13C-Photoassimilate Partitioning in Sweet Cherry on Dwarfing Rootstocks during Fruit Development

In sweet cherry (Prunus avium L.), fruit size and quality are dependent on photoassimilate synthesis and subsequent partitioning among different sink organs. As little was known about the relative importance of different sweet cherry leaf populations (i.e., fruiting spur leaves, FSL; non-fruiting spur leaves, NFSL; and current season shoot leaves, CSSL) as sources of carbon (C) for fruit and shoot development in dwarfing trees, it was hypothesized that the supply and partitioning C to canopy sinks during fruit development varies by leaf population within the fruiting branch unit on dwarfing precocious Gisela(R) rootstocks. The newly-fixed C contributions from specific leaf populations to fruit and shoot development during fruit growth stages I (SI), II (SII) and III (SIII) were studied in two different experiments using 2-year-old fruiting branches of 'Sam' on 'Gisela 5' (GI5) and 'Ulster' on 'Gisela 6' (GI6) rootstocks. The first experiment used (CO2)-C-13 to label, at 3 times during Sill, the NFSL on 'Sam'/'GI5' limbs having one of three different crop load treatments (quantified as leaf area [LA] to fruit [F] ratio, e.g., LA/F = 140, 75, or 40 cm(2) /fruit). Results indicated that C-13 fixed by NFSL was translocated both acropetally and basipetally. For all 3 pulsing dates, fruit were more highly enriched n C-13 than were young leaves. Fruit in closest proximity to the NFSL had the highest relative C-13 content. The second experiment quantified the relative C contributions of different leaf populations on 'Ulster'/'GI6' limbs to fruit and shoot development during SI, SII and Sill. Each leaf population on the fruiting branch was exposed separately to (CO2)-C-13 labeling on live dates (25, 40, 44, 56 and 75 days after full bloom). C-13 fixed by each leaf population was translocated to all sinks but not uniformly. Fruits were a priority sink vs. new shoot growth during the entire period of fruit development. Proximity to C-13 source influenced the amount of C-13 detected in fruit. The highest fruit sink strength was during SI and SIII. The terminal current season shoot provided a C source for fruit as early as SI.