Potato

Plant characteristics

The tuber or seed piece produces a main shoot within a week or so of planting, depending on soil temperatures and its physiological readiness to sprout. At about the time the main shoot breaks through the surface, rhizomes arise from the underground nodes on the main shoot. The rhizomes grow horizontally underground for 5 to 12 inches, then thicken at the tip to form a tuber. Since roots and rhizomes only develop from the stem between the seed piece and the soil surface, the seed piece must be planted deeply enough to permit adequate root and rhizome formation. The period of tuber initiation is fairly short and usually occurs 5 to 7 weeks after planting. At this time the above-ground portion of the plant is usually 6 to 8 inches high and just beginning to flower. Although flowering and tuber initiation occur at about the same time, the two processes are unrelated physiologically. The cells in the young tuber increase by division until the tip is ¸-inch thick. All further growth of the tuber is by cell enlargement. The tuber, like the rhizome, is stem tissue and has lenticels for gas exchange. 'Eyes' are rudimentary leaf scars. When potatoes sprout, the sprouts are lateral branches with several buds.

The number of tubers formed depends on the environment at tuber set while the size of the tubers depends on the subsequent growing conditions. The tuber competes with leaves and shoots for photosynthates so tubers can grow only when excess photosynthates are available. Thus high tuber number requires good conditions during the 1-to-2-week period of tuber initiation, but adequate tuber size requires good conditions during tuber development as well.

Most commercially grown potatoes are either the round, smooth-skinned white eastern types used for potato chips and boiling or the russetted western types used for baking and french fries. Russet types have a rough skin and are larger and longer and have a higher dry matter content and growing season than eastern types. Dry matter content (as measured by specific gravity of the tubers) must be high in potatoes used for baking and processing. Many more potato types than this are popular in South American markets where tubers may be round, oval, elliptical, long, oblong, or blunt in shape. In cross-section they may be round or flat. The skin can be white, cream, buff, red, purple, or yellow in color and smooth, flaky, netted or russetted in texture. Flesh can also be white, cream, buff, purple or yellow in color. The market for unusual potato colors is increasing in the United States as well. 'Yukon Gold' is a popular yellow-fleshed cultivar and in 1994, Ronniger's Seed Potatoes released 'Pinto' which has yellow skin with pink-to-red eye markings.

Climatic Requirements

Although the potato is considered a cool-season vegetable, the shoots are sensitive to hard frost and the tubers will freeze at 29 degrees F. The optimal temperature for shoot emergence is 72 degrees F. Tuber initiation. Tubers are initiated more rapidly under short days in most cultivars but under optimal temperature conditions (54 degrees F air temperatures at night) tuberization can occur even under long days. Best initiation is under cool, moist conditions. Under high temperatures or dry conditions the rhizome continues to elongate without tuberizing. Above 68 degrees F night temperatures, initiation is reduced and above 84 degrees F, it is inhibited. High nitrogen and low light, or shortage of photosynthates, also inhibit initiation.

Tuber growth. Best yield overall and highest tuber starch content occurs in regions with daily growing season temperatures averaging 60 to 65 degrees F. Tuber yield decreases 4 percent for each 1 degree F above the temperature optimum; dry matter content is also reduced. High soil temperatures also cause knobbiness and poor shape of tubers.

The amount of water received is also important. Too little water results in poor set and enlargement. Erratic water supply results in growth aberrations such as cracked tubers and knobs on the eyes resulting from secondary growth. Too much water leads to disease, excessive vine growth, low yield, enlarged lenticels and low dry matter content.

Exposure of tubers to light, either in the field or storage, results in sunscald and formation of both solanine, a colorless, toxic alkaloid, and chlorophyll, the green photosynthetic pigment. Although chloro-phyll is harmless, tubers with green patches cannot be sold because of the potential presence of solanine. In 1992, 1.35 million acres of potatoes were grown in the United States, making potatoes the top vegetable crop in terms of acreage. Potatoes are grown commercially in every state, but most of the crop is grown in the 'late states' where potatoes are harvested in the fall and stored. In the South, potatoes are mainly grown in the winter or spring and used immediately.

Production of potatoes has shifted from eastern states such as Maine to western states such as Idaho and Washington because consumption of the frozen french fries produced from western potato types increased more than consumption of the potato chips which are produced from eastern potatoes and because western russet types are more popular for fresh consumption than the round white potatoes grown in the eastern states.

Location of Production

The main potato-producing states based on harvested acreage in 1992 were Idaho, North Dakota, Washington, Maine, and Minnesota. Of the harvested potato acreage in the United States in 1992, 7.35 percent was in the Southern Region. The top states (by harvested area) in 1992 were Florida, 43,449 acres; North Carolina, 18,775; Texas, 10,487; Alabama, 10,355; and Virginia, 8,499.