Testing the trade-balance model: resource stoichiometry does not sufficiently explain AM effects.

Area of Science:

• Plant Science
• Mycology
• Ecology

Background:

• Arbuscular mycorrhizae (AM) symbiosis is crucial for plant nutrient acquisition.
• The trade-balance model (TBM) posits that AM effects on plant growth depend on carbon (C) cost and nutrient (e.g., phosphorus, P) benefit, influenced by plant stoichiometry (C:N:P ratios).
• Experimental validation of the TBM has been lacking.

Purpose of the Study:

• To experimentally test the assumptions of the trade-balance model (TBM) for arbuscular mycorrhizae (AM) symbiosis.
• To investigate the roles of carbon (C), nitrogen (N), and phosphorus (P) in regulating AM effects on plant growth (MGR) and nutrient uptake.
• To explore the interplay between N and P in AM symbiosis under varying nutrient limitations.

Main Methods:

• Oryza sativa (rice) plants were subjected to different N:P ratios under low light conditions to create C:P and C:N limitations.
• Parameters tested included plant growth (MGR), nutrient uptake, percentage of mycorrhizal colonization (%M), extraradical mycelium (ERM), photosynthesis, and shoot starch content.
• The study isolated and tested parameters central to the TBM framework.

Main Results:

• Carbon distribution to AM fungi did not influence plant growth (MGR).
• Nitrogen (N) was found to be essential for all AM effects, including phosphorus (P) nutrition.
• AM symbiosis effects and C distribution varied with the limiting nutrient (N or P), revealing significant N-P interactions.

Conclusions:

• The trade-balance model (TBM) was not supported by the experimental evidence.
• Results align with resource exchange driven by surplus resource availability and source-sink regulation between plants and AM fungi.
• Resource exchange in AM symbiosis appears to be governed by the needs of both symbionts, rather than complex exchange rate regulations.