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 mm inorganic 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 [13 C]glucose administered to the roots 7 d before harvest gave a negative correlation between root P content and 13 C enrichment in arbuscular mycorrhiza fungal storage lipids in the extraradical hyphae. Eighteen percent of the enriched 13 C in extraradical hyphae was recovered in the fatty acid 16:15 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 in G. 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.Arbuscular mycorrhizal (AM) association is the only way for fungi in the order Glomales to proliferate and reproduce (Bécard and Fortin, 1988; Smith and Read, 1997; Bago et al., 2000). It is well known that C is transferred from colonized plants to AM fungi (Ho and Trappe, 1973), whereas the plants in many cases receive most of their P through hyphal uptake and fungal transfer to the host root (Pearson and Jakobsen, 1993). Colonization by AM fungi increases the C sink strength of roots (Douds et al., 1988). The fungal C demand upon root colonization can constitute a significant cost to the host plant, as indicated by reduced growth at high P levels (Peng et al., 1993). This also implies that there is an important connection between external P supply and the regulation of C allocation to the fungal partner in the symbiosis.C metabolism of the AM fungus Glomus intraradices has been studied using 13 C-NMR in monoxenic cultures (Pfeffer et al., 1999). Although no hexose uptake occurs in the extraradical mycelium, intraradical AM fungal mycelium takes up C supplied as hexoses to the root. Triacylglycerols are synthesized from this C in the fungus and transported to the extraradical mycelium. These triacylglycerols are substantial sinks for C in the AM fungal mycelium (Bago et al., 2000; Olsson and Johansen, 2000).Formation of AM is important for P acquisition in most plants. The adverse effect of high soil P le...

Summary• 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 ° 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.

The aim of this work was to examine the response of wheat plants to a doubling of the atmospheric CO 2 concentration on: (1) carbon and nitrogen partitioning in the plant; (2) carbon release by the roots; and (3) the subsequent N uptake by the plants. The experiment was performed in controlled laboratory conditions by exposing fast-growing spring wheat plants, during 28 days, to a ~4CO 2 concentration of 350 or 700 p~L L 1 at two levels of soil nitrogen fertilization. Doubling CO 2 availability increased total plant production by 34% for both N treatment. In the N-fertilized soil, the CO s enrichment resulted in an increase in dry mass production of 41% in the shoots and 23% in the roots; without N fertilization this figure was 33% and 37%, respectively. In the N-fertilized soil, the CO s increase enhanced the total N uptake by 14% and lowered the N concentration in the shoots by 23%. The N concentration in the roots was unchanged. In the N-fertilized soil, doubling CO 2 availability increased N uptake by 32% but did not change the N concentrations, in either shoots or roots. The CO 2 enrichment increased total root-derived carbon by 12% with N fertilization, and by 24% without N fertilization. Between 85 and 90% of the total root derived-~4C came from respiration, leaving only 10 to 15% in the soil as organic ~4C. However, when total root-derived 14C was expressed as a function of root dry weight, these differences were only slightly significant. Thus, it appears that the enhanced carbon release from the living roots in response to increased atmospheric CO2, is not due to a modification of the activity of the roots, but is a result of the increased size of the root system. The increase of root dry mass also resulted in a stimulation of the soil N mineralization related to the doubling atmospheric CO s concentration. The discussion is focused on the interactions between the carbon and nitrogen allocation, especially to the root system, and the implications for the acquisition of nutrients by plants in response to CO 2 increase.

Two-year-old sweet chestnut trees were grown outside in normal or double CO2 atmospheric concentration. In spring and in autumn of two growing seasons, a six day labelling pulse of lac labelled CO2 was used to follow the carbon assimilation and distribution in the plant-soil system. Doubling atmospheric CO2 had a significant effect on the tree net carbon uptake. A large proportion of the additional C uptake was 'lost' through the root system. This suggests that increased C uptake under elevated CO2 conditions increases C cycling without necessarily increasing C storage in the plant. Total root derived material represented a significant amount of the 'extra-assimilated' carbon due to the CO2 treatment and was strongly correlated with the phenological stage of the tree. Increasing root rhizodeposition led to a stimulation of microbial activity, particularly near the end of the growing season. When plant rhizodeposition was expressed as a function of the root dry weight, the effect of increasing CO2 resulted in a higher root activity. The C to N ratios were significantly higher for trees grown under elevated CO2 except for the fine root compartment. An evaluation of the plant-soil system nitrogen dynamics showed, during the second season of CO2 treatment, a decrease of soil N mineralization rate and total N uptake for trees grown at elevated CO2 levels.