Dear Colleagues,

Citrus is an enduring crop that has been cultivated for centuries as a food source due to its nourishing fruit as well as nutritional, health, and other related benefits. Citrus species are indigenous to the subtropical/tropical regions of Asia, Melanesia, and Australia, with domestication being hypothesized to have occurred in ancient times. Subsequent introduction throughout the world likely resulted from human migration and settlement. Citrus remains an important commercial commodity in the modern world, conveying a high value in international trade, with Citrus sinensis (orange) fruit and processed products (e.g., juices, flavorings, cosmetics, air fresheners) accounting for the majority of worldwide production. More than 20 countries cultivate citrus, with Brazil, India, China, Mexico, and the United States typically representing the top orange-producing countries in terms of acreage. In the United States, citrus production is highest overall in Florida, followed by California, Arizona, and Texas, with a commercial value normally greater than $3 billion annually.

Huanglongbing (HLB), also known as citrus greening disease, has had a dramatic impact on the profitability of citrus worldwide by ruining millions of acres of citrus crops, initially by inducing the production of unmarketable fruit and ultimately resulting in tree death. HLB is known to be the result of an extremely damaging bacterial infection spread by an insect vector, the Asian citrus psyllid. HLB was identified in the United States initially in Florida in 1998 and has since spread rapidly across citrus producing states. Much of the recent progress in citrus research has focused on limiting proliferation and overcoming this crippling disease.

Citrus species have a chemical complement rich in nutrients and related compounds such as calcium, magnesium, potassium, phosphorus, ascorbic acid (Vitamin C), folate (Vitamin B₉), niacin (Vitamin B₃), pyridoxine (Vitamin B₆), riboflavin (Vitamin B₂), and thiamin (Vitamin B₁) that are beneficial for humans. Additionally, citrus contains a broad assortment of bioactive natural products such as alkaloids, carotenoids, coumarins, flavonoids, limonoids, and simple phenolics. Many of these secondary metabolites have identified antioxidant and anti-inflammatory activities as well as anticancer, cardiovascular protective, and neuroprotective benefits. These compounds can also have other critical medicinal implications. For example, furanocoumarins have been identified as the primary causative agents of the “grapefruit effect”, whereby certain citrus juices and products can inhibit the action of P450 enzymes, influencing the effective dosage of medications by slowing their normal metabolism to inactive forms. Flavonoid compounds have also been a particular focus for study as, in addition to their medicinal effects, they directly impact the marketability of citrus fruit and products by influencing taste and other organoleptic properties.

Considering the historic as well as modern versatility of citrus, this Special Issue of Plants will examine recent advances in research with these species across a variety of scientific disciplines.

Dr. Daniel K. Owens
Guest Editor

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• agrosecurity
• biosynthesis
• crop production
• metabolic organization
• natural products
• pharmacology