Gentiana (Gentianaceae) comprise 361 species distributed worldwide that are mainly associated with an alpine/mountain habitat (Ho and Liu, 2001; Mabberley, 2008). Most Gentiana species and their hybrids (gentians) are perennial herbs, which use preformed crown buds as part of the perennating structure. For use as an ornamental plant, floral productivity of gentians is associated with the growth and development of these crown buds (Samarakoon et al., 2010, 2012), and the timely initiation of an adequate number of crown buds on plants is perceived as a limiting factor during vegetative propagation in commercial nurseries. However, little is known about the origin and development of these crown buds.
In seedlings of gentians, crown buds arise in the transition zone between shoot and root (Samarakoon et al., 2013) as separate units comprising several associated buds with one such unit comprising two or more buds referred to as a bud cluster (Samarakoon et al., 2012). Bud clusters are also found in other families such as in Myrtaceae (Burrows et al., 2008) and Cucurbitaceae (Gerrath et al., 2008); however, crown bud clusters in gentians have barely been studied previously. An understanding of morphological features associated with the origins of crown buds and bud clusters, and plant ontogeny, is required to develop techniques for manipulation of bud initiation and shoot emergence for horticulture.
In terms of types of buds, axillary buds are typically of exogenous origin, whereas adventitious buds develop from either endogenous or exogenous tissue (Evert, 2006). Because adventitious shoots are more juvenile compared with axillary shoots (Del Tredici, 2001; Vesk and Westoby, 2004), even if shoots of these two types emerged at the same time, they are likely to develop differently, leading to variation in times at which shoots reach anthesis, a feature of interest in their production for ornamental use. However, in our previous study (Samarakoon et al., 2013), whether the buds within the cluster were axillary or adventitious was not addressed. The presence of a node and a leaf base is regarded as a prerequisite for axillary bud formation (Cutler et al., 2008; Evert, 2006; McConnell and Barton, 1998), whereas adventitious buds are not being related to a leaf axil (Cutler et al., 2008) and can develop from various plant organs (Evert, 2006; Kerstetter and Hake, 1997). In terms of vascular connections and alterations, the presence of an axillary bud at a node created gaps in the vascular cambium of the main shoot (Cutler et al., 2008), whereas adventitious buds could arise endogenously from the periphery of the cambium of the vascular cylinder or parenchyma (Bowes, 1996; Evert, 2006) or exogenously from more superficial tissue like the epidermis (Evert, 2006). In the current study, these differences between adventitious and axillary buds, their location of formation, presence of gaps in the vascular ring, vascular connections to the primary stem, and association with scale leaves were identified as strategies to demarcate axillary vs. adventitious buds.
Because individual crown buds colocated within a cluster are the source of floral shoots, their origin is of great importance in controlling the number of shoots, timing of emergence, and duration to flower. In our previous research with gentian, 27% of crown buds did not emerge within a growth cycle, presumably as a result of paradormancy (Samarakoon et al., 2012). Buds within the cluster could be axillary or adventitious and, if axillary, paradormancy could exist within the buds of the cluster. To identify the growth potential of crown buds within the cluster therefore, a prior understanding of their hierarchical arrangement was required.
We have been using species and hybrids of Gentiana triflora × G. scabra as a model to understand the morphology and ontogeny of crown buds in gentians (Samarakoon et al., 2013). Like with similar studies with other herbaceous (Chiatante et al., 2008; Myers et al., 1964) and woody perennials (Burrows, 2000; Saravitz et al., 1993; Woo and Wetzstein, 2008), the type of buds that was present was determined by tracing the vascular connection of the buds to the tissue of origin and the morphological identity of this tissue (i.e., shoot/stem, root or transition zone). A similar anatomical approach was applied to gentians to confirm whether bud clusters, and buds within them, are axillary or adventitious in origin; the current study detailed the anatomy and morphology of both bud clusters and the buds within these clusters.
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