Suzanne Simard and Colleagues Knew That the Same Mycorrhizal Fungal

Multiple Choice

Suzanne Simard and colleagues knew that the same mycorrhizal fungal species could colonize multiple types of trees. They wondered if the same fungal individual would colonize different trees, forming an underground network that potentially could transport carbon and nutrients from one tree to another (S. Simard et al. 1997. Net transfer of carbon between mycorrhizal tree species in the field. Nature 388:579-82) . Figure 31.2 illustrates the team's experimental setup. Pots containing seedlings of three different tree species were set up and grown under natural conditions for three years; two of the three species formed ectomycorrhizae (Douglas fir, birch) and the other (cedar) formed arbuscular mycorrhizae. For the experiment, the researchers placed airtight bags over the Douglas fir and birch seedlings; into each bag, they injected either carbon dioxide made from carbon-13 or carbon-14 (¹³CO₂ and ¹⁴CO₂, isotopes of carbon) . As the seedlings photosynthesized, the radioactive carbon dioxide was converted into radioactively labeled sugars that could be tracked and measured by the researchers.

Figure 31.2
-Referring to Simard et al.(1997) ,what is the result that would most strongly refute their hypothesis?

A) No movement;carbon-14 is found in the birch seedling's tissues and carbon-13 in the Douglas fir.
B) Reciprocal exchange;carbon-14 is found in the Douglas fir seedling's tissues and carbon-13 in the birch.
C) Either carbon-13 or carbon-14 is found in the fungal tissues.
D) Either carbon-13 or carbon-14 is found in the cedar seedling's tissues.