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[Effects of nitrogen application levels on translocation and distribution of 13C-photosynthate and 15N to fruit from leaves of apple tree].

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.