Rapeseed

Rapeseed, a yellow-flowered plant in the Brassicaceae family, is grown mainly for its oil-rich seeds. These seeds naturally contain mildly toxic erucic acid, but the term "canola" refers to specific cultivars bred to have very low levels of this acid, making them suitable for human and animal food. Globally, rapeseed ranks as the third-largest source of vegetable oil and the second-largest source of protein meal.

The plant, *Brassica napus subsp. napus*, typically reaches about 100 centimeters tall. Its lower leaves are hairless, fleshy, and deeply lobed with a bluish-green cast, and they have stalks; the upper leaves lack stalks. The bright yellow flowers are roughly 17 millimeters across, with four petals arranged in a cross shape and four alternating sepals. Flowering begins at the lowest bud and moves upward over several days. Each flower has two short-stamen lateral stamens and four longer median stamens, whose anthers split away from the flower's center when blooming. The developing seed pods are green and elongated, maturing to brown. They grow on short stems 1 to 3 centimeters long and range from 5 to 10 centimeters in length. Each pod has two compartments separated by a central wall, with a row of seeds inside. The seeds are round, 1.5 to 3 millimeters wide, with a net-like surface texture, and turn black and hard when ripe.

To tell *B. napus* apart from similar species: its upper leaves do not clasp the stem (unlike *B. nigra*), and its petals are smaller than 13 millimeters across (unlike *B. rapa*).

Taxonomically, rapeseed is a subspecies of *Brassica napus*, specifically *B. napus subsp. napus*. This group includes winter and spring oilseed types, as well as vegetable and fodder rape. A distinct leaf form, Siberian kale (*B. napus var. pabularia*), was once common as a winter-annual vegetable. The other subspecies of *B. napus* is *B. napus subsp. rapifera* (also called *subsp. napobrassica*), which includes rutabaga, swede, or yellow turnip. *B. napus* itself is a digenomic amphidiploid, resulting from natural hybridization between *B. oleracea* and *B. rapa*, and it is self-compatible for pollination, like other amphidiploid *Brassica* species.

The name "rape" comes from the Latin word for turnip, *rāpa* or *rāpum*, related to the Greek word *ῥάφη* (*rhaphe*).

In Northern Ireland, both *B. napus* and *B. rapa* are found as escapes along roadsides and on waste ground.

Cultivation of *Brassica* crops, including rapeseed, dates back at least 10,000 years. Rapeseed was grown in India as early as 4000 BCE and spread to China and Japan by 2000 BCE. In most of Europe and Asia, winter rapeseed is the dominant form because it requires vernalization to flower. It is sown in autumn, overwinters as a leaf rosette at ground level, then grows a tall vertical stem the following spring, followed by lateral branches. It flowers in late spring, with pod development and ripening taking 6 to 8 weeks into midsummer. In Europe, winter rapeseed is used as an annual break crop in three- to four-year rotations with cereals like wheat and barley, and with other break crops such as peas and beans, to reduce pest and disease carryover. Winter rape is less prone to crop failure than summer varieties because it is more vigorous and can recover from pest damage.

Spring rapeseed, which does not need vernalization and is not winter-hardy, is grown in Canada, northern Europe, and Australia. It is sown in spring, and stem development begins right after germination. Rapeseed grows well on a wide range of well-drained soils, prefers a pH between 5.5 and 8.3, and has moderate tolerance to soil salinity. It is mainly wind-pollinated, but bee pollination can nearly double grain yields, depending on the cultivar. The crop is currently grown with high levels of nitrogen fertilizers, whose production generates N₂O; an estimated 3 to 5 percent of the nitrogen applied as fertilizer is converted to N₂O. Rapeseed has high nutrient demands, especially for sulfur—the highest among all arable crops. Since atmospheric sulfur inputs dropped in the 1980s, sulfur fertilization has become standard. Among micronutrients, boron, manganese, and molybdenum require special attention.

Climate change is expected to shrink the area where rapeseed can be cultivated and to reduce both crop yield and oil quality. Some researchers suggest looking for alternative *Brassica* varieties.

Major diseases of winter rapeseed include canker, light leaf spot (*Pyrenopeziza brassicae*), and alternaria and sclerotinia stem rots. Canker causes leaf spotting and premature ripening and weakens stems during autumn and winter. Fungicide treatments with conazoles or triazoles are needed in late autumn and spring for canker, while broad-spectrum fungicides are used in spring and summer for alternaria and sclerotinia. Oilseed rape cannot be planted in close rotation with itself because of soil-borne diseases such as sclerotinia, verticillium wilt, and clubroot.