The structure and function of root systems
Authors: Jackson, RB, WT Pockman, WA Hoffmann
The structure and functioning of root systems is reviewed, recognizing that roots and shoots are functionally integrated. There are at least four broad root functions: anchoring the plant, capturing resources, storing resources, and sensing the environment. Roots anchor plants against windthrow and, in some cases, trampling. They take up the water and nutrients needed by plants, similar to the role that leaves play for carbon and energy uptake. Roots store carbohydrates and nutrients for times when resource availability may not coincide with plant demand. Roots also sense the environment and produce hormones, including the production of cytokinins in the spring to activate shoot buds and the feed-forward signaling of impending water stress by root-produced ABA. There are two important symbioses in which roots participate, mycorrhizae and the root/bacterial mutualism that fixes atmospheric nitrogen. More than 90% of forest and grassland plant species are mycorrhizal, a plant-fungus mutualism that increases the uptake of nutrients and water by plants. A much smaller group of plants participate in nitrogen fixation, adding 150 x 1012 g N to terrestrial ecosystems each year, approximately 1g N m-2 on average for all of the earth's land. Global patterns of root distributions are reviewed for different biomes and plant functional types, including a maximum confirmed rooting depth of almost 70 m for desert shrubs.