Plant hormones (Literature sources on phytohormones and plant signalling)

Scientia Horticulturae (2022)

Highlights: • ‘Orri’ mandarin has a low parthenocarpic ability, in accordance with its low CcGA3ox1 gene expression, but the application of GA3 does not increase fruit set. • Ringing branches performed 35-40 days after anthesis increases fruitlet growth rate. This effect correlates inversely with the fruitlet abscission rate, and positively with cell division, i.e., with the CcCYCA1.1 gene expression in the fruitlet, but not with the GA biosynthesis, i. e., with the CcGA3ox1 gene expression. • Ringing branches significantly increased CcPIN1 gene expression in the fruitlet. • Results suggest that ringing protects the fruitlet abscission zone mediated by polar auxin transport from the fruit, which allows the fruitlet to maintain carbohydrate assimilation and continue growing. 

Abstract: "Ringing branches is a technique which is widely used to increase the yield of Citrus cultivars with low parthenocarpic ability. When performed during the physiological fruitlet abscission stage it prevents fruitlet drop and increases the number of fruits harvested. This effect has been related with an increased carbohydrate supply, which requires an enhanced photosynthesis efficiency of leafy flowering shoots. Since ringing also reduces vegetative growth, both the number of shoots and the leaves per shoot, the mechanism by which the carbohydrate supply is increased should be revised. Our results show that ringing carried out at this stage maintains the ability of the ovary for cell division mediated by the availability of carbohydrates, as indicated by an increased CcCYCA1.1 expression. But this effect is not linked with an increase in GA1 biosynthesis (CcGA3ox1 expression), as this occurs during fruit set; hence, hormones other than gibberellin must be controlling the physiological fruitlet abscission in response to ringing. We found that an increased expression of the auxin efflux carrier CcPIN1 gene suggests that ringing induces the auxin export out of the fruitlet and transport to the abscission zone (AZ-C), thus inhibiting its activation and allowing carbohydrates supply to the fruitlet which, thus, prevents abscission and continues growth."