The carbon costs involved in nutrient absorption by a unit length of mycorrhizal hypha would generally be expected to be lower than that of a corresponding length of root, which is much thicker, and the amount of phosphorus uptake per unit of carbon allocated below-ground would consequently be expected to be higher in mycorrhizal than in non-mycorrhizal plants. On the other hand 14C studies have indicated that the maintenance costs of the mycobiont may be higher than expected from its biomass (Harris et al., 1985), perhaps due to costs imposed by translocation of phosphorus through hyphae and its transfer to the host.
A theoretical framework for studying the carbon efficiency in mycorrhiza was given by Koide and Elliot (1989). They defined the efficiency of below-ground carbon utilization, ACw/ACb, where ACW is the total carbon accumulation in the whole plant per unit time while ACb is the total below-ground allocation of carbon per unit time. This ratio, however, is the product of the efficiency of phosphorus utilization (ACW/APW) and the efficiency of phosphorus acquisition (APw/ACb), where APW represents the total phosphorus uptake per unit time. Arbuscular mycorrhiza had no influence on the efficiency of phosphorus acquisition by the roots of citrange seedlings (Douds et al., 1988). Below-ground carbon allocation was measured by 14C pulse-labelling and this was compared to the total plant phosphorus content at harvest of the labelled plants. The results would be easier to interpret if carbon partitioning and phosphorus uptake had been measured during the same time interval. This was attempted by Jones et al. (1991) in a similar study with ectomycorrhizal and non-mycorrhizal Salix viminalis L. The amount of phosphorus taken up by the plants over defined time intervals was related to the below-ground carbon allocation during the same intervals. The carbon allocation was estimated from 14C02 pulse-labellings performed once during each time interval. The efficiency of phosphorus acquisition was found to be highest in mycorrhizal plants during the initial 50-day growth period, while it was highest in non-mycorrhizal plants during the subsequent 48-day period. In order to obtain realistic results in these studies of efficiency it is important that root and fungal development occur at levels which are comparable to the field situation. The effects of mycorrhiza on phosphorus uptake are known to decrease with increasing root densities (Bââth and Hayman, 1984). Optimal conditions for root growth, combined with a limited soil volume often produce root densities in pots which are an order of magnitude larger than in the field, while hyphal lengths are likely to be proportionally reduced.
In a study of "mycorrhizal efficiency", below-ground carbon allocation measured by 14 C techniques was similar in leek plants inoculated with three different arbuscular fungi (Menge et al., 1985). In this case the production of external hyphae and the mycorrhizal effects on phosphorus uptake were also unaffected by the species of mycobiont. However, the total amount of hyphae per unit colonized root length and the spatial distribution of the hyphae may vary considerably with species of arbuscular fungi (I. Jakobsen, unpubl. res.); these fungi also differed in their capacity for uptake of phosphorus per unit length of hyphae. Consequently, there is scope for further comparisons of carbon allocation to the external hyphae of mycorrhiza when different species of mycobionts are involved. An hermetically sealed hyphal compartment containing 32P-labelled soil, in combination with 14C pulse-labelling, would facilitate simultaneous measurement of carbon flow to and phosphorus uptake by external hyphae of different fungi.
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