Chapter 47 - Plant Nutrition and Transport
Essential Elements
Most abundant elements in organisms
C,H,O = 96%
Mineral Elements
In addition to C, H, and O, there are thirteen essential inorganic substances needed by plants in ionized form ("mineral ions").
Macronutrients are each at least 0.1% of dry weight (R&J: approaches or exceeds 1%).
Micronutrients are required in trace amounts they compose a few parts per million of a plant's dry weight.
Essential nutrients may be translocated from parts to be shed; calcium cannot be translocated.
Function of mineral ions:
metabolic activities
They cause turgor pressure due to osmosis.
Plants cannot use molecular nitrogen (N2) as it is found in the atmosphere. Nitrogen must be converted to a form that can be used (N2 ? NH3). This process is called nitrogen fixation and is carried out by microorganisms. Many of these microbes live symbiotically in the nodules of the roots of legumes.
Nutrient Deficiencies
When macronutrients or micronutrients are limited to plants, they exhibit symptoms of the deficiency.
Fertilizer
N, P, K
numbers reflect percentage of N, P, and K
may be a source of pollution
requires energy to produce
Water Uptake, Transport, and Loss
Root
Annual grasses- fibrous root system, near the soil surface.
Dicots have a taproot system that penetrates more deeply.
Billions of root hairs may develop in a single system.
Roots grow toward regions of water and dissolved minerals.
mycorrhiza
A mycorrhiza is an association between a fungus and a vascular plant which increases the absorptive surface area of the plant.
The plant gets water and dissolved nutrients; the fungus gets carbohydrates produced by the plant.
Most water enters through root hairs; may also diffuse through cell walls, but must eventually enter cells due to Casparian strip.
Water uptake requires an osmotic gradient.
Solutes, especially mineral ions, are actively pumped into cells by membrane pumps that use ATP.
Selection of minerals that enters the vascular cylinder is controlled by endodermal cells.
Water uptake at night
Water loss is lower at night due to high relative humidity of air
active transport still occurs
osmosis causes root pressure which causes guttation
Cohesion Theory of Water Conduction
Xylem Structure
Cohesion Theory of Water Conduction
Xylem Structure
tracheids- pitted end walls
Vessel elements- no end walls; form a continuous tube
Properties of Water
Water molecules are polar; one side is positively charged and the other is negative. Due to their polarity, molecules are attracted to each other and hydrogen bonds form betwen the hydrogen atoms (which are positively chargeed) and the oxygen atoms (which are negatively charged).
Water and Mineral Transport
A vacuum can only pull water 10.4 ft. (3.17 m) at sea level
Transpiration- evaporative water loss from plants
Hydrogen bonds help to pull the continuous water column up the plant.
Evaporation in leaves (transpiration) causes the state of tension; results in movement
Transpiration and Carbon Dioxide Uptake
>90% of water moving taken in by the plant is lost by transpiration; 2% used for photosynthesis and other activities.
If transpiration > uptake, dehydration and death can result.
The cuticle conserves water, limits CO2 diffusion.
Stomata (sing. stoma) - transpiration and CO2 uptake
When guard cells are turgid- opens
When guard cells are flaccid- closes
Opens and closes according to H2O and CO2 conc.
Stomata close when the temperature is > 30 to 34? C.
CAM photosynthesis seen in many desert plants allows plants to conserve water by taking in CO2 at night and fixing it during the day.
Transport of Organic Substances in Phloem
Translocation
Translocation is the transport of food (mostly sucrose) in the phloem from one plant organ to another.
Example: site of synthesis ? site of use.
Occurs in sieve tube cells (phloem); alive at maturity; interconnected from leaf to root.
Observations of aphids provided translocation information (sucrose, pressure flow)
Pressure Flow Theory
"source-to sink" pattern
Source: leaves
Sinks: fruits, seeds, roots
Translocation through the phloem depends on pressure gradients between source and sink regions.
Active transport moves solutes into sieve tubes and water follows. ATP is supplied by companion cells
Active transport moves solutes into sink tissues and water follows. ATP is supplied by parenchyma cells
