29.1 Plant Nutrients and Availability in Soil Elements or molecules essential for an organism’s growth and survival Plants require sixteen elemental nutrients available from soil, water, and air Nine macronutrients, required in large amounts Seven micronutrients, required in trace amounts

29.2 How Do Roots Absorb Water and Nutrients? Mycorrhizae = Mutualism between root and fungi in which both species benefit Fungal hyphae share minerals absorbed from soil Root cells provide fungus with food Root hairs = Thin extensions of root epidermal cells that increase surface area for absorbing water and mineral ions New root hairs constantly form just behind the root tip Root nodules = Masses of root cells infected with bacteria that fix atmospheric nitrogen (N2) into a form usable by plants (nitrogen fixation) NH3, NH4+ and NO3-

How Roots Control Water Uptake Osmosis drives water from soil into the walls of parenchyma cells of the root cortex Water enters cell cytoplasm by diffusion or through aquaporins; active transporters pump dissolved mineral ions into cells Water and ions move from cell to cell through plasmodesmata

The Casparian Strip Endodermis between the cortex and vascular cylinder secretes a waxy substance which forms a waterproof band (Casparian strip) between plasma membranes of endodermal cells The Casparian strip forces water and ions to enter the vascular cylinder through plasmodesmata or through endodermal cell membranes (controlled by transport proteins)

Tracheids and Vessel Members

Water transport in plants Transpiration = water loss from a plant through its leaves Water leaves through open stomata Dry cell walls pull water from deeper in the leaf and pulls water from the roots, stem and leaf through xylem (creates negative pressure Water “sticks” to water (cohesion) and other molecules (adhesion) Even small trees can lose 100L of water/day What conditions increase transpiration??? Hot Dry Wind

Cohesion-Tension Theory

Controlling Water Loss at Stomata Cuticle = A translucent, water-impermeable layer coating the walls of all plant cells exposed to air Consists of epidermal cell secretions: waxes, pectin, and cellulose fibers embedded in cutin Stomata = Openings through the plant epidermis that regulate water vapor loss and gas exchange Guard cells open or close the stoma depending on the amount of water in their cytoplasm Swollen cells open stoma Collapsed cells close stoma

Homeostasis and stomata Plants keep stomata open just enough so that gas exchange can occur for photosynthesis but not so much that they lose too much water When water is abundant water flows into the leaf. This increases water pressure in the guard cells and opens them. When water is scarce, pressure decreases and the stomata close

Controlling Water Loss at Stomata Environmental cues open or close stomata Water availability (abscisic acid released by root cells binds to guard cells and solutes and water leave) Carbon dioxide levels in leaf (aerobic respiration) Light intensity (triggers potassium pumps that pump potassium into guard cells) Air pollution (prevents photosynthesis)