This study was designed to characterize the mechanisms of N-stimulated peach Prunus persica (L.) Batsch productivity. The effects of N fertilization on potential assimilate availability (source capacity) and on the growth capacity of individual fruit (sink capacity) were assessed. On heavily thinned trees, fertilization did not stimulate fruit growth rates relative to those on nonfertilized trees, suggesting that fruit growth rates were not assimilate-limited throughout the period of fruit development. However, N fertilization resulted in a longer fruit development period and increased the growth potential of individual fruit by 20% (fresh mass) and 15% (dry mass) vs. controls. In unthinned trees, N fertilization increased total fruit yield by 49% (fresh mass) and 40% (dry mass) compared to the unthinned, nonfertilized controls. N fertilization increased total fruit yield per tree in unthinned peach trees by extending the fruit development period and thus increasing the amount of assimilate accumulated for fruit growth. The fruit development period was prolonged both by assimilate deprivation associated with increasingly higher crop loads and by N fertilization. Thus, the prolongation of the peach fruit development period by N-fertilization appears inconsistent with the role of N in increasing assimilate availability for fruit growth. We conclude that N fertilization stimulates peach yields by increasing the period for fruits to use assimilates (sink capacity). The effect of N on assimilate availability was not directly evaluated. The timing of fertilizer N availability did not influence fruit growth potential.
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