Timing of the inhibitory effect of fruit on return bloom of 'Valencia' sweet orange (Citrus sinensis (L.) Osbeck).

Background: In Citrus the inhibitory effect of fruit on flower formation is the main cause of alternate bearing. Although there are some studies reporting the effect on flowering of the time of fruit removal in a well-defined stage of fruit development, few have investigated the effect throughout the entire fruit growth stage from early fruitlet growth to fruit maturity. The objective of this study was to determine the phenological fruit developmental stage at which the fruit begins its inhibitory effect on flowering in sweet orange by manual removal of fruits, and the role of carbohydrates and nitrogen in the process.

Results: Fruit exerted its inhibitory effect from the time it was close to reaching its maximum weight, namely 90% of its final size (November) in the present experiments, to bud sprouting (April). The reduction in flowering paralleled the reduction in bud sprouting. This reduction was due to a decrease in the number of generative sprouted buds, whereas mixed-typed shoots were largely independent of the time of fruit removal, and vegetative shoots increased in frequency. The number of leaves and/or flowers per sprouted shoot was not significantly modified by fruit load.

Conclusion: In 'Valencia' sweet orange, fruit inhibits flowering from the time it completes its growth. Neither soluble sugar content nor starch accumulation in leaves due to fruit removal was related to flowering intensity, but some kind of imbalance in nitrogen metabolism was observed in trees tending to flower scarcely.