THE ROOT STRUCTURE & FUNCTIONS

FUNCTIONS Water & nutrient absorption Anchor the plant to the soil Storage Perception of gravity (root cap)

Root Systems Taproot – long root extending deeply into the soil to store food and nutrients and absorb water from deep in the ground. Fibrous (adventitious) – an extensive mass of similarly sized roots.

Root Systems Prop roots – fibrous roots below ground and above ground that help support a tall upright stem. (Page 156, fig. 7.3 ) Rhizomes – adventitious roots grown from underground stems, also serve as vegetative, asexual, mean of reproduction for the plant. Where new plants develop from the rhizome, with out the fusion of gametes.

Root Cap and Tissues Tips of the root are covered and protected by a thimble-shape Root Cap. The cells that comprised the root cap produce a slimy substance known as Mucigel, which is a mixture of sugars, enzymes and amino acids. Mucigel functions for: Protection – prevents the roots from drying out, and inhibit growths of other plants around it. Lubrication Water Absorption – maintain continuity of between roots and soil, because soil clings the mucigel Nutrient Absorption – helps nutrients absorb in the same way it helps water.

Subapical Region Region behind the root cap is called the subapical region. Zone of Cell Division – consist of apical meristem cells, region where cells are dividing (~24-36 hours), producing upwards of 20,000 new cells a day. Zone of Cell Elongation – newly formed cells elongate by up to 150X. Primarily, by filling vacuoles with water, this action can push the roots at a downward rate of up to 4 cm a day. Zone of Cell Maturation – where immature cells take on a specific function, developing root hairs on the outside of the root. Root hairs form only in the mature, non-elongating regions of the root.

Development Primary growth From the root apical meristem primary tissues & increased root length Secondary growth (mostly in woody plants; it is limited in herbaceous plants From the vascular cambium secondary tissues & increased diameter

Root Tissues Hypodermis is the outermost layer of the cortex, which is typically coating in a protective layer called suberin, which is a waxy substance that prevents water loss in the roots closest to the surface by as much as 200X. Endodermis is the inner most layer of the cortex.

Root Tissues Stele of the root includes all tissues found in the middle of the roots and consist of the pericycle, and vascular tissues. Pericycle is a thin walled meristem tissue that produces branch roots.

Rhizosphere This is the narrow zone of soil that surrounds the roots, extending up to 5mm from the root surface. Very complex and constantly changing environment.

Rhizosphere Growth of the plant changes the Rhizosphere by Roots force their way down through the soil creating crevices, that help to aerate the soil when the plant dies. Alter chemical composition of the soil thru respiration, which lowers the oxygen in the soil and increases the CO2 concentration. This change the pH of the soil, increasing the acidity which increases the availability of ions such as iron and decreasing the availability of potassium.

Rhizosphere Roots enrich the soil with organic matter. Some plants can transport as much as 60% of the sugars not used, into the soil – primarily through the mucigel. Rhizosphere typically contains large amounts of energy rich organic molecules that microorganisms use to for their cellular processes. Upwards of 1010 microorganisms per cubic centimeter ~(10,000,000,000)

Root Growth Growth and Distribution of Roots is primarily controlled by; Gravity Genetic differences Stage of Plant Development Soil properties

Environmental Adaptations Storage Store sugars (ex. Sweet Potato) Water storage (ex. Hottentot – uses huge 350 kg / 770 lbs root to store water and starch) Vegetative reproduction Suckers – shoots that when separated from the parent plant become new plants.

Environmental Adaptations Aeration Plants that grow in low oxygen environments such as tidal marshes, avoid suffocation by growing roots that are adapted to import oxygen from the atmosphere. (ex. Black Mangroves) These roots can contain up to 80% aerenchyma tissues, contain large air spaces, and act as snorkels.

Environmental Adaptations Movement Contractile roots, contract the xylem and cortex which pull the plant deeper into the soil. (Ex. Dandelion) Nutrition Parasitic plants attach to the host plant and siphon nutrients from them – thus harming the host plant. (Ex. Mistletoe)

Epiphytes Epiphytes are plants that grow independently on other plants, these include orchids, stagehorn ferns and some cacti. Typically grow slowly and absorb nutrients through aerial roots.

Mycorrhizae Associations between plant roots and fungi in the soil. Mutualistic relationship. Fungi absorb nutrients in the soil that the plant uses and in turn the plant provides the fungi with sugars, amino acids and other organic material. Mycorrhizae dramatically increase plant growth and absorption. Wheat 200% Corn 100% Onions 3,000% Helps to improve a plants tolerance to drought, disease, temp. extremes, and lack of nutrients. % increase in plant growth

Nitrogen Fixation Plants have established a mutualistic relationship between N2 fixing bacteria, bacteria species that contain the enzyme nitrogenase that can convert atmospheric nitrogen in to ammonia (NH3) which the plants can utilize for growth and production of amino acids.

Soil Layers

Plant Nutrition Elements are essential for normal plants if they: Required for normal growth and reproduction No other element can replace it Has a direct / indirect action on plant metabolism

Plant Nutrition Macronutrients are elements that are required in large amounts, usually more that 0.5% of the dry weight of the plant. Nine required macronutrients Carbon Hydrogen Oxygen Phosphorus Nitrogen Sulfur Calcium Magnesium

Plant Nutrition Plant also require micronutrients, or trace elements, which are required for metabolism in very small quantities (~ppm, or part per millions or less). Iron Copper Zinc Chlorine Manganese Molybdenum Boron

Plant Nutrition Functions of essential nutrients Part of cellular structures and biological molecules. Involved in energy related chemical reactions Activate or inhibit enzymes. Alter osmotic pressure of a cell.

Importance and Uses of Essential Elements Some plant can concentrate certain elements in their bodies at much higher concentrations than needed; this is called hyperaccumulators. Helps plants to evade “weak stomached” animals and insects, acts as a defensive measure. Locoweed accumulates selenium, which affects animals nervous systems causing confusion, reduced mobility and death.

Environmental Importance of Root Systems and Hyperaccumultaion Phytoremediation – is the use of hyperaccumulators to remove toxic substances from contaminated soils. Hyperaccumulators also help geologist explore for ore deposits and contaminated areas and buried dump sites.