Abstract:The Stevia rebaudiana (Bert.) Bertoni is a perennial herbaceous plant and belongs to family of Asteraceae (Compositae). The contents of S. rebaudiana (Bert.) Bertoni consist mainly non-caloric stevioside and rebaudioside A sweeteners. Low seed germination is an important problem in stevia cultivation. Therefore in this study, the effects of light and temperature on germination performance of stevia seeds were studied. Stevia rebaudiana seeds were treated with two lights (light/darkness) and four different temperatures (15 ºC, 20 ºC, 25 ºC, 30 ºC). For each treatment, 50 seeds were counted and placed into petri dishes. Experiments were conducted as a randomized complete design method with three replicates. Germination was started within 6 to 10 days depending on the treatments. The highest seed germination rate (71%) was observed in darkness/25 ºC temperature and followed by darkness/20 ºC (68%) and light/25 ºC (67%). On the other hand, the lowest seed germination rate (31%) was obtained from 15 ºC in both light and dark conditions and followed by darkness/30 ºC (43%) and light/30 ºC (49%). The results showed that lower (15 ºC) and higher (30 ºC) temperatures significantly decreased the germination rates of stevia independently from the light treatments.

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IntroductionOriganum dubium Boiss. is an economically important wild oregano species in Turkey. The most important characteristic of this oregano is its high essential oil content (6%-8%). This herb is intensely harvested from wild uplands near the towns of Alanya and Gazipaşa in Antalya Province and is used to produce essential oil. Thus, natural populations have been decreasing annually. Various genotypes and chemotypes have been reported within wild populations, such as high carvacrol and high linalool types (Turgut et al., 2013). Therefore, cultivating O. dubium seems to be the most convenient way to conserve wild populations and produce stable quantities of drugs. O. dubium grows wild in Turkey, Greece, and Cyprus. O. dubium is composed of carvacrol, linalool, and linaloolcarvacrol chemotypes, as well as the thymol chemotype. The thymol, linalool, and carvacrol chemotypes have been reported in Turkish O. dubium populations, whereas those in Cyprus are only the carvacrol chemotype (Figuérédo et al., 2006). Lukas et al. (2013) reported taxonomic uncertainties concerning the section Majorana. They assessed the taxonomic status of O. onites, O. syriacum, O. dubium, and O. majorana and discussed the evolutionary relationships in the section Majorana after considering molecular, morphological, and phytochemical data. According to their results, the 'cymyl' chemotype of O. majorana L. is classified as O. dubium. The leaves and flowers of this species are used to treat gastrointestinal problems and as a diuretic, and the essential oil is used as an antirheumatic (Arnold et al., 1993). In addition, the essential oil of O. dubium has been evaluated as a pharmaceutical and industrial product (Baser et al., 1993;Vera and Chane-Ming, 1999) due to its antimicrobial and potential antioxidant activity (Karioti et al., 2006). The biological activities of O. dubium Boiss. essential oil, such as anticarcinogenic (Koparal and Zeytinoglu, 2003), antioxidant (Mezzoug et al., 2007), fungicidal (Ahmad et al., 2011;Dambolena et al., 2011), insecticidal (Tang et al., 2011), and antimicrobial properties (Nostro and Papalia, 2012 are associated with carvacrol content.

In this study, the efficiency of the different nitrogen doses (0, 5, 10, 15 and 20 kg ha-1) on biological activity levels (antioxidant and antimicrobial activity) of Stevia rebaudiana Bert. was investigated. In addition, methanol extracts were obtained by maceration method from different doses of fertilizer applied stevia. The components in methanol extracts of plants were determined by gas chromatography-mass spectrometry (GC-MS) method. Antimicrobial activities of stevia extracts were investigated by microdilution method. The antioxidant activity evaluated by 2, 2-diphenyl-1-picrylhydrazyl (DPPH) radical scavenging, ferric thiocyanate (FTC), thiobarbituric acid (TBA), reducing power, total phenol content (TPC), and total flavonoid content (TFC) methods. According to the results, the fertilizer doses effects on antimicrobial activity of stevia were not made much difference. But in antioxidant activity, there were some variations in the activity-dependent on fertilizer amount.