SCIENTIFIC NAME: Citrus species
FAMILY: Rutaceae

Although the following is written specifically for citrus (over time citrus have been intensively studied and documented) more and more we are realising that other fruit are often influenced in similar ways. The notes to follow are written to give you, the reader, some insight into factors which influence fruit characteristics of citrus and other tropical fruits.

Navel oranges reach ideal size in the warm, dry, sunny climate of California but become commercially too large in the humid climates of Florida and Brazil, while the Hamlin orange is too small in California, but reaches acceptable size in Florida.

Fruit form is influenced by climate. The length of the axis is longer in regions of low humidity and shorter under high humidities. This influences the shape of oranges. In grapefruit and mandarin this tends to produce a marked neck on fruit grown in dry climates. This may in fact be a temperate effect because day/night temperatures are more even in areas of higher humidity.

Colour development is markedly affected by temperature with the best colour developing in cold climate areas and the least colour in tropical areas. The difference between day and night temperatures is the major effect. This difference is usually greatest in the arid subtropics and least in the wet tropical areas.

Other fruit characteristics affected by humidity include smoothness, thickness, texture and adherence of the rind, texture of flesh, i.e. coarse or fine, and juice content. In the semi-tropics of Florida for mandarins, the skin is smoother, thinner, softer and more adherent to the flesh and the flesh is thinner and more tender than in fruit grown in the drier subtropical climate of California.

Flavour is influenced by the same climatic influences that affect rind colour i.e. a wide day-to-night temperature range gives more sugar and acid formation and therefore a better-flavoured fruit than in moist tropical areas.

Fruits which are generally very acid e.g. limes, kumquats, some mandarins and grapefruit are more pleasantly-flavoured when grown in the tropics. This last statement does, however, depend on individual taste preferences and the relative values will therefore differ with each individual palate.

The nipple of navel oranges is usually more developed in a cold climate than in the tropics.

The Nagpur mandarin of India flowers several times per year and there is wide flavour and texture differences between the Spring and Autumn-set fruit. Orchards are managed to give two crops per year from different blocks of trees within the orchard.

Rootstock affects the period of fruit maturity and the ability of the tree to hold fruit. Rough (Bush) lemon rootstock gives a less acid fruit than sweet orange rootstock, which usually gives more finely-textured fruit. The lower acid in fruit from trees grown on Rough lemon gives (by most people's standards) a less attractive flavour than fruit from the sweet orange rootstock.

Fruit from trees grown on rough lemon stock usually overmatures (loses quality) more quickly than from trees on sweet or sour orange rootstocks.

Fruit size, colour, rind thickness, juice content and flavour are all influenced by rootstocks. Rootstock effects are usually less than the variations caused by climate.

Heat units, i.e. the number of hours per day x the number of degrees of temperature (above a certain base level), profoundly affect the season of maturity i.e. early, mid-season or late-maturing varieties. Differences are large between species as well as between varieties within each species.

Some varieties have the ability to hold their fruit on the tree for extended periods with minimum loss of flavour. The Valencia Orange is the outstanding example of this.

I hope I have drawn your attention to some of the factors which influence fruit quality in citrus. No doubt similar effects will be noticed within many of the tropical fruits. This discourse should help you understand some of the factors which change the characteristics of fruit from district to district.

Reference: The Citrus Industry by W. Reuther, C.D. Bachelor and H.J. Webber,
University of California Press 1967.

Jim Wait, Cairns

DATE: November 1988

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