Girdling effects on fruitlet abscission, leaf chlorophyll, chlorophyll a fluorescence and carbohydrate concentration in various flowering and vegetative shoots were studied during natural fruit drop in two Citrus cultivars. Irrespective of shoot type, girdling delayed fruitlet abscission, but only fruitlets borne on leafy shoots had increased final fruit set. Chlorophyll a fluorescence analysis revealed differences in quantum yield efficiency of photosystem II of light adapted leaves (ΦPSII) among shoot types and in response to girdling. In young leaves of vegetative shoots, girdling decreased ΦPSII, whereas ΦPSII increased from Day 30 after girdling in young leaves of leafy flowering shoots; however,ΦPSII did not change in mature leaves during fruit set in either control or girdled trees.
Citrus is the main fruit tree crop in the world and therefore has a tremendous economical, social and cultural impact in our society. In recent years, our knowledge on plant reproductive biology has increased considerably mostly because of the work developed in model plants. However, the information generated in these species cannot always be applied to citrus, predominantly because citrus is a perennial tree crop that exhibits a very peculiar and unusual reproductive biology.
The application of the synthetic auxin 3,5,6-TPA at the cell enlargement stage increased hexoses in developing fruit from foliated and fully defoliated plants of Satsuma mandarin, cv. ‘Okitsu’ (Citrus unshiu Marc.). Although the sucrose concentrations also increased, in general the differences were not statistically significant. The plant growth regulator reduced fruit abscission in defoliated trees while it stimulated fruit growth in the foliated ones.
Like many fruit trees species, citrus trees blossom with high profusion and thereafter exhibit massive fruitlet abscission. Current evidence indicates that this process is under hormonal and metabolic regulation (Gillaspy et al. 1993). In citrus, it has been suggested that after hormonal activation of initial fruit growth subsequent development is mostly supported by nutrient supply (Talon et al. 1997). Thus, once mineral and water requirements are satisfied, competition for photoassimilates is thought to be responsible for fruit drop (Moss et al. 1972; Powell and Krezdorn 1977; Goldschmidt and Koch 1996).