Roselle (Hibiscus sabdarifa L.) is an annual herbaceous shrub grown in the tropical and subtropical countries belonging to the family Malvaceae, (Morton, 1987). It has more than 300 species grown all over the world and wider genome diversity is found in Sub-Saharan Africa (Mohamed et al., 2012). According to Murdock (1959), roselle is originated in West Africa and distributed to other parts of the world by African slaves. It is known by different names in different countries such as Jamaican Sorrel, karakade, roselle, bissap, zobbo, and Queens land jelly plant (El-Naimet al., 2012). It is a tetraploid (2n = 4x = 72) which has more related chromosome number with a diploid fibre crop (2n = 2x = 36) kenaf (Akpan, 2000). Roselle mainly adapts in areas with semi-humid to a subtropical climate in the altitudinal range of 600–2000 m above sea level and annual rainfall between 400–500 mm with an annual temperature ranging between 18–35ºC. It grows and performs well in well-drained, fertile soil with high organic matter and a pH of 4.5-8 (Babatunde et al., 2002; Olasantan, 2007).
Roselle is an economically important beverage, leafy vegetable, and medicinal plant used in several local dishes (Plottoet al., 2004; Atta et al., 2011). Its swollen fleshy calyx (sepals) is an economically important part which harvested by hand, dried, and sold completely into the herbal tea and beverage industry for food preparation in sauces, jams, juices, jellies, syrups, as a flavouring, and colouring agent for food and drinks; and also leaves as a leafy vegetable; seed as a valuable food resource on account of its protein, calorie and stem is as a substantial amount of fibre (McLean, 1973; Alegbejoet al., 2003). Its chemical composition of the calyx is approximately 15–30% made up of organic acids including citric, malic, tartaric, oxalic, stearic, and -Hibiscus acid which is most likely to contribute to the tartness of the herb and its teas and its health importance in the traditional medicine in the world are mainly attributed to the water extract of the roselle calyx (mainly as a mild laxative, antipyretic and diuretic) (Quisumbing, 1951; Mohamad, 2002). It is used in folk medicine for treating hypertension, pyrexia, liver damage, and cancer, as well as for its lipid-lowering and renal effects (Ibrahim and Hussein, 2006; Eltayeib and Elaziz, 2014).
Apart from nutritional and health importance, in developing countries, roselle plays an important role in income generation and subsistence among rural farmers and is relatively easy to grow and can grow as part of the multi-cropping system (Atta et al., 2011; Ibnouf et al., 2014). Its demand has steadily increased for roselle over the past decades. Approximately 15,000 metric tons enter into world international trades each year like Germany and the United States for consumption (Nath, 2007). However, in Ethiopia growth and yield of roselle are affected by various factors, such as the growing environment and sowing time (Khattak et al., 2016), application of organic and inorganic fertilizer (Alam et al., 2016; Sunday, 2016), and variety and irrigation managements (Khalil and Yousef, 2014), growing soil type and Soil nutrient status (Anyinkeng and Mih, 2011), insect pest, weeds and pathogens (Ansari et al., 2013) and post-harvest handling (Plotto et al., 2004). It has an overall growing period of 4–6 months from seed sowing to harvesting of the calyx depending upon the growing environment and variety (Giginyu et al., 2009).
Application of organic and inorganic fertilizers on roselle was reported as important and its performance increases with an increasing level of nitrogen application (Oyewole and Mera, 2010; El Naimet al., 2017). More importantly, the application of nitrogen significantly increases the growth and yield attributes of roselle by increasing till optimum farmyard manure or nitrogen level (Giginyu and Fagbayide, 2009). Its dry calyx yield ranges 388.48- 611.91 kg ha-1 for 0-100kg Nha-1 in Benin (Haruna et al., 2011). Moreover, 1200 kg ha-1 dried calyx yield was reported in Guinea (Giginyuand Fagbayide, 2009; Haruna et al., 2011; Mohamed, 2013). However, in Ethiopia, its yield is very low compared to those reports from other countries. Its Fresh calyx yield ha− 1 (kg) ranges between 1855.8 and 2597.98 and dry calyx yield ha− 1(kg) ranges from 184.2 to 257.8 for variety WG-Hibiscus-Jamaica, whereas Fresh calyx yield ha− 1(kg) ranges between 1464.4- 1594.7 and Dry calyx yield ha− 1(kg) between 148.6-161.7 for variety WG-Hibiscus-Sudan. These data suggest an economic yield of roselle is far below (about more than 50% lower) that obtained in other countries, despite the potential of the varieties. Thus, the need for improving the economic yield of roselle by appropriate nitrogen fertilization as one agronomic strategy from recent information in Ethiopia, there are no reports on optimum nitrogen level fertilization of roselle varieties. With these rationales, the main aim of this study was (1) to evaluate the Calyx yield response of roselle varieties to different levels of Nitrogen fertilizer application and (2) nitrogen use efficiency of roselle varieties at different levels of nitrogen application.