Short answers, pulled from the story.
Edible turnips were first domesticated in Central Asia several thousand years ago. Genetic studies show that Central Asian varieties are the most genetically diverse crops, supporting this origin story.
The turnip reached Japan by 700 CE. The average time from planting to harvest is 55 or 60 days depending on growing conditions.
Boiled green leaves of turnip top provide food energy and are a rich source of vitamin K at 350% DV along with substantial lutein at 8440 micrograms per 100 grams. Boiled turnip root supplies only vitamin C in moderate amount at 14% DV while other micronutrients remain low or negligible.
Charles Turnip Townshend promoted use of turnips in four-year crop-rotation system enabling year-round livestock feeding in England around 1700.
Turnips grow best in cool weather because hot temperatures cause roots to become woody and bad-tasting. They are typically planted in spring in cold-weather climates like northern US and Canada where growing season is only 3 or 4 months, or in fall for warm-weather climates with 7 or more month growing seasons.