A frequent response of fruit trees to deficit irrigation (DI) is a promotion of flowering. This response is often explained because of a lesser competition with exuberant vegetative growth. Here, we report the effects of regulated DI on loquat shoot growth and flowering and discuss the possible mechanisms involved in the promotion of flowering in water-stressed trees. Loquat is a subtropical tree crop that bloom in autumn after a period of summer dormancy. Loquat flowers develop in panicles formed at the apex of the new shoots, therefore shoot growth has to cease before flower initiation can take place. In our experiments, the flowering of fully irrigated trees of 'Algerie' loquat was compared with the bloom in trees undergoing three different levels of DI implemented at post-harvest from mid-June to the end of July. DI levels during these six weeks were: light (50% of the water applied to controls), moderate (25% of the water applied to controls) and severe (no watering). Minor effects due to DI were found on flowering intensity. In contrast, water-stressed trees reached bloom before controls (between 10 and 27 days, depending on treatment). The more severe the water stress was, the earlier the blooming resulted. Blooming advancement was produced despite final shoot length and leaf number remained essentially the same. On the contrary, DI profoundly altered the pattern of shoot growth that changed from a single sigmoid to a double sigmoid. This shift was the result of water stress causing an early, but transitory, cease of growth, which was reassumed up to the length of controls when water deficit ended. Observations carried out under scanning electron and conventional microscopy indicate that panicle initiation occurred days before the fully establishment of summer dormancy, when growth rate in the apical meristem slowed down. The advancements of summer dormancy and panicle initiation correlates well with blooming date advancement. Our results question the hypothesis of resource competition between flowering and vegetative growth and suggest that flowering promotion is the result of a diminished growth rate in the apical meristem due to hormonal changes that favour the process of flower induction. This theory is coherent with the promotion of flowering in response to growth inhibitors and with the negative effects that gibberellins have on tree blooming. The modification of reproductive phenology by the management of the agricultural water may represent a new avenue for improving profitability in tropical and subtropical fruit crops.
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