Source:sink relationship in the apple flower cluster


Inflorescences can be categorised into leafless ones (stone fruit) and leafy ones (pome fruit). The objective of the present study was to explore potential synergy or competition between the rosette leaves in the apple leafy inflorescence and the flowers for photo-assimilates, viz., carbon and nitrogen skeletons. In our deciduous fruit trees, these carbon and nitrogen skeletons are stored over winter and remobilised in spring. The second objective was to investigate the point when the leafy flower cluster becomes autarky. Six-year-old ‘Roter Boskoop’ apple trees on M9 spaced 3.5×1 m were selected at Campus Klein-Altendorf, Germany (50°N) in 2018. Crop load management (CLM) comprised either ATS and Flordimex applied at full bloom with an optional additional late BA treatment or mechanical CLM using the BAUM thinner operating at 320 or 380 rpm rotor speed at full bloom. Flower clusters were counted on whole trees before the treatments and fruit set related to the number of 100 flower clusters. Each treatment consisted of ten trees with adjacent untreated control and border trees to separate CLM resulting in 450 trees overall. The obtained results indicate that more than 50% of rosette leaves have to be removed to induce fruitlet abscission and this could then be a thinner as strong as chemical or mechanical thinning and that the carbohydrate reserves are exhausted at the end of fruit set. Overall, the results with apple suggest that the crucial role of stored carbohydrates and the conversion point, when the flower cluster becomes independent of stored carbohydrates, has to be clearly identified as the stage of fruit-set under our conditions.