Fruit growth, photosynthesis and starch accumulation are differentially affected by local variation in source/sink ratios

Tree crop load affects carbon assimilation, storage and fruit growth. Nevertheless, the impact of local variations in source/sink relationships on the within-tree variability of these processes are not fully understood. This study aims at investigating the effect of source/sink manipulations on these processes through leaf or fruit removal performed at different scales of tree organization. Experiments were performed in 2016 and 2017 on adult 'Golden delicious' apple trees planted in the south of France. On trees in either high or low crop loads, 11 different leaf or fruit removal treatments were set up in early June, at shoot or branch scales or one side of Y-shape trees. Photosynthesis was measured in August on the different parts of trees subjected to the different local treatments. Measured leaves and the wood supporting them were sampled for starch concentration. At harvest, mean fruit weight was determined for fruits in different locations on the trees. Our results show a strong impact of the tree crop load on photosynthesis without any clear effect of the local fruit presence. As observed for photosynthesis, starch concentration in leaves was strongly associated with the tree crop load without any local variation depending on local treatment. Conversely, in woody parts starch concentration was affected by the local treatment with greater starch concentrations in de-fruited or foliated parts of trees. Results on fruit weight suggest that assimilates could not be exchanged over long distances in the absence of leaves, whereas, long distant transport did occur between fruiting and non-fruiting parts of the tree. This study gives new insights on the effects of source-sink ratios and distances on functional (carbon acquisition and transport) and developmental (fruit growth) variables suggesting that the distance effects between sources and sinks could differ depending on the experimental setup.