Mycorrhizae are highly specialized associations between soil fungi and the roots of the vast majority of terrestrial plants. The fungal hyphae penetrate or wrap around the plant roots, vastly increasing the effective surface area for the absorption of water and scarce inorganic ions, such as phosphate. In return, the plant translocates photosynthesized sucrose and hexoses to the fungal partner.
Which of the following correctly identifies this ecological relationship and explains its overall impact on the plant's productivity?
Mutualism; it increases the plant's net primary productivity (NPPNPPNPP) because the enhanced uptake of essential minerals increases the rate of photosynthesis, resulting in a gross primary productivity (GPPGPPGPP) that outweighs the respiratory and carbon-transfer losses.
Commensalism; it increases the plant's gross primary productivity (GPPGPPGPP) by assisting mineral uptake, whilst the fungal partner remains metabolically unaffected and receives no chemical energy from the plant.
Mutualism; it decreases the plant's net primary productivity (NPPNPPNPP) because the substantial transfer of photosynthate to the fungal sink behaves strictly as a respiratory loss (RRR), leaving less chemical energy stored as plant biomass.
Parasitism; the fungus behaves as a biotrophic parasite that exploits the plant's roots, leading to a severe reduction in both the gross primary productivity (GPPGPPGPP) and net primary productivity (NPPNPPNPP) of the plant.