HYPHAL N TRANSPORT BY A VESICULAR-ARBUSCULAR MYCORRHIZAL FUNGUS ASSOCIATED WITH CUCUMBER GROWN AT 3 NITROGEN LEVELS

Cucumis sativus L. cv. Aminex (F1 hybrid) was grown alone or in symbio sis with Glomus intraradices Schenck and Smith in containers with two hyphal compartments (HC(A) and HC(B)) on either side of a root compart ment (RC) separated by fine nylon mesh. Plants received a total of eit her 100, 200 or 400 mg N which were applied gradually to the RC during the experiment. N-15 was supplied to HC(A) 42 d after planting, at 50 mg (NH4+)-N-15-N kg-1 soil. Lateral movement of the applied N-15 towa rds the roots was minimized by using a nitrification inhibitor and a h yphal buffer compartment. Non-mycorrhizal controls contained only trac es of N-15 after a 27 d labelling period irrespective of the amount of N supplied to the RC. In contrast, 49, 48 and 27% of the ap lied N-15 was recovered in mycorrhizal plants supplied with 100, 200 and 400 mg N, respectively. The plant dry weight was increased by mycorrhizal co lonization at all three levels of N supply, but this effect was strong est in plants of low N status. The results indicated that this increas e was due partly to the improved inflow of N via the external hyphae. Root colonization by G. intraradices was unaffected by the amount of N supplied to the RC, while hyphal length increased in HC(A) compared t o HC(B). Although a considerable N-15 content was detected in mycorrhi zal roots adjacent to HC(B), only insignificant amounts of N-15 were f ound in the external hyphae in HC(B). The external hyphae depleted the soil of inorganic N in both HC(A) and HC(B), while the concentration of soil mineral N was still high in non-mycorrhizal containers at harv est. An exception was plants supplied with 400 mg N, where some inorga nic N was present at 5 cm distance from the RC in HC(A). The possibili ty of a regulation mechanism for hyphal transport of N is discussed.