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Multiannual fruit bearing in fruit trees varies from regular bearing, with an almost steady yield from season to season, to alternate bearing, during which a year with a high fruit load (on-crop) is followed by a year of a low fruit load (off-crop). ‘Orri’ mandarin [Temple (Citrus temple hort. ex Y. Tanaka) × Dancy (Citrus tangerine hort. ex Tanaka)] exhibits a nonregular bearing pattern: steady for a few years and alternate for others. In nonregular or alternate-bearing cultivars, fruit load is thought to regulate flowering induction and development negatively the following year. In citrus, the bud meristem develops into a leafless inflorescence (generative), leafy inflorescence (mixed type, containing leaves and flowers in various ratios), or vegetative shoot, or remains dormant. Mixed-type inflorescences contribute to most of the final yield. Gibberellins inhibit flowering when applied at relatively high concentrations during the flowering induction period in the winter, but they are suggested to induce yield when applied at relatively low concentrations. We determined the effect of mild gibberellin application in an off-crop year on the productivity of ‘Orri’ mandarin, and whether it occurs through the induction of mixed-type inflorescences. Low-concentration, low-frequency gibberellin treatments were applied to trees during the winter when fruit load was high, resulting in a greater number of mixed vs. generative inflorescences at anthesis. An average increase of 40% in fruit number and yield weight was recorded in off-crop seasons at harvest, whereas no change was recorded in on-crop seasons. We discuss treatment efficiency in on-crop and off-crop years as a practical tool for increasing yield in low-crop seasons of ‘Orri’ mandarin. [ABSTRACT FROM AUTHOR]

In 'Hass' avocado (Persea americana), fruit presence reduces next season flowering. Recent fruit tree studies proposed that heavy fruit load (HFL) generates an auxin (indole-3-acetic acid, IAA) signal in the buds that represses flowering. However, the nature of this signal remains unknown. Here, we investigated the effect of avocado HFL on bud IAA accumulation and flowering transition. We found that IAA–aspartate and IAA–glutamate conjugate levels were significantly higher in buds from fully loaded ('on') than low-loaded ('off') trees, hinting that free IAA levels were higher in the former. Expression analysis showed that coinciding with flowering reduction, HFL induced the floral repressor PaTFL1 , and suggested that accumulation of IAA in buds as imposed by HFL was associated with its conjugation to aspartate and glutamate and resulted both from de novo IAA synthesis and from reduced IAA export. Accordingly, experiments involving radiolabelled [14C]IAA demonstrated that HFL reduced shoot basipetal IAA transport. Finally, we confirmed the negative effects of IAA on flowering, showing that IAA and polar auxin transport blocker (2,3,5-triiodobenzoic acid) treatments delayed 'off' trees' inflorescence development, reducing their inflorescence axis and inducing PaTFL1 expression. Together, our data indicate that avocado HFL generates IAA signalling in buds that induces PaTFL1 , leading to repression of inflorescence development. [ABSTRACT FROM AUTHOR]

In many fruit trees, heavy fruit load in one year reduces flowering in the following year, creating a biennial fluctuation in yield termed alternate bearing (AB). In subtropical trees, where flowering induction is mostly governed by the accumulation of chilling hours, fruit load is thought to generate a signal (AB signal) that blocks the perception of cold induction. Fruit removal during a heavy-fruit-load year is effective at inducing flowering only if performed one to a few months before the onset of the flowering induction period. We previously showed that following fruit removal, the content of the auxin indoleacetic acid (IAA) in citrus buds is reduced, suggesting that the hormone plays a role in the AB signal. Here, we demonstrate that fruit presence generates relatively strong polar auxin transport in citrus and olive stems. Upon fruit removal, polar auxin transport is reduced and allows auxin release from the bud. Furthermore, using immunolocalization, hormone, and gene expression analyses, we show that in citrus, IAA level in the bud and specifically in the apical meristem is reduced upon fruit removal. Overall, our data provide support for the notion that fruit presence generates an auxin signal in the bud, which may affect flowering induction. [ABSTRACT FROM AUTHOR]

Specific cultivars of many commercial fruit tree undergo cycles of heavy fruit load (ON-crop) one year, followed by low fruit load (OFF-crop) the next (termed alternate bearing). Fruit load may affect flowering at various developmental stages, and its presence is suggested to generate a flowering-inhibitory signal. In a previous report, we showed that the presence of fruit induces polar auxin transport from the fruit into the stem, interfering with indole acetic acid (IAA) release from the bud, and thus elevating its levels in the bud meristem. To better understand the relationship between auxin homeostasis in the bud and flowering, IAA or 2,4-Dichlorophenoxyacetic acid (2,4-D) were applied with the polar auxin transport blocker 2,3,5-triiodobenzoic acid (TIBA) to OFF-crop 'Murcott' mandarin trees during the flowering-induction period. The treatment reduced inflorescences number and delayed bud development. Transcriptome analysis following the treatment revealed a reduction in the expression of a few flowering-control genes, including LEAFY and SQUAMOSA PROMOTER BINDING PROTEIN-LIKE. In addition, genes related to carbohydrate metabolism were reduced. We suggest that the elevation of auxin levels in the bud by heavy fruit load directly affects the expression of flowering-control, flower-development, and developmental genes., (© The Author(s) 2025. Published by Oxford University Press. All rights reserved. For permissions, please e-mail: journals.permission@oup.com.)