ROOT SOLUBLE CARBOHYDRATES OF AFZELIA-AFRICANA SM SEEDLINGS AND MODIFICATIONS OF MYCORRHIZA ESTABLISHMENT IN RESPONSE TO THE EXCISION OF COTYLEDONS

Stem length, number of secondary lateral roots, shoot dry weight and r educing sugar concentrations of root were significantly reduced when t ranslocation of reserves from cotyledons to the roots of Afzelia afric ana seedlings was interrupted by complete or partial cotyledon excisio n. The sucrose but not the glucose concentration of lateral roots also decreased significantly after complete cotyledon excision. Hartig net development rather than fungal sheath formation was affected after in oculation with the early fungal isolate E1 and by both late-stage fung al isolates L1 and L2 after partial or complete cotyledon excision. Ho wever, mycorrhizal colonization by the early fungal isolate E2 was not affected by cotyledonary reserves, suggesting that this fungal isolat e has a lower carbohydrate requirement than fungal isolates E1, L1 and L2. The late-stage fungal isolates L1 and L2 induced a hypersensitivi ty reaction by epidermal cell walls of the host plant after complete c otyledon excision, suggesting they are more dependent than the early f ungal isolate E1 on available root carbohydrate substrates for ectomyc orrhizal colonization. These results are discussed in the light of the hypothesis that early- and late-stage fungi were different carbohydra te requirements, and that the time sequence of colonization was relate d to the root carbohydrate status, which increased with time.