Phosphorus relationships and production of extrametrical hyphae by two types of willow ectomycorrhizas at different soil phosphorus levels

Abstract

SUMMARYThere is much circumstantial evidence for a role of increased P uptake in the growth response of plants to ectomycorrhizas. Full response curves with and without mycorrhizal infection along a P gradient in soil are, however, required to test this hypothesis fully. In this experiment, rooted cuttings of Salix viminalis L. cv. Bowles Hybrid were grown in a 1 :2 mixture by volume of gamma-irradiated soil and sterile sand, with bicarbonateextractable P concentrations of 4, 6, 10, 21, 60 or 90 mg P kg'^. The cuttings were inoculated by mixing peat/vermiculite spawn oiLaccariaproximo (Boud.) Pat., or Thelephora terrestris (Ehrh.) Fr., or autoclaved spawn 1: 5 by volume with the soil: sand mixture. The plants showed a positive growth response to mycorrhizal infection by either fungus at the two lower P levels, and to L. proximo only at 10 mg P kg'^ At 21 mg kg^^ and above, infection was reduced and neither mycorrhizal inoculation nor further P additions caused significant growth increases. These results imply that the growth responses to ectomycorrhizas in this experiment were solely due to increases in P uptake. Cuttings infected with L. proximo tended to be larger than those infected by T. terrestris. Estimates of percent mycorrhizal infection did not diflFer between the fungi at the lower P levels. However, overall production of extramatrical hyphae per gram of soil was highest in soil inoculated with L. proximo. At 10 mg P kg~i the length of L. proximo hyphae per unit length of mycorrhizal root, P uptake per unit root weight, and total P content in plants infected with L. proximo were significantly higher than for T. terrestris. This study confirms that increased P uptake can be an important component of growth stimulation by ectomycorrhizas. It also presents the first quantification of extramatrical ectomycorrhizal hyphae in soil and suggests a role for them m the growth response.

Short‐term phosphorus uptake rates in mycorrhizal and non‐mycorrhizal roots of intact Pinus sylvestris seedlings

“…Ergosterol is a major sterol in fungal membranes, and not only correlates with active fungal biomass but probably also with total surface area of external mycelia involved in P i absorption. A possible causal relationship between the long-term P influx in pines and willows and the absorbing surface area of the external mycelia of their ectomycorrhizal associates was also found by Jones et al (1990) and Rousseau et al (1994). Ion influx rates in cells are determined not only by the quantity of available transporters, but also by qualitative aspects of the uptake systems.…”

“…Mycorrhizal colonization nevertheless enhanced P i uptake in P-limited pine seedlings 1.3, 2.6 and 3.3-fold in roots colonized with Laccaria bicolor, P. involutus and Pisolithus tinctorius, respectively, compared with non-mycorrhizal roots. Jones et al (1990Jones et al ( , 1991) obtained a mean P inflow rate of 0.2 nmol g −" s −" over a 50 d period in Salix viminalis plants colonized with T. terrestris, compared to only 0.1 nmol g −" s −" for nonmycorrhizal willow.…”

“…The possible mechanisms underlying the mycorrhizal effect on P uptake are diverse and have been discussed by Harley (1989), Bolan (1991) and Smith & Read (1997). Evidence for long-term increased P inflow rates in ectomycorrhizal plants grown under P limitation has been reported several times in the last decade (Bougher et al, 1990 ;Jones et al, 1990Jones et al, , 1991Rousseau et al, 1994 ;Cumming, 1996). External hyphae of mycorrhizal fungi can absorb P beyond the root-depletion zones.…”

Spatial distribution of ectomycorrhizas and arbuscular mycorrhizas in Korup National Park rain forest, Cameroon, in relation to edaphic parameters

“…E-mail : b.moyersoen!abdn.ac.uk scale and Janos (1980Janos ( , 1985 for tropical rain forests have both hypothesized that morphological and physiological differences between different types of mycorrhiza determine the success of the associated plants in different soils and lead to distinctive patterns of symbiosis in different biomes. Ectomycorrhizas and arbuscular mycorrhizas both possess extraradical hyphae which promote host uptake of poorly mobile nutrient ions (Sanders & Tinker, 1971, 1973Bougher, Grove & Malajczuk, 1990 ;Jones, Durall & Tinker, 1990). However, some ectomycorrhizas have rhizomorphs and greater hyphal development than arbuscular mycorrhizas (Bending & Read, 1995), which might result in different abilities of exploration and exploitation of soil compared with arbuscular mycorrhizas (Janos, 1985).…”

Genetics of ectomycorrhizal symbiosis

“…(Colpaert, Van Assche & Luijtens, 1992) can be used to estimate the amount of extramatrical mycelium. However, these methods are both expensive and laborious when compared to counting mycelial strand intersections on a grid as used in this study and in previous work by Jones, Durall & Tinker (1990). We believe that the video-imaging system for estimating the growth of mycelial strands growing on a delimited surface offers good accuracy and has been shown to be easy and rapid, but it needs to be tested against other accurate methods such as the ergosterol assay for measuring mycelial development on pine roots (Johnson & McGill, 1990).…”