CUCUMBER

Cucumber is herbaceous annual diploid plant with 14 chromosomes. It can be characterized as warm-season vegetable crop with an optimum temperature for growth about 25 °C and minimum around 15 °C that is extremely intolerant of cold weather. Exposure to cool conditions will slow growth even if temperatures remain above freezing. On the contrary, very high temperature causes poorer germination of pollen. Cucumbers are grown mainly for their fruits, which are derived from a single ovary containing many ovules or seeds. In some parts of the world, flowers and leaves of some species are also used for food.

Cucumbers have a relatively weak root system. Many feeder roots spread laterally in upper (20 cm) layer of soil. Above-ground part of the plant is coarse, prostrate and vining. Stems, petioles and leaves are hairy to spiny. At leaf axils unbranched tendrils develop. Vining begins after development of second or third true leaf. At this stage plant also starts to form branches and little bit later flowers also appear. Male and female flowers are on the same plant (monoecious plant). Monoecious cultivars first produce clusters of five male flowers at the leaf nodes on the main stem. Subsequently, the plant produces both male and female flowers. Most of modern varieties have female flowers only (gynoecious plant, all female flowers) and their fruits develop parthenocarpically without pollination and are seedless. If pollinated accidentally, fruit of these cultivars may be deformed by greater growth in the pollinated area. Gynoecious hybrids are widely used because they are generally earlier and more productive. The term all-female is somewhat of a misnomer, however, as 5 percent of the flowers are male under most conditions. In sensitive gynoecious cultivars, production of male flowers is promoted by the long days, high temperatures and high light of early summer. Production of male flowers also increases with high fruit load and other stresses on the plant. Production of female flowers is promoted by the short days, low temperatures and low light conditions of fall. Development of a fruit to the stage of consuming maturity can take two to three weeks only. Fruits of some varieties are harvested very early (gherkins) and are used for pickling. Fruits of other varieties are harvested after having finished their growth and are usually used for salads (slicing). Pickling cucumbers are usually blunt, sometimes angular, light green, warty, and covered by sparse rough or dense fine black spines. Slicing cucumbers are usually long, dark green, smooth and glossy but warty and spinny varieties also exist. Fruits are mainly eaten fresh as snack, as a part of salads or pickled. Nutritionally, cucumber has rather low value. It contents about 97 % of water. Energy content is very low, about 52 kJ/100 g of fresh matter. Most of vitamin A is removed with green rind and vitamin C content is moderate only, nevertheless one cucumber of average size can be source of about 13 % and 27 % of daily need of both vitamins, respectively. Cucumbers act as refreshing and basifying vegetable, but are not acceptable for all persons.

Varieties:
From the genetical point of view, existing cucumber varieties are either classic populations obtained by classic breeding or F1 hybrids which are obtained from special parental inbred lines by the crosspollination. In this case all plants coming from one crosspollination are genetically uniform. The advantage of F hybrids is in so called heterosis effect, it means that offspring of certain two inbred lines give much higher yield and quality than parental lines. To achieve this effect for each generation of seeds parental lines have to be crosspollinated (it is not possible to further cultivate plants from seeds from F1 generation, because in F2 generation properties of plants would segregate). That is why seeds of F1 hybrids are more expensive, but because of their properties they become to be prevalent in the assortment. Varieties and hybrids are also bred for resistance to some diseases, pests and absence of bitterness.

LIFE CYCLE

Crop establishment:

Cucumbers are usually sown directly into the field. Sandy and humus, permeable soils are preferred for cucumber production because they warm up faster in the spring. Soil should be well drained and rich in organic matter, pH should be between 6.0 and 6.5. In heavy and poorly drained soils the root system suffers from oxygen deficiency. At least perennial weeds should not be present on the field. Cucumber seedlings are very susceptible to residual herbicides. That is why the field for their cultivation should be carefully chosen. Good manure should be applied during the autumn. The optimal term for sowing is at the beginning of May so that the emergence occurs after last spring frosts. To ensure satisfactory stand establishment, soil temperatures should be at least 16 °C. The higher the soil temperature, the more rapidly seedlings emerge and the less vulnerable they are to seed corn maggots and damping-off diseases. Percentage of germination also increases with increasing temperature. At 16 °C, 9 to 16 days are required for seedlings to emerge. At 21 °C, only 5 to 6 days are required. Like most cucurbits, cucumbers do not transplant well and transplant costs would be hard to recover. Only on small plots it is possible to practise planting of transplants because they are rather vulnerable to be damaged both by handling and by wind after planting. In this case, 2-3 weeks old plants in containers are the best suitable and should be planted in mid-May. Emerged or transplanted plants are very susceptible to cold soil, wind erosion and chilling. If cold weather is forecast white textile can be used for covering of smaller plots.

Maintenance:

Irrigation and proper fertilization are essential for good yield and quality. Potassium and phosphorus are important for good fruit shape, and nitrogen for good fruit colour. Low potassium prevents the stem end of the fruit from developing properly. Low phosphorus will give the fruit a dull bronze-green coloration. Low nitrogen will result in light green leaves and fruit. There is some evidence that excessive nitrogen before flowering can delay the onset of flowering, but low N during fruiting can also reduce the number of fruits that develop on the vine. Fruit set and fruit quality are also highly dependent on water availability. On the average, cucumbers need 30 - 40 mm of water every week, with more needed in hot, dry weather. Under stress conditions, fruits nearing harvest form bitter-tasting cucurbitacins in the skin near the stem attachment. Some types of cucumbers have a 'bi' or bitter-free gene and never produce cucurbitacins. Unfortunately, these bitter-free cultivars are also less resistant to spider mite damage. Cucumber vines can be trained on trellises to save space and improve yield and fruit quality but the high cost of trellising makes commercial production by this method uneconomical in most cases. Greenhouse cucumbers must be trellised, however, because the long fruit bend if they rest on the ground. First fruits are normally harvested at the end of June or beginning of July and harvest could last to the end of August or beginning of September but usually it is stopped earlier by downy mildew. Setting of new fruits is blocked by older ones, so frequent harvests lead to higher number of harvested fruits. Seeds can be obtained from fully ripened fruits. Ripening can take additional three weeks after consuming maturity.