Girdling effects on fruit set and quantum yield efficiency of PSII in two Citrus cultivars.

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 (Phi(PSII)) among shoot types and in response to girdling. In young leaves of vegetative shoots, girdling decreased Phi(PSII), whereas Phi(PSII) increased from Day 30 after girdling in young leaves of leafy flowering shoots; however, Phi(PSII) did not change in mature leaves during fruit set in either control or girdled trees. Girdling altered leaf carbohydrate concentrations and the photosynthetic performance of the various shoot types. Our results indicate that, in Citrus, several carbohydrate-based regulatory mechanisms of photosynthesis coexist during carbohydrate accumulation brought about by girdling. It is concluded that the delay in fruitlet abscission and the increase in Phi(PSII )observed in girdled leafy flowering shoots are the mechanisms underlying the enhancement of fruit set after girdling.