Comparison between IAD and other maturity indices in nine commercially grown apple cultivars

To maintain storage potential as long as possible, it is important to harvest fruit at optimal maturity. Different maturity indices have been developed, including flesh firmness, soluble solids content, starch degradation, ethylene production, and respiration rate. However, many of them are destructive, time consuming, and may be require some laboratory equipment to perform. The portable device DA-meter (measuring index of absorption difference; IAD) can monitor the chlorophyll decline non-destructively in the field, and could potentially save time. To evaluate the IAD in comparison with other maturity indices, nine common commercial cultivars of apple were investigated in a three-year trial. Correlations between IAD and other maturity indices, especially starch degradation and ripening index by Streif were strong in most cultivars, though variation between years lead to weaker correlations were found last year of the trial. The strongest correlations were found between IAD and harvest date showing that IAD decreased with time in all investigated cultivars. Comparison between IAD and ripening index by Streif showed in some cases that the two indices decreased at the same time, suggesting that IAD could be used to monitor maturity when it is rapid. The suitability to use IAD as a maturity index seems to be cultivar-dependent. For cultivars having a more consistent pattern between years in the decrease of IAD, combined with relatively low variation in IAD at any given time, it could be a good complement to other commonly used maturity indices.