**1. Introduction**

*Stevia rebaudiana* Bertoni, also known as 'candy leaf', 'honey leaf' and 'sweet herb', is a zero-calorie source alternative to sugar or artificial sweeteners as a natural sweetener [1]. It is primarily grown in forests, mountainous areas, dry valleys and on the banks of rivers [2, 3]. There are ~230 species of the genus Stevia have been identified throughout the world based on their growth behavior and chemical compounds. Out of 230 species, *S. rebaudiana* and *S. phlebophylla* are two species that have been identified with higher steviol glycosides and a sugary taste [4–8].

SG (Steviol glycosides), including stevioside, Rebaudioside-A, B, C, D, M, and dulcoside, are secondary metabolites (diterpene glycosides), which are extracted from the leaves of the *Stevia rebaudiana* plant that are approximately 250–300 times more sweet than sucrose or cane sugar [9, 10]. According to a WHO survey, approximately 500 million people worldwide will be diabetic by 2030 [11, 12]. The use of stevia extract in high concentrations results in a licorice aftertaste [13]. Most sugar consumers prefer low-calorie, natural sweeteners in their food to reduce the risk of cardiovascular disease, obesity, diabetes, and tooth decay [14, 15]. Because of its pH stability, stevia has no influence on blood glucose and insulin levels [16]. Nonetheless, no harmful effects have been documented with its usage [17, 18].

Steviol Glycosides (SGs) have been reported as safer to use as a sweetener in Japan after the demonstration of about 40,000 studies. Extract of stevia has also been reported to be antioxidant, reduce hypertension and reduce blood pressure [7]. Stevia was formerly prohibited in the United States/Nations for commercial usage as a food ingredient in food items or food industry, but in 1995, it was approved as a dietary supplement by Food and Drug Administration (FDA) throughout the world [19]. SGs have wide use in herbal medicine to make sugarfree tonics for diabetic patients, cosmetic industries for making face creams, mouth-wash, toothpaste, and food industries for making ketchup, drinks, fruit juices sauces, desserts, and energy drinks [13]. Due to the high demand for natural sweeteners, farmers are growing *Stevia rebaudiana* on a large scale as commercial cultivation in different parts of the world from Asia to America [20]. In Europe, undefined varieties of stevia are propagated through traditional method, and they show high genetic diversity, which does not assure the production of good quality steviol glycosides.

Most of the steviol glycosides content is found just before the plant transitions from the vegetative phase to the reproductive stage and initiation of flower buds [21, 22]. Some efforts have been undertaken to alter the flowering time in stevia and the long day—short day relationship, leading to an increase in steviol glycoside content [21, 23]. According to certain research, the temporary production of SGs is also controlled by nutrient availability, temperature, and the plant's requirement for their GAs/steviol glycoside with inter/intra genotypic variation [21, 23].

In the late 1990s, the Stevia plant was first imports to India by the University of Agricultural Sciences in Bangalore, where research on plant adaptation began. After that, the Institute of Himalayan Bioresource Technology (CSIR), introduced two accessions of *Stevia rebaudiana* for cultivation and domestication in Himachal Pradesh. The production of enhanced stevia varieties/cultivars is vital for boosting steviol glycoside chemicals (Rebaudioside-A, D, & M) with the breeding process to meet the industry's continuously expanding demand. Selection of possible genotypes based on vegetative development and adequate characteristic for maximum accumulation of steviol glycoside with an increase in ontogenetic period provide for high metabolic flux. Although incredible approaches are taken in the field of agronomic practices [24–26], purification and extraction of steviol glycoside [27], seed germination, self-incompatibility, cross-pollination, and a lack of wild germplasm to access inhibit breeding approaches to achieve genetic changes in [28].

#### **1.1 Botanical description and systemic classification**

*S. rebaudiana* is a 60–80 cm tall perennial herb that is part of the Asteraceae family. However, under certain climatic conditions, it behaves as both an annual and a perennial. It is typically grown in subtropical and tropical climates. It has a brittle stem, elliptical leaves with an alternate pattern of leaf arrangement on the stem [29], and a broad root system; yet, are more vigorous when compared to wild plants [2, 30].

#### *Genetic Improvement of Stevia: A Natural Non-Calorie Sweetener DOI: http://dx.doi.org/10.5772/intechopen.105510*

Stevia plant bears small sessile leaves that are 1.2 cm wide and 3–8 cm long, serrated, lanceolate to oblong, irregularly curled upwards and elevated in the center [31]. In addition to growing procedures, the quality of stevia leaves is affected by environmental factors such as air purity, sunshine intensity, irrigation method, cleanliness, soil type during the cultivation and processing, and storage of dry leaves after harvesting. For the best growth of stevia plants, the soil pH ranges from 6.5 to 7.5 [32]. Stevia leaves have sweetness with a pleasant flavor that lasts for hours, but they also have an aftertaste of bitterness due to the presence of bitter constituents in leaf veins [33].

Stevia has a white inflorescence with pale purple corollas that are small in size and placed in an unequal sympodial cyme, loose paniculate head on the opposite side of the bract [31, 34]. "Stevia flowers have both reproductive organs are present in corymbs and coated with small 2–6 white florets" with "Stevia flowers have both reproductive organs present in corymbs coated with small 2–6 white florets" [35]. Because of the several stages of flower development, the plant takes more than 30 days to achieve its full blooming [36, 37]. The stevia flower has five little anthers and carries extremely allergenic pollen with a viability of 65 percent [38], while, the stigma is bi-lobed/bifurcated in the middle part and style is covered with anthers. Due to self-incompatibility, stevia is a cross-pollinated (insect-pollinated) crop [39, 40]. For optimum seed production in the stevia crop, three to four hives per hectare with a high density of bees are recommended [41]. Stevia is a short-day plant whose blooming is affected by the photoperiod. As a result, a photoperiod of at least 12 hours is thought to be best for stevia flowering.

In the northern hemisphere, the months of September to December are favorable for blossoming whereas, in the southern hemisphere, it begins in January and lasts through March. Flowering takes 54–104 days following seedling transplanting in the field, depends on the cultivar's day-length sensitivity, which ranges from 8 to 14 hours under short-day circumstances [39, 40]. A photoperiod of 8 hours is more favorable for flowering but does not allow for full vegetative growth due to the long dark period [42]. Stevia seeds are roughly 3 mm in size, with very little endosperm and 20 hairy pappus bristles that aid in seed distribution through wind [35]. Stevia seeds germinate poorly [43, 44] viable seeds are usually dark colored, whereas pale yellow/transparent seeds are sterile [35, 45, 46].

*Stevia rebaudiana* is one of the most important plants in Asteraceae family [44, 47–49]. In 1888, Moises Santiago Bertoni discovered *S. rebaudiana* for the first time in Paraguay [50]. In the honor of Paraguayan chemist Rebaudi, this plant was known as *Eupatorium rebaudianum* Bertoni. The name was later changed to *S. rebaudiana* and also recommended in pharmaceutical as well as in food industries [51].

A systemic classification of *Stevia rebaudiana* according to hierarchy is given below [52].

Kingdom—Plantae Division—Magnoliophyta Class—Magnoliopsida Group—Monochlamydae Order—Asterales Family—Asteraceae Tribe—Eupatorieae Genus—*Stevia* Species—*rebaudiana* Botanical name: *Stevia rebaudiana* Berto

#### **1.2 Origin and distribution**

Stevia is a South American native plant, primarily found in Paraguay, Brazil, and Argentina [53]. It is grown commercially in Canada, India, China, Brazil, Japan, The United Kingdom, the United States, Spain, Australia, Belgium, South Korea, Taiwan, Israel, and Thailand [54, 55]. Japan and China are the world's leading producers and exporters of stevia [53].

For the first time, Paraguayans and Brazilians employed the leaves of *S. rebaudiana* as a sweetener [56]. Dr. Rebaudi reported various sweetening agents such as stevioside and rebaudioside (white crystalline substance) in 1905 [57]. England attempted to grow stevia as a commercial crop in 1942, but they were unsuccessful. However, Paraguay became the first country to cultivate it commercially in 1964 [58, 59]. Following that, Japan made significant efforts to establish stevia and its cultivation as a commercial crop, as well as to conduct various studies to assess stevia's potential [60]. Nowadays, Japan is the major market for the consumption of stevia all over the world [37] and China is the largest producer as well as exporter of stevia (approx. 2–3 billion/year) in the market [17]. Furthermore, it has become well-known as a crop in several nations, including the United States, India, Korea, Canada, Indonesia, Brazil, Mexico, and Tanzania [10, 61].
