The citrus tree is a member of the flowering plant genus Citrus, which belongs to the Rutaceae family, commonly known as the rue family. These plants are native to the subtropical and tropical regions of Southeast Asia, East Asia, Melanesia, and Australia. Citrus fruits, such as oranges, lemons, and limes, are among the most widely cultivated and commercially important fruit crops globally. They are valued for their culinary uses and nutritional content, supporting major agricultural industries across the world.
Defining Characteristics of the Genus Citrus
Citrus plants are small to moderate-sized trees or shrubs that are typically evergreen. The height of these plants can range from about 16 to 49 feet, though many cultivated varieties are kept smaller to facilitate harvesting. Their leaves are simple, often shiny and dark green on the upper surface, and remain on the tree throughout the year.
A defining feature is the presence of specialized oil glands found throughout the foliage, wood, and fruit rind. When the leaves are crushed, these glands release aromatic compounds, primarily limonoids and flavonoids, which give the plant its characteristic fragrance. This strong scent is thought to function as a defense mechanism against certain plant parasites and herbivores.
The branches of many wild and some cultivated citrus varieties are armed with sharp, stiff thorns, especially on new growth. The flowers of the citrus tree are highly fragrant and usually appear singly or in small clusters. These blossoms are generally white and waxy, featuring five petals and numerous stamens, and their intense fragrance attracts pollinating insects.
The Unique Anatomy of Citrus Fruit
The fruit produced by a citrus tree is botanically classified as a hesperidium, a specialized type of berry. This fruit structure develops from the ovary of the flower and is composed of a leathery rind and a fleshy, segmented interior. The rind, or pericarp, is divided into two main layers: the exocarp and the mesocarp.
The outermost layer is the exocarp, known as the flavedo, which contains the pigments and the majority of the aromatic oil glands. This is the colored part of the peel, often referred to as the zest, and the volatile oils within it are responsible for the fruit’s familiar scent. Beneath the flavedo is the mesocarp, or albedo, which is the white, spongy layer commonly called the pith.
The edible interior of the fruit is the endocarp, separated into multiple segments by thin membranes. Inside these segments, the juice is held within numerous small, teardrop-shaped structures known as juice vesicles or sacs. These vesicles contain the pulp and liquid that give citrus fruits their juicy texture and characteristic sharp flavor.
Common Varieties and Hybridization
The vast array of citrus fruits available today, including lemons, sweet oranges, and grapefruits, are the result of extensive natural and cultivated hybridization. The diversity seen in commercial citrus originated from a small number of core ancestral species. Scientists generally identify three primary foundational species: the citron (Citrus medica), the pomelo (Citrus maxima), and the mandarin orange (Citrus reticulata).
These ancestral species originated in distinct regions of Asia and are genetically unique, but they can interbreed freely. Most commercially grown citrus varieties are complex hybrids derived from the interbreeding of these three types over thousands of years. For instance, a sweet orange is a hybrid of a pomelo and a mandarin, and a lemon is a hybrid between a sour orange and a citron.
The process of hybridization has led to the creation of many distinct fruit types. Some modern fruits are the result of backcrossing, where a hybrid is crossed again with one of the original ancestral species. This continuous mixing of genetic material explains the wide range of sizes, colors, and flavors found across the Citrus genus.