Changes in sugars during ripening transition and after harvest.
Sweetness is strongly associated with the general concept of fruit
quality. Thus, it is important to understand how this trait is affected
by both fruit ripening stages and their specific interaction with the
environmental after harvest. The improvement of flavour and fruit
quality at harvest may be possible, through greater understanding of the
ripening stages that are particularly sensitive to sucrose accumulation
and have a positive impact on sensorial quality. The present results
show increased sucrose content in DR, which is in accordance with
previous works that reported higher sucrose levels during the ripe stage
(stage 6, Zhang et al., 2011). In a similar way, in other varieties,
glucose and fructose were also found to be the major sugars followed by
sucrose and myo-inositol (Macias-Rodriquez, et al., 2002). Previous
works reported an increase in myo-inositol during the ripe stage,
followed by a decrease during the over-ripe stage (stage 7, Zhang et
al., 2011). Similar sucrose levels, however, were quantified at the AR
and FR stages, suggesting that another role may be important other than
the contribution to sucrose. With respect to FOS during ripening
transition stages, our results show that DP3, 1-kestose, 6G-kestose and
DP4 show similar trends, with significant increases being seen in fruit
during FR relative to strawberries at the AR stage. We previously
reported a correlation between FOS and changes in fruit water status
(Blanch et al., 2012). The present results about dynamic changes in FOS
during ripening point to this contribution. In consideration of the high
levels of sucrose observed, alongside those of other major sugars during
the DR stage, it would be interesting to analyse the molecular
mechanisms involved in the synthesis and degradation of sucrose.
Furthermore, it is essential to analyse the dynamics of sucrose
accumulation and to explore the way in which sucrose reserves might be
controlled following harvest through modifications to the environment
surrounding fruits.
One of the most effective technologies for delaying biochemical and
physiological processes after harvest is to apply low temperature
storage at 0 ºC. Indeed, the negative correlation between the storage
duration at LT and taste quality requires application of coadjutant
technologies such as pre-treatment with high CO2.
Present results indicate maintenance of sucrose levels in
CO2 pre-treated fruit during low temperature storage,
whilst significant decreases occurred in fruit at the early stage of
storage at 0 ºC. Moreover, the decrease in hexose sugars detected is
also coupled with the decrease in sucrose previously mentioned,
indicating that a net consumption of sugars mainly occurs during the
early stage of LT storage. However, a decrease in glucose or fructose
was not observed during the late stage of LT in either regular or
CO2-treated fruit. An increase in sucrose due to
CO2 treatment has already been reported and the opposite
trend was observed during postharvest storage at low temperature (6 ºC)
in several cultivars of strawberries (Drake et al., 1997; Cordenunsi et
al., 2003). Further, a decrease in myo-inositol has been observed during
low temperature in air-stored fruit in a similar way as was previously
reported under unfavourable storage conditions (Blanch et al 2015b). We
suggest that myo-inositol could be a precocious marker of a relevant
metabolic condition for measuring optimal storage conditions. Likewise,
the increase in RFOS observed, particularly in air-stored fruit, could
be linked to the response of strawberries to storage at 0 ºC. Several
authors have indicated that RFOS seems to play an important role in
plant responses to osmotic stress (Ishitani et al., 1996; Loewus and
Murthy, 2000; Sengupta et al., 2015). Given the present findings of a
marked increase in RFOS in air-stored fruit at during early and late
storage at 0 ºC, we suggest that RFOS was linked to cold stress. In the
case of trehalose levels, CO2-treated fruit showed
higher levels that fruit stored in air, only at the end of low
temperature storage. These results agree with those reported in the skin
tissue of CO2-treated table grapes. In this case, higher
levels of trehalose were quantified in comparison with untreated fruit
after prolonged low temperature storage (Vazquez-Hernandez et al.,
2018). In fact, a protective role has been attributed to trehalose
through the stabilization of membranes and proteins (Garg et al 2002,
Fernandez. et al. 2010). According to present outcomes regarding
sucrose, trehalose and RFOS, we speculate that treatment with high
levels of CO2 seems to favour the accumulation of
sucrose during the early stage of LT, as opposed to accumulation of
trehalose and RFOS.