Discussion 60

In order to develop an accurate, rapid and resource-efficient way to compare the efficacy of various postharvest treatments on spathe re-greening, a primary aim of this experiment was to determine what if any relationship existed between changes in L*, C* and Hº and changes in chlorophyll or carotenoid content. In the current experiment, re-greening on the abaxial surface of the spathe of ‘Best Gold’ resulted from a 10-fold accumulation of chlorophyll a and b and a less than 1-fold decrease in

the content of total carotenoid (Figure 2.2). The change in content of these pigments was shown to be highly correlated with the changes in value of L*, C* or Hº, with values of |r| ranging from 0.66 to 0.98 (Table 2.1). These results are in agreement with

those reported for leaves of Petroselinum sativum L. and fruits of L. esculentum, in

which there was a significant correlation between the colour coordinates (particularly Hº) and pigment content (Berset and Caniaux, 1983; Thiagu et al., 1993). These horticultural commodities contained a relatively simple pigment composition (i.e. one or two pigment groups) similar to the situation that occurred in ‘Best Gold’. Thus significant correlations may in part be explained by the change in colour primarily resulting from changes in one of the predominant pigments, e.g. chlorophyll in the case of ‘Best Gold’. Conversely, in some fruits, e.g. Malus×domestica, where colour

was determined by more than two pigment groups including chlorophyll, carotenoid and anthocyanin, the change in colour during fruit ripening resulted from significant and simultaneous changes in all these pigments (Lancaster et al., 1994; Singha et al., 1991). As a consequence, no or only a poor correlation was found between the values of the colour coordinates and pigment content with these fruits. Additionally, in the current research, the predominant pigment, i.e. chlorophyll, was concentrated in the subepidermis of the spathe (Figure 2.1), and not spread throughout the mesophyll

layers. As such, the accumulation of chlorophylls during re-greening can be readily discerned by a tristimulus colorimeter. Hence, the simplicity in dynamics of both the pigment composition and distribution in spathes of ‘Best Gold’ during re-greening, likely contribute to the highly significant correlations between content of chlorophyll and carotenoid with colour coordinates achieved in this study.

Among the three colour coordinates, the change in Hº showed the strongest correlation with the accumulation of chlorophyll during the re-greening of ‘Best Gold’, with a r value of 0.98. Hence, measuring the change in Hº can be used to infer a

change in chlorophyll content and, therefore, evaluate the progress of re-greening for future research in spathe re-greening of ‘Best Gold’. However, to use this method with confidence, its limitations need to be acknowledged.

One of the limitations for this method is that its sensitivity is not consistent at different stages of re-greening. As shown in the current experiment, within the range of data examined, the relationship between Hº _{and chlorophyll content was not linear} but curvilinear (Figure 2.5). For example, an increase in Hº by 7 units from 85º to 92º during the early stage of re-greening (i.e. crossing the break-point for the colour change from yellow to green) correlated with a change in total chlorophyll content of 2.6 μg·cm-2 (Figure 2.5). In contrast, a similar increase in Hº from 92º to 99º, during the later stage of re-greening, was associated with a greater change in chlorophyll content of 4.5 μg·cm-2 (Figure 2.5). This suggests that Hº reflects the change of chlorophyll content more sensitively during the early stages of re-greening, where chlorophyll content is relatively low, than at later stages when there is a higher total chlorophyll content in tissues. In other words, a subtle increase in chlorophyll content

during the early stages of re-greening can result in a rapid change in Hº. In the later stages of re-greening, i.e. beyond 7 days in this study, increases in the content of chlorophyll did not result in as large a change in Hº, but mainly contributed to a reduction in the lightness of the colour. Hence, Hº is likely to be a less sensitive indicator for the change in chlorophyll content during this later stage of re-greening. This reduced sensitivity of Hº in reflecting the change in pigment content, after it reaches a saturated level (i.e. curvilinear instead of linear relationship between Hº and pigment content), has also been reported with fruits of Malus×domestica (Dixon,

1993; Iglesias et al., 2008), L. esculentum (Thiagu et al., 1993) and, V. vinifera (Peppi

et al., 2006). This inconsistency in sensitivity of Hº in inferring changes in chlorophyll content at different stages of spathe re-greening however, is not perceived as reducing the validity of this method when applied in the future research on re-greening in spathe tissue of ‘Best Gold’. This is because it is the initial stage of re-greening, reflected by the values of Hº around 90º, which are most relevant to the research interest of this thesis. Therefore, in the subsequent chapters of this thesis, after horticultural harvest-maturity, an increase in Hº _{can be used to infer an accumulation} in chlorophyll. In addition, values of Hº greater than 90º can be used to indicate that the content of chlorophyll has reached the level that humans perceive green colouration (Iglesias et al., 2001).

As a further limitation of the methodology, since the relationship between chlorophyll content and Hº was developed based on the data from the abaxial surface of the spathe of ‘Best Gold’, measuring the change in Hº can not be used to infer the change in chlorophyll content (if any) on the adaxial surface. In the current experiment no chlorophyll accumulation was detected on the adaxial surface of the

spathe, and yet a slight increase in Hº from 86º to 90º was observed within the 14-day period of observation (Figure 2.2). This change in Hº on the adaxial surface might partially result from a decrease in the content of total carotenoid, as evident by a significant correlation between Hº and the content of total carotenoid for this surface (Table 2.1). Therefore, the method developed in the current experiment is only suitable for evaluating re-greening on the abaxial surface.

Compared with other experiments, (refer Chapter 5) the range of Hº values examined in this study (i.e. 85º to 98º) was comparatively narrow. Due to variation in environmental conditions (e.g. light intensity), flowers harvested from different seasons are likely to vary in their initial colour (Ben-Tal and King, 1997; MacKay et al., 1987) and pigment content, hence presenting a wider range of Hº values than those encountered in the current experiment. As a result, the quadratic relationship between Hº and chlorophyll content derived from the current experiment must be considered limited for an accurate estimation of chlorophyll content across a wider range of Hº values. To solve this limitation, repeating the current experiment under different growing environment conditions is required. Since resources did not permit this to occur, with regard to the subsequent experiments presented in this thesis, application of the correlations found will be limited to only inferring changes in chlorophyll content have occurred.

Carotenoids remained as a key pigment group in the spathe of ‘Best Gold’ during the 14-day period of monitoring re-greening. By day 7, they still accounted for about 46% of total pigment content in the abaxial surface (Figure 2.2). Compared with total chlorophyll however, during re-greening the change in the content of total

carotenoid appears to contribute less to the colour change of the spathe. This is partially due to the fact that within the layers of the subepidermis where chlorophyll and carotenoids coexist, the newly-synthesized chlorophyll appears to mask the visibility of the carotenoids. This kind of masking effect is common in green leaves, e.g. Spinacia oleracea L. and fruits, e.g. A. deliciosa and Mangifera indica L.

(Alkema and Seager, 1982; Ketsa et al., 1999; Kidmose et al., 2001; McGhie and Ainge, 2001).

The change in the content of total carotenoid on both abaxial and adaxial surfaces during re-greening of the spathe of ‘Best Gold’ was more closely reflected by the change in C* than in Hº (Table 2.1). This is also in agreement with the correlation reported for total carotenoid in fruits of C. moschata (Itle and Kabelka, 2009). In the

current experiment however, the two surfaces differ in magnitude of the parameters describing the linear relationship between total carotenoid content and C* (Figure 2.5; Figure 2.6). Hence, a given value of C* cannot be inferred to predict the same content of total carotenoid for both surfaces. This is because the presence of chlorophyll on the abaxial surface also partially contributes to the saturation of colour viewed. The pigment composition, therefore, likely determines the relationship between the change in pigment content and colour. In other words, the method developed in this study to evaluate spathe re-greening of ‘Best Gold’ cannot simply be used for other hybrids of