety, a fingerling midseason specialty potato with smooth, vibrant red skin and bright red flesh. Credit: Stephen Ausmus.

Brown carefully bred the three unique red- and purple-pigmented potatoes at ARS's Vegetable and Forage Crops Production Research Unit in Prosser.

"Getting them to consumers has taken decades," says Brown. "The cost in labor alone to bring a new variety to market is considerable." The steps to market included selections, crosses, evaluations, and extensive field-testing around the nation. Growers, buyers, and industry representatives make further postharvest evaluations based on consumer-acceptability features prior to release.

The three great-tasting potato varieties with colored flesh that are now available for consumption are TerraRosa, AmaRosa, and Purple Fiesta (also known as Purple Pelisse). They perform well across a variety of preparation methods such as baking, roasting, microwaving, steaming, and mashing.

AmaRosa is a "fingerling" (long and thin) specialty potato with smooth, vibrant, red skin and bright-red flesh. TerraRosa, described by growers as sweet and creamy, is a full-sized, oblong variety with red skin and pinkish flesh that sometimes has white marbling inside.

Purple Fiesta is a small, smooth fingerling with purple skin and dark-purple flesh. "Purple Fiesta has ranked better in taste, color, and nutrition than any other blue or purple potato I've explored," says Dan Chin, who holds the exclusive license to grow and sublicense seed. "These varieties were carefully bred to enhance all the unique qualities found in a colorful potato, including uniform size, striking color, rich vitamin and mineral content, sweet flavor, and versatility."

Brown worked on developing and evaluating the varieties as a contributing partner with the Northwest (Tri State) Potato Variety Development Program—a team that includes university researchers in Washington, Idaho, and Oregon commissioners of the three states and the potato industry. The Tri State Program created the Potato Variety Management Institute, a nonprofit based in Bend, Oregon, that administers licensing agreements for new varieties.

All three potatoes have ranked well for use in making potato chips and fries. "One of the sensory evaluations ranked AmaRosa highest among 10 contenders when prepared as fried chips," says Brown. "The chips retained their rosy red color and resisted fading, showing great potential for the chipping snack sector as well."

Articles describing the attributes of AmaRosa and Purple Fiesta were published in the American Journal of Potato Research in 2012.

The Power of Pigments

While the new potato varieties are festively colored and taste good, their value is far from skin deep. Foods rich in antioxidative bioactive compounds have been linked to reduced incidence of chronic diseases. So Brown and geneticist Duroy (Roy) Navarre, who is also at the ARS Prosser laboratory, worked with lead author Boon Chew and colleagues at Washington State University on a human nutrition study. They tested whether the carotenoids and anthocyanins from pigmented potatoes decreased oxidative stress, inflammation, and improved immune status. Their study was based on USDA-ARS colored-potato breeding lines developed at Prosser.

For the study, 36 healthy male volunteers were randomly assigned to consume white-, yellow-, or purple-fleshed potatoes for 6 weeks. Every day, the three groups (12 volunteers each) consumed two-thirds of a cup (150 grams) of the type of cooked potatoes assigned to them. All the potatoes were prepared and served at the research site.

Fasting blood samples were collected from the participants at the start and end of the 6-week intervention. Their blood was tested for cytokines, including interleukin-6, which is secreted by lymphocytes (immune system white blood cells), while initiating an inflammatory response. Blood samples were also tested for lymphocyte proliferation, C-reactive protein (another biomarker of inflammation), and phenotype.

The researchers found that the participants who were fed the carotenoid-rich yellow potatoes and anthocyanin-rich purple potatoes had lower plasma IL-6 than those fed white potatoes. Volunteers consuming the purple potatoes showed a heart-healthy decrease in C-reactive protein from the study's start. And those who ate the yellow-pigmented potatoes showed an enhanced immune response, as indicated by increases in specific lymphocyte populations. The study was published in the American Journal of Advanced Food Science and Technology in 2013.

With this many positive features, if you decide to serve these colorful potatoes this holiday season, you might want to be sure to have enough to send home with guests and for leftovers the next day.


Propagation

Pollination

If you want to keep growing potatoes from TPS, you will need to produce more seed. There are many ways to go about this. You can plant a group of varieties that you like and allow them to open pollinate. You can perform hand pollinations between varieties. With tetraploid varieties, you can also isolate varieties that you like and let them self pollinate.

The first thing that you will need is some potato flowers. In favorable weather conditions, flowers appear two to three months after tubers or seedlings are planted out. The most favorable conditions for flowering and berry production are daytime temperatures in the 60s F and nighttime temperatures in the 50s F, accompanied by high humidity. Some varieties will flower in warmer, drier conditions, but very few will flower when daytime temperatures rise above 85° F and berries tend to drop early in warm weather. Flowering lasts about two weeks for most varieties. Certain pesticides may inhibit flowering in potato. Rio (2012) found that application of carbofuran, a nematocide, reduced flowering significantly in a number of wild potato species.

Diploid potatoes are self incompatible they require cross pollination with another variety in order to form seed. In most cases, seed set is much better with hand pollination than relying on insect pollination.

Tetraploid potatoes are usually self compatible and will set seed that has been pollinated by their own flowers, if they have fertile pollen. That is a big “if” most tetraploid potatoes of modern origin have pollen sterility problems. If they have sterile pollen, they require cross pollination to produce seed. But, there is another problem: the seeds collected from varieties that have sterile pollen will usually also have sterile pollen. Even when self pollination works and produces progeny with fertile pollen, the practice eventually results in inbreeding depression, which means that you are more likely to get good results by cross pollinating tetraploid potatoes than by allowing them to self pollinate. This is not always true, but the details are beyond the scope of this guide. Roughly 80% of the seeds produced by self compatible tetraploids with fertile pollen, even in mixed plantings, are self pollinated. In order to maximize hybrid TPS production, you must emasculate flowers and hand pollinate.

When hand pollinating, you have the choice of performing open pollination or controlled pollination. Open pollination is the easier choice. You simply collect pollen from one set of flowers and transfer it to another. One easy way to go about this is to use an electric toothbrush to vibrate the flowers, with a small container below to catch the dislodged pollen. You can then use a small brush to paint the pollen onto the flowers that you want to pollinate, or if you have collected a lot of pollen, simply dip the flowers into the container. Because you have not blocked other pollinators from reaching the flower, it is likely that the pollination you perform won’t be the only one. The flower might have been pollinated before you get to it and it might be pollinated again after. This is the nature of open pollination. A potato berry can form several hundred seeds and each seed is the result of a separate pollination. It is rare that a single application of pollen will fully pollinate a potato berry. When hand pollinating potato flowers, repeating the pollination at least once produces significantly better seed set than a single pollination (Pallais 1985).

If you want to be certain about which variety is the pollen parent, then you need controlled pollination. Controlled pollination requires emasculating the flower: removing the stamens before their pollen matures. This procedure is beyond the scope of this guide, but you can find instructions and even videos on the Internet. In brief, you open the flower just before it would first open naturally and pluck out the stamens. This prevents self-pollination. You can then pollinate the flower and bag it to prevent access by other pollinators.

You can store the pollen that you have collected for later use, which may be important if the varieties that you wish to cross flower at different times. Dry the pollen for 24 hours in a container with silica gel desiccant and it can then be stored at refrigerator temperatures for up to two months without significant reduction in viability.

For natural pollination, bumble bees appear to be the most active pollinators in the Pacific Northwest, followed by hoverflies and other small, native bees.

Collection

It takes a minimum of six weeks for berries to mature (Simmonds 1963). The optimum time for berries to mature varies from one variety to another and I would generally prefer to not harvest berries that are less than eight weeks old. In warmer climates, berries can mature more quickly. The berries closest to the main stem produce the largest yields of seed (Almekinders 1995). These are typically also the earliest berries. If you can wait until the berries drop from the plant naturally, that is the best approach, but pests may make that difficult. In that case, harvest the berries while they are still firm and allow them to ripen in a protected place. I typically store berries in plastic bags until they are fully ripe and begin to break down. They are very easy to process at that point. A quart of potato berries weighs about a pound and produces an average of 2 grams of seed (about 2,200 seeds), but different varieties here have produced a range of almost zero seeds per quart to as much as 9 grams, which is nearly 10,000 seeds.

Extraction

Once you have harvested mature berries, you need to extract the seeds. There are a number of ways to go about this. For small quantities, you can merely cut berries in half and squeeze out the seeds. This has the advantage of causing little damage, but would become quite tedious if you are planning on extracting hundreds or thousands of berries.

For large extractions, a blender is often used to break up the berries for about 30 seconds. This works very well, but it does cause damage to some of the seeds. The number of seeds that you will get from a blender pitcher full of berries is so large that it probably doesn’t matter, but it has been reported by the International Potato Center that a meat grinder provides a superior result, breaking up berries with very little damage to the seeds. Another study found that using a mixer with rubber beaters produced the highest percentage of germinable seed (Gallagher 1984).

Once the basic extraction is done, you have two choices for how to clean the seed: fermentation or detergent.

Fermentation for 4 days at 86° F (30° C) produces clean seed (Pallais 1985). The only problem with fermentation is that you often don’t completely get rid of the mucilage and the seeds have a tendency to clump when drying. You can speed things up by adding some dry yeast.

Detergent generally produces TPS with less mucilage, which clumps less during drying as a result. This may be an additional layer of protection against surface contamination. Potato breeder Tom Wagner recommends the following procedure: Rinse seeds in warm (90° F) water until it runs clean, without any slimy strands. Then soak for 20 minutes in a 10% solution of trisodium phosphate (TSP). TSP can be purchased at hardware stores. It is a detergent that is used for cleaning surfaces before painting, among other uses. A 10% solution can be made by adding 24 grams of TSP to a cup of water. At the end of 20 minutes, rinse the seed clean.

Once your TPS is clean, there are other treatments to consider. To eliminate surface contaminants, such as bacteria and fungi, you can bleach the seed. Soak for 10 minutes in a solution of 1 tablespoon (15 ml) of household bleach per 1 cup (237 ml) of water. You can also possibly reduce the odds of transmitting viruses by heating the seeds. I don’t usually do this, as I’m not sure about the long term effect on germinability or the efficacy for that matter. Soak the seeds in 50 C (122° F) water for 25 minutes. You will need a hot plate or other appliance that can hold a constant temperature. If the temperature goes higher than 50 C for very long, it will kill the seed. The objective here is to heat the seeds enough to kill the viruses contained within them, without harming the seed.

For additional information about extracting and cleaning potato seed, follow the link.


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Chapter 12 Potatoes

-Usually small and round in shape, but some varieties can be large, and some may be elongated.

-Flesh is white, yellow, or even blue or purple.

-Skin is white, red, yellow, or blue.

-Holds shape well when cooked.

-Use for boiling whole, for salads, soups, hash browns, and any preparation where the potato must hold its shape.

2. All-purpose potatoes (sometimes called chef potatoes).

-Ideal choice for the traditional baked potato.

-Best potato for French fries because the high starch content produces an even, golden color and good texture.

-Also, the regular shape means little trimming loss.

-May be mashed, but is generally too expensive for that purpose.

-Less expensive than russets.

-Suitable for most purposes, but not usually used for baking because of irregular shape.

-Especially useful for puréeing or mashing, or any preparation in which the shape of the whole potato is not important.

-Not usually baked due to its shape.

They have a regular, elongated shape, brown or reddish-brown, rough skin, and white flesh.

-Sprouting potatoes are high in sugar.

-Green areas develop on potatoes stored in light.
-These areas contain a substance called solanine, which has a bitter taste and is poisonous in large quantities.
-All green parts should be cut off before cooking.

-Do not refrigerate. Temperatures below 45°F convert potato starch to sugar.
(Refrigerated potatoes must be stored at 50°F for two weeks to change the sugar back to starch.)

-New potatoes do not keep well.
(Purchase only one week's supply at a time.)

-Potatoes begin to turn brown as soon as they are peeled.
(To prevent browning place peeled potatoes in cold water immediately.)

-Potatoes may be peeled in advance and stored a short time under water, although some nutrients will be lost.


Potato types explained

Confused by the different potato types? We explain the differences between first early, second early, salad and maincrop potatoes in this short guide.

Published: Wednesday, 13 January, 2021 at 7:02 am

There are three main types of potato to grow, named according to when you plant and harvest them: first earlies, second earlies and maincrop.

First early or ‘new’ potatoes are so-called because they are the earliest to crop, in June and July.

Second earlies (also called ‘new’ potatoes) are hot on their heels, taking a few more weeks to mature. They are ready from July.

These two types are expensive in the shops and taste better when just been harvested, so many people grow these. They also take up less room and are less prone to potato blight, as they are generally harvested before the disease has taken hold.

Maincrop potatoes take the longest to mature and are harvested from late August to October. They are suitable for baking, roasting and mashing, and can be stored for a few months.

Growing potatoes is easy – plant one seed potato, and you dig up lots of potatoes a few months later. Find out all you need to know about growing potatoes in our potato Grow Guide.

Find out when and how to plant the main potato types, below.

First early potatoes

First early or ‘new’ potatoes are so-called because they are the earliest to crop, in June. They take 10-12 weeks to mature. Plant 30cm apart, with 60cm between rows, around 12cm deep.

Plant: March (with frost protection)
Harvest: June and July
Recommended varieties: ‘Red Duke of York’, ‘Lady Christl’, ‘Orla’ and ‘Rocket’.

Second early potatoes

Second early potatoes take 14-16 weeks to mature. As with first earlies, plant them 30cm apart, in rows 60cm apart, 12cm deep.

Plant: March (with frost protection)
Harvest: July and August
Recommended varieties: ‘Charlotte’, ‘Maris Peer’, ‘Ratte’

Maincrop potatoes

Maincrops take 16-22 weeks to mature. Plant 40cm deep, in rows 75cm apart, at a depth of 12cm.

Plant: mid to late April
Harvest: August to October
Recommended varieties: ‘Pink Fir Apple’, ‘Desiree’, ‘Cara’, ‘Maris Piper’, ‘King Edward’, ‘Sarpo Mira’

Salad potatoes

Salad potatoes share a firm, waxy texture and many have an intense, nutty flavour, perfect with a knob of melted butter or cold in a potato salad. They are usually first earlies or second earlies. Discover the 10 best salad potatoes to grow.

Growing potatoes in a small space

Potatoes can take up a lot of room on the veg plot but they grow brilliantly in large containers or even large bags. Find out how to grow potatoes in a pot.


Potatoes and Related Crops: Role in the Diet

South America (Center of Origin)

Potato ( Solanum tuberosum L.) is a versatile staple crop with a unique and historically significant story of domestication. Originating in the Andean highlands almost 10 000 years ago, potato has since spread to all continents with a vast range of climate conditions, ranging from temperate to tropical regions. Even Charles Darwin commented on the wide adaptation of potato in The Voyage of the Beagle writing, “It is remarkable that the same plant should be found on the sterile mountains of Central Chile, where a drop of rain does not fall for more than six months, and within the damp forests of the southern islands.”

Potato domestication began in South America, where potatoes were planted on high-altitude lands in raised beds to protect from frost damage. Initially, cultivated potatoes were adapted to long-day conditions, but converted to short-day conditions following hybridization with a wild species. This adaptation was crucial for the spread of potato production to other continents, particularly regions of North America, Europe, and Asia, where most production occurs today.


How Potatoes Grow

Potatoes are usually grown from other potatoes. You plant a whole, small potato, or a piece of a larger one for a new plant. The whole potato or cut piece has several slightly recessed, dormant buds or "eyes" on the surface. When conditions are right, these buds will sprout, whether the potatoes are in the ground or in a kitchen cupboard. The sprouts then develop into independent plants.

The cut potato piece or "seed" piece provides the new sprout or seedling with nourishment from its supply of stored starch.

After you've planted a seed piece, it usually takes one to two weeks for the main stem and first leaves to appear above ground. The root system develops quickly and begins to absorb nutrients as the food supply in the seed piece is used up.

The top, leafy part of the plant puts on a lot of growth in the first four to five weeks after planting. Then the main stem of the plant stops growing and produces a flower bud. When that happens, the plant will have as many leaves as it will ever have.

With proper sunshine, the leaves eventually produce more food than the plant needs, and the excess energy is channeled downward to be stored in the "tubers" -- thick, short, underground stems -- which we simply call potatoes. Irish potato tubers develop above the original seed piece, rather than below it like many other underground vegetables.

In general, the storage process starts five to seven weeks after planting, often when the plants have flowered. Some varieties will produce great potatoes with no flowering at all, but usually flowering is a sign that something is definitely happening underground.

Incidentally, potato flowers don't produce any nectar, so they're not visited much by bees or insects. The flowers are self-fertilized, and many potato plants produce small green seed balls about 1-inch in diameter, which contain up to 300 seeds. These seeds are mostly used by potato breeders.

When the tubers start forming, cooler temperatures are a plus. Years ago, research showed that fewer and fewer tubers were formed on the plants as the temperature went from 68° to 84°F. In fact, none formed at 84° F.

The best potato crops are produced when the daytime temperature is in the 60° to 65° F range, and when night temperatures are below 57° F. When the weather is hot, the top part of the plant respires heavily, reducing the amount of food material that can otherwise be put into storage in the tubers below ground. This helps to explain that while potatoes may be a summer crop up North, they're a late winter, spring or fall crop in the South.

In a big potato-producing state like Idaho, for example, cool summer days and nights keep energy losses to a minimum. Plenty of starch is stored in the tubers, helping to make the Idaho potatoes terrific, big, mealy bakers.

As potatoes enlarge underground, the outside layer of the tuber gets tougher and tougher, keeping moisture within the potato and protecting it from outside attacks by organisms that can cause rot.

This toughening of the skin continues even as the plant tops die, the signal to the gardener that the harvest is at hand. Potatoes can remain underground for a little while after the tops die, so that the last energy in the tops can be transferred to the tubers. If the outer skins can't be rubbed off after the potatoes have been dug, they'll store well