Mineral nutrition: essential minerals and their uptake

Question 1

Defining features of essential mineral elements

Answer 1

• Plant cannot complete its life-cycle without the element
• Function of the element cannot be performed by another
• Element must be directly involved in plant metabolism
• Essential macronutrients are needed in large amounts
• Essential micronutrients in very small amounts

Question 2

Leiblig’s law of the minimum:

Answer 2

The element present in the least amount, relative to its requirement by a plant, sets the limit to that plant’s growth and yield

Question 3

Mengle & Kirkby’s mineral groups

Answer 3

(For details see Table 5.2, p 109 of Taiz & Zeiger)

Nitrogen and Sulphur: incorporated into organic compounds by forming covalent bonds – proteins, nucleic acids etc.

Phosphorous, Silicon and Boron: important in energy storage reactions or structural integrity - often as esters bound to an organic molecule

Potassium, Calcium, Magnesium, Chlorine, Manganese and Sodium: remain as ions – cofactors in enzymes, regulation of osmotic potential etc.

Iron, Zinc, Copper, Nickel and Molybdenum: also cofactors- involved in redox reactions in electron transport chains. (very little required and excess can cause issues)

Question 4

Mineral sources

Answer 4

Majority of plant minerals are obtained through water drawn up from the soil ( a nutrient reservoir)
Often must work against a concentration gradient requiring active transport
-> What is happening in the roots decides how much minerals can be uptaken

Question 5

Mineral uptake from roots: the soil reservoir

Answer 5

Mineral uptake from roots: the soil reservoir
* Solid phase – K, Ca, Mg and Fe (inorganic particles) plus associated organic compounds (N, P, S etc).
* Colloidal phase – suspended particles
* Liquid phase – dissolved mineral ions. Acts as medium for ions to move to root surface.
* Gas phase – O2, CO2, N2 dissolved in soil solution and present in air spaces between particles

Microorganisms in the soil may deprive the roots of minerals or work in symbiosis with the plant and provide minerals in exchange for other nutrients

Question 6

Mineral uptake from roots: Cation exchange

Answer 6

Organic materials in soil tend to dissociate affecting the soil pH

Soil particles are left with a negative charge

This affects what attracts to them and how well they bind to the soil particles

Cations (+ charged) are attracted to the soil particles so they are not washed out easily by rainfall and this high cation exchange capacity is useful to retain them for plants to obtain

Anions (- charge) do not bind to soil well so they wash deeper into the soil or if applied as fertiliser can be washed off the surface into nearby water sources as ‘chemical run off’

Question 7

Nutrient bioavailability in soil

Answer 7

as rocks weather they break down and release ions into solution

Acids help to release them however if the soil is too acidic phosphorous begins to precipitate, bound to iron or aluminium

If pH is too high phosphorous can bind to calcium

These phosphorous compounds cannot be absorbed by roots – this is a common problem in rainforest soils

Question 8

Root systems: monocots and dicots

Answer 8

Monocots tend to have a complex fibrous root system and are able to penetrate soil more effectively when it is irrigated

Dicots tend to have a main taproot with offshoots and this makes them more efficient in low water conditions

Minerals are absorbed in different areas of the root

Question 9

Root tip structure

Answer 9

(see diagram in notes)
Root tip has a protective cap and a mucigal sheath to lubricate growth path through soil
Endodermis (casparian strip) in elongation zone separates inner and outer tissue
Phloem develops first to fuel growth and xylem develops after
At the maturation zone root hairs begin to develop to anchor the plant and take up minerals

Question 10

Water is drawn in for cell expansion by minerals

Answer 10

Water is necessary to stretch the cell wall to drive expansion of the cell, first minerals are pumped into the cell, this generates osmotic pressure and water is drawn in by osmosis

Question 11

The area around the root may become depleted of nutrients

Answer 11

this region is referred to as the depletion zone. The root hairs need to be close to phosphates to draw them in whereas nitrates are more mobile and can be drawn in from further afield

Question 12

The role of root cap exudates

Answer 12

Root cap exudates have a role in:
protection, lubrication (mucilage) and increasing availability
of minerals (exozymes)
(From Ridge, 2000)

Exudates improve availability of iron and aluminium phosphates by:
*reducing pH (increases solubility)
*releasing phosphates from clay by anion exchange
*releasing phosphates from Al and Fe phosphates (by binding to Al3+ and Fe2+)

Acid phosphatases also release phosphates from organic material.

Question 13

Examples of beneficial traits that could be incorporated from crop wild relatives to improve agricultural practices and food security.

Answer 13

see diagram in notes

*Increased release of volatile organic compounds (VOCs) and non-volatile secondary metabolites (e.g. phenolics, alkaloids) to attract predators of root pests or directly inhibit herbivores/weeds, increasing pest resistance/reducing pesticide use.
*Increased exudation of organic acids and more beneficial associations with microbes (e.g. mycorrhizae, rhizobia) to increase soil nutrient availability [especially (N) and (P)], reducing dependence on fertilisers. (Preece & Peñuelas, 2020)

Question 14

Mycorrhizal association:

Answer 14

Mycorrhizal association:
“Symbiotic association, essential for one or both partners, between fungus and root of a living plant, primarily responsible for nutrient transfer. Occur in specialised plant organs where intimate contact results from synchronised plant-fungus development.” (Brundrett, 2004)

• 90% land plants have these associations
• Plants provide upto 30% of nutrients formed for minerals from the fungi

see: Entangled Life – Merlin Sheldrake

A plant can interact with several fungi and a fungi to many plants providing interaction pathways

Mycorrhizal associations provide:
- Ability to regenerate (e.g. in orchids)
- Vigour in vegetative state - most species grow much faster with mycorrhizas than without
- Increased SA and penetration for roots
- Help with nutrient foraging
*acidification (to obtain P)
*exoenzymes (for organic N)
- Resistance to pathogens

Glomalin – protective coating on mycelium which remains after mycelial death acting as a ‘glue’ holding soil structure together

fungal hyphae fungi are very thin making them able to move between soil particles and secretes organic acids to release minerals – between 1/3 and ½ of soil tends to be fungal matter

Question 15

Mycorrhizal association: endo/ectomycorrhizal

Answer 15

endomycorrhizal fungus grows inside the cell wall but stays outside the membrane

Ectomycorrhizal fungus stays on the outside of the cells forming a thick coat

Question 16

Ectomycorrhizal: The wood wide web (WWW)
(Simard et al 1997)

Answer 16

Using the fungal network, plants can:
* share information with neighbouring plants about the need for defences
*Share nutrients
*Prevent the growth of neighbouring plants by spreading toxins

http://www.bbc.co.uk/earth/story/20141111-plants-have-a-hidden-internet

Question 17

Endomycorrhizal association

Answer 17

Funghi is inside root cell walls but not inside cell membranes, root mass is no more than 10% funghi which form arbuscles in the root tissue where nutrients are transferred between the plant and fungal hyphae

Question 18

Ways to reduce mycorrhizal damage:

Answer 18

No till agriculture, cover plants, avoiding excessive use of fertiliser (causing plants not to form associations)

Question 19

Once inside the plant water has an apoplastic or symplastic path

Answer 19

apoplastic - through cell walls
sympoplastic - through cytoplasm

see ‘xylem loading’ diagram

Question 20

sympoplastic path: crossing the cell membrane

Answer 20

two main options to cross a membrane
– through a channel by diffusion (if conc. Gradient is right) may be gated
-through a channel protein against a concentration gradient

Carrier molecules also used but this is a much slower process

Question 21

root biomass depends on nutrient availability

Answer 21

when soil is nutrient rich the plant allocates less carbohydrates to growing the roots (as it does not need to search further afield for nutrients)

Question 22

Relationship between leaf nitrogen and photosynthesis

Answer 22

Nitrogen is a limiting factor as it is necessary to form proteins including RuBisCo