Influence of calcium on the activity of enzymes involved in kiwifruit ripening

Flesh firmness and total soluble solids content (SSC) are widely used to define post-harvest quality of kiwifruit during ripening. It is also interesting to investigate the other potential factors involved in this process. In this research, the role of calcium ions (Ca2+) on kiwifruit ripening (Actinidia deliciosa 'Hayward') was investigated. It is known that Ca2+ and other divalent ions delay ripening and senescence of kiwifruit. However, the way in which divalent ions act is not yet completely understood. The aim of this study has been to assess the effect of Ca2+ on some enzyme activities. Fruit quality parameters, such as fruit total calcium content, firmness, SSC and some enzyme activities were determined after harvest and during cold storage of fruits from plants grown in two different conditions of light exposition, namely exposed (E) and shaded (S). The results show that fruit firmness throughout the post-harvest period could be related with calcium content that was greater in exposed fruit than in shaded ones. During the whole storage period, amylase content was also higher in exposed fruits than in shaded ones while a slight difference between SSC respective trends was observed. A direct inhibitory effect of calcium on PG and on 23 KDa proteases was revealed. Even if pectinolytic proteins are considered the enzymes directly responsible for tissue softening, we also formulate an hypothesis in which kiwifruit proteases may be involved indirectly in ripening process.