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Difficulty: Very hardLogical Inferences

Specifically, in nutrient-poor subarctic soils, *Betula nana* (dwarf birch) establishes symbioses with ectomycorrhizal fungi to acquire organic nitrogen. Recent studies show that when *B. nana* is shaded, reducing its photosynthetic carbon production, the fungi typically restrict nitrogen delivery to that individual. However, under elevated soil temperatures, the fungi maintain high nitrogen transport to shaded *B. nana* individuals, even when their carbon allocation is minimal, provided that neighboring unshaded *B. nana* plants in the same fungal network have surplus carbon. Which choice most logically completes the text?

  1. A
    ectomycorrhizal fungi prioritize nitrogen delivery to plants that are experiencing the greatest environmental stress.
  2. the resource exchange between the fungi and a host plant is mediated by the collective carbon contributions of the network rather than only the individual plant's input.Answer
  3. C
    *Betula nana* plants respond to elevated soil temperatures by increasing their photosynthetic rate to support neighboring plants.
  4. D
    elevated soil temperatures generally enhance the photosynthetic efficiency of shaded subarctic vegetation.

Answer

The resource exchange between the fungi and a host plant is mediated by the collective carbon contributions of the network rather than only the individual plant's input.
The correct answer logically completes the text because the passage establishes that while fungi normally restrict nitrogen to a shaded plant that provides minimal carbon, this restriction is bypassed under elevated temperatures if neighboring plants connected to the same fungal network have surplus carbon. This indicates that the resource exchange is determined by the total carbon resources available within the shared fungal network rather than strictly by the individual plant's input.

Step-by-Step Solution

1
Identify the baseline relationship described in the passage.
Normally, a reduction in a host plant's carbon allocation (due to shading) causes ectomycorrhizal fungi to restrict nitrogen delivery to that individual.
This establishes that the resource exchange is typically localized and pairwise between the fungus and the specific host.
2
Analyze how elevated temperatures modify this relationship.
Under warming conditions, a shaded plant with minimal carbon output still receives nitrogen, provided neighboring plants in the same network have a surplus of carbon.
This indicates that the presence of other carbon sources in the network overrides the restriction based on the single plant's low carbon contribution.
3
Synthesize the findings to draw a logical conclusion.
The fungi's resource allocation under elevated temperatures depends on the total carbon pool of the shared network rather than just the individual host's carbon input.
This directly matches the assertion that the resource exchange is mediated by collective network contributions.

Key Concept

Logical Inferences
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Logical Inferences

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