We have used Alnus incana (L.) Moench (Betulaceae) as a representative of intracellularly infected actinorhizal plants and Discaria trinervis (Hooker et Arnot) Reiche (Rhamnaceae) and Hippophae rhamnoides L. (Elaeagnaceae) as representatives of intercellularly infected actinorhizal plants. For comparison we have used the intracellularly infected legume Trifolium pratense L. Seedlings raised from surface sterilized seeds were grown in growth pouches (Mega, Minneapolis, MN, USA) or in pots with coarse sand or perlite. A nutrient solution (the modified Evans solution (Huss-Danell 1978) at 1/10 of full strength) had the P concentration according to recipe (medium level, 0.1 mM P) or was reduced 10 times (low P) or increased 10 times (high P). Ammonium nitrate was added to the solution to give low, medium or high N level (0.071, 0.71 or 7.1 mM N). Combinations of the N and P levels gave N/P ratios in the solution ranging from 0.071 to 710. Nodulation was evaluated as number and biomass of nodules. Growth of plants was recorded as their biomass at harvest. To distinguish between local and systemic effects plants were grown with split-root systems.
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