Nutrition and Transport in Plants CO 26 Chapter 26 Nutrition and Transport in Plants

Plant Nutrition and Soil 17th Century Dutchman Van Helmont conducted an experiment. He planted a 5 pound willow tree in a pot with 200 pounds of soil. After five years of watering, the tree weighed 170 pounds; only a few ounces of soil was missing. He concluded the increase in tree weight came from water; he was unaware of substances in air.

Essential Inorganic Nutrients Essential inorganic nutrients (e.g., carbon, hydrogen, oxygen) comprise 96% of plant dry weight. Carbon dioxide is the source of carbon for a plant. Water is the source of hydrogen. Oxygen can come from either atmospheric oxygen, carbon dioxide, or water. A plant requires other inorganic nutrients that are absorbed by the roots (minerals) Minerals are an inorganic substance usually containing two or more elements (ex. SO42-)

Essential Inorganic Nutrients Essential inorganic nutrients must fulfill the following criteria. They have an identifiable nutritional role. No other element can substitute and fulfill the same role. A deficiency of the element causes the plant to die. These elements are divided into macronutrients and micronutrients by concentration in plant tissue

Macronutrients & Micronutrients Carbon Hydrogen Oxygen Nitrogen Potassium Calcium Phosphorus Magnesium Sulphur Micronutrients Chlorine Iron Manganese Zinc Boron Copper Molybdenum Chlorine Iron Manganese Zinc Boron Copper Molybdenum

The Nutritional Function of Soil Soil is defined as a mixture of soil particles, decaying organic matter, living organisms, air and water that together support the growth of plants. Best soil includes particles of different sizes; this provides critical air spaces. Soil Particles: (particles vary by size) Sand particles are larger Silt particles are medium sized Clay particles are smallest

Soil Particles Soils lose water too readily; clay packs tight to hold water and clumps. Clay particles are negatively charged and attract positively charged ions (e.g., calcium Ca2+ and potassium K+). Loam (a mixture of the three soil particles = 1/3 of each) retains water and nutrients; roots take up oxygen in the air spaces.

Figure 26.3

Uptake of Water and Minerals In contrast to water, minerals are actively taken up by plant cells. Mineral nutrient concentration in roots may be 10,000 times more than in surrounding soil. During transport throughout a plant, minerals can exit xylem and enter cells that require them. Mineral ions cross plasma membranes by a chemiosmotic mechanism.

Uptake of Water and Minerals Minerals follow the same path as water. Some mineral ions diffuse in between the cells. Because of the impermeable Casparian strip, water must eventually enter the cytoplasm of endodermal cells. Water can move directly into cytoplasm of root hairs and is transported across cortex and endodermis.

apoplastic symplastic

Uptake of Water & Minerals

Adaptations of Roots for Mineral Uptake Two symbiotic relationships Legumes have nodules infected with the bacterium Rhizobium Rhizobial bacteria reduce atmospheric nitrogen (N2) to ammonium (NH4+)(nitrogen-fixation) Most plants have mycorrhizae Mycorrhizae are a mutualistic symbiotic relationship between soil fungi and plant roots Fungus increases surface area for mineral and water uptake and breaks down organic matter. The root furnishes fungus with sugars and amino acids

Transport Mechanisms in Plants Xylem vascular tissue passively conducts water and mineral solutes from roots to leaves; it contains two types of conducting cells: tracheids and vessel elements. Tracheids are hollow, nonliving cells with tapered overlapping ends; thinner and longer than vessel elements; water crosses end and sidewalls because of pits in secondary cell wall. Vessel elements are hollow, nonliving cells that lack tapered ends; wider and shorter than tracheids; lack transverse end walls; form a continuous pipeline for water and mineral transport.

Transport Mechanisms in Plants Phloem is vascular tissue that conducts organic solutes in plants mainly from leaves to roots; contains sieve-tube cells and companion cells. Sieve-tube cells lack a nucleus, are arranged end to end and have channels in end walls (thus, the name “sieve-tube”) through which plasmodesmata extend from one cell to another. Companion cells connect to sieve-tube cells by numerous plasmodesmata, are smaller and more generalized than sieve-tube cells; they have a nucleus.

The Concept of Water Potential Water flows from a region of higher water potential (the potential energy of water) to a region of lower water potential. Water potential is a measure of the capacity to release or take up water; in cells, water potential includes the following: Pressure potential, the effect that pressure has on water potential; water will move from a region of higher pressure to a region of lower pressure; Osmotic potential, the effect that solutes have on water potential; water tends to move by osmosis from an area of lower solute concentration to area of higher solute concentration.

Water Transport Movement of water and minerals in a plant involves entry into roots, xylem, and leaves. 3 processes: Osmosis Capillary Action Cohesion-Tension Theory

Water Transport Osmosis - Water entering root cells creates a positive pressure called root pressure. Root pressure (primarily at night) tends to push xylem sap upward in plant. Guttation is appearance of drops of water along the edge of leaves, it is result of root pressure. Root pressure is not a sufficient mechanism for water to rise to the tops of trees

Water Transport Capillary Action – is the rise of liquids in narrow tubes. Adhesion – Molecular attraction between UNLIKE substances. Capillary Action is also not a sufficient mechanism for water to rise to the tops of trees

Water Transport Cohesion-Tension Theory Transpiration – evaporation of water from plants Cohesion – water molecules attracted to other water molecules. (polarity & hydrogen bonds) Bulk Flow – water movement from roots to leaves as water molecules evaporate from the leaf surface.

Opening and Closing of Stomates Each stomate has two guard cells with a pore between them. Stomates OPEN - when guard cells take up water = increase in turgor pressure Stomates CLOSE - when guard cells lose water = decreases in turgor pressure . Guard cells are attached to each other at their ends; inner walls are thicker than outer walls. As they take up water, they buckle out, thereby creating an opening between cells.

Opening and Closing of Stomates When stomates open, there is accumulation of K+ ions in guard cells. A proton pump run by breakdown of ATP to ADP and P transports H+ outside the cell; this establishes an electrochemical gradient allowing K+ to enter by way of a channel protein. Blue-light component of sunlight is a signal that can cause stomates to open. ???? - flavin pigments absorb blue light. This pigment sets in motion a cytoplasmic response activating the proton pump that causes K+ ions to accumulate in guard cells.

Organic Nutrient Transport Role of Phloem Marcello Malpighi (1679)-bark transferred sugars from leaves to roots Girdling – Removing a strip of bark Radioactive tracer studies 14C-labeled carbon is supplied to mature leaves, radioactively labeled sugar moves to roots Aphids used in study The aphid body is cut off; the stylet becomes a small needle from which phloem is collected.

Pressure-Flow Model of Phloem Transport Transport of sap through sieve tubes by a positive pressure potential. Buildup of water creates a positive pressure potential within sieve tubes that moves water & sucrose from a source (leaves) to a sink (roots). Pressure exists from leaves to roots; at roots, sucrose is transported out and water follows. Consequently, the pressure gradient causes a flow of water from leaves to roots.

Pressure-Flow Model of Phloem Transport Leaves produce sugar Sucrose is actively transported into phloem by an electrochemical gradient established by a H+ pump. Water flows into sieve tubes by osmosis. A SINK can be at the roots or any other part of the plant that requires nutrients. Because phloem sap flows from SOURCE to SINK, sap can move any direction within phloem.

Organic Nutrient Transport