Root Functions

Most root functions take place in the youngest part of roots, usually 10 centimeters from the growing root tip. Four activities are accomplished in that area:

Roots Stabilize the Plant. Roots anchor plants in soil by pressing between soil particles. Loose or wet sand or mud makes it much harder for roots to

secure mechanical anchorage. Trees topple when high winds sweep sandy areas, wetlands, or rainsoaked slopes. In free-floating aquatic plants, such as duckweed and water hyacinth, roots stabilize the plant by dangling into the water, acting as a keel to keep the leaves upright and preventing plants from toppling over. When roots are experimentally removed from water hyacinth, the plants tip over and die.

Roots Absorb Water and Minerals. Roots absorb water and minerals and transport them to the shoot. In a root system, each root elongates from its tip, entering new soil that can be exploited as an undepleted source of minerals and water. Delicate single-celled root hairs grow from the epidermis and greatly increase the root surface area devoted to absorption. Conditions that harm root hairs in the soil will impede nutrient uptake and harm the entire plant as leaves turn yellow and plant growth slows. Soil compaction and salt accumulation are two environmental factors that harm plants by killing root hairs. In 1999, decline in fruit quality from date orchards in Southern California was slowed by reducing tractor traffic near the trees. This decreased the likelihood of crushing root hairs on the shallow fibrous root systems of date palms.

There are more than fifteen elements needed by plants to grow. Of these, roots absorb nitrogen (N), phosphorus (P), and potassium (K) in the largest amounts. The soil minerals most commonly available for root uptake are those that are stuck onto tiny colloid particles in the soil. Colloids are abundant in clay and in broken-down compost. They enhance soil fertility by anchoring minerals in the soil and preventing them from being washed (leached) down and away from plant roots. Colloids are negatively charged particles, allowing minerals with positive charges (potassium, calcium, iron, etc.) to stick to them. Roots can free these stuck minerals by releasing protons (H+) from inside the root and exchanging them for positively charged minerals on the colloid surface in a process called cation exchange. The cation exchange capacity of a soil is a strong indicator of soil fertility, the ability of a soil to provide roots with essential minerals over long periods of time. Acid rain (high in H+) in Europe and Northeastern North America (Canada and the United States) decreases soil fertility by displacing minerals from soil colloids, thereby lowering the soil's exchange capacity.

Roots Attract Microbes. The roots of many species attract beneficial soil microbes by secreting a paste (mucigel) rich in sugar. The sugars support large populations of soil bacteria and fungi that help roots absorb minerals, especially nitrogen and phosphorus. The bacteria involved are nitrogen fixing bacteria. The fungi are mycorrhizae, long threadlike fungi that attach to plant roots and form a bridge connecting roots to minerals that would have been out of reach of the shorter root hairs. Almost all major agricultural crops have fungi or bacteria associated with their roots. Plants invest up to 10 percent of all food made by leaves to the paste they secrete out of roots. In some cases, the mucigel can feed more than just microbes. In lava tubes such as the Kaumana caves of Hawaii, entire populations of insects live in total darkness, fed only by mucigel from roots pushing through the cave ceiling.

Roots Store Food. Since roots are underground and away from light, making their own food by photosynthesis is impossible. Instead, sugar made by impede slow down or inhibit compaction compacting of soil, leading to loss of air spaces impede slow down or inhibit compaction compacting of soil, leading to loss of air spaces

leaves is transported to roots for storage as starch. The stored food can power root growth, or it can enlarge roots in some cases, turning them into economically important commodities such as cassava (Manihot) from West Africa and the Caribbean, sweet potato (Ipomoea), and ginseng (Panax) from China and North Carolina.

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