Plant Nutrition And Transport

Plant Nutrition and Transport

Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings

Plants That Clean Up Poisons • This healthy fern, pictured with University of Florida researcher Lena Ma, contains high levels of arsenic – Arsenic is a chemical element toxic to most plants and animals Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings

• Plants like this are commonly known as “metal accumulators” – They are able to absorb large amounts of lead, zinc, and other heavy metals

• Unable to move about in search of food, plants have adapted to survive in the presence of toxins

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• Metal accumulators can be used in phytoremediation – the use of plants to help clean up polluted soil and groundwater

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• Sunflowers have been used to clean up radioactive wastes – Sunflowers were set adrift on foam rafts in a pond near the destroyed nuclear power plant in Chernobyl to absorb strontium-90 and cesium-137

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• Although phytoremediation has had some successes, it does pose several problems – Disposal of plants that have accumulated the toxic substances – Evaporation of accumulated toxic substances into the air – Animal ingestion of toxin-laden plants – The length of time it takes for the plants to reduce soil toxicity

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THE UPTAKE AND TRANSPORT OF PLANT NUTRIENTS 32.1 Plants acquire their nutrients from soil and air • As a plant grows, its roots absorb water, minerals (inorganic ions), and some oxygen from the soil – Its leaves take carbon dioxide from the air

Figure 32.1A Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings

• Photosynthesis makes use of the uptake of water, carbon dioxide, and minerals to produce sugars – These sugars are composed of carbon, oxygen, and hydrogen

• The nitrogen and magnesium absorbed from the soil are components of chlorophyll • Phosphorus, also absorbed from the soil, is a major component of nucleic acids, phospholipids, and ATP Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings

– The ability to move water from roots to leaves and to deliver sugars to specific areas of the body are remarkable feats of evolutionary engineering

Figure 32.1B Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings

32.2 The plasma membranes of root cells control solute uptake • Root hairs greatly increase a root's absorptive surface

Figure 32.2A Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings

• In order for upward transport, water and solutes must enter the xylem • Water and solutes move through the root's epidermis and cortex by two main routes – Through cells (intracellular route) – Between cells (extracellular route)

• Water and solutes typically follow a combination of routes and passages through numerous plasma membranes and cell walls en route to the xylem Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings

Epidermis

Root hair

Cortex

Phloem

Xylem

Casparian strip Endodermis

EXTRACELLULAR ROUTE, via cell walls; stopped by Casparian strip

Casparian strip

Xylem

Root hair

INTRACELLULAR ROUTE, via cell interiors; through plasmodesmata Epidermis Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings

Cortex

Endodermis

Figure 32.2B

• The Casparian strip stops water and solutes from entering the xylem via cell walls – Thus water and ions that travel the extracellular route can enter the xylem only by crossing a plasma membrane into an endodermal cell

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32.3 Transpiration pulls water up xylem vessels • Xylem sap is the solution of inorganic nutrients conveyed in xylem tissue from a plant's roots to its shoots • Root pressure can push xylem sap up only a few meters – Solute transport raises water pressure in the xylem

• Plants pull xylem sap upward from the soil through the transpiration-cohesion-tension mechanism Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings

• Transpiration is the loss of water from the leaves – It exerts a pull on the xylem sap

• Cohesion causes water molecules to stick together – It relays the pull of transpiration along a string of water molecules all the way to the roots

• The adhesion of water molecules to xylem cell walls helps counter the effect of gravity Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings

Figure 32.3 Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings

32.4 Guard cells control transpiration • Guard cells surrounding stomata in the leaves control transpiration – The opening and closing of stomata is an adaptation to help plants regulate their water content and adjust to changing environmental conditions H2O Guard cells

H2O

H2O

H2O

H2O

H2O

K+ Vacuole

H2O

H2O Stoma opening

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H2O H2O Stoma closing

Figure 32.4

32.5 Phloem transports sugars • While xylem sap flows upwards from the roots, phloem sap moves throughout the plant in various directions • The main function of phloem is to transport the sugars made by photosynthesis

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• Phloem contains food-conducting cells arranged end-to-end as tubes

Sievetube member

Sieve plate

Figure 32.5A Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings

• Phloem transports food molecules made by photosynthesis by a pressure-flow mechanism – Sugar is loaded into a phloem tube at the sugar source, raising the solute concentration inside the tube Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings

Figure 32.5B

– Water is drawn into the tube by osmosis, raising the pressure in the tube – Sugar and water leave the tube at the sugar sink

Figure 32.5B Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings

– The increase in pressure at the sugar source and decrease at the sugar sink causes phloem sap to flow from source to sink

Figure 32.5B Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings

• Plant biologists have used aphids to study phloem sap – These studies have supported the pressure-flow model

Honeydew droplet

Stylet of aphid

Figure 32.5C Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings

PLANT NUTRIENTS AND THE SOIL 32.6 Plant health depends on a complete diet of essential inorganic nutrients • A plant must obtain nutrients from its surroundings • Macronutrients, such as carbon, oxygen, nitrogen, and phosphorus, are needed in large amounts – They are used to build organic molecules Figure 32.6B Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings

• Micronutrients, including iron, copper, and zinc, act mainly as cofactors or enzymes • Growing plants in solutions of known composition enables researchers to determine nutrient requirements – Hydroponic culture Complete solution containing all minerals (control)

Solution lacking potassium (experimental) Figure 32.6A

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32.7 Connection: You can diagnose some nutrient deficiencies in your own plants • Stunting, wilting, and color changes indicate nutrient deficiencies – Compared to the healthy tomato plant on the left, the plant on the right is not getting enough nitrogen

Figure 32.7A, B Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings

– Phosphorus deficiency is sometimes indicated by a purplish leaf color – Yellow leaves can result from potassium deficiency

Figure 32.7C, D Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings

32.8 Soil contains rock particles, humus, organisms, water, and crucial solutes • Soil characteristics determine whether a plant will be able to obtain the nutrients it needs to grow • Fertile soil contains a mixture of small rock and clay particles – They hold water and ions and allow oxygen to diffuse into plant roots

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• Humus is decaying organic material – It provides nutrients, holds water and air, and supports the growth of organisms that enhance soil fertility

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• Soil horizons are distinct layers of soil • Horizon A, or topsoil, contains rock particles (sand and clay), humus, and living organisms • Horizon B contains fine clay particles and nutrients that have drained down from Horizon A • Horizon C is composed mainly of partially broken-down rock Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings

Figure 32.8A

• A plant's root hairs are in direct contact with the water that surrounds the tiny particles of topsoil • The root hairs take up dissolved oxygen, ions, and water from the film of soil water that surrounds them

Soil particle surrounded by film of water Root hair Water

Air space

Figure 32.8B Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings

• Anions, such as nitrate (NO3-), are readily available to plants because they are not bound to soil particles – But they tend to drain out of the soil quickly – This reduces soil fertility

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• Cations, such as Ca2+ and Mg2+, adhere to soil particles – In cation exchange, root hairs release H+ ions, which displace cations from soil particles – The root hairs then absorb the free cations Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings

K+ K+

H

K+ Clay particle

+

K+

K+ K+

K+

K+

Root hair

Figure 32.8C

32.9 Connection: Soil conservation is essential to human life • Good soil management includes – water-conserving irrigation – erosion control – the prudent use of herbicides and fertilizers Figure 32.9A, B Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings

32.10 Connection: Organic farmers avoid the use of commercial chemicals • Organic farmers rely on the principles of ecology rather than the use of synthetic chemicals or pesticides that can damage the environment – Organic farmers try to restore as much to the soil as is drawn from it Figure 32.10 Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings

32.11 Fungi help most plants absorb nutrients from the soil • Relationships with other organisms help plants obtain nutrients • Many plants form mycorrhizae – A network of fungal threads increases a plant's absorption of nutrients and water – The fungus receives some nutrients from the plant Figure 32.11 Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings

32.12 The plant kingdom includes parasites and carnivores • Some plants have evolved parasitic ways of obtaining food from other plants – Dodder obtains organic molecules from other plant species using specialized roots that tap into the host’s vascular tissue Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings

Figure 32.12A

– Mistletoe supplements its diet by siphoning sap from the vascular tissue of its host plants

Figure 32.12B Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings

• Carnivorous plants obtain some of their nutrients from animal tissues – The sundew and Venus flytrap use insects as a source of nitrogen – This nutritional adaptation enables them to thrive in highly acidic soil

Figure 32.12C, D Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings

32.13 Most plants depend on bacteria to supply nitrogen • Plants cannot use atmospheric nitrogen, gaseous N2, although it is very plentiful – Instead, nearly all plants depend to some extent on nitrogen supplies in the soil

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• Bacteria in the soil convert N2 from the air and nitrogen compounds from decomposing organic matter into forms that plants can take up and use – Nitrate ions (N03-) and ammonium ions (NH4+)

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• This process of converting atmospheric nitrogen to ammonium is called nitrogen fixation ATMOSPHERE N2 Amino acids Nitrogen-fixing bacteria

N2

NH4+ NH4+ (ammonium)

Soil Ammonifying bacteria Organic material

Nitrifying bacteria

NO3– (nitrate)

Root Figure 32.13

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32.14 Legumes and certain other plants house nitrogen-fixing bacteria • Legumes and certain other plants have nodules in their roots that contain nitrogen-fixing bacteria

Shoot

Nodules

Roots

Figure 32.14A Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings

• Most of the nitrogen-fixing bacteria in legume nodules belong to the genus Rhizobium

• The relationship between the plant and the nitrogen-fixing bacteria is mutually beneficial Bacteria within vesicle

Figure 32.14B Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings

PLANT NUTRIENTS AND AGRICULTURE 32.15 Connection: A major goal of agricultural research is to improve the protein content of crops • Plants are the main nutritional source for most people in the world – Therefore, improving the protein content of crops is an important research goal Figure 32.15A Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings

• One of the most promising lines of agricultural research is directed toward improving the output of the Rhizobium bacteria that inhabit the root nodules of legumes Rhizobium DNA

Genes for nitrogen fixation

TURN OFF GENES Nitrogen compounds in root nodules

n ge tro ion Ni xat fi Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings

Nitrogen-fixing enzymes

N2

Figure 32.15B

32.16 Connection: Genetic engineering is increasing crop yields • Using both gene guns and plasmids for gene transfer, researchers are developing new varieties of crop plants Gunpowder

Gun

“Bullet” Plant cells

DNA-coated pellets Figure 32.16

Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings