Variability of Index of Absorbance Difference (IAD) to indicate fruit maturity at harvest for major apple cultivars in Ontario

The overall objective of this study was to evaluate the index of absorbance difference ( I AD ) of four major apple cultivars in Ontario (‘Honeycrisp’, ‘Ambrosia’, ‘Gala’, and ‘McIntosh’) during the harvest window over multiple seasons (≥4 years), as well as its relationship with fruit firmness, internal ethylene concentration, and starch index values. I AD values differed among the four cultivars, with ‘McIntosh’ having the highest I AD (1.03–1.33) overall and ‘Gala’ having the lowest (0.19–0.56). Principal component analysis showed that the cultivars were separated into distinct groups. ‘Honeycrisp’ was clustered with starch and ethylene, while ‘Gala’ and ‘McIntosh’ were mainly clustered with firmness and I AD , respectively. Variable correlations between I AD and other maturity indices were found over the years. The negative relationship between I AD and ethylene for ‘Gala’ showed variability with R 2 ranging from 0.008 in 2012 to 0.47 in 2018. The correlation between I AD and starch for ‘Gala’ was very strong ( r s = −0.82 **** ) in 2018, whereas it was not significant in any year for ‘McIntosh’. Overall, I AD may relate to harvest maturity, but it did not correlate closely or consistently with other maturity indices, varied greatly year to year, and was cultivar dependent. I AD measures are not consistently related to fruit maturity every year, making reliability difficult to attain.