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.