Leaf N is generally highest in top leaves and decreases with depth in leaf canopies. Although such a gradient contributes to efficient use of canopy N for photosynthetic production, the gradient of leaf N is suboptimal in actual canopies.
Hikosaka et al. present a meta-analysis of the vertical gradient of leaf N which reveals that in most canopies except for wheat, the slope of the leaf N was half of the optimal slope.
Kouki Hikosaka, Niels P. R. Anten, Almaz Borjigidai, Chiho Kamiyama, Hidemitsu Sakai, Toshihiro Hasegawa, Shimpei Oikawa, Atsuhiro Iio, Makoto Watanabe, Takayoshi Koike, Kazuya Nishina, Akihiko Ito, 2016, 'A meta-analysis of leaf nitrogen distribution within plant canopies', Annals of Botany, vol. 118, no. 2, pp. 239-247 http://dx.doi.org/10.1093/aob/mcw099