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.

Water Requirements

Citrus trees are evergreens and thus transpire throughout the year. Water requirements for high production vary with climate, ground cover, clean cultivation or no weed control, species and rootstock. The water requirements for grapefruit -are somewhat higher than for the other citrus species. In general total water requirements vary between 900 and 1200 mm per year.

The crop coefficients (kc) relating ETm citrus to the reference evapotranspiration (ETo) for the subtropics with winter rainfall are:

The relationships between relative yield decrease (1 - Ya/Ym) and relative evapotranspiration deficit for the total growing period of citrus are shown in the figure below.

As a perennial crop the response of citrus to water supply at a particular period of development will depend greatly on the level of water supply prior to that period during the same growing season and also the level of water supply during previous growing seasons.

In general, when water is insufficient, growth is retarded, leaves curl and drop, young fruits fall and fruits that mature are deficient in juice and inferior in quality. When the soil water depletion reaches permanent wilting point, tree growth is terminated and subsequently affects fruits and leaves, followed by twigs, branches and eventually the whole tree.

New vegetative growth in any year is influenced by residual effects of growth in previous seasons. The vegetative growth of young trees determines their final tree size and future fruit-bearing capacity. For mature trees, the growth vigour determines the replacement rate of fruit-bearing branches. Any effect of water deficit on root and leaf development may impair the number and size of fruits later in the season. Water deficits must be avoided when vegetative growth is most rapid. Prior to flowering and fruit set, however, too vigorous, luxurious growth may impair production of high quality fruit.

In citrus a rest period appears to be essential for flowering. The duration of the rest period determines the amount of flowers produced. The rest period, preferably of 2 months duration, can be induced either by low temperatures in winter (about. l0° C) in the subtropics and in the tropics by a period of water deficit (monthly rainfall or irrigation £ 50 to 60 mm). The flower bud initiation occurs during this rest period when vegetative growth is minimum. Water deficits can have, however, some harmful effects for long-term crop production as compared to when dormancy is caused by a cold period. Once the rest period is ended, an adequate water supply is necessary because prolonged water deficits will not only delay flowering but also lead to over-production of flowers. This can result in lower yields during the next season and possibly in subsequent seasons to a biennial fruit-bearing cycle. For lemons, water deficits in summer are commonly used to start off-season flowering for year-round production.

The flowering period is very sensitive to water deficits. Water deficits directly reduce fruit set; also during this period nutrition, especially nitrogen, is essential and adequate water is necessary to make the nutrients available to the crop. Moreover, water deficit during fruit set reduces yield by causing a heavy June or December fruit drop.

Water deficits during June or December (early yield formation) can increase fruit shedding and reduce the rate of fruit growth. After June or December drop, water deficits can affect the final fruit size. The increase in fruit size from June or December to maturity is highly dependent on water uptake, and the rate of enlargement of immature fruit is an indication of the need for irrigation. However, soil water depletions resulting in moderate water deficits after July or January (yield formation and ripening) can be desirable because the content of soluble solids and acids in the fruit is increased; also, tree growth is reduced which facilitates picking. In addition, a slight reduction in fruit size is often commercially desirable. When soils are fine textured, moderate water deficits after early yield formation provide better soil aeration, and diseases such as root-rot (Phytophtora spp.) may be prevented.

A more severe water deficit during summer followed by irrigation may induce out-of-season flowering which generally results in a worthless second fruit production and causes a possible reduction of yield in the following main crop. Only lemon can produce all year round without harmful effects on tree growth or yield. For other citrus species, the production of a second crop is a fairly reliable indication that the tree has been short of water at some stage.

Because of the carry-over effects of water deficits on tree growth and later yields, the relation between yield decrease and relative evapotranspiration deficit only applies when year to year water deficits are of similar magnitude . Since data are largely obtained from subtropical climates with winter rainfall and where winter rainfall is sufficient to meet the crop water requirements in winter and early spring. The relationship in the figure presenting the relationships between relative yield decrease (1 - Ya/Ym) and relative evapotranspiration deficit for the total growing period applies to water deficits only during the period just prior to flowering to early harvest. However, variation in yield per tree is always likely to be considerable.

Most citrus species develop a single tap root. The lateral roots ford a horizontal mat of feeding roots with weakly developed root hairs. Root development is largely dependent on the type of rootstock used and on the characteristics of the soil profile. Rooting depth varies between 1.20 and 2 m. In general, 60 percent of the roots are found in the first 0.5 rn, 30 percent in the second 0.5 m, and 10 percent below 1 m. Where water supply is adequate, normally 100 percent of the water is extracted from the first 1. 2 to 1. 6 m (D = 1. 2-1. 6 m) but under dry conditions the depth of water extracted below this depth increases. During prolonged periods of water deficit, soil water in a deep and well-drained soil may be utilized up to a soil depth of 2 or 3 m.

Peak water requirements are reached between flowering and June or December drop. In this period frequent irrigation is necessary. When ETm is 5 to 6 mm/day, the fraction of available soil water (p) in this period equals about 0.4 but soil water depletion may be 60 to 70 percent from July or January to the end of autumn. During the latter period less frequent irrigation is advisable because during this period citrus is less sensitive to water deficits.

Irrigation scheduling requires great caution. Citrus trees demand good soil aeration and over-irrigation is highly detrimental, particularly to young trees. Too frequent and heavy irrigations may affect root development and yield and lead to leaching of nutrients. In climates where winters are too mild to induce a rest period, irrigation should be withheld for 2 to 3 months.

The most common surface irrigation methods are furrow irrigation (several furrows between the tree rows), check irrigation (basins containing one or more trees) or flood irrigation (where citrus trees are planted on beds or ridges). Because of uneven water distribution and the difficulty of applying small amounts of water the importance of surface irrigation for citrus is decreasing.

Sprinkler irrigation may provide a more uniform distribution of water and the possibility of applying the exact depth of required water. With the drip or micro-jet systems, water savings may be obtained because water is applied only to the root zone, leaving the remaining part of the soil dry. Sprinkler irrigation is also frequently used for frost protection.

Within an orchard yield varies greatly from tree to tree, while for a single tree yield varies from year to year. Sometimes a two-year fruit bearing cycle occurs.

Good yields of citrus are: orange - between 400 and 550 fruits per tree per year corresponding to 25 to 40 tons per ha per year; grapefruit - 300 to 400 fruits per tree per year and 40 to 60 tons per ha; lemons - 30 to 45 tons per ha per year; mandarin - 20 to 30 tons per ha per year. The water utilization efficiency for harvested yield (Ey) for citrus fruits is about 2 to 5 kg/m³ with a moisture content of the fruits of about 85 percent, except for lime which contains about 70 percent moisture.