Variability of index of absorbance difference (I_AD) to indicate fruit maturity at harvest for major apple cultivars in Ontario

Abstract

The overall objective of this study was to evaluate the Index of Absorbance Difference (IAD) 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. IAD values differed among the four cultivars, with ‘McIntosh’ showing the highest IAD (1.03 to 1.33) overall and ‘Gala’ having the lowest (0.19 to 0.56). Principal component analysis (PCA) 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 IAD, respectively. Variable correlations between IAD and other maturity indices were found over the years. The negative relationship between IAD and ethylene for ‘Gala’ showed variability with R2 ranging from 0.008 in 2012 and 2019 to 0.47 in 2018. The correlation between IAD and starch for ‘Gala’ was very strong (rs= -0.82 ****) in 2018, whereas it was not significant in any year for ‘McIntosh’. Overall, IAD 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. IAD measures are not consistently related to fruit maturity every year, making reliability difficult to attain.