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

2. 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.

4. 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-)

5. 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

6. 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

10. 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

12. 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.

13. Figure 26.3

16. 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.

17. 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.

19. apoplastic
symplastic

20. Uptake of Water & Minerals

21. 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

25. 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.

26. 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.

27. 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.

32. 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

33. 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

35. 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

36. 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.

38. 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.

41. 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.

42. 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.

45. 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.

47. 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.

48. Organic Nutrient Transport