Fisiologia della maturazione e suscettibilità della fragola a malattie post-raccolta

Strawberry fruits (Fragaria x ananassa Duch.) are very perishable because of their susceptibility to Botrytis cinerea Pers.:Fr. (grey mould) and Rhizopus stolonifer (Ehrenb.:Fr.) Vuill. (Rhizopus rot). Grey mould rot is the most damaging. It attacks green fruit as well as ripe fruit; after successful flower infection remains quiescent in immature strawberry fruits. Further funga1 development occurs only when the fruits become mature. Ripe fruit rapidly rots, becoming covered with a grey mouldy growth. In this study, changes in some biochemical markers during ripening in field and cold storage are investigated in relation to quality and rot-susceptibility of fruits. Strawberry fruits (cvs Clea, Chandler and Pajaro) in four stages of ripening (green, white, pink and red) and red-ripe fruits, stored at 3 °C for 6-9 days. were analysed for their sugar. anthocyanin. flavonoid and tannin content, phenylalanine ammonia-lyase (PAL), polyphenoloxidase (PPO) and peroxidase (POD) activity. Among soluble sugars, glucose, fructose and sucrose were detected; their content increased during ripening (from green to red stage). There proved to be a ratio of about 1:l in each stage of ripening for glucose and fructose (78.88% of total); their content remained constant during storage, while the content of sucrose decreased. The red color in strawberry fruit is derived from three anthocyanins: pelargonidin-3-glucoside, the predominant pigment (84.88% of total in ripe fruit), pelargonidin-3-rutinoside and cyanidin-3-glucoside. Anthocyanin content ran parallel with the increase in PAL activitv; this latter was firstly detectable in white stage and reached the maximum value in ripe berries. Anthocyanin pigments increased in berries also after harvest during storage both at room temperature (20 °C) and 3 °C. Among flavonoids, catechin, quercetin-3-glucoside and an unidentified quercetin glycoside were found. Their content decreased during ripening. This was especially evident for catechin, the most abundant flavonoid, the content of which markedly decreased starting from the white stage . Hydrolyzable and condensed tannins were also detected and their content was highest in green berries. At this stage of ripening the greatest differences between cultivars were found. with cv Pajaro being the highest in total tannin content and cv Clea the lowest. In all the assaved cultivars condensed tannins (proanthocyanidins) accounted for 61-75% of the total tannins. Tannin content drastically decreased during ripening and this finding corroborates the potential role of tannins in the defence: the overall tannin content of strawberry fruits was negatively correlated with fruit susceptibility to gray mold rot. The reduction of total tannins was greater in Pajaro (-96%) than in Chandler (-84%) and Clea (-70%). During ripening PPO activity decreased unti1 it became about half in the ripe fruits. Furthermore, PPO activity tended to increase with storage to a certain maximum in three days and then decreased again. POD activity remained constant during ripening and successively increased during storage reaching the maximum value in two days. No differences among cultivars during ripening were found, otherwise during storage POD activity in Clea reached values of about twice those of Chandler and Pajaro. These results provide useful information for defining the «biochemica1 potential» of three studied cultivars in relation to their susceptibility to B. cinerea during storage. This biochemical potential changes with the changing composition of plant tissues during storage and that explains why the hitherto quiescent fungus can progress further into the fruit tissue. Knowledge about the metabolism of phenolic compounds during cold storage, the physiological role of PPO