In recent years, new gentian cultivars (particularly Gentiana triflora Pall and Gentiana scabra Bunge) have been released for production of cut flowers or potted plants. As a herbaceous perennial, gentians overwinter as a crown comprised of an underground stem, various sized buds (crown buds), storage roots, and feeding roots (Ohkawa, 1983). Crown buds arise as separate units comprising several associated buds, with one such unit comprising two or more buds arranged in a spiral pattern along a compressed stem axis referred to as a bud cluster (Samarakoon et al., 2014). The hierarchical arrangement of the crown buds from the proximal to distal end of the cluster defines the ontogenetic timing of development of buds from the earliest to latest within the cluster and, therefore, their degree of maturity (Samarakoon et al., 2014). One of the crown buds within the cluster would be an apical bud capable of producing more buds during the growth cycle, and the other crown buds would be axillary and capable of developing into flowering shoots. Flowering shoots of gentian develop from the overwintering crown buds produced in the previous growing season. Crown buds are dormant in winter until growth recommences in spring when they emerge, with floral initiation evident once axillary shoots become visible in leaf axils of the developing flowering shoot (Samarakoon, 2012), and anthesis in later summer through early fall (Samarakoon et al., 2010, 2012). While greater shoot emergence leads to a greater number of shoots at harvest, not all crown buds present at the beginning of a growing season emerge as shoots leading to loss of yield (Samarakoon et al., 2012).
Shoot emergence, development, and flowering, only occur in some temperate perennials when requirements for prior exposure to cold temperature (chilling) have been satisfied, i.e., vernalization and/or breaking of endodormancy (Iversen and Weiler, 1994). Inadequate accumulation of chill units can result in delayed or uneven shoot emergence, reduced budbreak, and slow or weak shoot growth in a range of plant species (Lang et al., 1987), and while evidence exists that prior exposure to chilling is required for crown buds of gentian to emerge in spring (Samarakoon et al., 2012), peer-reviewed articles defining the specific timing and requirements for floral induction, initiation, and shoot development to flowering, are limited. In regions with mild winter temperatures, such as New Zealand (NZ), inadequate accumulation of chill units was hypothesized as a possible reason for variation in timing and quality of shoot emergence and flowering.
Exogenous application of GA3 is known to replace (Dahanayake and Galwey, 1999), or partially replace (Zieslin and Geller, 1983) chill unit requirements in a range of herbaceous perennials. In a summary of unpublished technical reports based on older cultivars of gentian, the application of GA3 (100 ppm) resulted in shoot emergence in dormant plants, but these shoots did not produce flowers (Ohkawa, 1983). The role of GA3 in substitution of chilling with application to the current new cultivars remained to be investigated.
Because of the hierarchal arrangement of buds within clusters, paradormancy (correlative inhibition) is likely to exist within the cluster (Samarakoon et al., 2014) and, based on the comparative growth potential of individual buds, could lead to differences in response to dormancy-breaking treatments. This could also influence the time needed for a bud to reach horticulturally relevant developmental endpoints such as shoot emergence or flower harvest maturity and, therefore, spread in duration to harvest maturity of the flowering shoots within a single plant. Such spread in duration of harvest maturity has previously been identified as a concern by commercial growers (Samarakoon et al., 2013).
In commercial practice, dormancy-breaking treatments would ideally be applied early in the growing season, potentially in winter or spring, before shoot emergence. In hybrid cultivars of gentian like Showtime Diva, at the beginning of the annual growth cycle in spring crown buds/shoots are likely to be at different stages of development, i.e., from nonemerged crown buds through to developing shoots (Samarakoon et al., 2014). With these stages of development in mind, in this paper we describe the effects of adequate and inadequate chilling and the use of GA3 as a substitute for chilling on shoot emergence and development for the cultivar Showtime Diva. The potential differential response to chilling and/or GA3 was investigated by applying these treatments to plants with buds and shoots at different stages of development. To develop an understanding of the developmental hierarchy within crown bud clusters, the correlations between timing of shoot emergence and flowering and the position of buds within the cluster was investigated.
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