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(.) In this study we investigated the effects of temperature on fungal growth and tested whether the differences in fungal growth were related to the effects of temperature on carbon movement to, or within, the fungus. (.) Growth curves and C uptake-transfer-translocation measurements were obtained for three arbuscular mycorrhizal fungi (AMF) isolates cultured within a 6-30 degrees C temperature range. A series of experiments with a model fungal isolate, Glomus intraradices, was used to examine the effects of temperature on lipid body and 33 P movement, and to investigate the role of acclimation and incubation time. (.) Temperature effects on AMF growth were both direct and indirect because, despite clear independent root and AMF growth responses in some cases, the uptake and translocation of 13 C was also affected within the temperature range tested. Root C uptake and, to a lesser extent, C translocation in the fungus, were reduced by low temperatures (< 18 C). Uptake and translocation of 33 P by fungal hyphae were, by contrast, similar between 10 and 25 C. (.) We conclude that temperature, between 6 and 18 C, reduces AMF growth, and that C movement to the fungus is involved in this response. (Less)

The effects of elevated CO2 concentrations upon carbon allocation in mycorrhizal (M) and non-mycorrhizal (NM) birch (Betula pendula) seedlings were investigated. M plants, colonised by the fungus Paxillus involutus, and NM plants, were exposed for 3 months to ambient (350 μl l−1) or elevated (700 μl l−1) CO2 environments. The assimilation and distribution of carbon within the different compartments of the plant–substrate–fungal system were investigated using radioactive carbon as a tracer. In addition, the impact of elevated CO2 upon extension growth of the ectomycorrhizal mycelium of the fungus was determined in transparent observation chambers. Yields of whole plants and of shoots were significantly decreased under elevated CO2 whether they were grown with or without their fungal symbionts. Neither the dry mass production of roots of mycorrhizal plants, nor the amount of carbon allocated to shoots, roots and mycorrhizal tips were affected by elevated CO2. While the number of mycorrhizal root tips was decreased with CO2 enrichment, their relative importance in the total root system was unchanged. There was a significant increase in the extent of development of the external mycelium under elevated CO2. A greater proportion of the radioactive carbon was allocated to the soil compartment under elevated CO2. This increase, probably arising through increased rhizodeposition, was greater in NM than M plants. The responses are discussed in terms of nutrient availability in the growth media and the possible role of increased carbon allocation to mycorrhizal mycelium in nature.

Plantago lanceolata L. was grown for 104 d with (M) or without (NM) arbuscular mycorrhizal colonization under conditions of ambient (CAMB=350 μl l−1) and elevated (CELEV=540 μl l−1) CO2. Sequential harvests (H) were taken at 41 (H1), 76 (H2) and 104 d (H3) to determine the time-course of mycorrhizal influence on the response of the plant to CO2 enrichment. Total yields of M plants were greater than those of NM from H2 onwards. Plants in the MELEV treatment were significantly larger than those in the MAMB at 104 d. There were significant but much smaller differences in yield between NMELEV and NMAMB. The differences in total yield arose through impact of CELEV on both shoots and roots. Total root length was greater in MELEV than in MAMB only at H3, but total length of mycorrhizal root was greater at H2 and H3. The percentages of root length colonized and that occupied by arbuscules and vesicles were greater in MELEV than in MAMB at the last two harvests, indicating increased sequestration of carbon in internal fungal structures. Though extraradical hyphal lengths were greater in MELEV than in MAMB at H2 and H3, the differences were not significant. Phosphorus inflow and P content of M plants were higher than those of NM plants at H2 and H3, and were higher in MELEV than in MAMB at H3. anova revealed no significant interactions between CO2 and mycorrhizal treatment. The results are discussed in relation to carbon sequestration in mycorrhizal systems and likely impacts of CO2 enrichment on P. lanceolata grown under field conditions. The importance of sequential harvesting for realistic determination of responses to CO2 is stressed.

Partitioning of 14C was assessed in sweet chestnut seedlings (Castanea sativa Mill.) grown in ambient and elevated atmospheric [CO2] environments during two vegetative cycles. The seedlings were exposed to 14CO2 atmosphere in both high and low [CO2] environments for a 6-day pulse period under controlled laboratory conditions. Six days after exposure to 14CO2, the plants were harvested, their dry mass and the radioactivity were evaluated. 14C concentration in plant tissues, root-soil system respiratory outputs and soil residues (rhizodeposition) were measured. Root production and rhizodeposition were increased in plants growing in elevated atmospheric [CO2]. When measuring total respiration, i.e. CO2 released from the root/soil system, it is difficult to separate CO2 originating from roots and that coming from the rhizospheric microflora. For this reason a model accounting for kinetics of exudate mineralization was used to estimate respiration of rhizospheric microflora and roots separately. Root activity (respiration and exudation) was increased at the higher atmospheric CO2 concentration. The proportion attributed to root respiration accounted for 70 to 90% of the total respiration. Microbial respiration was related to the amount of organic carbon available in the rhizosphere and showed a seasonal variation dependent upon the balance of root exudation and respiration. The increased carbon assimilated by plants grown under elevated atmospheric [CO2] stayed equally distributed between these increased root activities. ei]H Lambers

The influence of external phosphorus (P) on carbon (C) allocation and metabolism as well as processes related to P metabolism was studied in monoxenic arbuscular mycorrhiza cultures of carrot (Daucus carota). Fungal hyphae of Glomus intraradices proliferated from the solid minimal medium containing the colonized roots into C-free liquid minimal medium with different P treatments. The fungus formed around three times higher biomass in P-free liquid medium than in medium with 2.5 mminorganic P (high-P). Mycelium in the second experiment was harvested at an earlier growth stage to study metabolic processes when the mycelium was actively growing. P treatment influenced the root P content and [13C]glucose administered to the roots 7 d before harvest gave a negative correlation between root P content and13C enrichment in arbuscular mycorrhiza fungal storage lipids in the extraradical hyphae. Eighteen percent of the enriched13C in extraradical hyphae was recovered in the fatty acid 16:1ω5 from neutral lipids. Polyphosphate accumulated in hyphae even in P-free medium. No influence of P treatment on fungal acid phosphatase activity was observed, whereas the proportion of alkaline-phosphatase-active hyphae was highest in high-P medium. We demonstrated the presence of a motile tubular vacuolar system inG. intraradices. This system was rarely seen in hyphae subjected to the highest P treatment. We concluded that the direct responses of the extraradical hyphae to the P concentration in the medium are limited. The effects found in hyphae seemed instead to be related to increased availability of P to the host root.