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2.2.1 Morphological structure

A mature perennial gentian plant is composed of erect flowering shoots, swollen crown buds, fleshy rhizomes, thick storage roots and fine feeding roots (Figure 2.1). These organs can be grouped into two parts, i.e. above-ground shoots and underground crowns. In horticulture, the crown of a gentian plant, comprising crown buds, rhizomes, storage roots, and feeding roots, is the perennating structure supporting re-growth in the next growth season (Ohkawa, 1989). Crowns of gentians are able to be lifted, divided, traded and replanted.

Crown buds are located at the top of crowns, typically being presented as covered with purple scale leaves. Crown buds commonly develop in clusters of axillary buds, and the first crown bud within a cluster is able to be initiated as an adventitious bud (Samarakoon, 2012). The fleshy rhizomes of the crown are the compressed stems, being vertical or horizontal. Within the cultivars used in this thesis, the rhizomes are normally vertical. Storage roots are stout taproots or linear-cylindrical roots arising from the rhizomes. Shoots originate from crown buds and typically develop to form an inflorescence. Although the underground crowns are an important structure for the perennial lifecycle of gentians, knowledge of the function and features of the form and structure of a crown is very limited. One aspect proposed for this thesis was, therefore, to detail the features pertaining to the form, structure and development of crowns.

Shoots are normally erect and, composed of stems and leaves. Leaves are sessile in an arrangement of decussate opposite leaves, with lower leaves of shoots being scale-like (Ohwi, 1965; Ohkawa, 1989; Ho & Liu, 2001). The spike-type inflorescence with leaves is a cyme, comprising few to many florets which are terminal and/or axillary. The corolla

Chapter 2 – Literature review

is sympetalous and usually funnel-form, campanulate, tubular, or salverform, with five lobes. In this thesis, with the shoots being able to develop inflorescences, they are termed “floral shoots” and, when the floral shoots achieve anthesis, are termed “flowering shoots”.

Figure 2.1 Photograph and comparative schematic diagram presenting the morphological structure of a mature perennial gentian plant.

2.2.2 Growth cycle

Perennial gentians are typical geophytes, and their growth is seasonally controlled (Figure 2.2). In annual growth cycles, gentian plants sprout in spring, flower in summer/autumn, with above-ground floral shoots typically senescing in winter; relying therefore on the underground crowns (storage roots, rhizomes and crown buds) to survive winter and initiate re-growth in the next growth season (Ohkawa, 1989).

Floral differentiation of gentians cultivars used for cut flower production, occurs after sprouting in spring (Ohkawa, 1989). In Japan, floral differentiation of early-season flowering cultivars commenced from late May to early June (seasonally being equivalent to late November to early December in the southern hemisphere) when shoots had reached 30 to 40 cm in length, and 15 to 17 pairs of extended leaves. In contrast, with late-season flowering cultivars, floral differentiation started when shoots were 60 to 70 cm long and with 24 to 28 pairs of fully expended leaves. Warm temperature (20-25 °C) and long-day conditions accelerated floral development of early-season cultivars, while late-season cultivars were promoted by low temperature and short-day conditions (Ohkawa, 1989). In Japan, the timing of anthesis of early-season flowering cultivars is late July (being

equivalent to January in NZ), mid-season flowering cultivars from August to September (February to March in NZ), and late-season flowering cultivars in late October (April in NZ). In NZ, Japanese-bred cultivars flower naturally between January and April, depending on the cultivars and weather. The timing of anthesis of the new cultivars and genotypes used in this thesis such as; ‘03/04-114’, ‘Showtime Diva’ and ‘Showtime Spotlight’, is approximately between February (late summer) and April (middle autumn), depending on weather, under the natural condition in Palmerston North (40.38° S 175.60° E), NZ (E. Morgan, personal communication, 2009).

Figure 2.2 Diagram illustrating the annual growth cycle of perennial gentians, with focus on the seasonal changes associated with dormant crown buds in winter, shoot emergence in spring, flowering in summer/autumn, and progressing through to shoot/foliage senescence in autumn/winter.

The new cultivars developed in NZ are derived primarily from G. triflora and G. scabra (Morgan et al., 2003). The distribution of latitude for the parent species G. triflora range

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from approximately 34° N through to 54° N (Ohwi, 1965), with photoperiods of 14:25 h to 17 h in June and between 7:22 h to 10 h in December (Lammi, 2005). The other dominant parent, G. scabra, is naturally distributed approximately between latitudes 30° N through to 54° N, with photoperiods of 14 h to 17 h in June and between 7 h to 10 h in December (Lammi, 2005). Since the critical photoperiods of short and long photoperiods for any physiological response in gentians have not previously been reported, the shortest and longest day length for the centres of origin, i.e. 7 h and 17 h were referenced as the basis of short and long photoperiods within this thesis, respectively (refer to Chapter 5).

2.3 Horticultural Production

2.3.1 Propagation

Gentian can be propagated by seeds, cutting, division, and tissue culture. Their seeds need chilling to break dormancy. For autumn sowing, seed dormancy can be broken by the natural cold of winter; however, for spring sowing, seeds need exposure to 0-5 °C for two weeks, a treatment with 50 ppm GA3 for three days, or a combination of these two

treatments (Ohkawa, 1989).

Recently hybrid cultivars of gentians have become more important for supply to commercial cut flower and potted flowering plant production, so vegetative propagation has been widely used to deliver genetically uniform plants. One method of vegetative propagation is dividing the crowns and replanting, but low rates of multiplication and disease infection are noted problems (Anonymous, 1984). Cuttings are a highly efficient method for multiplication, but can only be applied to some cultivars in which it is relatively easy to form crown buds. It is anecdotally reported that difficulty forming crown buds before winter limits the wide application of this method, within Japan at least.

Plant tissue culture provides an efficient method of propagation to produce disease-free plants (Jomori et al., 1995; Hosokawa, 1996 ; Morgan et al., 1997). However, these young clonal-plants of some cultivars, also encounter a similarly low number and rate of crown bud formation after being de-flasked and planted in the field, as well as a relatively long period (approximately 3 years) to reach a commercial level of productivity of flowering shoots (Sato, 1988a). To achieve uniformity of experimental material, gentian plants used in the series of experiments within this thesis, were clonally propagated by tissue culture.

2.3.2 Cultivation

Since gentian cultivars used for commercial cut flower production are native of alpine and cool-climate regions, cool summer conditions are considered favourable to their growth. The optimum temperature is reputed to be between 15-18 °C (Ohkawa, 1989), although no scientific literature is available to verify this. If temperatures exceed 30 °C, shade may be used to reduce temperatures (Yamanaka & Kujii, 1978; Anonymous, 1984), but commercially in NZ shaded structures may be used so as to eliminate pollinating insects more than for temperature control. Using Japanese cultivars, the higher night temperature (15 °C) and longer day length (16 h) has been found to promote the timing of flowering, while shorter day length (8 h) and lower night temperature (11 °C and 7 °C) increased the floral shoot length and the number of nodes (Tsukada, 1984).

Acid soil (pH 5.0-5.5) with good drainage and high organic matter is considered preferable for growing gentian plants. Irrigation is required, especially for young plants that may dry out easily, and poor drainage often leads to root rot. Young plants appear to need extra care, in order to reduce mortality.

Gentian plants in cultivation are usually planted in two rows on a raised bed (100 cm wide and 6 to 15 cm high), with a row distance of 40 cm and between plant distance of 20 cm. To prevent floral shoots from bending, support-netting needs to be provided as the plants become tall and are prone to collapse. The first net is at around 30 cm high, with a second or third net raised up as the floral shoots grow. After the third year, the number of floral shoots per plant increases to such an extent that thinning is normally carried out from the 4th year to prevent over-crowding, and maintain a high quality of flowering shoots. When thinning, the common method followed is, as the floral shoots reach around 30 cm height, eight to ten of the best shoots are left to produce a harvestable crop of flowering shoots, and the rest are pinched out to remove their florets, but leaving foliage at their bases. Under this regime the harvest of flowering shoots can continue for 4-5 years from the third year after planting (Ohteki, 1982; Anonymous, 1984).

2.3.3 Harvesting of flowering shoots

Commonly, the harvest of flowering shoots in gentians is carried out when the floral buds at the top of the floral shoot is fully coloured but, still tightly closed. Harvesting flowering shoots at less mature stages usually results in failure of undeveloped florets to open or

Chapter 2 – Literature review

light colouration of petals (Ohteki, 1982; Eason et al., 2004). Over-maturity decreases the vase life, and may require removal of the old senescing florets from the base of shoots before being marketable.

Leaf wilting is another factor affecting the vase life of gentians. Cultivars with larger leaves (e.g. ‘Late Blue’) are more prone to wilt compared with those which have smaller leaves (e.g. ‘Ashiro No-Ake’; Eason et al. 2004). Sucrose pulses can reduce leaf wilting and enhance pigmentation of the apical florets when harvesting at less mature stages. Pollination also accelerates petal senescence. Moreover, cross pollinated florets senesce more rapidly than self-pollinated or un-pollinated flowers. Growers and florists are recommended to avoid using fluoridated water for handling flowering shoots after harvest, because fluoride in water can lead to leaf softening and browning (Eason et al., 2004). The new cultivars developed by Plant & Food Research (Palmerston North) are considered suitable as a cut flower for export from NZ, in part because they have a long vase life. For example, ‘Showtime Diva’, ‘Showtime Spotlight’, and ‘Showtime Starlet’ were reported to have a vase life of 15 days (after 8 days in cool storage), 10 days (after 3 days cool storage), and 8 days (after 5 days cool storage), respectively (Eason et al., 2006). Also these cultivars are sterile (the mechanism is still not clear) and have small leaves, which are characteristics believed to be beneficial to increase vase life.