Effects of phosphorus and ectomycorrhiza on maritime pine seedlings (Pinus pinaster)

The purpose of this study was to determine how ectomycorrhizal infection and phosphorus nutrition affect biomass, photosynthesis and root respiration in the host plant. Maritime pine (Pinus pinaster Soland. in Ait.) seedlings grown in containers filled with perlite-vermiculite were inoculated with the ectomycorrhizal fungus Hebeloma cylindrosporum (strain D3.25.9) and given 0 or 0.5 mM phosphate in the nutrient solution. Hebeloma cylindrosporum infection increased net photosynthesis and root respiration rates compared with those of non-mycorrhizal plants, but there was an accompanying 35 % depression in growth. The addition of phosphorus to non-mycorrhizal plants induced a rise in tissue phosphorus content which made them similar m that respect to mycorrhizal plants but did not result in increased photosynthesis. The nitrogen content of mycorrhizal plants was, moreover, lower than that of the control group. The data recorded were consistent with the photosynthate source-sink hypothesis. The increase in fixed carbon in mycorrhizal plants was unable to compensate for the increased carbon cost of the mycorrhizal root system.

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

There 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 bicarbonate-extractable P concentrations of 4, 6, 10, 21, 60 or 90 mg P kg^-1 . The cuttings were inoculated by mixing peat/vermiculite spawn of Laccaria proxima (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. proxima only at 10 mg P kg^-1 . At 21 mg kg^-1 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. proxima tended to be larger than those infected by T. terrestris. Estimates of percent mycorrhizal infection did not differ 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. proxima. At 10 mg P kg^-1 the length of L. proxima hyphae per unit length of mycorrhizal root, P uptake per unit root weight, and total P content in plants infected with L. proxima 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 in the growth response.