Auxin treatment reduces inflorescences number and delays bud development in the alternate bearing Citrus cultivar Murcott mandarin.

This article investigates the role of auxin, specifically indole-3-acetic acid (IAA) and the synthetic auxin 2,4-dichlorophenoxyacetic acid (2,4-D), in regulating alternate bearing (AB) in 'Murcott' mandarin (Citrus reticulata × Citrus sinensis), a phenomenon characterized by alternating heavy (ON-crop) and light (OFF-crop) fruit loads. The study demonstrates that applying auxin together with the polar auxin transport (PAT) blocker 2,3,5-triiodobenzoic acid (TIBA) during the flowering-induction period reduces the number of generative (leafless) inflorescences and delays bud development. Transcriptome analyses reveal that this treatment down-regulates key flowering-control genes, including LEAFY (LFY) and SQUAMOSA PROMOTER BINDING PROTEIN-LIKE 5 (SPL5), as well as genes involved in carbohydrate metabolism and flower organ development, suggesting that elevated auxin levels in buds—induced by heavy fruit load—directly repress flowering. These findings support a model in which fruit presence elevates auxin accumulation in buds via enhanced PAT, thereby inhibiting flowering induction and contributing to alternate bearing in citrus.