Essential Oils Bearing Plants of Cameroon – An Update

In Cameroon, several plants are used both for their medicinal properties and as food. Many of them contain essential oils as one of their secondary metabolites. Although traditional healers rarely, if ever, use essential oils as the basis for their treatments, it is thought that the latter would be found in their products in view of the preparation processes used. And therefore several activities attributed to their drugs are linked to the presence of essential oils. Many studies have revealed the presence of essential oils in several plants found in Cameroon, and validated their traditional use. The purpose of this study is to identify these plants on the basis of studies carried out in order to Review Article Fogang et al.; JPRI, 33(42A): 163-207, 2021; Article no.JPRI.72567 164 make a review which will serve as support for future studies. This work reviewed available and accessible original articles in EBSCO, Ovid MEDLINE ® , PubMed ® , ScienceDirect TM , Scopus ® and Web of Science databases on Cameroon plants studied for their essential oils. We have identified ninety plants that have been the subject of proven studies and scientific publication. They are mainly distributed in the families of Lamiaceae (14: 16%), Annonaceae (11: 12%), Rutaceae (10: 11%), Asteraceae (10 : 11%) and Myrtaceae (09 : 10%). We believe that an important work remains to be done in the search for new essential oils in Cameroon, because the country is endowed with a very great botanical diversity.


INTRODUCTION
Covered, for its southern half, by the forest of the Congo Basin, Cameroon is an important reserve of global biodiversity and is home to hundreds of endemic species. The country has one of the most important biological diversity in Africa in terms of variety, quantity, ecosystem and genetic resources. With a high rate of endemism, Cameroon ranks 7 th in the world in terms of flora richness [1]. The country is home to 92% of Africa's ecosystems and the rich biodiversity contains 8260 identified plant speciesincluding 156 endemic species [2]. Plants play an essential role in the life of the Cameroonian populations. Either consumed or sold, they represent a significant part of their income. In Cameroon, several plants are used both for their medicinal properties and as food. Many of them contain essential oils as one of their secondary metabolites. Although traditional healers rarely, if ever, use essential oils as the basis for their treatments, it is thought that the latter would be found in their products in view of the preparation processes used. And therefore several activities attributed to their drugs could be linked to the presence of essential oils. Many studies have revealed the presence of essential oils in several plants found in Cameroon, and validated their traditional use.
Essential oils are complex mixtures of volatile compounds produced by living organisms and isolated by physical means only (pressing and distillation) from a whole plant or plant part of known taxonomic origin [3]. They are important aromatic components of herbs and spices and their biological activities have been known and utilised since ancient times in perfumery, food preservation, flavouring, and medicine. They act biologically according to several mechanisms that justify their use as an antibiotic, antiinflammatory, antioxidant and many others.
The market value of essential oils worldwide is expected to grow from around 17 billion U.S.
dollars in 2017 to about 27 billion dollars by 2022 [4]. The five major producers of essential oils across the world are China, India, indonesia, Sri Lanka and Vietnam [5]. The production and sales in Africa are not very important. Nevertheless, some African countries are recognised worlwide as essential oils producers. In 2019, the major african countries producers of essential oils were Egypt, Madagascar, South Africa, Morocco, Tunisia, Comoros, Kenya, Ethiopia, Tanzania and Eswatini [6]. In this ranking, Cameroon is far behind. The Cameroon trade of essentials oils are as follow ; 111000 dollars of exportation versus 346000 dollars of importation, with therefore a loss of trade value of 335,000 dollars [6]. Knowing the floristic richness of Cameroon as well as its climate favorable to the growth of numerous species of plants, we believe that a concise inventory of essential oils bearing plants would make it possible to identify the most important ones in order to develop their production on a large scale and thus rank in the first ranks of African and world producers of essential oils. It is a real part of the national economy which remains very underexploited.

MATERIALS AND METHODS
This work reviewed available and accessible original articles in EBSCO, Ovid MEDLINE ® , PubMed ® , ScienceDirect TM , Scopus ® and Web of Science TM databases on recent literature, which cover the years 2000 and 2021 on Cameroon plants studied for their essential oils.
Traditional Medicine : Diuretic, antifungal, wound healing, and in the treatment of respiratory andinflammatory disorders.
Traditional Medicine : the seeds are used to eliminate lice the stem bark to kill intestinal worms and the root bark to treat cancer. In Congo, the plant is prescribed as an additive in medicine for asthma and adecoction of the stem barks is used to cure cephalgia [9].
Spice, heart and gastric troubles, stomach ache, carminative help and reduce the effects of alcohol, and reduces asthma attacks

Spice
In Kenya, South Africa and Tanzania, the decoctions of the leaves and roots are used against venereal disease, eye injury, skin rash and pimple, snake bite, wound and as astringent, detoxifying, antalgic and aphrodisiac. The root is used for chest complaints and the twigs for gonorrhoea and syphilis. The smoke of the leaf and the root are used for pulmonary tuberculosis [32] Gramineae / Poaceae herpes, headache, bleeding, hyperglycemia, insect bites, cleening blood, kidney infections [52] Bioactivity : Antibacterial Cymbopogon giganteus Chiov. Spice remedy for child s cough, heart rate, worms, constipation, hernia abscesses, and boils. Antimalarial [58] [57] [58] Hyptis lanceolata Poit. [59] Hyptis spicigeraLam       Thymus vulgarisL.
Bioactivity :Antimicrobial, antimalarial   Abdominal pain, asthma, appendicitis, toothache, spice [15] Bioactivity : Some components were found to be moderately active against lung carcinoma cells (A549), colorectal adenocarcinoma cells (DLD-1) and normal cells (WS1) [15] [94]       Traditional Medicine : sterility in women, schizophrenia, and is believed to have a positive effect on the sympathetic nervous system. There is a general agreement between local Cameroonian populations that the plant brings strength and peace to families with twins [101].
The Table 1 also presents the main traditional uses of these plants as well as the demonstrated biological activities. We have identified 84 situations (diseases, symptoms, syndromes and food) for which these plants are used. Fig. 2 shows the percentage of 25 recurrent situations for which these plants are used or for which biological activities have been demonstrated. Infectious diseases occupy the first rank of uses with 12.62%, followed by spices (6.46%), fungal infections (6.15%) and malaria (5.85%). These four uses alone represent 31.08%.

CONCLUSION
This review presented an exhaustive report of essential oil plants found in Cameroon, as well as their use in traditional medicine and their demonstrated biological activities. However, many of them remain extremely underexploited in terms of their essential oil content. The latter represent a real advantage, both medicinal and economic, for the populations in particular and for Cameroon in general. A good selection of the most important plants from the point of view of the composition of their essential oils, with the objective of cultivating them on a large scale, would allow Cameroon to rank among the African and world producers of essential oilsand develop its pharmacopoeia.

DATA AVAILABILITY STATEMENT
The data presented in this study are available on request from the corresponding authors.

CONSENT
It is not applicable.

ETHICAL APPROVAL
It is not applicable.