Abstract: A field experiment was carried out in a six-year old ‘Fuji3’/M26/Malus hupehensis Rehd. apple with ¹⁵N and ¹³C labeled tracers, to understand ¹³C assimilation capability and the characteristics of translocation and distribution of ¹³C-photosynthate and ¹⁵N to fruit under different nitrogen application levels (urea 0%, 0.6%, 1.2%, 1.8%, 2.4%, CK, N₁, N₂, N₃, N₄, respectively) to smear the leaves within 20 cm around the fruit at late stage of fruit enlargement. The results showed that, with the increases of urea application, the chlorophyll content, nitrogen content, net photosynthetic rate, sorbitol and sucrose content, sorbitol 6-phosphate dehydrogenase (S6PDH) and sucrose phosphate synthase (SPS) activities, ¹³C assimilation capability of leaves were first increased and then decreased, with the highest value in 1.8% urea smear treatment and the lowest value with the treatment of clear water. The ¹³C of self retention (self leaves and self branches) was the highest in clear water (81.6%) and the lowest in 1.8% urea smear treatment (63.5%). The ¹³C was mainly allocated to fruit, followed by unlabeled perennial branch, and the lowest in unlabeled leaves. With the increases of urea application, the ¹³C absorption of fruit was first increased and then decreased, with the highest value in 1.8% urea smear treatment (1.21 mg·g^-1) and the lowest value in clear water (0.51 mg·g^-1). The ¹⁵N absorption of fruit was enhanced with the increases of urea application. These results indicated that foliage application of urea solution improved translocation and distribution of leaf photosynthate and nitrogen to fruit with varying degrees, which was the highest in 1.8% urea smear treatment and could avoid excessive intake of nitrogen to fruit.

Key words: translocation, ¹⁵N-urea foliar application, ¹³C-photosynthate, apple, distribution