Challenges to interpreting internal and external factors limiting apple fruit growth, set and abscission

The development of the apple fruit over a season in managed orchards is a complex interaction of genetic, physiological, environmental and management factors. The inherent genetically-limited growth habit of apple growth begins with cell division, then transitioning to a cell expansion as typical for many fruits. Determining the potential growth allows the evaluation of when environmental factors or management treatments affect fruit growth. Monitoring of fruit growth at frequent intervals is a valuable method to identify key periods limiting fruit development. Fruit abscission related to a reduction in growth rate appears to be a general mechanism in fruit species. An integrated approach with simulation modeling is also helpful in these cases. Diurnal fruit growth shows an interesting pattern with little growth in the morning, then beginning growth in mid-afternoon. Shoot growth surprisingly shows the same pattern. Improvements in physiological and molecular genetic understanding support our understanding of fruit development and can provide opportunities to improve apple crop management, however, we must understand fruit growth to correctly interpret our observations and experiments.