Citrus species are perennial in growth habit. The most commonly cultivated species are Citrus aurantifolia (lime), Citrus aurantium (sour or Seville orange), Citrus grandis (pummelo, shaddock), Citrus limon (lemon), Citrus medica (citron), Citrus paradisi (grapefruit), Citrus reticulata (mandarin, tangerine) and Citrus sinensis (sweet orange). Present world production of citrus is about 98.7 million tons of fresh fruit, of which 62 percent is orange, 17 percent mandarin and tangerine, 5 percent citron, 11 percent is lime and lemon and 5 percent grapefruit.(FAOSTAT, 2001). The quantity of fresh fruit entering international trade is only exceeded by banana.
Citrus originates from the wet tropics in Southeast Asia, but large-scale commercial production is found in the subtropics under irrigation. In addition to fresh fruit and juice, citrus is grown for production of oil and citric acid.
Citrus trees normally start bearing fruit from the third year after planting, but economic yields are generally obtained from the fifth year onward. For flowering in spring a period of rest or reduced growth is needed. In the subtropics the low winter temperature induce:, this rest period, but in the absence of sufficient chilling, the rest period can be induced by water deficits.
Only a small percentage of the flowers produce mature fruits; during the flowering period fall of the weaker younger fruits occurs naturally and this is called 'June drop' in the northern or the 'December drop' in the southern hemisphere. Fruits take 7 to 14 months from flowering to maturity, corresponding to a harvest season from October /November to May/June in the northern hemisphere and from April/May to November/December in the southern hemisphere. Lemons, however, have a longer flowering period and are harvested throughout the year. For most cultivars, pollination is necessary for fruit development.
During ripening the amount of acid decreases while the sugar and aromatic substances increase. The fruit is of prime quality when sugar content is high. Picking takes place when the fruits are fully mature. Colour is not always an indication of fruit maturity. Degreening is conditioned by a period of cool weather. Green, mature fruits are obtained in the humid tropics, and with early or late season harvests in the subtropics. In the subtropics, fruits that have attained full colour, yellow or orange, in late autumn or early winter have been known to turn green again in spring, when not harvested. Fruits in the humid tropics tend to be large with thin, smooth rinds, a high juice content and lower total soluble solids and acid concentration.
Citrus is cultivated between 40°N and 40°S, up to 1800 m altitude in the tropics and up to 750 m altitude in the subtropics. For large-scale production geared toward export markets the crop is not suited to humid tropics because in addition to the difficulty of achieving the right fruit colour, humidity increases the incidence of pests and diseases. Only mandarins will tolerate humid conditions to a certain extent.
The optimum mean daily temperature for growth is 23 to 30°C. Growth is markedly reduced above 38°C and below 13°C. Active root growth occurs when soil temperatures are higher than 12° C. Most citrus species tolerate light frost for short periods only. Injury is caused by a temperature of -3°C occurring over several hours. Temperatures of -8°C cause branches to wither and -10°C generally kills the tree entirely. Flowers and young fruits are particularly sensitive to frost and are shed after very short periods of temperatures slightly below 0°C. Dormant trees are less susceptible to frost. Strong wind is harmful to citrus trees because flowers and young fruits fall easily; windbreaks are provided where necessary.
Citrus is grown on soils that are sufficiently aerated and deep to allow tap roots to penetrate to the desired depths (1-2 m). Light to medium textured soils, free from stagnant water and sticky impervious layers are preferred. Areas with a high water table should be avoided. Soil physical structure is of greater importance than the chemical properties, provided sufficient magnesium and minor elements such as zinc, copper and manganese are present in an available form. Soils with pH between 5 and 8 are preferred. The annual fertilizer requirements of citrus are 100 to 200 kg/ha N, 35 to 45 kg/ha P and 50 to 160 kg/ha K. Adequate fertility is important for both fruit quality and yield.
Citrus trees are sensitive to a high salt concentration in the soil. Yield decreases due to soil salinity are: 0% at ECe 1.7, 10% at 2.3, 25% at 3.3, 50% at 4.8, and 100% at ECe 8 mmhos/cm.
Propagation of citrus trees is done mostly by bud grafting, i. e. the insertion of buds of a desired variety on to a stock grown from seed of another variety. Normally citrus trees are transplanted. Planting distances vary according to soil conditions, the general topography, the variety and the type of tree to be planted, and are generally from 4 x 4 to 8 x 8. Planting may be square, rectangular, triangular or hexagonal. On steep slopes trees are planted in terraces or along contours. Tree density varies from 200 to 800 trees/ha. Young citrus orchards are often intercropped. In high rainfall areas permanent cover crops or broad-leaved weeds may be desirable, but in drier areas the soil is often kept bare. If an orchard is interplanted the companion crop should not strongly compete with the citrus trees for water and nutrients. A legume crop is often preferred.
The table below summarises the main crop coefficients used for water management.