Citrus limon
The lemon tree thrives best in warm Mediterranean climates with mild winters, reaching a height of about 20 feet. However, it is prone to collar rot and must be planted with the graft union well above the soil and kept away from mulch. The C. limon ‘Eureka’ cultivar is likely the most widely grown, producing fruit and flowers year-round. It is an attractive, almost thornless tree and the best variety for temperate locations and coastal gardens.
The smaller and hardier ‘Meyer’ cultivar produces smaller fruit and is the ideal variety for growing in pots. It is a popular indoor plant as it bears fruit that can be consumed. Additionally, the ‘Lisbon’ cultivar is popular with commercial growers and is a reliable and heavy fruiting variety. It adapts well to hot areas, but it is thorny.
Citrus medica
Citron, also known as cedrat, is a large fruit resembling a giant lemon that measures about 6 to 12 inches in length. It has a more irregular shape and a rougher, highly fragrant skin. Citron is native to North-Western India and is widely distributed in South and South-East Asia, including Sri Lanka, Thailand, Vietnam, China, Japan, and Taiwan. Citron requires well-drained sandy soil in full sun with consistent and regular moisture.
Citron has little juice and is mainly used for marmalade and candied peel. It is a tall shrub with an ungainly habit that has young foliage with a purple tinge as well as flowers. Citron was the first citrus fruit to reach the Mediterranean and had many claimed medicinal virtues in earlier times. It was also used in perfumery. The smaller-fruited variety ‘Etrog’ has been grown in Israel for many centuries and has become part of Jewish religious custom.
The elliptic fruit is unique with finger-like projections as it ripens. It has a coarse surface that can be white or pale yellow. The seedless, pale, milky yellow sarcocarp with an acidic to slightly sweet taste is surrounded by a soft and thick pericarp. Historically, C. medica L. var sarcodactylis Swingle has been used as a temple offering in the Buddhist religion. It gained its common name “Buddha’s hand” because of its morphology that resembles a closed hand in prayer. The fruits are used in culinary applications such as candied, salad dressing, marinades, cooked foods, drinks, jams, and marmalades.
Citrus australasica
This species is endemic to the northeast coast of New South Wales and southern Queensland, Australia. Its scientific name is Citrus australasica F. Muell., also known as Microcitrus australasica var. australasica (F. Muell.) Swingle. It is commonly called finger lime, as the fruit looks like a finger in size and shape, and comes in different colors. It can be easily grown in temperate areas with slightly acidic soil rich in nutrients and high levels of organic matter. It is a shrub or small tree, growing 2 to 6 meters in height, with a compact crown and straight spines in the axils. The leaves are borne on short, wingless petioles and have an aromatic smell when crushed due to the presence of many oil glands on the leaves. The bisexual flowers are white or pale pink in color. After five months from successful pollination, the fully ripen fruits are required to picked and harvested to avoid sunburn.
The peel of ginger limes has a powerful floral lime flavor and aroma. The fruit contains many spherical juice vesicles that are effervescent and resemble caviar. The “pulp”, vesicles, or “pearls” are known as “lime caviar” and have a flavor slightly sweeter than lemon. It is becoming increasingly popular as gourmet food, seafood garnish, as well as a cocktail component. It is the most lucrative fruit as it is highly sought after by chefs and foodies of top restaurants and for the production of cordials, marmalades, and desserts. The demand for finger lime is growing, and its price has reached up to 125 Euros per kg of good quality product.
This Australian finger lime are continually being developed and selected with color varying from blue-green to pink-red. For instance, Citrus australasica var. sanguinea. Commercially, the hybrids have been raised between the C. australasica and C. limonia (rangpur lime) to give rise ‘blood lime’ as well as between C. australasica and C. macrocarpa (calamondin) to obtain ‘sunrise lime’. It was reported that finger lime can be pollinated naturally by bee (Hymenoptera), flies (Diptera), beetles (Coleoptera) and other small infects.
It is susceptible to bacterial infection such as citrus greening disease, viral infection through the vector like psyllid. A fungal infection by Diaporthe citri that will cause dark brown to black spots to develop on the foliage, twigs and fruit. Then, Phytophthora sp. is another fungi that cause root rot. The presence of psyllid only found in Asia as the psyllid that attacks finger lime is Asian citrus psyllid (Diaphorinia citri). Citrus melanose is one of the important diseases causing degradation of finger lime in Australia with small and black raised lesions often surrounded by yellow halos and cause leaf distortion. Furthermore, it is susceptible to other pests such as Australia scale insects, spined citrus bug (Biprorulus bibax), bronze orange bug (Musgraveia sulciventris), mealybugs, aphids, caterpillars of moth and butterflies, katydids, and grasshopper.
Citrus aurantifolia
Commonly known as lime, it is best suited to tropical and subtropical climates. The flesh and juice have a stronger acidity and flavor than lemon. It is an erect tree that can grow up to 15 to 20 feet. The canopy of a lime tree is more irregular and less ornamental than that of a lemon, with spiny branches. Among all varieties, Tahitian lime is the most commonly grown variety, which bears fruit all year round. On the other hand, Mexican lime has smaller fruit with high acidity, stronger flavor, and is a thornier tree.
Citrus fruits are generally consumed fresh or as juice, and are a popular refreshing drink globally. The peels of citrus fruits contain high concentrations of essential oils, which can be further processed into perfumes, soaps, and cleaning solutions due to their natural antimicrobial properties. Citrus-flavored candies are also a popular sweet treat.
In food industry, lemon-flavored marmalade is a spread enjoyed with toast and other breakfast foods. Citrus zests, such as those from oranges and lemons, are used in a variety of recipes, including cakes, pies, and marinades. Citrus fruits are rich in vitamin C and other antioxidants, making them a popular dietary supplement. Extracts from citrus fruits are often used in a variety of cosmetic products, such as lotions, shampoos, and body washes.
Lemon oil contains furocoumarin derivatives, which can cause phototoxicity, a severe skin reaction when exposed to sunlight. In one case, a woman suffered from burns while gardening because she had squeezed lemonade for the summer and then went to gardening without washing her hands. When the active compounds in lemon oil on her hands reacted with sunlight, it caused a severe burn. It is advised to thoroughly rinse hands when they are exposed to citrus. Therefore, it is recommended to use some whitening products only at night to avoid phototoxicity and achieve a better effect.
Further reading:
Abbas, F., & Fares, A. (2009). Best management practices in citrus production. Tree and Forestry Science and Biotechnology, 3(3), 1-11.
Delort, E., & Yuan, Y. M. (2018). Finger lime/The Australian Caviar—Citrus australasica. In Exotic fruits (pp. 203-210). Academic Press.
Lim, T. K., & Lim, T. K. (2012). Citrus australasica. Edible Medicinal And Non-Medicinal Plants: Volume 4, Fruits, 625-628.
Naganuma, M., Hirose, S., Nakayama, Y., Nakajima, K., & Someya, T. (1985). A study of the phototoxicity of lemon oil. Archives of dermatological research, 278, 31-36.
Sottile, F., Del Signore, M. B., & Barone, E. (2019). Ornacitrus: Citrus plants (spp.) as ornamentals. Folia Horticulturae, 31(2), 239-251.
Wang, Y., Liu, X. J., Chen, J. B., Cao, J. P., Li, X., & Sun, C. D. (2022). Citrus flavonoids and their antioxidant evaluation. Critical Reviews in Food Science and Nutrition, 62(14), 3833-3854.
