This paper discusses the enzymatic hydrolysis of organic phosphorus with reference to the classical inorganic and organic compartmental analysis of phosphorus compounds in ecosystems. Also discussed are: the potential use of soil organic phosphorus by plants and microorganisms; properties of phosphohydrolases; inhibitors and activators; phosphohydrolase activity in soil; rhizosphere and mycorrhizal aspects of phosphohydrolase activity; and the regulation of the phosphohydrolase activity in the rhizosphere.
Nitrate uptake rates were determined for nonmycorrhizal and mycorrhizal maritime pine (Pinus pinaster) associated with the ectomycorrhizal basidiomycete Hebeloma cylindrosporum and for this fungal species grown in vitro. Different [NO3−] (0–1.3 and 10 mM) were supplied to nonmycorrhizal or mycorrhizal P. pinaster plants grown axenically in test tubes for 21 weeks. Uptake of NO3− by the plants was determined by measuring NO3− depletion from the medium that was added to the culture tubes without root pertubation. For all applied [NO3−] the uptake rates for NO3− in mycorrhizal plants were increased about twofold compared with nonmycorrhizal plants. The same range of [NO3−] was supplied to the fungus in pure culture, and uptake rates were calculated either from the measurement of NO3− depletion from the medium or by 15N accumulation in the mycelium. For the same external nitrate concentrations the results obtained with these two methods were not significantly different. The amount of fungal biomass in mycorrhizal root systems was determined with the chitin assay. The contribution of each partner to nitrate uptake in the mycorrhizal plant was calculated from the kinetic parameters of nitrate uptake by the fungus and host plant alone given by the Lineweaver–Burk plots. For external [NO3−] higher than 0.1 mM the measured uptake rates for NO3− in mycorrhizal plants were greater than the calculated sum of the individual symbiotic partners. The significance of this excess nitrate uptake is discussed in relation to the role of ectomycorrhizae in nitrogen accumulation. Key words: NO3− uptake rates, Km, Vmax, ectomycorrhizal symbiosis, Hebeloma cylindrosporum, Pinus pinaster.
SUMMARYThe 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. Hebetmna cylindrosporum infection increased net photosynthesis and root respiration rates compared with those of nonmycorrhizal 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 in 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 sourcesink hypothesis. The increase in fixed carbon in mycorrhizal plants was unable to compensate for the increased carbon cost of the mycorrhizal root system.
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