W5, Roots, nutrient uptake, & plant-microbe interactions

Question 1

If you could only know one chemical property of a soil, what would it be?

Answer 1

pH, because it provides so much information about the nutrient availability and likelihood of constraints.

Question 2

If you could only know one physical property of a soil, what would it be?

Answer 2

Particle size distribution, because it determines the texture of the soil and provides clues about other properties (e.g. CEC, nutrient mobility, etc.).

Question 3

Fill in the gaps

Nutrient availability is a function of the ________, ________, and __________ __________ of the ___________.

Answer 3

Nutrient availability is a function of the physical, chemical, and biological properties of the environment.

Question 4

How is N fertiliser produced?

Answer 4

The Haber-Bosch process.

Requires high temperatures (350-550°C) and pressures (150-350 atm), and accounts for ~1% of the world’s total energy consumption.

Question 5

Explain what is meant by anatomical and physiological foraging for nutrients.

(Another way to ask the same question would be: Describe some anatomical and physiological adaptions that plants have to make them more competitive at foraging for nutrients.)

Answer 5

ANATOMICAL

• Growing more roots to acquire more nutrients
  
  • ↑ root length
  • ↑ number of fine roots
  • ↑ number of surface roots (consider nutrient stratification, particularly P)
  • more dispersed (spread out) laterals
• Grow longer, denser root hairs
  
  • particularly beneficial under low nutrient conditions (e.g. P)
• Proteoid roots
  
  • also a physiological adaption (citric acid exudation)
  • produced in response to low P availability (only)
  • only members of the Proteaceae family (includes lupins)

PHYSIOLOGICAL

• Increasing nutrient uptake without growing more roots
  
  • e.g. expression of sulfate transporters (get up-regulated when S supply is low, and down-regulated when S supply is adequate).
    
    • could also make more transporters, but its easier (more energy conservative) to just increase their activity.
• Producing root exudates
  
  • Citric acid (proteoid roots)
  • Enzymes (e.g. acid phosphatase produced by chickpeas)

Question 6

Explain the difference between mutualism, parasitism, competition, and predation (the functional continuum of organism interactions).

Answer 6

MUTUALISM

• Both organisms enounter costs and benefits, but the benefits outweigh the costs for both of them (i.e. they both benefit).

PARASITISM

• One organism obtains nutrients and carbon at the expense of another organism.
• The parasite don’t want to kill their host - they want it to survive so they can keep drawing resources from it.

COMPETITION

• Both organisms share a requirement for a resource that is in limited supply = both organisms suffer.

PREDATION

• One organism consumes another = clear winner and loser.

Question 7

Briefly explain how arbuscular mycorrhizae form associations with plants.

Answer 7

• AM spore comes into contact with root cell
• AM spore germinates and hyphae invade the root (growing between and within the cells)
• AM hyphae penetrate the cell walls and invaginate the cell membrane (i.e. don’t break it) and form arbuscules.

It’s a really invasive process for the plant, but it allows it (there’s lots of communication happening between the plant and fungi to allow it) because it works out beneficially for the plant (and the fungus) in the end.

Question 8

Roughly how much carbon (photosynthate) will a plant allocate to establishing and nurturing a symbiotic relationship with arbuscular mycorrhizas?

Answer 8

~20%.

Question 9

How would you study the effect of AM in the field if they’re so ubiquitous?

Answer 9

• Mycorrhiza defective mutants (i.e. plant mutants where the genes that control AM associations have been deleted).
• Can’t use soil fumigation or sterilisation, because that’s basically just comparing +soil biology and -soil biology rather than +myco and -myco

Question 10

What are some of the effects (benefits) of plants being associated with arbuscular mycorrhizal fungi?

Answer 10

• ↑ ability to forage for nutrients
  
  • = ↓ fertiliser requirements
• ↑ harvest index (same vegetative biomass in tomatoes, but +AMF produced more fruit (not bigger fruit))
• ↑ nutrient density
• ↑ water use efficiency
• ↓ nutrient losses (leaching, denitrification, etc.)

Question 11

Why is it not necessarily worth importing AMF (i.e. adding them), and what are some things you can do to encourage AMF in soil?

Answer 11

WHY ITS NOT WORTH IMPORTING AMF

• most soils already have sufficient populations of AMF present
• field-propagated nursery stock will have already formed associations (and so will be imported already)

ENCOURAGING AMF

• plant cover crops (or just have living roots in the ground as much as possible)
  
  • AMF need roots to complete their life cycle, so growing a cover crop can increase the spore and hyphae load in the soil
• cut rather than cultivate the cover crop
• plant field-grown nursery stock