WATSON, MAXINE A.* AND CYNTHIA S. JONES. Department of Biology, Indiana University, Bloomington, IN 47405. - Developmentally-dependent interactions between mayapple, Podophyllum pelatum L. (Berberidaceae) and its associated mycorrhizal fungi.
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