Sinks for plant surplus carbon explain several ecological phenomena

Abstract

AbstractPlants engage in many processes and relationships that appear to be wasteful of the high-energy compounds that they produce through carbon fixation and photosynthesis. For example, living trees keep leafless tree stumps alive (i.e. respiring) and support shaded understory trees by sharing carbohydrates through root grafts or mycorrhizal fungal networks. Plants exude a diverse array of organic compounds from their roots and leaves, which support abundant rhizosphere and phyllosphere microbiomes. Some plants release substantial amounts of sugar via extra-floral nectaries, which enrich throughfall and alter lichen communities beneath the canopy. Large amounts of photosynthetically fixed carbon are transferred to root associates such as mycorrhizal fungi and N-fixing micro-organisms. Plants also respire fixed C through an alternative pathway that does not generate ATP. Rates of each of these processes appear to be highest when plants are growing under mild-to-moderate deficiencies of nutrients or water. During this stage of deficiency, aboveground plant growth is curtailed more than photosynthesis, causing leaves to produce surplus carbohydrates. Each of the above phenomena provide a sink for these surplus carbohydrates, thereby preventing feedback inhibition of photosynthesis, and perpetuating the influx of C. Because these processes incur little cost to the source plant, they need not provide a benefit beyond the removal of surplus carbohydrates.