Critical Crop Load Threshold in Nutrition and Biennial Bearing of Apple Trees

Crop load regulation is vital for achieving excellence in orchards, particularly in terms of consistent yields and high-quality fruit. It also has a direct impact on tree nutrition. The objective of this study was to investigate the relationship between crop load and tree nutrition using segment linear regression models. The focus was on identifying any breakpoints in this relationship and exploring the connection between leaf nutrient contents and fruit quality characteristics. Additionally, the study aimed to determine the critical crop load level that influences biennial bearing. The research was conducted in a high-density 'Golden Delicious'/M.9 apple orchard located in the Lake Region of Turkey over three consecutive years (2013-2015). Twenty-four different crop load levels were examined to assess the impact of the number of fruits on leaf nutrient contents. The critical threshold levels were determined as follows: potassium [0.91 kg cm(-2) Trunk CrossTCSA), manganese (0.99 kg cm(-2) TCSA), zinc (1.0 kg cm(-2) TCSA), and iron (1.15 kg cm(-2) TCSA). This suggests that a crop load ranging from 3.71 to 4.69 fruit/cm(2) TCSA could be considered critical depending on the specific nutrient in tree nutrition. The results revealed significant negative correlations between leaf mineral contents and overall fruit quality characteristics. Moreover, the critical crop load threshold for biennial bearing (0.77 kg cm(-2) TCSA) was found to be lower than the nutrient threshold. Building on previous studies, this research significantly contributes by clarifying the critical crop load level at which a sudden change occurs in macro- and micro-nutrients, as well as biennial bearing.