Among all the numerous ornamental bedding plants, due to its bright colors and long florescence, S. splendens belonging to Labiatae family, is most widely planted in gardens, flowerbeds, and by roadsides (Soundararajan et al., 2013). With the development of research and technology, the breeding of S. splendens has been largely improved (Liu et al., 2012), and many new varieties have been cultivated, such as S. splendens ‘Zhongchuanhong’, a tall variety breed (Hui et al., 2004), and S. splendens ‘Shenzhouhong’, a variety produced through space breeding (Fu et al., 2009).
It has been reported that, wild type S. splendens as well as its new varieties need to remove the terminal bud three or four times to meet the product requirements (Zhi et al., 2013). Through years of selection, we have found a new variety with weak apical dominance and named it as ‘Cailinghong’. Its original lateral branch develops into the first lateral branch during vegetative growth phase, which further develops into multilevel lateral branches. ‘Cailinghong’ grows and develops without the need of manual removal of the terminal bud during its whole life. At last, it forms a naturally low and spherical type plant with multilevel lateral branches and blossom exuberant, which is more suitable for landscaping. As far as we know, there is no other variety of S. splendens having such powerful branching ability as ‘Cailinghong’ does. In addition, related report about its tissue culture remains rare.
However, there are still many problems in the development of S. splendens, such as limited colors, serious pests, and diseases, which will inevitably lead to a decrease in yield and a decline in flower quality. What is more, S. splendens shows higher requirements on temperature, which adds difficulties on its reproduction (Lai et al., 2001). Thus, it is of great importance to investigate proper rapid propagation technique of S. splendens.
Recently, an indirect regeneration method that uses the callus of S. splendens has been reported. Unfortunately, the regeneration cycle is comparatively long. Moreover, under in vitro conditions, because of long-term exposure to PGRs and chemicals, the callus cells are very sensitive (Maruthi Rao et al., 2012), and they can exhibit abnormal morphological characteristics and somaclonal variation. Direct regeneration by organogenesis is an alternative method, referring to the culture of different tissues in vitro such as buds, leaves, stems, and roots, to develop new shoots (Mallón et al., 2011). In all, it is an effective method of mass propagation of plants, while there is no need for callus as an intermediate stage.
In this study, explants (receptacle, leaf, petiole, stem segment, and stem node) were selected to establish an effective method for regeneration of S. splendens ‘Cailinghong’. Then the effects of PGRs on adventitious bud induction and shoot regeneration were evaluated. The results may provide clues for further application of biotechnology on regeneration of S. splendens ‘Cailinghong’.
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