Discussion ·······································································································································

NSCs in plants play an important function as a source of energy and metabolites, as well as being involved in essential physiological processes; therefore it is a key area of study for crop production (Farrar, 1999). While numerous studies on NSCs have been conducted in many crops (Lewis, 1984a), the previous literature in terms of NSCs in gentians was very limited. The current series of experiments have for the first time identified and quantified the distribution and seasonal changes of NSCs in gentian plants. Similar to other geophytic plants, the data in this chapter confirm that gentian plants accumulate substantial amounts of NSCs within the crown for overwintering and supporting initial spring re-growth. As discussed above, the concentration of the unique NSCs, i.e., gentianose, gentiobiose, as the most abundant NSCs in various organs of gentian, significantly fluctuated in different organs over the growing season, implying that both gentianose and gentiobiose potentially play important physiological roles related to growth and development, e.g. dormancy, cold tolerance, morphogenesis of crown buds, and the development of floral shoots, and are worth further study.

Geophytes rely on resting buds attached to underground storage organs to survive adverse conditions and initiate new growth in spring. The prerequisite for this growth strategy is sufficient accumulation of reserve substances (primarily carbohydrates) within the underground organs for respiration and demand arising from initial growth in spring. Similar to other geophytes (Bradbury & Hofstra, 1977; Woolley et al., 1999; Orthen & Wehrmeyer, 2004; Walton et al., 2007), including G. lutea (Bridel, 1911), the concentration of TNC in rhizomes and storage roots of ‘Showtime Spotlight’ reached their maximum concentration in autumn, declined during winter and spring, followed by gradual replenishment over summer and autumn (Figure 6.4A and B). The point of inflection where the concentration of TNC increased from its lowest, can be interpreted as that point in time when photoassimilate supply started to exceed the demand for growth consumption in January during the 2007-2008 growth cycle, as compared with November in the 2010-2011 growth cycle (Figure 6.4A and B). While the growth cycle evident in both the 2007-2008 and 2010-2011experiments presented a similar pattern of change in TNC, the lowest concentration of TNC occurred earlier in 2010-2011 than 2007-2008. This difference may have been due to the monthly mean daily temperature in spring during 2010-2011 being 1.0 °C higher than in 2007-2008 (Figure 6.10). As mean

Chapter 6 – Seasonal changes of carbohydrates

temperature is regarded as the major environmental factor influencing the phenology in geophytes of temperate origin (Rees, 1992; Le Nard & De Hertogh, 1993b), it seems plausible that differences in temperature could account for the differences in timing of changes in NSCs concentration between the two years. This hypothesis is supported by contrasting the phenological calendar, wherein flowering occurred one month early in the 2010-2011 season compared to the 2007-2008 season. In contrast, Samarakoon (Samarakoon et al., 2012b) reported that compared to the outdoor environment in Palmerston North, cultivation of ‘Showtime Spotlight’ in a protected environment (higher mean temperature) resulted in longer shoot length and increased appearance of crown buds, but did not alter the timing of flowering within the current growth season. This contrary result may be because the growth rate of gentian plants only responds to a narrow scale of temperature, i.e., in Samarakoon’s experiments the temperature within the protected environment was higher than the linear temperature range (Arnold, 1959) that gentian plants respond to for changes in growth rate, thus did not affect the timing of flowering. Therefore, how temperature affects the growth and development of gentian plants needs further study, utilising temperature treatments including the linear response range.

In many other perennials the importance of stored NSCs such as starch and fructans etc, for winter respiration, cold tolerance, and early re-growth in spring has been widely recognized (White, 1973; Pressman et al., 1993; Tamura & Moriyama, 2001; Anderson et al., 2005; Walton et al., 2007; Zhang et al., 2011). Keller & Wiemken (1982) suggested that the high concentration of gentianose in the vacuoles of storage roots may have been involved in the cold tolerance for G. lutea in alpine areas, although there were no data to clearly support this hypothesis. The results of previous experiments utilising defoliation (refer to Chapter 4) supported the potential role of gentianose (a predominant NSC in storage roots) as a cold protectant and a carbon source, via the highly positive correlations between the concentration of gentianose stored in autumn and both the proportion of plants surviving winter, and re-growth of crown buds in spring. Similarly in the current study, the changing pattern of concentration of gentianose in the rhizome and storage roots of ‘Showtime Spotlight’ (i.e., accumulation during summer and autumn, and decreasing in spring, coordinated with the breakdown of the oligosaccharide, gentianose), logically supports the above hypothesis as to the ecological importance of stored gentianose in underground organs. In order to test these hypotheses, however, plants with differing contents of gentianose in conjunction with cold treatments at various severities would be

required to determine the role of gentianose with cold tolerance. In future experiments, research using labelled carbon for tracking the carbon flux would help to determine if gentianose in the roots and/or rhizomes are remobilized to above ground organs, and could quantify the amount of NSCs used for respiration, or as structural components of re- growth. In addition, cold tolerance would also be affected by other factors. For example, Takahashi et al (2006) identified that some proteins might be related to the cold tolerance and dormancy of crown buds of G. triflora. In the current experiments the high level of L- bornesitol concentration found during winter in crown buds and rhizomes (Figure 6.11A), may also be interpreted as relevant to cold tolerance. All these hypotheses, however, require confirmation by further experiments.

In most geophytes studied so far, NSCs stored in underground storage organs are only partially used, i.e., the storage of NSCs is typically greater than the demand for normal plant growth (Wyka, 1999; Kleijn et al., 2005; Walton et al., 2007). The primary hypothesis in this regard is that the unused storage NSC serves as an adaptive buffer, allowing the plant to survive unexpected adverse conditions such as herbivore grazing (Stamp, 2003). In the current two growth cycles of measurement using ‘Showtime Spotlight’, it was found that at least 30% of the accumulated NSCs (approximately 50 mg g-1 FW; mainly gentianose) within storage roots, and 40% (approximately 30 mg g-1 FW; mainly gentianose and sucrose) in rhizomes was not consumed in the normal annual growth cycle and presumably provided this adaptive buffer as long storage NSCs. As evident in the defoliation experiment (refer to Chapter 3), if an adequate leaf canopy was not re-established/maintained, there is a risk of utilizing too much of the reserves within the underground storage structures, potentially resulting in plant mortality. This long-term storage of NSCs, mainly gentianose, can be explained as an evolutionary adaption in the pattern of carbon partitioning in response to adverse conditions, such as grazing (Miller et al., 1999; Huhta et al., 2000).

As discussed within Chapters 3 through to Chapter 5, carbohydrate supply can influence the morphogenesis as well as the dormancy of crown buds; therefore, it is logical to speculate that the concentration of NSC in rhizomes and/or storage roots is able to affect the morphogenesis, dormancy, and re-growth of crown buds. Supporting evidence for this is provided by exogenously supplied sucrose in vitro having been reported to increase the number of crown buds both in vitro and subsequently in the field after being de-flasked of

Chapter 6 – Seasonal changes of carbohydrates

G. triflora (Sato, 1988a). It was also identified within in vitro experiments (refer to Chapter 5) that a high concentration of carbohydrate was a requirement for the formation of crown buds. As the macroscopic formation of crown buds occurred from early summer through autumn (refer to Chapter 3), this period coincides with the increase in concentration of TNC, gentianose and sucrose in storage roots and rhizomes (Figure 6.4A and B). This coordination of timing therefore further supports the hypothesis that the NSCs in storage roots and/or rhizomes may be directly, or indirectly, involved in the morphogenesis of crown buds. The determination of the mechanism of NSCs participating in the morphogenesis of crown buds is particularly complex due to the large number of factors, either physical, chemical or biological, taking part in plant morphogenesis through space and time. Further experiments using molecular technologies, exploring the effect of carbohydrate metabolism on the expression of genes involved in morphogenesis, would greatly contribute to the understanding of the role of NSC in the regulation of the morphogenesis of crown buds (Rolland et al., 2006; Chao & Serpe, 2010; Sairanen et al., 2012).

From an ecological perspective, it is understandable that it is necessary to maintain the dormancy of crown buds during the cold winter, so as to prevent damage to new growth before the arrival of warmer temperatures in spring (Anderson et al., 2005). During summer and autumn, apical dominance may play a key role in maintaining the state of dormancy (paradormancy) of crown buds, while with the senescence of shoots in winter, other internal physiological factor(s) are necessary to regulate dormancy (endodormancy) of crown buds (Lang et al., 1987; Chao et al., 2007). It is recognized that carbohydrates may be directly involved in signal(s) regulating the dormancy status of vegetative buds (Anderson et al., 2001; Anderson et al., 2005; Chao et al., 2006; Chao & Serpe, 2010), or perhaps indirectly via osmotic potential (Green & Jane, 1983; de Fay et al., 2000; Morisset et al., 2012). This hypothesis is supported in part by the concentration of TNC (mainly gentianose) in storage roots during winter, although gradually decreasing, still presented a comparatively high internal concentration (15% FW; 150 mg g-1 FW; Figure 6.4A and B). In addition, the concentration of TNC (mainly sucrose) in crown buds was also at its highest level (5% FW) in winter, with a sharp decrease in early spring when re-growth started in September (Figure 6.2; Figure 6.6A and B). These patterns of change in TNC in storage roots, rhizomes and crown buds are, therefore, consistent with the changes in dormancy status of crown buds, and support the hypothesis that a high concentration of

NSCs may be involved in maintaining the dormancy of crown buds. In addition, considering the occurrence of the high peak of concentration of sucrose in crown buds in winter when no photosynthesis is producing new assimilate (Figure 6.6A and B), the conversion of carbohydrate forms and remobilisation may be involved in the regulation of dormancy and cold tolerance in crown buds, e.g. sucrose in crown buds may originate from the breakdown of gentianose in storage roots and/or rhizomes, and then move to crown buds. It is also suggested that carbohydrates (mono or oligosaccharides such as glucose, fructose and sucrose) play an important role as signals in regulating vegetative bud dormancy, through regulating gene expression or cross-talking with plant hormones, e.g. inhibiting perception of gibberellic acid and/or, enhancing perception of abscisic acid (Horvath et al., 2003; Anderson et al., 2005; Chao et al., 2006; Chao et al., 2007; Chao & Serpe, 2010). The mechanism of carbohydrates regulation of bud dormancy, however, either as osmotica, signals, and/or through the synthesis of proteins (Takahashi et al., 2006), still needs further study.

The changes in concentration of NSCs in flowering shoots differed between the two experiments (Figure 6.7A to C); primarily this may be due to the differences in the organs sampled (e.g. leaves measured separately or not) and the time of sampling (e.g. sampling in early morning would lead to lower values for the concentration of NSCs). In contrast, sampling later in the day would result in higher concentrations (refer to Section 6.2.2.1). Considering the likely effects arising from changes to sampling procedures, the data reported from 2010-2011 is considered to be more reliable than that from 2007-2008. The results over the 2010 to 2011 growth cycle (Figure 6.8B and C) indicated that the concentration of TNC in both floral-shoot stem tissue and leaves followed similar seasonal changes. The increase in concentration of TNC (mainly gentiobiose) during spring, and its highest concentration achieved in early summer (December), coincided with when initiation of florets were evident in ‘Showtime Spotlight’ under microscopic examination (Samarakoon, 2012), and the continuing rise of temperature (Figure 6.3), which could potentially result in increased photosynthesis (Roca et al., 2005). In contrast, the subsequent decrease in concentration of TNC cannot be interpreted only as being due to the changes in weather conditions, because similar temperatures also occurred during summer through autumn. As an alternative hypothesis for this latter period therefore, it has been reported that temporarily accumulated NSCs in floral-stem tissue and leaves during vegetative growth and/or through to pre-anthesis, can be remobilised for anthesis and

Chapter 6 – Seasonal changes of carbohydrates

subsequent reproductive growth (Foulkes et al., 2002; Monteiro et al., 2002; Ruuska et al., 2006; Fu et al., 2011; Slewinski, 2012). The development of flowering shoots in gentian, including the extension of shoots and the development of the individual florets are key to determining the pre-harvest quality of the shoot as a cut flower. Hence, it is logical to hypothesize that accumulation of NSCs, particularly gentiobiose, in leaves and floral- shoot stem tissue of ‘Showtime Spotlight’ during spring to early summer, are used for the later development of the flowering shoots. To test this hypothesis, and/or to quantify how much NSCs reserves are remobilized to the florets, further studies using a radioactive carbon tracer could be utilised (Cliquet et al., 1990; Woolley et al., 1999). In addition, the patterns and possible roles of the accumulation and utilization of NSCs during the development of individual florets have been explored in this thesis (refer to Chapter 7). In terms of postharvest quality, NSCs are widely known to provide a source of carbon for respiration, inhibiting responsiveness to ethylene, and maintaining water uptake (Adachi et al., 2000; Monteiro et al., 2002; Verlinden & Garcia, 2004; Ekman & Worrall, 2005). Similar to other cut flower crops, the function of NSCs in improving the postharvest quality of flowering shoots of gentian has been reported using sucrose pulsing treatments (Zhang & Leung, 2001; Eason et al., 2004; Eason et al., 2007); however, until the current study the internal NSCs in stems and leaves of floral shoots was not reported. In order to maximise the postharvest display life, in commercial production of gentian, flowering shoots are normally harvested when the apical floret is fully pigmented but tightly closed (Eason et al., 2004), i.e. a substantial proportion of the florets are still undeveloped buds. Therefore, abundant gentiobiose accumulated at this pre-anthesis period (Figure 6.7B and C), is likely to become an important carbohydrate source for the continued development of these florets after harvest. To better understand the role of gentiobiose in the postharvest quality of gentian, further investigation is required to examine changes of gentiobiose in leaf and stem tissue of flowering shoots under various postharvest conditions (Bieleski et al., 1992). In support of the hypothesis that such carbohydrates may be involved, it has been found that the vase life of cut flowers in Helianthus annuus and Lilium was correlated with carbohydrate status under different preharvest conditions, including light and temperature. In particular, if the recognised roles of carbohydrates in improving postharvest quality of cut flowers, as discussed above, are also applicable in gentian, the production practices of plants to increase NSCs reserves, particularly gentiobiose, in the

floral-shoot stem tissue and leaves, should contribute to the improved postharvest quality of flowering shoots in gentian.

It is well known that NSC accumulates in leaves of some plant species during the light period when photo-assimilate production exceeds utilization (Sicher et al., 1984; Morin et al., 2011). In the present study, the concentration of TNC in leaves demonstrated a clear diurnal pattern when shoots were at harvest maturity, i.e. TNC reached its maximum at sunset (8:00 pm; summer daylight savings time NZ; Figure 6.8A). In ‘Showtime Spotlight’, this increase of TNC was mainly due to an increase in sucrose concentration during daylight hours (Figure 6.8 A), being similar therefore to the result reported in barley (Hordeum vulgare L.) (Zeeman et al., 2007). Although the TNC concentration (mainly gentiobiose) was as much as three-fold higher in floral-shoot stem tissue than leaves, there was little diurnal change in the concentration of TNC within the floral-shoot stem tissue, nor the individual NSCs (Figure 6.8 B). In order to maximise the concentration of NSCs within floral shoots at harvest, the best time during the day to harvest is clearly at the end of day, i.e., at sunset. While current commercial production practices emphasize the desire to harvest cut flowers early in the morning when plant water status is high and field heat is low (Halevy & Mayak, 2011 ), the current results lead to the inference that the harvest of flowering shoots of gentians later in the day should be considered. To confirm that this increased content of sucrose does in fact result in an improved postharvest performance for gentian, further experiments are required to examine the vase life and/or the sensitivity to ethylene of flowering shoots when harvested at different times during the day (Rapaka et al., 2007).

Depending on the position along the flowering shoot, the TNC concentration either in leaves or floral-shoot stem tissue decreased 33% and 50% respectively from upper to lower segments (Figure 6.8A and B; P < 0.05). This may be due to the distribution and density of the leaf area within the canopy, which determines the light environment influencing photosynthesis, and/or the age of leaves influencing their photosynthetic capacity (Bradbury & Hofstra, 1977; Xie & Luo, 2003). Another possible explanation is that the distribution of NSCs may be related to the competition for carbon between different positions along the shoot, i.e. due to their closer physical proximity to developing florets, the upper segments of the shoots have stronger competency than the lower segments. For example, it has been demonstrated in Italian ryegrass (Lolium multiflorum

Chapter 6 – Seasonal changes of carbohydrates

Lam.) that the quantity of carbon assimilate translocated to the inflorescence within the upper segments of shoots, was considerably greater than that exported to rhizomes underground (Bradbury & Hofstra, 1977). The pattern of NSCs distribution along flowering shoots, i.e., upper segments store more NSCs, may result in the stored NSCs being more easily transported to florets due to their close physical distance, and thereby benefiting the maintenance of postharvest longevity of flowering shoots.

In the current study, where shoot removal was applied, there was a significant reduction in reserves of gentianose within storage roots, and both gentianose and sucrose within crown buds (Figure 6.9A and B). This was consistent with the result reported in the defoliation experiment (refer to Chapter 4). As evident within the current experiment, a reduction in concentration of NSCs resulted from complete shoot removal at any time up until June, i.e. at the end of the annual growth cycle when shoots were naturally senescing (Figure 6.7A and B). In order to achieve maximum NSCs reserves in storage roots and crown buds therefore, allowing the natural senescence of shoots is the optimum strategy. However, within a commercial context, to determine how much/many shoots, should be left on the plants when harvesting, requires further experiments to compare the effect of different severities of harvest on NSCs reserves, development of crown buds, and subsequent commercial yields.

Carbohydrates are typically stored as polysaccharides or oligosaccharides (Lewis, 1984a). Many studies on storage carbohydrates and their seasonal changes in perenial crops have