Apple fruitlet thinning and photosynthate supply

The effects of reduced photosynthate supply on apple fruit retention was investigated during the fruit setting period in two years with mature 'Jonagold'/M.9 apple trees. Shading (90%, 7-12 d after petal fall, DAPF) caused fruitlet drop by 23 DAPF and strongly reduced the yield. Carbaryl did not enhance the abscission caused by shading. The photosynthesis inhibitor terbacil resulted in significant abscission, primarily during June drop. Two days after the beginning of shading and one day after terbacil treatment fruitlets net assimilation of CO2 was reduced (i.e. CO2 loss was greater compared with controls). Further, at the end of the shading or 8 d after terbacil application, fruitlet size was smaller. These smaller fruitlets had a diminished CO2 exchange and were more prone to abscission. Fruitlets in competition, king fruitlets in competition with laterals and lateral fruitlets in competition with kings and other laterals, abscised more than comparable fruitlets without competition in the cluster.