Doubts regarding carbohydrate shortage as a trigger toward abscission of specific Apple ( Malus domestica ) fruitlets

Abscission of young fruitlets is a widespread phenomenon in fruit trees termed ‘physiological fruitlet drop’ (PFD). For some fruit crops, the rate of PFD is agriculturally sufficient, and in many cases too intense. In apples (Malus domestica) PFD is insufficient since without additional fruitlet thinning, fruits will not reach commercial size and trees will enter a cycle of alternate bearing. An apple inflorescence contains 5–6 flowers, the terminal king flower is the first to initiate, the first to reach anthesis, and is considered the fruitlet with the lowest chance to go through PFD. The last flower to initiate and later reach anthesis is termed lateral 1 (L1), and it has the highest probability to enter PFD. A better understanding of the PFD process might lead to more precise thinning procedures. The current hypothesis is that the ‘sink strength’ of the L1 fruitlet is weak compared to king fruitlet, thus L1 will enter a nutritional shortage which will lead to its developmental arrest and abscission. Based on this hypothesis, we assumed that the concentration of carbohydrates (soluble sugars and starch) would be highest in king and lowest in L1. We traced the level of different soluble sugars and starch in the different flowers/fruitlets in the apple inflorescence, before anthesis and during the early development of the apple fruitlet. Sugar levels were indeed higher in king compared to L1 at initial stages, even before anthesis. While this result nicely fit the consensus hypothesis, we considered it might be an artifact caused by the different developmental stages reached by each flower within the inflorescence. When we normalized the collection of different flowers within an inflorescence to a certain developmental stage and not to a date, the differences in sugar level were reduced or non-existent. While carbohydrates are clearly essential for young fruitlet survival, our finding suggests that the ability of L1 fruitlets to collect sugars is not reduced compared to other fruitlets in the cluster, just slightly delayed. If a nutritional shortage indeed occurs in L1 fruitlets, the type of chemical in shortage, or the cells that suffer from this shortage, are still unknown.