Dynamics of arbuscule development and degeneration in onion, bean, and tomato with reference to vesicular–arbuscular mycorrhizae in grasses

A quantitative light and electron microscopic study of developing and degenerating arbuscules of the vesicular–arbuscular mycorrhizal fungus Glomus fasciculatum in onion, bean, and tomato was carried out to estimate three parameters during the colonization cycle and to compare these parameters with those in maize, oats, and wheat. The parameters are (i) Vv(a,c) the fraction of the host cell volume (c) occupied by the arbuscule (a); (ii) VV(cy,c) the fraction of the host cell volume occupied by host cytoplasm (cy); and (iii) SV(p,c) the ratio of the surface area of the host protoplast (p) to the volume of the whole host cell. Uninfected cortical cells contained 3.4% cytoplasm in onion, 3.1% in bean, and 3.5% in tomato. In cells with mature arbuscules, cytoplasm increased to 9.9% in onion, 14.2% in bean, and 13.6% in tomato. Cells with mature arbuscules contained 11.4% fungus in onion, 20.3% in bean, and 20.5% in tomato. The initial SV(p,c) in onion was 0.10 μm2/μm3 and in bean and tomato 0.11 μm2/μm3. This increased to 0.37 μm2/μm3 in onion, 0.82 μm2/μm3 in bean, and 0.54 μm2/μm3 in tomato by the time arbuscules were mature. Development of the arbuscule was estimated to take 2.5 days and occupied 33% of the total cycle time. The variation seen across host species can be used as an indicator of fungal and (or) host control for each parameter. Arbuscular parameters of onion were compared with those obtained by other authors.