Arbuscular mycorrhizal fungi alter nitrogen allocation in the leaves of Populus x canadensis 'Neva'

Intra-leaf nitrogen allocation plays a pivotal role in plant growth performance; however, the effects of arbuscular mycorrhizal (AM) fungi on the allocation of nitrogen within a leaf remain poorly understood. A pot experiment was conducted with different nitrogen levels and Populus x canadensis 'Neva' with or without Rhizophagus irregularis inoculation. The fractions of leaf nitrogen allocated to water-soluble protein, membrane-bound protein, cell wall protein and photosynthetic apparatus were used to estimate nitrogen allocation strategy. AM fungi increased the light-saturated photosynthetic rate (P-max) and photosynthetic nitrogen use efficiency (PNUE) under low nitrogen levels, whereas the nitrogen content per unit area (N-area) and nitrogen content per unit mass (N-mass) were unaffected. AM inoculation decreased leaf mass per area (LMA) and the fraction of leaf nitrogen allocated to cell walls but increased the fraction of nitrogen allocated to photosynthesis under low nitrogen levels. The increased fraction of photosynthetic nitrogen allocated to carboxylation (P-C) and bioenergetics (P-B) in response to AM inoculation under low nitrogen levels contributed to the higher P-max and PNUE values. The improved photosynthesis and PNUE resulting from AM inoculation were mostly determined by the relative allocation of nitrogen to photosynthesis and not by the leaf nitrogen concentration. AM fungi may enhance nitrogen allocation to photosynthesis at the expense of cell walls.