Chapter 47 - Plant Nutrition and Transport

C,H,O = 96%

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.

Nitrogen fixation

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 ® NH₄). 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.

N, P, K

numbers reflect percentage of N, P, and K

may be a source of pollution

requires energy to produce

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

Xylem Structure

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.

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