Citrus

Citrus plants are trees and shrubs.

They grow in warm, sunny lands. Many of these plants come from Asia.

Some fruits are very large. Others are small. Many fruits are made by mixing different plants together.

This mixing makes oranges, lemons, and limes.

These fruits have a thick skin. Inside, they are full of juice.

Citrus fruits smell very good. They also have a sharp taste. People all over the world love to eat them.

Citrus plants are shrubs or trees. They grow in warm, sunny places. Most citrus plants come from parts of Asia.

Many fruits we eat are hybrids. A hybrid is a plant made by mixing two different kinds. Most common fruits, like oranges and lemons, come from mixing three main types. These are the mandarin, the pomelo, and the citron.

The fruit has a special part called a rind. This is the thick skin. The outer part is the flavedo, which is the zest. The middle part is the white, spongy pith.

Inside the fruit are juice sacs. These sacs hold the juice that gives the fruit its sharp taste. Citrus fruits also have a strong smell. This smell comes from oils in the skin.

People grow many citrus crops today. In 2016, the world grew 124 million tonnes of citrus. Oranges made up about half of that amount. Brazil and Mexico are large producers.

Citrus plants are special trees and shrubs that produce many fruits we love. They belong to a group called the genus Citrus. These plants are famous for their bright colors and strong smells. Most of the fruits we eat, like oranges and lemons, are actually hybrids. A hybrid is a plant made by mixing different species together. This mixing has created a huge variety of flavors and looks.

Every citrus fruit has a unique way it works inside. The fruit is a special kind of berry called a hesperidium. It has a thick skin known as the pericarp. The outer layer is the flavedo, which is the colorful zest. Below that is the mesocarp, which is the white, spongy pith. Inside, the fruit is split into segments called carpels. These segments hold juice vesicles, which are the little sacs of pulp.

Scientists believe these plants have a very long history. They likely evolved from small berries millions of years ago. About 15 million years ago, they split from a common ancestor. A DNA study in 2018 found they likely began in the foothills of the Himalayas. This area includes parts of India, China, and Myanmar. Over time, they spread across Asia and into places like Australia.

Humans have moved citrus plants all around the world. Long ago, travelers carried them to islands in the Pacific. Later, trade routes brought them to the Middle East and the Mediterranean. We find very old evidence of citrus seeds in Cyprus from 1200 BCE. In the 15th and 16th centuries, people brought sweet oranges to Europe. Spanish colonists eventually brought oranges to the state of Florida.

Today, citrus is a huge part of how the world eats. In 2016, the world grew 124 million tonnes of citrus fruit. About half of that total was just oranges. Large countries like Brazil, Mexico, and China lead the way in growing them. Many farmers use a method called grafting to grow better trees. This means they attach a desired fruit branch onto a strong rootstock.

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.