Citrus

Citrus is a diverse genus of flowering trees and shrubs within the Rutaceae family. These plants are responsible for producing a wide range of famous fruits, including citrons, mandarins, and pomelos. Many of the most common crops we eat today, such as oranges, lemons, grapefruits, and limes, are actually the result of extensive hybridization. This means different species were bred together to create new varieties. Because these species hybridize so easily, the taxonomy—or the scientific naming and classification—of the genus is very complex.

To understand how a citrus fruit works, we must look at its unique structure. The fruit is a specialized type of berry called a hesperidium. It features a leathery rind known as the pericarp. The outermost layer of this rind is the exocarp, often called the flavedo or the zest. Below the zest lies the mesocarp, which is the white, spongy pith known as the albedo. The innermost layer is the endocarp, which surrounds several radial segments called carpels. Inside these carpels are locules filled with juice vesicles, which we call pulp.

The biology of these fruits provides their distinct sensory qualities. The fragrant aromas come from essential oil glands in the rind containing flavonoids and limonoids. The sharp, acidic taste is caused by a high concentration of citric acid and ascorbic acid, also known as vitamin C. Different flavors are driven by specific biochemistry; for example, the bitter taste in grapefruit comes from a flavanone called naringin. During the ripening process, the fruit undergoes chemical changes. Starches convert into sugars, acid levels decrease, and the fruit softens and changes color.

Evolutionary history shows that citrus fruits began as small, edible berries millions of years ago. Genetic mapping of plant chloroplasts suggests that citrus species diverged from a common ancestor about 15 million years ago. Around 7 million years ago, the lineage split into the Citrus genus and the Poncirus group. Some scientists consider Poncirus a separate genus, while others include it within Citrus. A 2018 DNA study published in Nature suggests the genus originated in the foothills of the Himalayas. This region includes eastern Assam in India, northern Myanmar, and western Yunnan in China.

As the climate changed during the Late Miocene, a sudden speciation event occurred. This led to the creation of several distinct species, such as the citron in South Asia and the pomelo in Mainland Southeast Asia. Other species, like the mandarin and kumquat, emerged in southeastern China. As these plants moved into Taiwan and Japan during the Early Pliocene, the tachibana orange developed. Later, during the Early Pleistocene, citrus spread into Papua New Guinea and Australia, where Australian limes formed.

Humans have played a major role in spreading citrus across the globe. During the Austronesian expansion around 1500 BCE, voyagers carried plants like the pomelo into Micronesia and Polynesia. The citron reached the Mediterranean via ancient trade routes through Persia and Egypt. Archaeobotanical evidence, such as seeds from Cyprus dated to 1200 BCE, supports this early movement. Later, Arab traders introduced lemons and sweet oranges to the Mediterranean around the 10th century CE. In the 15th and 16th centuries, the Genoese and Portuguese brought sweet oranges to Europe.

Today, citrus is a massive global industry. In 2016, world production reached 124 million tonnes, with oranges making up about half of that total. In the 2019–20 period, orange production alone was estimated at 47.5 million tonnes. Major producers include Brazil, Mexico, the European Union, and China.

To maintain high-quality crops, farmers often use grafting. This involves attaching a desired cultivar onto a hardy rootstock. However, the industry faces threats from pests and diseases. Bacteria like citrus greening and fungi like citrus black spot can damage entire plantations.