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

In peach fruit (Prunits 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, I-AD) 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 I-AD in different years (2003 and 2004). In 2005, the relationship I-AD/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 I to 2 was marked by increased ethylene production, and reduced flesh firmness (FF) and fitratable 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 I-AD 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 I-AD also differed in their postharvest ripening behaviour: fruit with higher I-AD produced lower amounts of ethylene, began to soften later, and maintained higher TA than those with lower I-AD. Present data demonstrate that the I-AD 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 I-AD 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. (C) 2008 Elsevier B.V. All rights reserved.