THE LEMON FRUIT

GENERAL INFORMATION

Origin and History

THE LEMON has existed for so long that its origin is not known. Swingle (see Webber and Batchelor, 1943, p. 399) has the following to say with reference to its origin: "... Probably the lemon should be considered as a satellite species [a species of doubtful validity] of the citron; possibly it may prove to be of hybrid origin, perhaps having the citron and lime for parent species. As is true with the grapefruit, it is difficult to explain the origin of the lemon as a hybrid, as it crosses readily with other species of Citrus and yet, when self-pollinated, reproduces itself from seed with only very small variations. . . ."

That the first-known habitat of the lemon was Southeastern Asia, probably Southern China and Northern Burma, appears to have become fairly well established. It was introduced by the Arabs into Persia and Palestine where it was widely grown by the beginning of the twelfth century. From these countries it was apparently taken into Spain, North Africa and the Canary Islands. The lemon entered the United States, probably indirectly, from the island of Haiti where it had grown from seeds brought from the island of Gomera, one of the Canary Islands group, by Columbus on his second voyage, 1493. A much more detailed discussion of the interesting historical story of the spread of the lemon from its natural habitat into other parts of the world is given by Webber and Batchelor, 1943 (see especially pages 6, 7—9, 10, 20).

Distribution and Production
Lemons are grown in limited amounts for home use in almost every area where citrus can be grown but their commercial production is confined principally to three countries, Italy, Spain, and the United States.

Since 1938 the United States has been the world's largest producer of lemons. Their production in the United States is confined almost exclusively to California. According to the California Fruit Growers Exchange (1947, 1950), the five-year-average number of boxes of lemons produced in California and Arizona between 1924 and 1944 has been as follows:
Since 1944 the annual yields of California and Arizona lem¬ons, in round numbers, have been as follows: During the last three of these years, approximately two-thirds of the crop was consumed as fresh fruit. The standard shipping weight of a box of California or Arizona lemons is 79 pounds; previous to 1943 it was 76 pounds.

The lemon acreage in Arizona is small but is increasing. According to a recently published survey (June 2,1949) the present plantings total 814 acres. Whole groves of lemon trees in Arizona are rather rare. Most of the trees are planted in single or double rows in or bordering groves of other varieties of citrus (for reference see "Literature Cited," under "Arizona Citrus"). The State of Texas also produces a limited supply of lemons, most of which are consumed locally.

Variety
The Eureka is the principal variety of lemon grown in California. Of the total acreage of lemons in California in 1946 approximately 88 per cent consisted of Eurekas, 8 per cent of Lisbons, 2 per cent of Villafrancas, and 2 per cent of all other varieties. The Eureka variety gained predominance because of its superior quality and productiveness in most areas, and because it matures a considerable quantity of its fruit during late spring and in the summer when demand and prices are at a maximum. The Lisbon variety is preferred in south central California and some new plantings of this variety have been made in Arizona. Having a relatively dense foliage its fruit is less susceptible to such factors as sunburn and freeze injury.

As a result of their studies on the relative susceptibility of different species and varieties of citrus to freeze injury in California during the winters of 1947-48 and 1948-49, Hodgson and Wright (1950) found that the Eureka lemon was more susceptible to freezing temperatures than either the Lisbon or the Villafranca. Because of its other predominant favorable qualities it is not probable, however, that the Eureka's greater susceptibility to freeze injury will prevent it from being the principal choice for future plantings.

Fruit Set and Harvest
The lemon tree in California blooms and sets its fruit more or less continuously the year around, but the heaviest sets occur in the spring and fall. In southern California pickings are usually made every six to eight weeks throughout the year, but in the south central California and Arizona areas the pickings are mostly limited to a period of only four months, October to January.

Maturity Defined
In most species or varieties of fruits it is not difficult to distinguish by looks or taste the differences between those that are immature and those that are mature. As applied to lemon fruits, however, the terms "immature" and "mature" are rather vague. The generally accepted maturity test is based on the availability of extractible juice. According to State regulations in California and Arizona a lemon that contains a minimum of 25 per cent of juice by volume is considered to be mature, regardless of size or color. Supply and demand would not make it profitable to pick all fruits just as soon as they contain that amount of juice; they are therefore picked according to size. The rings used to deter¬mine the size of the fruit range from 22/32" to 29/32" in diameter, but the ones most commonly used are 22 6/32", 22 7/32" and 22 8/32" in diameter. The name of the sizing ring is indicated by its number of 32nds of an inch in excess of 2 inches, thus the five rings just mentioned would be called Nos. 2,9, 6, 7 and 8 picking rings. The size of the ring used for any one picking depends on local conditions and on the market's demand for fruit. When brought to the packinghouse the fruits are segregated according to color. The color designations may include all or part of the following: dark green, green, light green, silver, light silver, light yellow, yellow, and dark yellow (tree ripe). A lemon is said to be "tree ripe" when it has remained on the tree for at least several weeks after it has changed from light yellow to dark yellow in color. Some fruits are picked before they have reached the usual picking size. They are those that have turned light silver to dark yellow and have stopped or practically stopped growing. Sizes smaller than will pack 588 fruits (1.750 inches in diam.) to the box are usually sent to the products plant.

All lemon fruits used in the experiments described on the fol¬lowing pages were commercially mature, unless specially designated as immature, young, very young, or very small.

Storage
The nature of the lemon fruit is such that it can be safely stored much longer than other commercial species of citrus. Since lemons are usually picked according to size rather than color they may be green when put into the storage rooms. The demands of the market usually determine the length of time that they are kept in storage. If the demand is strong, a portion or all of the green lemons picked and taken to the packinghouses are treated at once with ethylene gas which causes them to change from green to yellow in color within a few days, generally 5 to 7. They are then ready to be placed on the market. If there is no immediate demand, the lemons are placed in storage rooms where, after several weeks, they become yellow without the ethylene treatment. Citrus fruits themselves give off a small amount of ethylene while in storage but probably not enough to cause degreening of the fruit (see section on "Respiration," p. 46).

When lemon fruits are kept in storage over extended periods, sometimes 3 to 4 months or longer, every precaution must be taken to keep them in as healthy a condition as possible. After picking, and especially if they are green, they are allowed to remain in the open on the packinghouse floor for at least 2 or 3 days until they have lost some of their turgidity, to avoid the swelling and breaking of the oil glands. They are then washed in warm or hot water containing substances which remove dirt and which kill a large portion of the surface molds and other fungi. They are then dried and placed in standard wooden packing boxes which are stacked 9 to 11 high. The standard packing box contains two compartments, each of which is 13" x 12 1/2" x 9 7/8" deep, inside measurements. Spaces are left between the stacks to permit good ventilation. The best storage rooms are equipped for producing forced-air circulation and for controlled temperature (56° to 60° F) and relative humidity (86 to 88 per cent). The ideal storage temperature for lemons varies slightly, depending on locality and age of fruit, but, in general, if it goes below 56° F it will cause such troubles as bronzing of the peel and darkening of the segment membranes. Temperatures above 60° F are conducive to other physiological disorders and to the growth of decay-producing organisms. Sufficient fresh air is introduced to approximate pure air conditions. C02 content of air should not be allowed to rise above 0.10 per cent. See MacRill, Nedvidek and Nixon (1946) for a recent description of the most favorable storage conditions of citrus fruit.

Even under the most favorable conditions, storage of lemons over long periods gives rise to many problems of fruit decay and to external and internal physiological breakdown. Most of these do not fall within the scope of this discussion. Some of the storage disorders, however, that are directly or indirectly related to respiration will be presented in the section on "Respiration." For a detailed discussion of citrus storage maladies see Fawcett (1936) and Fawcett and Klotz (1948).

Structure
The gross anatomical structure of the lemon fruit is so well known that it will not be discussed here in detail. In general, it may be said to consist of the peel and the pulp or edible portion. In the peel of the mature fruit, the first few layers of cells under the epidermis are usually called the "flavedo"; the remainder of the peel, the white portion, is called the "albedo." The oil glands which contain the essential oil of commerce are embedded primarily in the flavedo, but the larger glands may extend into the outer layers of the albedo.

The pulp, the edible portion of the fruit, is composed of segments (locules) and the central axis (sometimes called the "core"). The number of segments per fruit may vary but there are usually 9 or 10. Each segment has a membrane which covers the juice sacs (vesicles) and the seeds. The interior of the juice sac is composed of numerous thin-walled cells which contain the juice of the fruit. Each juice sac terminates at its basal end in a slender, almost threadlike, long, medium or short stalk by means of which it is attached to the segment wall. The length of the stalk depends on the position of the juice sac in the segment. All stalks are attached to the inner wall of the outer portion of the segment, the portion that is in contact with the peel.

The vascular bundles which conduct water, food materials and foods are confined to the peel and central axis. Unlike in most other fruits, no vascular bundles penetrate the fleshy portion of the lemon except where branches from the bundles in the central axis connect with the seeds in the segments. This presents a rather unique physiological situation under which liquids and substances must be transferred for relatively great distances without the aid of the usual conducting tissues.
Collectively, the tissues of the segment walls and of the central axis, in commercial terms, are called the "rag" of the fruit. A high percentage of rag is undesirable.

Citrus fruits are harvested by being cut from the branch with a "clipper." The cut is made at the distal end of the pedicel so that the calyx, receptacle and disc remain attached to the base of the fruit. Collectively these three organs constitute what, in commercial terms, is called the "button" of the fruit. A lemon fruit with the button attached is apt to survive storage conditions longer than one from which the button has been removed. (For detailed structure of citrus fruits in general, refer to Bartholomew and Reed, 1943 and, particularly of the lemon fruit, to the work of Ford, 1942-43.)

Relative Proportions of Peel and Pulp
The peel of the lemon varies a great deal in thickness, as influenced by variety, rootstock, vigor of growth, and environ¬mental factors. If the fruit is allowed to remain on the tree until it begins to change in color from green to yellow the peel slowly commences to decrease in thickness, and continues to do so as long as the fruit remains on the tree. The decrease in thickness proceeds more rapidly, of course, when the fruit is picked and placed in storage.
Chace, Wilson and Church (1921) measured the thickness of a large number of peels within a short time after the lemons were picked. They considered the peel to be thin if it measured less than 3 mm, medium if 3 to 5 mm, and thick if over 5 mm. They seldom found a peel that was more than 7 mm thick.
Instead of measuring its thickness, several investigators weighed the peel separately and expressed its weight as a percentage of the weight of the whole fruit. They report the follow¬ing minimum and maximum percentages on a fresh-weight basis: In working with Eureka lemons grown at Homestead, Florida, Stahl (1935) found that the peel weighed 57 per cent of the weight of the whole fruit.

The preceding values subtracted from 100 give the percentages in terms of pulp.

Age vs. Size
In making biochemical and physiological studies of lemon fruits, both their age and their size should be considered. Because of differences in the twigs to which they are attached, the month in which the fruit is set (Reed, 1919; Bartholomew, 1923;
Furr and Taylor, 1939), and many other factors, the length of time it takes the individual fruits of a given set to attain picking size may vary from 7 to 14 months; some may never reach standard picking size (2 1/16" diam., or larger). Reed's results are shown in table 1 and those of Furr and Taylor in table 2.
Joslyn and Marsh (1937), Harding and Lewis (1941), Sinclair and Bartholomew (1944), and others have found that small oranges contain a higher concentration of soluble solids than large oranges. Some recent unpublished data of the authors show that the same holds true for lemon fruits. For these reasons, the true biochemical and physiological characteristics of lemon fruits cannot be determined unless both their age and their size are taken into consideration.

By tagging a large number of fruits soon after they are set, samples can be obtained at intervals in which the fruits are not only all of the same given size but which are also all of the same age. By this method reliable results are assured.


Growth-Promoting Substances and Fruit Size
Recently interest has centered around the possibility that the size of citrus fruits may be increased by spraying the trees at the proper time with certain growth-promoting substances. While trying to prevent abnormal amounts of leaf and fruit drop, Stewart and Hield (1950) found that spraying a plot of lemon trees with the isobutyl ester of trichlorophenoxyacetic acid (2,4,5-T) caused a 31.9 per cent increase in yield of fruit. In another plot of lemon trees, dichlorophenoxyacetic acid (2,4-D), applied as the triethanolamine salt, caused a similar increase in yield, 24.4 per cent. These increases in yield were found to be due to an increase in fruit size. They also found that an increase in size could be obtained even though there was no increase in yield., The isopropyl ester of 2,4-D failed to cause a significant increase suits appear to support the findings of Stewart and Hield.

Chemical Changes
The chemical changes which occur in lemon fruits during growth and maturation may be very different from those occurring in the orange under similar growth conditions. The lemon fruit, unlike the orange, does not show an increase in soluble carbohydrates with the advance of the season. The organic acids are the chief soluble constituents of the juice of mature lemons but the sugars predominate in the juice of mature oranges. As lemons mature, the free acids of the juice increase and the pH decreases, but as oranges mature, the free acids in the juice decrease and the pH increases. These are some of the commonly encountered physiological differences between the two species.

Interpretation of Data
The degree of accuracy with which biochemical data can be interpreted depends to a marked degree on the terms in which the results are expressed. For example, a value may be significant if expressed on a fresh-weight basis but may not be if expressed on a dry-weight basis. Likewise, neither of these values necessarily reveals the progressive changes in the amounts of a given constituent during the processes of growth and maturation. In the carbohydrate fractions of plants, an increase in the actual amount of one constituent may so affect the dry weight as to cause an apparent, but not an actual decrease in another constituent.

Before drawing general conclusions from limited data, one should bear in mind that different environmental factors in various localities and annual variations may noticeably affect the actual and relative quantities of the chemical constituents of the fruit. For example, lemon juice contains a maximum per cent of citric acid when lemons are grown on non-retentive soil having a relatively low moisture content. As a further illustration, Cheema (1927) found the following variations in the amounts of sugars and acids in the juice of lemon fruits grown in different provinces of Italy. The water content of the lemon fruit consists of more than 70 per cent of its total weight, and a large portion of the total solids is water soluble. The total soluble constituents of the juice, there¬fore, are reported as the percentages of the fresh weight of the juice. The ash, mineral constituents, and water and alcohol-soluble carbohydrates of the peel can be satisfactorily reported as percentages of its dry weight. Calcium pectate, uronic acid, and other alcohol-insoluble components can be best expressed as percentages of the total alcohol-insoluble fraction.

If such precautions as those just indicated are taken into consideration, a more nearly accurate evaluation of analytical results is ensured.

Elbert T. Bartholomew and Walton B. Sinclair: THE LEMON FRUIT Its Composition and Product. University of California Press. Barkeley and Los Angeles. 1951