Apple fruitlet physiological characteristics and their influence on diffuse visible/near-infrared reflectance spectroscopy.

Background and aims: The recent rise in digital technologies to inform agronomic practices has necessitated further understanding of how plant physiological characteristics affect light reflectance measurements that these technologies employ. A young apple fruitlet undergoes rapid changes in size, chlorophyll content and trichome density as it develops. Our objective here was to characterize these changes in 'Fuji' and 'Honeycrisp' fruit to understand how those changes affect diffuse spectral reflectance.

Methods: Visible/near-infrared reflectance spectra were captured with a portable visible and near-infrared spectrometer on individual fruit from 20 to 36 d after full bloom. From these reflectance spectra principal component analysis (PCA) was performed and vegetation indices were calculated. A method was developed to quantify trichome density using image analysis. Water content, trichome density and chlorophyll concentration were determined for each fruitlet. Regression analysis and correlation coefficients were calculated to understand how these measures relate during fruitlet development.

Key results: PCA revealed the first principal component contributed to 86.56 % of the variation in the reflectance data. Along this component there was a gradient of fruit size. These size effects led to lower reflectance at wavelengths associated with chlorophyll and water absorption. Chlorophyll concentration and trichome density decreased with increasing fruit size. The total amount of chlorophyll in the fruit increased as fruit grew larger. Total chlorophyll content had a stronger relationship with fruit size than with trichome density.

Conclusions: Fruit size and in turn total chlorophyll content had the greatest effects on reflectance spectra. When measuring reflectance with a portable visible and near-infrared spectrometer that has a fixed lens size, the amount of tissue covering that lens is a major influence on the reflectance spectra. There were differences in chlorophyll content between the two apple cultivars studied that may have larger implications for the photosynthetic capacity of fruit.