1. INTRODUCTION
Jatropha (Jatropha curcas L.) is a succulent shrub or small tree, which belongs to the large Euphorbiaceae family. It is a multi-purpose plant which has been exploited for various purposes such as soil erosion control, firewood, hedges, green manure and traditional medicines (Carels, 2013). On the other hand, the seed oil of Jatropha is also used as soap manufacturing ingredient, paints and as a biodiesel to substitute kerosene (Kumar and Sharma, 2008). Among many other attributes and importance of Jatropha, in recent years it has gotten special attention for being a priority feed stock in production of biodiesel.
Biodiesel is an alternative diesel fuel made from different types of renewable sources such as plant oils and animal fats. It is environmentally friendly fuel with low emission profiles and also non-toxic and biodegradable (Abdulla et al., 2011). Among the plant species producing raw materials for biofuels, Jatropha is one of the plant species that stimulates the highest interest in tropical and subtropical regions. It has been identified as most suitable oil seed bearing plant due to its various favorable attributes like high oil content, hardy nature, adaptability in a wide range agro-climatic conditions, need for less irrigation and less agricultural inputs, pest resistance, short gestation periods and suitable traits for easy harvesting (Heller, 1996; Edrisi et al. , 2015).
Because of the increased interest in the potential of Jatropha as an energy plant, more attention is given to methods that allow the mass production of elite material. However, several challenges remain before that plant biomass can be commercially exploited. Its supply on a large scale requires massive production of phenotypically uniform plant material of a very high quality within a short time-frame that is adapted to the growth conditions of the plantation areas (Medza Mve1et al. , 2013).
Traditionally Jatropha is propagated through seed and vegetative cutting. The most common method to obtain Jatropha plantlets is by seed germination, which can be severely limited by poor seed viability, low germination percentage, inadequate rooting in growth plants in small pots and the delayed rooting of seedlings (Openshaw, 2000). Vegetative propagation of Jatropha through stem cuttings has been achieved however the established plants are not deep rooted and hence, they easily get uprooted when cultivated in lands with poor top soil (Openshaw, 2000). Therefore, to improve cultivation of this crop, the traditional inefficient mode of propagation should be changed and proper techniques need to be studied and put in place for mass production of the Jatropha plants.
The in vitro multiplication would be a useful alternative method for mass production of plant. It offers an opportunity for large scale production of uniform disease free planting material in a relatively short period of time and independent of the season (George, 2008). Callus induction, culture and regeneration optimization in vitroare important steps in the process of plant propagation and genetic transformation (Oliveira et al. , 2017). Therefore, efficient callus induction and in vitro regeneration system is highly required to enhance the use of modern techniques in genetic improvement (Pan et al ., 2010; Jose et al., 2012). Callus also can be used for long term conservation of plant tissues by maintaining them under slow growth conditions, and as target tissue for genetic transformation (Ali et al ., 2007).
Among factors influencing the process of in vitro callus induction, the plant growth regulators (PGRs) can be considered as one of the most important ones. Two relevant classes of PGRs that are used to control organ and tissue development are the auxins and cytokinins (Ismail et al ., 2013). Both callus induction and plant regeneration from explants require the presence of appropriate concentrations and combinations of plant growth regulators in the culture media (Kalimuthu et al. , 2007). A sub-optimal culture medium may cause physiological disorders or death of tissue. Studies of auxins and cytokinins separately or their combinations to initiatein vitro callus induction and regeneration in Jatropha were reported (Sujatha et al., 2005; Deore and Johnson, 2008; Oliveiraet al. , 2017). In addition, genotypic differences in shoot organogenesis have been observed in a wide range of plant species including Jatropha. It has been reported that regeneration in Jatropha is highly genotype dependent (Kumar and Reddy, 2010; Kumar et al ., 2010; Mweu et al ., 2016). Keeping in view of the importance of the crop and its propagation methods, the present study was designed to optimize the concentrations and combinations of different growth regulators on callus induction and proliferation of shoots from three Jatropha accessions using leaf explants.