S16 - Session P1 - What makes the difference between apple cultivars which are `easy´ and `hard´ to thin with metamitron?

Authors: Mekjell Meland *, Milica Fotiric Aksic, Tomislav Tosti, Zivoslav Tesic, Frank Maas

The objective of this experiment was to establish the source-sink relationship between sugar levels of leaves (source) and fruits (sink) in two apple cultivars (`Summered` and `Rubinstep`) showing a difference in thinning response to Brevis® (15% metamitron). During 2016-2019, at NIBIO Ullensvang, western Norway, both apple cultivars were treated with different concentration of this photosynthetic inhibitor after flowering. Four different treatments were applied: 1.1 and 2.2 kg/ha Brevis® when fruitlets were 8-10 mm; 1.1 and 2.2 kg/ha Brevis® when fruitlets were 8-10 mm and when fruitlets were 12-24 mm and compared with unthinned control and handthinned. Leaves from the different treatments were collected before and after the treatments [3 rd , 6 th and 9 th day after Brevis® application (DAA)]. The contents of sugars (glucose, fructose, and sucrose) and sugar alcohol (sorbitol) in leaves were analyzed by ion chromatography. The different treatments of Brevis® thinned 'Summerred' to the level of handthinned, but had no effects on 'Rubinstep'. Average number of mature seeds per fruit was 3.6 for 'Summered' and 9.5 for 'Rubinstep' and some fruits had up to 14 seeds per fruit. For 'Summered' sugar metabolism in leaves was altered, and in 3 rd DAA, levels of sorbitol and glucose significantly decreased, level of sucrose significantly increased, while level of fructose remained stable. Sixth day all sugars dropped under the control level except for sucrose. Ninth day, the levels of sorbitol was higher while sucrose dropped compared with the control. For 'Rubinstep' sugar metabolism was disrupted in the exactly same way 3 rd day DAA, but with some slight differences for glucose 6 th DAA and sorbitol and sucrose 9 th DAA. A much higher number of seeds/fruit in `Rubinstep` made a stronger sink strength and seed activity which resulted in high enough transport of sugars to the fruitlets to prevent their dropping.