Evaluation of fluorescence and remission techniques for monitoring changes in peel chlorophyll and internal fruit characteristics in sunlit and shaded sides of apple fruit during shelf-life

The objective of the present study was to assess the potential of laser-induced fluorescence (LIF) and light remission techniques for detection of senescence-induced changes in apple peel chlorophyll content and internal fruit quality characteristics under shelf-life conditions. Results obtained with ‘Jonagold’ and ‘Golden Delicious’ fruit indicate that fruit ground colour alterations due to chlorophyll breakdown can be successfully monitored by LIF and light remission techniques. Normalized difference vegetation index (NDVI) and LIF at 730 nm (F730) showed strongest correlations with chlorophyll content in the apple peel with r in the range of 0.87–0.93. The intensity of red pigmentation of apples could be estimated by a light remission normalized anthocyanin index (NAI). Since the occurrence of anthocyanin pigmentation was accompanied by increased concentration of underlying chlorophyll, red patches of ‘Jonagold’ fruit displayed higher NDVI, F690 or F730 and lower F690/F730 values than those which were apparently green. The multipoint scanning mode of LIF provides information on fruit colour heterogeneity. Among internal fruit quality parameters, the strongest correlation with the apple peel chlorophyll content was found for fruit firmness. For green ‘Golden Delicious’ fruit, there were no differences in the Pearson's coefficients calculated for the data from sunlit (r = 0.78), shaded (r = 0.77) or both sides (r = 0.77). For ‘Jonagold’ fruit, in contrast, r calculated for the data from both fruit sides was lower (r = 0.65) compared to those from sunlit (r = 0.74) or shaded (r = 0.76) sides, due to the different chlorophyll content and same firmness values on the sunlit and shaded sides of fruit of this cultivar. The correlation coefficients between the non-destructively evaluated indices of apple peel chlorophyll content and the Streif fruit maturity index could be significantly improved in both cultivars by considering differences in pigment contents and flesh characteristics on the sunlit and shaded apple sides. Thus, the methods are ideal for sensitive and rapid monitoring of senescence-induced changes in peel chlorophyll and may enhance the accuracy of non-invasive external and internal fruit quality evaluation.