Source-sink relationship in processing tomato as affected by fruit load and nitrogen availability

Field experiments were carried out in three years on processing tomato (Lycopersicon esculentum Mill.) grown with two N-fertiliser rates (0 and 400 kg N ha(-1)) and two different fruit loads (full and reduced by weekly partial defloration), in order to investigate the effects of source and sink limitations on source-sink relationships. Biomass and N accumulation and partitioning, leaf photosynthesis, crop light balance and light use efficiency were determined. N fertilisation increased biomass and N accumulation whereas the reduced fruit load did not alter total biomass yield and N uptake. However, reduced fruit load increased biomass and N allocation towards vegetative organs. At reduced fruit load N fertilisation enhanced dry matter allocation to the vegetative plant part, whereas at full fruit load no significant effect could be observed. N fertilisation and reduced fruit load enhanced LAI, LAD and light absorption. However, the influence of N supply was much stronger. Leaf photosynthesis rate (Amax) was slightly increased by N supply and the reduced fruit load. Light use efficiency (RUE) was little affected by treatments. However, at high N supply reduced fruit load decreased RUE. It is suggested that this was due to increased respiration costs related to increased dry matter partitioning towards leaves that are known to have higher respiration rates than fruits. Results could be used for modelling crop growth and N nutrition.