ALLOCATION OF DRY MATTER AND N TO FRUIT AND SHOOTS IN DWARF APPLE IN RESPONSE TO SINK SIZE AND N AVAILABILITY

Precision application of N to improve growth and fruit quality requires information on the magnitude and timing of N demand. Two field studies were undertaken to determine 1) the source and timing of N and dry matter acquisition by fruit and shoots in 2-year-old Fuji/M.9 and 2) dry matter and N accumulation in response to N availability (rate and timing) and crop load in 4-year-old Gala/M.9. In both experiments, accumulation of N and dry matter were highly related for whole tree vegetative and reproductive tissue, but less so for individual leaves and fruit. Remobilization supplied most of the N for fruit growth during cell division. During the same period shoot leaf growth was supported by N remobilization and root uptake. Both N and dry matter accumulation in fruit were low during cell division, relative to accumulation in the shoot leaf canopy, supporting the hypothesis of differential dominant sinks at different phenological stages. All trees were unthinned, and fruit number did not affect individual bud or fruit dry matter or N accumulation, suggesting that the number of buds was coupled to the amount of stored resources. Shoot and fruit responded differently to sink size (crop load) and N availability (rate and timing). Shoot growth was limited by low N supply and high crop load after, but not before the start of fruit cell expansion. Trees adjusted to lack of resources by reducing leaf number rather than area. Total fruit growth was limited during cell division and expansion by a large total fruit sink size. In addition to reducing dry matter accumulation, high crop load decreased the availability of soluble carbohydrates to individual fruit. Nitrogen availability did not affect fruit growth but fruit N content was higher in trees receiving high N immediately after fertigation. High N availability later in the season accelerated conversion of starch to sugars at the same time negatively affecting fruit color.