Backhousia myrtifolia

Backhousia myrtifolia was first documented and subsequently named after an English botanist James Backhouse (1794–1869), in his report on Australian landscape and wildlife titled A narrative of a visit to the Australian colonies.[10] James Backhouse was renowned not only for his publications in the field of botany, but also the friendships he formed with the indigenous Australians, whose land he was conducting research on.[10] However, prior to this B. myrtifolia was used extensively by indigenous Australians as a spice in cooking, due to its unique aroma and widespread accessibility.

However, despite these seemingly ideal properties for domestic cultivation, the species is particularly susceptible to a condition termed "post-harvest browning syndrome", which involves the discolouration, browning and wilting of the plant's stereotypically dark green leaves.[12][8][2] This browning syndrome was first documented between July and September 2008, on a 4-year stock of B. Myrt folia plants located at the University of Queensland.[13] The discoloration was visible initially as small black dots on the surface of the leaves, progressing later into larger brown stained regions.[14] This condition affected approximately 30% of the harvested plants, signifying it infects populations of {B. Myrt folia randomly, in non-specific quantities.[15]

Although research is still being conducted into its causes, "post-harvest browning syndrome" has been attributed to two primary factors:

• Fungal and/or bacterial infection[16][12]
• Prolonged heat exposure and lack of temperature control[2]

Infection of B. myrtifolia by the fungus Alternaria alternata was analysed and confirmed via Koch's postulates (a series of tests used to analyse the pathogenicity of certain bacteria and fungi).[17] Alternaria alternata was found to release certain low-molecular weight host-specific toxins (HSTs) such as tenuazonic acid, tentoxin and zinniol .[17] These toxins not only damage plant cell membranes but also cause permeability changes in the cell wall, that may contribute to plant browning and death .[16][17] In addition to this, these HST toxins may also cause disruptions in the oil gland structure of the plant's leaves, resulting in oil leakage.[12] The leakage of the plant's 4 oil chemotypes damages the efficiency of sunlight capture by chlorophylls (decreased chlorophyll fluorescence) resulting ultimately in the development of black and brown spots along the leaves' surface.[14] On the other hand, prolonged exposure to heat stress, was also found to produce free radicals in the leaves of B. myrtifolia plants.[2][15] These radicals oxidise the lipoprotein membranes surrounding cells, resulting in a loss of cellular compartmentalisation and structure, once again damaging chlorophyll content and chlorophyll fluorescence.[2] This is thought to be another significant contributing factor to the high rates of browning observed for this particular plant.[15] Therefore, it is recommended that B. myrtifolia be kept at temperatures above  in the presence of above 20mL of water, to prevent the development of post-harvest browning syndrome.[15]

This browning of leaves not only damages the visual appearance of B. myrtifolia leaves, but also eliminates the recognisable cinnamon aroma produced by the oils of the plant.[8] Therefore, since no treatment options currently exist for "post-harvest browning syndrome" (despite its high frequency), the viability of B. myrtifolia as a domestic product is decreased considerably.

Backhousia myrtifolia is used extensively as an aromatic/spice, due to the unique properties of its natural oils; methyl eugenol and elemicin. These oils combine to produce a natural cinnamon-like fragrance, and as a result B. myrtifolia can be used as a flavouring agent in biscuits, stews and deserts.[18] In addition to this the B. myrtifolia plant is rich in fibres, vitamins (Vitamin A, C, E, K and pyridoxine) and minerals (potassium, calcium and copper) which are important in maintaining homeostatic body function.[18] Vitamins function as co-enzymes in the body, accelerating the rate of biological reactions, whilst minerals such as potassium are useful in preserving cellular electrochemical gradients, and thus maintaining musculoskeletal function.[19]

Despite this, research is also being conducted into B. myrtifolia's application as an anti-bacterial or anti-fungal agent, due to the bactericidal properties of its oils. The methyl eugenol and elemicin components of B. myrtifolia's natural oils are very potent anti-septic agents, capable of inhibiting the growth of over 100 species of bacteria cumulatively. The methyl eugenol oil of B. myrtifolia is capable of breaking down the thick peptidoglycan wall of gram positive bacteria whilst, elemicin creates small pores in the outer lipopolysaccharide layer of gram negative bacteria. Through both these mechanisms, B. myrtifolia is capable of exposing the interior of bacterial cells, to the external environment, therefore killing them. Consequently, the Aqueous and methanolic leaf extracts of B. Myrtifolia were found to inhibit the growth of both Proteus mirabilis and Escherichia coli bacteria. Infection by the P. mirabilis bacterium is responsible for between 1-10% of urinary tract infections and has also been linked as a causative agent of rheumatoid arthritis, whilst E. coli is generally responsible for gastro-intestinal infections such as food poisoning.[20] In addition to this, the oils produced by the B. myrtifolia plants are non-toxic (as determined by an Artemia franciscana nauplii bioassay) further supporting their use as either an internal or topical treatment for bacterial/fungal infections.

In this same vein of thought, the anti-septic properties of Backhousia myrtifolia's oils also make it suitable for use as an insect repellent against common disease-carrying arthropods such as mosquitoes, flies and beetles.[21] Elemicin, a compound derived from the oils of B. myrtifolia leaves can replace substances such as citronella oil, which is the common active ingredient in candle and torch insect repellents.[22] The resultant naturally sourced insect repellents, would be especially useful to individuals who suffer allergic reactions to the chemicals present in conventional repellents (diethyl phthalate, dimethyl carbate etc.).[22] In countries such as Indonesia and Papua New Guinea, where the rates of mosquito spread malaria and West Nile virus are high, B. myrtifolia based repellents would be especially useful.[21]