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Summary

Mycorrhizal roots of beech absorb [14C]glucose, fructose and sucrose at rates some two and a half times faster than uninfected roots. In the latter, sucrose is synthesized from all three sugars despite the presence of an active surface invertase, clearly suggesting that uptake from sucrose involves prior hydrolysis. There is only a small incorporation into polysaccharides by uninfected roots, this being into structural polymers. In contrast, mycorrhizal roots synthesize principally trehalose and glycogen from glucose, and mannitol from fructose. Sucrose feeding results in synthesis of all three storage carbohydrates. Whatever sugar is supplied to mycorrhizas, host sugars incorporate little radioactivity and dissection shows that 70% of the activity, following long term glucose feeding, remains in the sheath and, of the 30% in the core, most is in fungal metabolites of the Hartig net. When fed separately, core tissue more closely resembles the behaviour of uninfected roots.

From the totally different destinations of the two hexoses in mycorrhizas, it is concluded that the fungus, unlike host tissue, has a restricted ability to interconvert hexoses. From experiments with high specific activity [14C]glucose supplied to mycorrhizas in tracer quantities, it appears that alternative pathways of carbohydrate breakdown such as the pentose phosphate cycle and Entner-Doudoroff scheme may possibly be operative in addition to glycolysis and so supplement the breakdown of sugars to the level of pyruvate.