Physiology at work to model apple expansion growth and skin pigment changes

Fruit sizing is a major factor in determining yield while non-destructive monitoring of the fruit skin colour and internal quality attributes on the tree can provide valuable maturity and quality information for precision horticulture. Repeated spectral scanning and fruit sizing data of ‘Braeburn’ apples were collected on the tree from about 60 days after flowering until harvest. Assessed variables were: fruit diameter, dry matter (DMC), soluble solids content (SSC), a normalised difference vegetation index (NDVI) and a normalised anthocyanin index (NAI) analysed using indexed non-linear regression based on an adapted von Bertalanffy model (diameter, DMC, SSC), or a logistic model (NDVI, NAI). The reaction rate constants in the models were estimated in common for all fruit in a selection, while the biological shift factors ($\Delta$t) estimated the development stage or fruit maturity per individual fruit. Explained parts (R²_adj) range from 85 to 97%. Tree location or crop load treatment only minimally affected the rate constants but did affect the estimated $\Delta$t values that describe almost all variation in the data. There is a close relationship between the $\Delta$t values for diameter, DMC and SSC but less with those of NDVI and almost none with the NAI. These data support the assumption that there is only one stage of fruit maturity, but it is estimated slightly differently depending on the measured variable. The actual relative growth rate strongly depends on the current size. Understanding apple expansion growth will therefore require a closer focus on the cell production period.