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CHAPTER 4
Horticultural Varieties of Citrus
BY ROBERT WILLARD HODGSON

Climatic Effects.
Among tree characters, period of fruit maturity is undoubtedly of greatest importance and is determined by the nature of the climate. Thus, the Valencia orange is horticulturally mature by January in the very hot, dry, low-elevation desert of southern California, but does not attain maturity until the following June in the cool, mild, equable coastal valley regions. Ability of trees to hold their fruit is also affected by the nature of the climate during and after maturity; this character is minimized by warm night conditions after maturity. Even certain aspects of tree growth habit —compactness of tree and density of foliage— are influenced by climate; these tendencies are accentuated in dry and very cool climates.

The effects of climate factors on fruit characters are much more striking, however, and are sometimes remarkable.

While there are numerous factors that may affect fruit size, climatic conditions characterized by high heat and humidity during the growing season make for large fruit. Thus, the large-fruited Washington navel orange, which attains ideal size in California, reaches undesirably large size in the humid, semitropical climates which characterize Brazil and Florida. In contrast, the small-fruited Hamlin orange never attains a commercially acceptable size in the arid, cooler subtropical climate of the southern California coastal region but does become sufficiently large in Florida and Brazil. In this general connection, it may be pointed out that the "small fruit" problem is restricted to medium-sized varieties in arid regions and in seasons of marginal or below average heat, whereas the "large fruit" problem is confined to large-fruited varieties in regions or in seasons of above-average heat or humidity.

Fruit form may be profoundly modified by climatic factors. In general, the axis is longer in regions of low atmospheric humidity, and vice versa. Thus, an oval variety, such as Shamouti orange, may range from short-oblong (almost round) to long-elliptic depending on climate. Likewise, the average shape, within any given round variety of orange, may range from subglobose to oblong, with accompanying differences in diameter-height (D/H) index from well above 1 to considerably below that value. The factors which function to increase length of the fruit also appear to favor or accentuate the tendency to develop a neck in the mandarins and a pyriform shape in the grapefruits. Therefore, the same variety may exhibit significantly different fruit forms in regions of different climatic conditions.

It has long been recognized that color is markedly affected by the temperature regime during the ripening period and thereafter. Maximum color intensity develops when the fruit is subjected to considerable chilling—normally the result of cold nights. In arid, subtropical climates, this is assured by the prevalent cool nights (associated with the wide diurnal fluctuations in temperature) which characterize the fall and winter months. Primarily because of warmer nights (associated with small diurnal temperature variations), color development in semitropical climates is much slower and the intensity ultimately attained considerably lower, with the possible exception of some of the mandarins, notably Dancy tangerine.

Other fruit characters materially affected by atmospheric humidity during the growing season include rind surface, thickness, texture and adherence, texture of the flesh (juice vesicles and carpellary membranes), and juice content. Thus, in semitropical regions such as Florida, the rind is smoother, thinner, softer, and more tightly adherent, the flesh and carpellary membranes are tenderer, and the juice content is higher than in such subtropical regions as California.

Flavor is markedly influenced by the same conditions that are primarily responsible for the intensity of color development, namely, degree of fluctuation between day and night temperature. Wide diurnal fluctuation appears to promote sugar accumulation and acid formation, and vice versa. In general, therefore, the fruit grown in arid subtropical climates is more strongly and richly flavored than that produced in semitropical or tropical climates. Fruits or varieties that are characterized by relatively high acidity, however, such as the kumquats and some of the mandarins (for example, King and Kara) and their hybrids (for example, Temple tangor and Minneola and Seminole tangelos) are more pleasantly flavored in semitropical or tropical climates, and vice versa. The same seems to be true with respect to the bitterness that characterizes most of the grapefruits and some of their hybrids (notably Sampson tangelo). Since individual tastes differ so greatly, it is meaningless to argue as to the superiority or inferiority of one over the other. The important fact is that they exhibit distinctive differences.

Likewise, the characteristics of the areole, mammilla, and navel are subject to climatic modification. In general, these modifications are more pronounced and prominent in arid than in humid climates. In the hot semitropics, the areolar furrow, which may be prominent in the arid subtropics, is usually absent and the nipple much less well developed or even suppressed to the degree that it is scarcely discernible, if at all. In subtropical regions, it has long been recognized that the size and prominence of the navel in the navel oranges varies considerably among climatic zones, from season to season, and even between the exterior and interior parts of the tree.

A most remarkable illustration of the interactions between climatic factors and fruit characteristics is afforded by the Nagpur mandarin in central India, where climatic conditions induce the production of several periods of bloom per year and, thus, provide a choice of the bloom to be employed—spring or fall. Many of the growers use the spring bloom for part of the orchard and the fall bloom for the remainder. The characteristics of the fruit in the two crops are remarkably different. The spring-bloom fruit is larger, flatter in form, paler in color, and notably less acid. Its sugar-acid ratio averages approximately double that of the fall-bloom fruit. Bonavia (1888-90) has described numerous other more striking illustrations in India, where there is doubtless a wider range of citrus species and forms and of climatic conditions than exists elsewhere.

Another less spectacular illustration is exhibited by the lemon, most varieties of which have two principal periods of bloom—spring and late summer or early fall—but are more or less everflowering and everbearing. In Italy, the crops resulting from the flowers produced at various times during the year have been named and accurately described (Casella, 1935b). There are clearly greater differences between some of them, notably Primofiori (spring) and Verdelli (summer) than between many lemon varieties.

Rootstock Effects.—Although rootstocks may effect growth habit to some degree, their principal effects on tree characters are concerned with period of fruit maturity and ability of the tree to hold or store the fruit. On certain rootstocks, notably the rough lemon and Indian or Palestine sweet lime, the acidity attained by most citrus fruits is significantly reduced as compared to such common rootstocks as sour or sweet orange. As a consequence, on these rootstocks maturity is advanced as judged by taste or measured by the sugar-acid ratio. The flavor is not rich, however, for the total soluble solids content of the juice is likewise usually low. On certain other rootstocks, notably trifoliate orange, the soluble solids are significantly increased, with the result that although a corresponding sugar-acid ratio is attained at approximately the same period the fruit has a richer flavor. Earliness of commercial maturity, as measured by either the sugar-acid ratio or acid content of the juice, may therefore be markedly affected by the rootstock employed. Closely associated with these effects is the ability of trees to hold or store the fruit. The rough lemon as a rootstock causes the fruit to lose quality—both juice content and flavor—and to drop earlier than normal, whereas the bitter orange as a rootstock seems to prolong the period of satisfactory storage on the trees.

The principal fruit characters that may be affected by rootstocks include size, color, rind thickness, juice content, and flavor. Thus, orange size is usually somewhat larger on the trifoliate and bitter orange rootstocks and smaller on sweet orange. Color is sometimes paler on the rough lemon and Palestine sweet lime rootstocks and the thickness of rind is generally greater, with a corresponding reduction in juice content. And, as brought out above, flavor is richer on certain rootstocks than on others. With the exception of flavor, however, rootstock effects on fruit characters are usually less marked than those caused by climatic factors.

Soil Influences.—That the soil may affect tree characters and fruit characteristics is a belief of long standing that finds support in observational evidence. It appears that average fruit size is larger and maturity is slightly earlier on the lighter-textured soils, but that the color is likely to be paler, the rind thicker, and the flavor poorer. And it is generally agreed that the fruit holds on the trees better on the finer-textured soils.

Soil differences are usually small, however, unless accentuated by rootstock effects. Thus, the undesirable influences of the rough lemon rootstock are increased by sandy soils and reduced slightly by silty soils.