Presented via Trans Texas Videoconference Network

Cross listed as:

PLSS 6390 (Texas A&M University-Kingsville)
PLSS 4390 (Texas A&M University-Kingsville)
HORT 422 (Texas A&M University-College Station)

Fall 2004

OBJECTIVES: Learn to distinguish between various members of the Citrus spp. by the differences in fruit anatomy. The fruits of Citrus spp. are hesperidium berries.
I. The Pericarp (Reuther, et al; 1968)
A. The Exocarp (Flavedo)

The flavedo, which contains the flavoproteins which make up the pigments that give each fruit its distinctive color. Each of the species has certain distinctive characteristics, but all of the fruits share taxonomic parts in common. The exterior surface of the fruit is the epidermis which is composed of tabular parenchyma cells, guard cells, accessory cells, and oil-gland-cover cells. A thick cuticle covers the surface and intrudes part way between the anticlinal walls of cells. Cell division continues in the epidermis until the fruit is ripe and that cuticle is thin over recently divided cells. Thin-walled parenchyma cells underlay the epidermis of the mature fruit. Within this parenchymatous tissue occur crystal idioblasts and oil glands. Parenchyma cells of the exocarp contain the chloroplasts, which give immature fruits their green color. During ripening, the chloroplasts are converted to chromoplasts - also called chromatophores. This conversion does not happen during the maturation process unless it is accompanied by cool temperatures. Thus it is entirely possible for the fruit to be mature and still green in color. A breakdown of the internal membrane system and the grana-fretwork system occurs in the chloroplasts; the osmiophilic (fat) bodies increase in size and number. The resulting loss of chlorophyll unmasks the variously colored compounds, principally carotenoids, which increase during ripening; the fruit then take on their characteristic colors. The charotenoids are probably in solution in the fat globules in the plastids. The red color in blood oranges results from materials dissolved in the vacuoles.
B. The Mesocarp (Albedo)
The mesocarp is located in the middle of the rind and is white in most citrus fruits, hence the name albedo. During Stage I, its cells are meristematic, polygonal in shape, and compactly arranged, and most of the increase in fruit size during Stage I is due to increase in thickness of the mesocarp with locular size increasing very little. Cell divisions cease in the mesocarp during Stage II, bulges develop on parenchyma cell walls, and schizogenous air spaces begin to form. The bulges become cylindrical arms as they continue to elongate, and thus the albedo cells become deeply lobed and the air spaces large. Distal ends of the cylindrical protuberances are joined to the distal ends of protuberances of other cells, and conspicuous pit fields occur in the walls where they join.
C. The Endocarp
The endocarp is the inner layer of the pericarp or the adaxial face of the hypothetical carpellary leaf. It is composed of an epidermis that lines the locules and a few adjoining layers of compactly arranged parenchyma cells. When the ovary is young, just before the petals open, the inner epidermis is a conspicuous meristematic layer with anticlinal divisions. Carpellary emergences and juice sacs appear as the petals open. The carpellary emergences, like the juice sacs, grow into the locules; the terminal cells enlarge, the middle lamella swells, and the cells become detached. Carpellary emergences occur on sepal walls and on the endocarp. Before the locules enlarge, the juice sac primordia, discussed earlier, are densely arranged. However with expansion of the locules to full size, the stalks of the juice sacs become scattered. The endocarp cells (epidermal and subepidermal) become greatly stretched, and a thick cuticle appears on the epidermis.

D. The Septa
The septa is the thin partitions between the locules. Each septum is composed of the locular membranes of adjacent fruit segments with intervening spongy mesocarp-like parenchyma. Segments are readily broken apart in C. reticulata, easily in C. sinensis, and with some force in C. paradisi, and with more force in C. grandis . Tearing occurs in the spongy tissue.
Each juice-sac-filled locule is enclosed by a single locular membrane, the entirety of which should be called the endocarp. The membrane is hypothetically the adaxial surface of the infolded carpellary leaf. The locular membrane at the septum consists of a cuticle-covered epidermis and adjacent, compactly arranged parenchyma cells. In the young ovary, periclinal divisions (meaning divisions that go around) were observed in the epidermis, but only anticlinal divisions (meaning divisions that are perpendicular to the curved surface of the fruit) were seen in the endocarpal region.
E. Central Axis
The central axis is derived from the floral axis, and the margins of the carpellarly leaf are attached to it. A cylinder of axillarly bundles occur near the middle of the fruit, ovule traces diverge, and further acropetally, the marginal carpel traces diverge. Spongy parenchyma cells with long arms occur between the vascular bundles.
F. The Seed
The ovules are short funiculus located near the upper end of the cup-shaped ovular wall. The outer part of the ovular wall is called the outer integument. The funiculus is attached to one side of it, and the integument at the point of attachment is much thickened and contains a vascular bundle running from the funiculus to the chalaza at the bottom of the cup. The bundle terminates as a bowl-shaped cup of vascular tissue. From the chalaza grow the integuments and the nucellus. The opening in the ovule wall (the micropyle) is formed by slit-shaped openings in the integuments, with the slits usually radially oriented. Most ovules of seedless fruit abort which do not enlarge, their integuments may develop seed coats.
II. Individual Citrus Fruits
A. C. medica [Citron] has fruit which is lemon yellow in color, large, 6-9 inches (15-23 cm) long, oblong, rough or warty, sometimes ridged, apex blunt pointed; rind thick, white except for the outer colored rim; pulp sparse; juice scant, acid, and somewhat bitter, or sweetish; juice sacs small, slender; seeds oval, plump, light colored, smooth.
B. C. limon [Lemon] has fruit which ripen at all seasons; ovoid or oblong and pointed at both apex and base, about 3 inches (7.6 cm) long, either smooth or rough, light yellow in color; rind thin; flesh light colored; pulp acid, juice sacs long and pointed; seed oval, pointed at micropylar end, quite smooth.
C. C. aurantifolia [Lime] has fruit which is rounded or oblong, frequently mammilate, light yellow to green; maturing irregularly throughout the year; rind thin; pulp greenish, acid; juice sacs small, slender, pointed; seeds small, oval, pointed.
D. C. grandis [Pummelo] has very large fruit, globose or pyriform, light lemon to orange in color; rind very thick, white, spongy, smooth, dotted on surface with large conspicuous oil glands; segments large, commonly 10 to 14 in number, covered with thick, leathery tissue; juice vesicles large, long tapering, only loosely adhering; pulp light colored or pink, tough; flavor acid and bitter with scant sweetness; seeds monoembryonic, numerous, large, flattened or wedge-shaped.
E. C. paradisi [Grapefruit] has large fruit, oblate, globose, pyriform from out of season bloom, light lemon or orange colored; flesh grayish or pink; juice sacs large, spindle shaped, closely packed together; flavor a mingling of acid, bitterness, and sweetness or subacid; seeds polyembryonic, large, light-colored, wedge-shaped or irregular, ridged; cotyledons white.
F. C. aurantium [Sour Orange] has orange or reddish fruit, rough, slightly aromatic, bitter; pulp acid; juice sacs spindle-shaped, rather small; core hollow when fully ripe; seed flattened and wedged toward micropylar end, marked with ridge lines.
G. C. sinensis [Sweet Orange] has fruit globose or oblate, light orange to reddish; rind smooth; pulp juicy, subacid; juice sacs spindle-shaped; seeds few or many, oblong-ovoid, plano-convex, generally broad, wedged or pointed at micropylar end, marked with oblique ridges.
H. C. reticulate [Mandarin] has fruit flattened or depressed globose, yellow to reddish orange; segments 10-14, separating readily from one another; rind loosely attached, easily removed; seeds small, beaked; cotyledons green.
III. Exercise
1. Draw and label the citrus fruits displayed in the lab. Labels should include vascular bundles, carpel walls, vesicles, epidermis, flavedo, oil glands, and albedo.
2. Make notes on each fruit comparing and contrasting by anatomical and other visual characteristics.
3. Where does the essential oil originate?
4. Where is most of the vitamin C located in citrus fruits?
5. What actually happens when citrus fruits change from green to yellow or orange?
Davies, F. S. and L. G. Albrigo. 1994. Citrus. CAB International. Wallingford, Oxon OX10 8DE, United Kingdom. Chapter 7.
Hume, H. H. 1957. Citrus fruits. Macmillan Co. N.Y.
Reuther, Walter, L. D. Batchelor, and H. J. Webber. 1968. The citrus industry. A Centennial Publication of the University of California. Revised. Volume II.