Floral induction, fruit growth and photosynthesis are differentially affected by local variation in source/sink relations
Fares Belhassine is a PhD candidate studying alternate bearing of apple trees (Malus × domestica Borkh.), at the French National Institute for Agricultural Research (INRA), in the Architecture and Functioning of Fruit Tree (AFEF) team, Montpellier, France. Fruit trees are prone to alternate bearing and the inhibition of floral induction (FI) under high crop load conditions. The competition for non-structural carbohydrates (NSC) between vegetative and reproductive organs and the inhibition by gibberellins (GA) produced by seeds, are the most likely mechanisms that determine alternate bearing. However, their relative influence on FI remains unclear. My research focuses on the effect of distances between FI promoting organs, such as leaves and fruits, and targets, i.e. meristems, within ‘Golden Delicious’ apple trees on FI. I examined how leaf and fruit removal treatments that were performed at different scales of tree organization, e.g., shoot, branch, half-tree, affect FI and mean fruit weight (MFW). Photosynthesis was associated to starch concentration in leaves and regulated by crop load at the tree scale, independently on distances. FI proportion in shoot apical meristems (SAM) and MFW were sensitive to competition and distances among organs. Both FI and MFW of remaining fruit decreased when leaves were removed and increased when other fruit were removed, the effect increasing with the distances to the SAM and remaining fruit. SAM of fruiting and non-fruiting tree parts presented different FI rates and GA9 content but no variation in NSC content. Similarly, different FI rates were found in SAM of foliated versus defoliated tree parts, but there was no localized variation in NSC content. This suggests that GA rather than carbon could be a main regulator of FI and that a signal other than NSC originated from leaves to promote FI. This other signal could be the flower integrator FLOWERING LOCUS T, known for that action in many other species. These results are currently integrated in a functional-structural plant model for apple tree and used to calibrate carbon and signal transport distance effects and antagonistic impacts on FI. Studying the interactions between model components is expected to improve our understanding of apple tree alternate bearing under different conditions.
Fares Belhassine won an ISHS Young Minds Award for the best oral presentation at the International Symposium on Understanding Fruit Tree Behaviour in Dynamic Environments at the IHC2018 in Turkey, in August 2018.
Fares Belhassine, UMR AGAP Amélioration Génétique et Adaptation des Plantes Méditerranéennes et Tropicales, INRA - CIRAD - Montpellier SupAgro, Montpellier, France, e-mail: fares.belhassine@.inra.fr
The article is available in Chronica Horticulturae