EFFECT OF SOIL CONDITIONS ON THE EXTENT OF ARBUSCULAR

ARBUSCULAR MYCORRHIZAL ROOT COLONIZATION AND ON THE DEVELOPMENT OF EXTRARADICAL HYPHAE

The importance of mycorrhizal fungi in the mineral nutrition of plants depends on the ability of the fungi to exploit sources of non-mobile nutrients in the soil (Becerra et al., 2007). Therefore, the symbiotic efficiency depends not only on plant and fungal genotype, but also on soil factors such as forms and amounts of non-mobile nutrients in the soil, and on the interaction between all these factors. For example, soil fertility is also an important factor in the control of mycorrhizal colonization (Correnho et al., 2007).

At high soil P concentration, the extent of AM root colonization is usually inhibited, because carbon allocation to the AM fungi from the host plant is reduced (Gosling et al., 2013). In agreement, in the present study the extent of AM root colonization was higher when plants were supplied with low mineral P than with high mineral P (Chapter 2; Fig. 2.6).

It has to be kept in mind though, that plant nutrient distribution in soil at natural sites or in practical agriculture is usually not homogeneous.

In one of the experiments described in this thesis, root AM fungal colonization byG. intraradices was not affected by differences in P supply to the two halves of the root systemin a split-root experiment (Chapter 3; Tab. 3.2.A-D). This indicates that the P status of the host plant (shoot) rather than local soil P supply determined colonization rates. In that experiment, shoot P concentration of plants inoculated with G. intraradices was not affected by the distribution of P supply in the root compartment (Tab. 3.3.A). Notably, even root P concentrations were not affected by variations of the P distribution in the soil (Tabs. 3.3.B and C). Jarosch et al. (2008) also suggested that the extent of root colonization dependsprimarily on the P status of the host plant. This is not surprising as the plant P status regulates the carbohydrate allocation to the root (Hammond and White, 2008).

In many cases, AM colonization is highest at moderate soil P concentrations (Olsson et al., 2006). The highest level of P supply in the root compartments in the experiment reported in Chapter 3 was 85 mg P per kg dry soil, and this amount was apparently not high

127 enough toinduce a local reduction of AM fungal colonization by G. intraradices. From the results of this thesis, it is unclear whether extremely high P concentrations in soil patches would result in locally decreased colonization of the root by G. intraradices.

In contrast to G. intraradices, in the present study AM fungal colonization of G mosseae was significantly affected by P supply in the root compartment. In a split-root experiment (Chapter 4), both a higher and a lower level of P supply in aroot compartmentslightly decreased the extent of AM fungal colonization by G. mosseae compared to the treatment with equal P supply in both root compartments (Tabs. 4.2.B and C). Decreasing AM fungal colonization in the root compartment that received a higher amount of P might be caused by increasing root growth which will decrease the ratio of colonized to un-colonized roots in that part of the root (Smith et al., 2011). Decreased AM fungal colonization at low local P supply might be caused by competition between the host plant and the AM fungus for the limited amount of P in supply (Peters and Habte, 2001).

In the present study, the activity of the AM fungal mycelium in P uptake from hyphal compartments wasapparently also not specifically affected by different P supply distribution in soil. Thisapplied to both G. intraradices and G. mosseae (Tabs. 3.2.B and C; 4.2.B and C).

Phosphorus uptake by hyphae was not measured directly, but the activity of the AM fungal mycelium in P uptake was estimated from the ratio of coarse (runner hyphae) to thin (absorbing hyphae) hyphae according to Olsson et al. (2006). Previously published evidence also suggests that plant P status controls the rate of AM fungal P uptake (Nagy et al., 2009).

It should be noted, however, that in the present study measurements of the ratio of coarse to thin hyphae had in many cases high standard errors, so that the lack of a significant treatment effect is not good evidence of a general lack of influence of local P supply on hyphal uptake activity.

The development of extraradical hyphae of the AM fungus G. Intraradices may be more sensitive than the development of intraradical colonization of that fungus to low versus high soil P concentration. Alower P supply increased the weight of mycelium (significantly) and the hyphae length (in tendency) in the hyphal compartment (Tab. 3.2.B). Under decreasedsoil P supply, carbon flow to the fungus can beincreased, and lipid transport from intraradical to extraradical mycelium can also be increased (Olsson et al., 2002). Lipids are important for energy storage and the main component of fungal biomass (Olsson, 2009).

In contrast, in most other measurements in this thesis the length of extraradical hyphae and the weight of the mycelium were not significantly affected by a local increase in P supply. This probably indicated that the growth of AM fungi was not limited by P

128 deficiency in the part of the soil that received alower amount of P, and that both AM fungi did not specifically forage in the root compartment that received the higher amount of P.

The extent of AM root colonization and the weight of extraradical mycelium from both isolates were also not affected by the N supply ratio (Chapter 3 and 4; Tabs. 3.2.E and F, Tabs. 4.2.E and F). The most important factor influencing mycorrhizal symbiosis is usually soil P availability (Nogueira and Cardoso, 2007). However, when compared to homogeneous soil N distribution in the present study the number of spores per unit mycelium dry weight was decreased in the soil half with higher N supply in one of the split-root experiments (Tabs.

3.2.D-F) while the number of spores per unit mycelium length was decreased in the soil half with lower N supply in another experiment (Tab. 4.2.E). Thus, spore production per unit mycelium was affected by N supply in the present study, but the effect was not consistent.

The decreasing number of spores in the soil zone that received the higher amount of N might be caused by increasing N assimilation of the fungus (Wallenda et al., 1996). Conversely, the decreasing number of spores in the soil zone that received a lower amount of N might be caused by lower N uptake for spore production. Nitrogen is the main component of chitin which is a component of spore walls (Bago et al., 2004; Roesti et al., 2005).

In summary, in the split-root system used in experiments described in Chapter 3 and4, the effects of soil P and N distribution on mycorrhizal colonisation and spread were not very distinct and not consistent. It should be kept in mind that variations of nutrient distribution to two halves of a root system (as in experiments described in Chapter 3 and 4) may have different effects from small-sized local soil patches ("hotspots" of nutrients) with high nutrient supply intensity.

In the present study, the extent of root colonization by G. mosseae was particularly low when marigold plants were grown in peat (Chapter 5; Tab. 5.3). Although peat has superior physical and hydraulic properties for plant growth (Raviv and Lieth, 2008), high organic matter, high soluble P contents, high ammonium concentrations and the acidity of peat may all be reasons for the inhibition of AM root colonization in peat-grown plants (Linderman and Davis, 2003).

129