Fig. 2. Development of Rhizophagus irregularis hyphae and spores in Experiment 1 as affected by mineral- (ammonium chloride) or organic- (chitin) nitrogen addition to the labelling compartment (marked with asterisk) in comparison to a nitrogen-free control.

Growth of the R. irregularis was supported by Ri T-DNA transformed Cichorium intybus roots in rhizoboxes (small dishes delimited from the main dish volume by a 42 µm nylon mesh), visible in the upper row (A–C), where they are labelled with handwritten plate numbers. The AM fungal hyphae and spores developed in the main dish volume filled with the standard MSR medium [35] and supplemented with 1% sucrose after emerging from roots pre-colonised with Rhizophagus and inoculated into the rhizoboxes. After 2 months of growth, labelling compartment was filled with MSR medium supplemented with 1% sucrose and devoid of nitrogen (A, D, G, J, M), or with added ¹⁵N-labelled ammonium chloride (B, E, H, K, N) or ¹⁵N-chitin extracted from Zygorhynchus sp. cell walls (C, F, I, L, O). Fungal development was then observed and photographed at the edge of the labelling compartment under an Olympus SZX10 stereomicroscope at different time points (see left edge of panel for details).