All the cells in a plant’s body contain mitochondria that carry out respiration to provide a constant supply of ATP. Typically, about 50% of the carbohydrates produced by photosynthesis in one day is converted back to CO2 by respiration within 24 hours.
Carbohydrates that are not immediately consumed in respiration can be used as raw materials for growth, or they can be stored for later use. Carbohydrates stored within roots and stems as starch, or in tubers (specialized storage organs such as potatoes), can support new growth in the spring or following a period of drought. Stored reserves can also be used to repair mechanical damage or replace leaves consumed by insects or grazing mammals.
What determines how carbohydrates become distributed within the plant? Phloem transport to reproductive organs appears to have priority over transport to stems and developing leaves, and these have priority over transport to roots. In Chapter 31, we discuss the role that hormones play in controlling the growth and development of plants and how these hormones may influence the ability of different sinks to compete successfully for resources transported in the phloem.
Phloem also supplies carbohydrates to organisms outside the plant. A fraction of the carbohydrates transported to the roots spills out into the rhizosphere, the soil layer that surrounds actively growing roots. This supply of carbohydrates stimulates the growth of soil microbes. As a result, the density of microbial organisms near roots is much greater than in the rest of the soil. These soil bacteria decompose soil organic matter rich in nutrients such as nitrogen and phosphorus. Thus, by releasing carbohydrates into the soil, roots are thought to acquire more nutrients from the soil.