Visible spectroscopy IAD measures in apple: impact of stress and shading on maturity indexing

Various spectroscopy techniques show promise for the non-destructive assessment of quality and maturity in tree fruit. However, as with all new technologies, operating parameters must be tested and evaluated to ensure reliability of use. There have been many reports on the appropriate set up and analysis approaches for both visible and near infrared spectroscopy of intact fruits, however, little effort has been placed on the understanding of the fruit physiology and how spatial differences across a fruit might impact the interpretation of resultant data generated from the instrumentation. We have performed numerous evaluations of the use of a DA meter (T.R. Turoni srl, Forli, Italy) for assessing maturity of apples using loss in peel chlorophyll as maturity index. It became clear in early tests that the instrument could generate data having extremely large variances if the user were not aware of potential causes for variation associated with factors other than maturity. We have previously reported that sun-and shade-exposed sides of apples can produce quite different values simply based on the general differences in chlorophyll content of the peel associated with exposure. More recently we have been able to determine that sun exposure of fruit in extreme cases can cause significant losses in chlorophyll and lead to artificially low levels based solely on sub-lethal sunscald as opposed to maturity-related changes. Therefore, we continued to focus on taking measures from the shaded side of fruits, hoping this approach would more reliably estimate maturity of the fruit. However, further analysis showed that areas of the fruit where no blush developed, the fruit was so deeply shaded that dark-induced chlorophyll loss occurred, leading to artificially low chlorophyll values. In summary, our work shows that when using visible spectroscopy to assess maturity of apples, there must be a protocol established that avoids potentially sun-stressed or deeply-shaded portions of the fruit. With such a protocol, visible spectroscopy can reliably index apple maturity in the field.