Direct evidence for modulation of photosynthesis by an arbuscular mycorrhiza-induced carbon sink strength

It has been suggested that plant carbon (C) use by symbiotic arbuscular mycorrhizal fungi (AMF) may be compensated by higher photosynthetic rates because fungal metabolism creates a strong C sink that prevents photosynthate accumulation and downregulation of photosynthesis. This mechanism remains largely unexplored and lacks experimental evidence. We report here two experiments showing that the experimental manipulation of the mycorrhizal C sink significantly affected the photosynthetic rates of cucumber host plants. We expected that a sudden reduction in sink strength would cause a significant reduction in photosynthetic rates, at least temporarily. Excision of part of the extraradical mycorrhizal mycelium from roots, and causing no disturbance to the plant, induced a sustained (10-40%) decline in photosynthetic rates that lasted from 30 min to several hours in plants that were well-nourished and hydrated, and in the absence of growth or photosynthesis promotion by mycorrhizal inoculation. This effect was though minor in plants growing at high (700 ppm) atmospheric CO₂ . This is the first direct experimental evidence for the C sink strength effects exerted by arbuscular mycorrhizal symbionts on plant photosynthesis. It encourages further experimentation on mycorrhizal source-sink relations, and may have strong implications in large-scale assessments and modelling of plant photosynthesis.