**1. Introduction**

Plants are a valuable source of various secondary metabolites used as pharmaceuticals, agrochemicals, flavors, fragrances, colors, biopesticides, and food additives [1]. Plant secondary metabolites (PSM) are low molecular weight organic compounds that do not directly affect plant growth and development but have essential roles as a defensive tool in interacting with the environment and adapting to environmental conditions. Under natural conditions, many PSMs accumulate in different parts of plants (vacuoles, specialized glands, trichomes, and sometimes only at certain developmental stages) to provide functional flexibility under the influence of environmental factors without affecting cellular and physiological developmental pathways [2]. Since these compounds have many properties, such as antioxidant, antimalarial, antifungal, antimicrobial, and antiviral, they are essential in protecting the plant's defense system due to their toxicity. Thus, removing other microbes and herbivores protects them from all kinds of pathogens [3]. PSMs are diverse and numerous chemical compounds derived from primary metabolic pathways by the plant cell. There are over 100,000 known PSMs in the plant kingdom. These compounds are divided into three major classes according to their chemical structures: terpenes, nitrogencontaining compounds (e.g., alkaloids and glucosinolates), and phenolic compounds (e.g., phenylpropanoids and flavonoids) [4].

Terpenes are the largest and most structurally diverse group of natural products, with more than 80,000 characterized compounds [5, 6]. In addition to being a pigment, flavoring and solvent, terpenes have many functions, such as medical thermoprotectant and signal transduction processes. Although there are many types and varieties, it is impressive that different organisms use terpenes for common purposes. Many living organisms, such as microorganisms, fungi, and plants, are protected from abiotic and biotic stresses thanks to their synthesized terpenes [7]. Terpenes are aromatic metabolites found in plants and can improve plants' adaptation to the environment. Some terpenes are of enormous value to humanity due to their application in medicine, industry, and agriculture. To date, 52 antimicrobial terpenes have been identified, including carvacrol, thymol, menthol, geraniol, carnosic acid, quercetin, and allicin [8, 9]. Other terpenes, such as beta-myrcene limonene, pinene, and caryophyllene, may be safe and cost-effective alternatives for treating malaria in the pharmaceutical industry because they have antiplasmodial potential [10, 11]. Recent studies suggest that another terpene, Tanshinone IIA, can prevent the occurrence of atherosclerosis and damage and hypertrophy of the heart [12]. Perillyl alcohol is a monocyclic monoterpene and is of great interest due to its potent antitumor activity.

On the other hand, geraniol has been found to have therapeutic effects on cancer diseases, such as lung, colon, prostate, pancreas, and liver. Artemisinin and its derivatives have been reported to affect tumors significantly and have specific inhibitory effects at low costs [13]. However, terpenes are challenging to produce in large quantities due to their complex chemical structure and low content [14]. Plant production *in vitro* conditions is preferred because it allows the production of plantspecific metabolites using elicitor and precursor compounds and even the increase in the number of metabolites and the synthesis of new metabolites. Elicitors can be defined as a substance that, when delivered to a living cell system in a small concentration, initiates or increases the biosynthesis of specific compounds. Different types of stress used as elicitors may promote or inhibit terpene production [15]. Abiotic elicitors, such as salinity, UV light, temperature, pH, and heavy metals, can stimulate the accumulation of terpenes [16].

Salinity is one environmental factor limiting growth, development, and productivity among the abiotic stress variety. Under salinity conditions, terpenes protect cells from ion-induced oxidative damage [17]. It can also increase the tolerance of biological activity of some plant species, especially *in vitro* conditions. This trend is noteworthy because of the interesting biological properties of terpenes. This approach, which leads to the overproduction of terpenes, is highly desirable, especially in some medicinal plants. Plant production of terpenes against biotic and abiotic stress has been widely studied [18], and in some studies, it has been proven that salinity causes the accumulation of terpenes [16, 19, 20].

Recently, various strategies have been developed to synthesize terpenes, such as optimization of culture media, elicitation, use of precursors, bioreactor cultures, metabolic engineering, immobilization, and biotransformation methods [1]. This section focuses on the effect of salinity on terpene production in medicinal plants grown *in vitro* conditions.
