Sinks for plant surplus carbon explain several ecological phenomena

Plants 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 variety 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. In roots, some fixed C is respired through an alternative non-phosphorylating pathway that oxidizes excess sugar. Each of these processes is most prevalent when plants are growing under mild-to-moderate deficiencies or nutrients or water, or under high light or elevated atmospheric CO2. Under these conditions, plants produce more fixed carbon than they can use for primary metabolism and growth, and so have ‘surplus carbon’. To prevent cellular damage, these compounds must be transformed into other compounds or removed from the leaf. Each of the above phenomena represents a potential sink for these surplus carbohydrates. The fundamental ‘purpose’ of these phenomena may therefore be to alleviate the plant of surplus fixed C.