A new index based on vis spectroscopy to characterize the progression of ripening in peach fruit

In peach fruit (Prunus persica L. Batsch), establishing the optimal harvest time is a crucial issue, since fruit shelf-life potential and quality are closely related to the ripening stage at harvest. In order to develop a non-destructive index for monitoring the progression of ripening, the difference in absorbance between two wavelengths near the chlorophyll-a absorption peak (670 and 720 nm; index of absorbance difference, IAD) was related to the time course of ethylene production during on-tree ripening of peaches (cv. ‘Fayette’) and nectarines (cvs. ‘Laura’ and ‘Stark Red Gold’). For each variety, consecutive stages of ripening, as defined according to ethylene production (pre-climacteric, climacteric, post-climacteric), occurred in the same ranges of IAD in different years (2003 and 2004). In 2005, the relationship IAD/ethylene production was used to classify fruit at harvest according to their ripening stage (class 0: pre-climacteric; class 1: onset of climacteric; class 2: climacteric). For each cultivar, the transition from class 1 to 2 was marked by increased ethylene production, and reduced flesh firmness (FF) and titratable acidity (TA). In contrast, fruit quality traits did not discriminate between fruit belonging to classes 0 and 1. In ‘Stark Red Gold’ nectarines, the robustness of the IAD was further corroborated by changes in transcript levels of genes which are either up- or down-regulated during peach fruit ripening. Class 0 fruit had the lowest transcript amount of the up-regulated genes and the highest of the down-regulated ones, while the opposite occurred in class 2 fruit. Moreover, mRNA abundance of some marker genes discriminated class 0 and 1 fruit. Peaches and nectarines graded at harvest according to the IAD also differed in their postharvest ripening behaviour: fruit with higher IAD produced lower amounts of ethylene, began to soften later, and maintained higher TA than those with lower IAD. Present data demonstrate that the IAD identifies physiological changes occurring during ripening regardless of the fact that they might have or not led to appreciable modifications in fruit quality. Therefore, the IAD can be regarded as a very promising tool both for practical and scientific applications, since it allows to monitor on-tree fruit ripening, to establish accurately the optimal harvest time, and to reduce the variability which is present in fruit batches.