Nutrient balance and salinity stress in arbuscular mycorrhizal fungi

Abstract: Earth’s primary production depends to a considerable extent on the mycorrhizal symbiosis, since mycorrhizal fungi supply their host plant with important mineral nutrients. I examined the influence of different nutrient regimes on the fungal partner in the arbuscular mycorrhizal symbiosis. I especially focused on salinity problems, as they occur in southern Tunisia, where I performed several field studies. The following main results were obtained: • The spores of arbuscular mycorrhizal fungi (AMF) may contain high amounts of mineral nutrients. Therefore, our results indicate that AM fungal spores do not only serve for carbon storage, but also as mineral nutrient reservoirs. Both high environmental P concentrations and low C status of the plant partner led to an accumulation of P and other mineral nutrients in the fungus. We suggest that AMF can control the transfer of P and hold it back if not provided with sufficient C from the host, and that there must be a feed-back mechanism in the symbiotic C-P exchange. • I found AMF in saline ecosystems in southern Tunisia with at least 17 morphospecies per 40 g soil. The AMF biomass in soil declined with rising salinity Under elevated salinity, AM fungal hyphae and spores contained high amounts of Ca, Cl and K, while Na was found at low levels. The same findings hold true for a salinity stressed G. intraradices grown in in vitro cultures. This led us to suggest that AMF could act as an ion filter for the plant under high salinity, which could explain the high K/Na rates that are often reported in mycorrhized plants growing in saline soils. • Sole osmotic stress had different effects on G. intraradices than salinity stress, which includes both osmotic and toxic ion stress. Sole osmotic stress impaired the growth of the external mycelium and spores more than salinity stress, while the uptake of nutrients from the medium was more reduced by salinity stress caused by NaCl or KCl. This indicates that sole osmotic stress constitutes a high energetic cost in osmoregulation in AMF. • NaCl stress triggered a strong rise in production of glomalin, a protein important for soil structure, while sole osmotic stress did not. Thus, glomalin seems to be a stress response, but not a general one. • Growth of AM external mycelium was strongly stimulated by organic matter addition to the soil, and also the C flow from the plant to the mycelium was higher under organic matter addition. Mineral nutrient addition to the soil also stimulated AMF growth, though to a lesser extent than organic material. Organic matter addition to subtropical soils can therefore be an important soil remedy, since by stimulating AMF growth, it supports the symbiotically associated vegetation, improves soil structure and increases C storage in the soil. • In a revegetation project, double inoculation of Acacia saplings with AMF and Rhizobium in the desert border of the Sahara did not lead to measurable effects on the plants after five years, but the overall survival of the saplings in field after having been raised in a nursery was satisfactory. We suggest that the inoculum needs to be very carefully chosen in order to be compatible with the ecosystem, or that the native soil inoculum should be considered in management practices.

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