Photosynthetically assimilated carbon (C) is transported from source leaves to fruits depending on their development stage. To clarify the behavior of the assimilated C in a shoot with a fruit throughout its development, we constructed a 13CO2 exposure chamber system for a fruit-bearing shoot of apple and measured assimilation of 13C in leaves and its translocation to the fruit in an early maturing apple cultivar (Malus domestica cv. ‘Tsugaru’). Fruit-bearing shoots were exposed to 13CO2 for 1 h in the chamber and collected 72 h after the exposure, followed by analysis of 13C in each plant part (leaves, a branch and a fruit). The exposures were done three times during fruit developing: at the early developing, the developing and the mature stages. We evaluated the behavior of 13C by following two indices: 13C remaining ratio, (13C inventory in each plant part 72 h after the exposure)/(net assimilated 13C during exposure); and 13C distribution ratio, (13C inventory in each plant part)/(13C whole inventory in the shoot) at sample collection time.
Although the 13C remaining ratio in the fruit in the early development stage was slightly lower than that in the fruit developing and mature stages, the ratio in the mature stage was similar with that in the fruit developing stage, indicating that the ability of source leaves to allocate photoassimilates to fruit was maintained until maturation of fruit. The 13C remaining ratio in the whole shoot was lower in the early fruit development stage than in the later stages, possibly due to higher translocation of assimilated 13C from the shoot in the early stage. The 13C distribution ratio in the whole shoot was similar regardless of fruit development stage, though the biomass growing of fruit was different among the stages. This suggests that the sink strength of ‘Tsugaru’ fruit at the fruit-bearing shoot level may not be altered by fruit development.