Question

Under what conditions would you expect mycorrhizal symbiosis to have the most NEGATIVE (least beneficial) fitness consequences for bean plants?

	A.	When they are planted in tropical soils with very low nutrient availability
	B.	When they are planted in Nebraska soils that have abundant soil pathogens
	C.	When they are planted in Nebraska soils to which urea fertilizer has been added
	D.	When they are planted in Nebraska soils to which NPK fertilizer and pesticides have been added
	E.	When they are planted in tropical soils to which NPK fertilizer has been added that have abundant soil pathogens

Rhizobial bacteria are able to live freely in soil as well as within nodules of legume plants. When they live alone in soil, they never fix nitrogen. What is the best explanation for this phenomenon? Pick one answer.

	A.	They don't have all the genes for nitrogen fixation and need genes provided by their host plant
	B.	Nitrogen is readily available in the soil so there is no need to fix N
	C.	Unlike nodules, N2 is not available for fixing in the soil
	D.	Rhizobia can only fix N when they are housed in a safe environment away from predators
	E.	The costs outweigh the benefits of fixing N2 when a plant host is not there to supply sugar in exchange

Answer 1

Nitrogen fixation requires a high energy expenditure as well as anaerobic or microaerobic conditions, both of which can be provided by plants, and this explains the prevalence of symbiotic N₂ fixation. The rhizobial bacteria have all the genes required for nitrogen fixation. The right option is E