Jatropha curcas (jatropha, physic nut) is a shrub producing seeds with high oil for biodiesel production. The center of origin of jatropha is in the Central American region around Mexico. It was introduced to Africa and Asia and cultivated worldwide in the tropics and subtropics, largely on waste lands under harsh climatic conditions (Openshaw, 2000). Most of the current jatropha cultivars were obtained from naturally grown plants; thus, they are still wild and giving low seed yield with uneven fruit maturity. Progress in jatropha breeding is still limited, attributed mainly to its low genetic diversity (Tar et al., 2011). Germplasm improvement is initially the main pre-breeding work before an effective jatropha breeding program can be launched. Presently, most jatropha trees are medium-sized bushes with canopy height of 3 to 6 m and canopy diameter of 2 to 4 m when fully grown. The farmers need to regularly prune them to reduce the plant size to ease fruit harvesting. Decreasing the canopy size to facilitate production and increase seed yield per unit area is an important objective to domesticate this shrub species.
Jatropha curcas has a shorter juvenility period (≈1 to 2 months shorter) in plants grown from stem cuttings than those grown from seedlings. Jatropha can flower 4 to 6 months after transplanting with profuse fruit set. The fruits mature 2 to 3 months after flowering and the seeds contain ≈35% to 45% oil content. J. integerrima (peregrina) is a strictly ornamental species bearing bright red flowers in inflorescences. The plants grown from cuttings can flower within 6 to 8 months with very few fruits set giving a similar seed oil content as J. curcas. J. integerrima seeds have ≈40% oil but the quality is inferior to jatropha oil as a result of lower oleic-linoleic acid ratio (Popluechai, 2010).
A number of scientists have reported inheritance of plant architecture such as basal branching type in guar [Cyamopsis tetragonoloba (Liu et al., 2006)], compact dwarf plant in pigeonpea [Cajanus cajan (Dhanasekar et al., 2007)], and pillar (columnar), compact, and dwarf plants in peach [Prunus persica (Hu and Scorza, 2009; Scorza et al., 2002)]. Plant height and columnar (canopy angle) in peach were each independently controlled by a single gene (Hu and Scorza, 2009). For jatropha, there has been no report on inheritance of plant type, possibly as a result of low variability of this character available in its germplasm. However, genetic and phenotypic variability in jatropha can be created, particularly through interspecific hybridization between jatropha with a dwarf J. integerrima. Crossing between the two species can produce hybrid seeds (Dhillon et al., 2009; Muakrong et al., 2014; Parthiban et al., 2009; Sujatha and Prabakaran, 2003). Among these four reports, Muakrong et al. (2014) produced the hybrids for woody purpose, whereas Sujatha and Prabakaran (2003) produced ornamental hybrids. The other two articles reported the success in obtaining the interspecific hybrids without further pursuit.
In this study, we made interspecific crosses between tall-erect J. curcas and dwarf-spreading J. integerrima and obtained several F1, F2, BC1F1, and BC1F2 plants. The progenies were observed on canopy height and canopy angle with the objective to determine the inheritance of these two traits.
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