‘Gala’ apple trees tend to produce small-sized fruit resulting from a combination of several factors (Robinson et al., 2005). First, Gala is a small-fruited cultivar. Second, ‘Gala’ trees differentiate flowers readily, have heavy fruit set, and are difficult to thin, especially when tree vigor is low, and as a result, ‘Gala’ trees often carry heavy crop loads. Finally, heavy cropping reduces tree vegetative growth (Avery, 1970; Buwalda and Lenz, 1992; Lenz, 1986; Wünsche and Palmer, 2000), which further decreases source (leaves) to sink (fruit) ratio. However, large-sized ‘Gala’ fruit is preferred over small fruit at a significant premium on the market. This has prompted growers to strive for large fruit size.
Apple fruit size at harvest is the result of an early exponential cell division in the first 4 to 6 weeks after bloom and subsequent cell expansion during the remainder of fruit growth (Al-Hinai and Roper, 2004; Blanpied and Wilde, 1968; Denne, 1960; Goffinet et al., 2005; Lakso et al., 1995). Fruit growth is affected by genotype and many internal and environmental factors (e.g., carbon supply, crop load, hormones, mineral nutrients, temperature, and light). For a given cultivar, crop load is perhaps the single most important factor that determines fruit growth and final fruit size (Haller and Magness, 1925; Magness, 1928; Magness and Overley, 1929; Preston, 1954; Wünsche and Palmer, 2000). The effect of crop load on fruit size exemplifies the central role of the source-sink relationship in determining apple fruit growth, which also provides a very useful framework for understanding the effect of many other factors on apple fruit growth and final fruit size.
Increasing nitrogen (N) supply has been tried to improve ‘Gala’ fruit size, but the results are inconsistent. Robinson et al. (2005) reported that increasing the rate of N fertilization from 62.8 to 188.4 kg·ha−1 did not significantly increase ‘Gala’ fruit size. Similarly, Forshey (1982) did not detect any significant effect of N fertilization on the fruit size of ‘Empire’, another small-fruited apple cultivar, although leaf N was increased by N fertilization. We have conducted field trials for 3 years to determine the effect of N fertilization on ‘Gala’ fruit size and found that when crop load was maintained at a moderate level, increasing the rate of N fertilization led to an increase in fruit size; but under heavy crop load, the effect of N fertilization on fruit size was not significant. The lack of significant responses of fruit size to increasing N supply observed in the field could arise in two situations. First, tree N status is not limiting to fruit growth, and therefore further elevating tree N status through increasing N supply does not affect fruit size in a significant way. Second, tree N status is limiting, but heavy crop load may override the effect of increasing N supply on fruit size. Flower and fruit-thinning experiments have clearly demonstrated that when a tree is source-limited resulting from too much competition between fruit in a heavy cropping situation, fruit is typically small and thinning improves fruit size by lessening the source limitation through reducing the number of competing fruit. However, it is not known whether low-N trees are source-limited in terms of carbon supply. Under low N supply, apple trees generally have less vegetative growth, smaller leaf area, and lower CO2 assimilation capacity, indicating the carbon supply capacity of source leaves to fruit is lower compared with trees grown under adequate N supply. However, at the same time, low-N supply could directly decrease cell division and fruit growth, leading to low fruit set and low sink activity. The objective of this study was to understand how N supply affects the source-sink relationship of ‘Gala’ apple trees, leading to differences in final fruit size.
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