Plant-mycorrhizal interactions are generally considered to be examples of a mutualism in which both plant and mycorrhizal fungi benefit. Plants gain increased access to soil nutrients, particularly phosphorus, while mycorrhizal fungi gain access to the carbon fixed by plants. However, there are exceptions to this pattern. Numerous workers have noted that some plants shed mycorrhizal fungi when soil nutrients are supplemented, suggesting that under nutrient rich conditions, the costs to the plant of maintaining the symbiosis outweigh the benefits. In such systems the symbiosis is facultative. Much less is known about plant-mycorrhizal fungi interactions in woodland as opposed to grassland, prairie or agricultural systems. Here we report the nature and properties of the symbiosis in mayapple, Podophyllum pelatum L. (Berberidaceae), a long-lived perennial of the eastern deciduous forest floor. Our data indicate that the mayapple-fungal interaction is facultative; addition of soil phosphorous reduces the intensity of colonization of mayapple roots by mycorrhizal fungi. Of greater interest, we find that not all roots are equally colonized. Roots at young rhizome nodes contain few if any mycorrhizal fungi. Fungal loads increase to their highest level at two to four year old nodes before dropping again. The pattern of soil phosphate depletion mirrors the pattern of mycorrhizal fungi distribution, with the greatest depletion of soil P observed under the most heavily colonized nodes. Hypotheses relating to the regulation of plant-mycorrhizal fungal interactions will be discussed.

Key words: Berberidaceae, fungal symbiosis, mycorrhizal fungi, phosphorous nutrition, Podophyllum pelatum