Vesicular-arbuscular mycorrhizal fungi can affect the water balance of both amply watered and droughted host plants. This review summarizes these effects and possible causal mechanisms. Also discussed are host drought resistance and the influence of soil drying on the fungi.

Water relations, drought and vesicular-arbuscular mycorrhizal symbiosis

https://www.researchgate.net/profile/Rupam_Kapoor/publication/26881549_Arbuscular_mycorrhizal_fungi_in_alleviation_of_salt_stress_A_review/links/0c960523fd2cc5ec7f000000.pdf

Arbuscular mycorrhizal fungi in alleviation of salt stress: a review.

Publisher Summary This chapter discusses the ecological implications of mycorrhizal associations in natural ecosystems and the role of soil or environmental factors, mycorrhizal fungus characteristics or host plant properties. Mycorrhizal associations are regulated by features of the host plant and mycorrhizal fungus, as well as by soil conditions and environmental factors. Factors which can influence the occurrence and effectiveness of mycorrhizal associations include (1) root properties (2) edaphic or climatic factors (3) soil organisms, (4) soil disturbance, and (5) host-fungus compatibility. Several other complex ecological topics also discussed in the chapter include (1) mycorrhizal phenology , (2) factors responsible for varying degrees of mycorrhizal dependency in host plants, (3) the role of mycorrhizal hyphae in soil (4) nutrient competition involving mycorrhizal and non-mycorrhizal plants, and (5) mycorrhizal interactions involving pollution and other stresses, the rhizosphere, soil properties and allelopathy. Finally, the chapter also discusses the population ecology of mycorrhizal fungi and the influence of their associations on plant population ecology.

Mycorrhizas in Natural Ecosystems

Microbial interactions in the mycorrhizosphere and their significance for sustainable agriculture

Role of soil microbes in the rhizospheres of plants growing on trace metal contaminated soils in phytoremediation.

Spores of four species of VA mycorrhizal fungi were unable to germinate when first formed. The change in their capacity to germinate was examined by using populations of newly-formed spores with a narrow age range. Percentage germination and length of hyphae produced by the spores when incubated in soil or on nutrient agar media was measured during storage at constant temperatures between 5 and 37 °C in both wet (−0.15 MPa) and dry (−300 MPa) soil. The results supported the hypothesis that spores of these species were innately dormant when first formed and suggest that changes occurred with close synchrony. The dormancy period in wet soil was approximately 6 weeks for Glomus caledonium and G. monosporum , and 12 weeks for Gigaspora calospora . This period was significantly reduced to 1 week for the Glomus spp. and 6 weeks for Gigaspora calospora in dry soil. For Acaulospora laevis the dormancy period was 6 months under all conditions. Plant roots had no effect on the change to quiescence of spores of any species, however newly-quiescent spores were effective propagules.

Spore dormancy in vesicular-arbuscular mycorrhizal fungi

Spores of Glomus caledonium, Gigaspora calospora and Acaulospora laevis from the same source and grown under the same conditions had different temperature limits for germination. Within these limits, temperature affected the duration of the period between imbibition and the onset of germination. Over most of the range, for each fungus, temperature caused no change in either the proportion of the spores which germinated, the time (after onset of germination) taken for the maximum proportion to germinate or the rate and extent of hyphal growth. At temperatures close to the limits, the rate of both spore germination and hyphal extension were decreased. The upper temperature limits for germination were less than those in moist soils in the natural habitat of the fungi, a feature which may have an ecological role.

Temperature relations of spore germination and hyphal growth of vesicular-arbuscular mycorrhizal fungi in soil

Spore germination, in three vesicular arbuscular mycorrhizal fungi growing in the field and in two of the three in a controlled environment, was not changed by soil water potentials between those of waterlogged soil and −1.4 MPa. At soil water potentials less than −1.4 MPa, the time lapsing before hydration and the onset of germination increased, and the rates of germination and hyphal extension decreased. The four developmental phases of spore germination appear to have different requirements for water. The state of hydration of spores prior to incubation at low water potentials changed the proportions that germinated. Changes in spore germination at various soil water potentials can be explained by concepts similar to those used to interpret relationships between the seed-soil interface and soil water. Germination occurred in dry soils which would not be conducive to plant growth and hence to subsequent phases of the life cycle of the fungi; this may have an adverse effect on the inoculum reserves in field soil.

Effect of soil water potential on spore germination by vesicular-arbuscular mycorrhizal fungi

In situ germination of quiescent spores of Glomus caledonium, Gigaspora calospora and Acaulospora laevis was prevented in soils under crops or pastures, and in pot cultures of the fungi. Quiescent spores also failed to germinate during long-term storage in these soils when they were at temperatures and water potentials appropriate for germination. Germination was also prevented in other crop soils. Soil from a virgin forest did not prevent germination but did so after it had carried one subterranean clover crop to maturity. In all soils, prevention of germination was relieved by seed germination and seedling growth or by treatment of soils with heat or methyl bromide. Water-soluble, heat-labile compounds which prevented germination were extracted from the crop and pot culture soils which prevented germination, but not from soil from under a virgin forest. In addition to compounds in the bulk soil, self-inhibitors seemed to be involved in regulating germination. Bacteria from the spore surfaces promoted germination on sterile extracts of soils in which germination was blocked. A multiplicity of factors appeared to have a role in preventing germination, and these included mid to late stages of crop growth and associated changes in microbial activity. The absence of in situ germination of quiescent spores during growth of the crop under which the spores formed or during storage in the soils may have a role in survival of the fungi in agricultural or range land ecosystems, or in maintaining inoculum levels in pot culture systems.

Inhibition of spore germination of vesicular-arbuscular mycorrhizal fungi in soil

Spores of Glomus caledonium and Acaulospora laevis , which had been germinated in untreated or steamed field soil without plants, retained their capacity to infect plants for periods up to 4 months. Retention of infectivity was greater for A. laevis than G. caledonium in either soil. Decreased infectivity appeared to be due to the loss of the capacity of the hyphae to form pre-penetration and penetration structures. For equivalent amounts of inoculum (the same number of germinated spores that were similar in size), A. laevis had a greater capacity than G. caledonium to colonize roots. Germination of the spores of either species prior to sowing increased the extent of colonization of young roots.

I.C. Tommerup

Papers

Spore dormancy in vesicular-arbuscular mycorrhizal fungi

Temperature relations of spore germination and hyphal growth of vesicular-arbuscular mycorrhizal fungi in soil

Effect of soil water potential on spore germination by vesicular-arbuscular mycorrhizal fungi

Inhibition of spore germination of vesicular-arbuscular mycorrhizal fungi in soil

Persistence of infectivity by germinated spores of vesicular-arbuscular mycorrhizal fungi in soil