Abstract
Bicolored apples are valued for the intensity of the red color of the skin, which is mainly due to the presence of anthocyanins. Among the different environmental factors, temperature and light regimes in the tree canopy play a pivotal role for the anthocyanin synthesis. Established practices like summer pruning, reflective mulches in the interrows laid on the soil in the orchard alleys, evaporative cooling and defoliation by hand are sometimes used in apple orchards to increase fruit skin color. However, in the last five years, a new technique called pneumatic preharvest defoliation has emerged in commercial apple orchards. This PhD thesis deals with three different approaches of preharvest defoliation of apple trees, namely summer pruning, pneumatic defoliation, and the combination of both. It attempts to investigate their diverse effects on fruit quality, yield, carbohydrate reserves, nitrogen content and light dynamics in apple orchards. Therefore, this study was structured into four work packages. i) In the first work package (Chapter 2) preliminary data was used to investigate different strategies to improve color and quality of two apple varieties: ‘Nicoter’/’Kanzi®’ and ‘Cripps Pink’/Pink Lady®. The treatments included manual and pneumatic leaf removal, with different air pressure settings and were conducted in the fall of 2018. The preliminary results were promising, although the results varied depending on the apple variety. Trees that were pneumatically defoliated achieved 90.1% first quality fruit using the 0.7 bar setting of air pressure, while the untreated control group only achieved 53.3%. This led to a significant increase in the potential revenue of the harvested apple fruit. No significant differences in ripening and starch degradation were observed in the trials, although significant effects of the treatments on reduced fruit size were observed. Analysis of this data allowed to set the best parameters for driving speed and air pressure, to be determined for the different varieties in the other experiments. The work packages 2, 3 and 4 refer to two experiments carried out between 2019 and 2023 in two apple orchards, where manual summer pruning was applied approximately three weeks before the estimated harvest and pneumatic defoliation was performed with a pneumatic leaf blower (Vortex, Olmi, Asti, Italy) approximately two weeks before the estimated harvest. As a standard setting, a speed of 1 km h-1 with an air pressure of 0.8 bar was used for both varieties. ii) In the second work package (Chapter 3) the focus was on the light environment. The study unveiled how the treatments affected the light conditions within the canopy. The decrease of the total leaf area (up to 60% compared to the untreated control), resulting both from summer pruning and pneumatic defoliation and especially from their combination, increased the amount of light inside the canopy. Pneumatic defoliation was in general more effective than pruning in increasing light penetration, particularly in the middle section of the trees and on both cultivars. iii) In the third work package (Chapter 3) a special focus was on fruit quality. The study revealed that the defoliation treatments had notable effects on fruit quality. A reduction in fruit weight, particularly pronounced in 'Nicoter' was observed. Also, a negative impact of pneumatic defoliation on the total soluble solids content of apples was detected in two years on the cultivar ‘Nicoter’. A positive influence of both summer pruning and pneumatic defoliation on the red color and anthocyanin content of fruit skin was recorded, especially from the fruits sampled in the inner part of the canopy. iv) In the fourth work package (Chapter 4) the impact of summer pruning, pneumatic leaf removal and their combination on the storage of carbohydrates (starch) and nitrogen in apple twigs and roots was examined. These reserves are crucial for tree growth and fruit development, particularly in the spring when root nitrogen (N) uptake and leaf photosynthesis may be a limiting factor. As treatments were carried out just before fruit harvest, considering the different harvest dates of ‘Nicoter’ (on average beginning of September) and ‘Rosy Glow’ (on average end of October) different effects of leaf removal on starch and N storage in the two varieties was expected. The research found that both summer pruning and pneumatic leaf removal reduced starch concentration in roots and nitrogen concentration in twigs. The apple trees starch content in roots and nitrogen content in twigs were reduced up to 44% and 28%, respectively, compared to untreated trees. Moreover, the regression analysis proved that the smaller the tree leaf area (after treatment application) the lower the root starch content and the lower the nitrogen content in twigs. This study suggests that the reductions should be seen as initial evidence of the physiological impact of pre-harvest leaf removal. Further research over multiple years is needed to gain a deeper understanding of the long-term effects of these techniques on apple trees, providing valuable insights for optimizing tree growth and fruit quality in commercial orchards. In conclusion, the pre-harvest defoliation can be a useful tool to increase fruit skin color on bicolored apple varieties. Some minor influences on fruit size and soluble solid content in the apple fruits were found over the course of this study. No negative influences on return bloom, fruit set and yields were seen, despite the reduction of starch content in roots and the reduction of nitrogen (N) in twigs. This work underscores the importance of a balanced approach when implementing summer pruning and pneumatic defoliation in apple orchards, considering the trade-offs between different aspects of the fruit quality. It establishes a foundation for further research and practical implementations, with a focus on fine-tuning defoliation techniques for optimal fruit production and retaining the storability of apples.