Ferttility

Theme2.2:

Nutrient Uptake

• Taiz et al. pp. 144– 165

Selected pages

Importantterminology

• Absorption (taking in mineral ions into the root cells)

• Transport (Molecular and ionic movement from one location to another, into or

within cells)

• Membrane proteins regulate the transport of solutes across cells

• Translocation (large-scale transport of molecules from one plant part to another)

• Transport of sucrose from leaves to roots

• Is regulated and driven by membrane transport into the phloem cells of the leaf

and from phloem to the storage organs

• Accumulation (concentration of mineral ions inside the cell sap)

• Solute

Movement of nutrients to the roots

• Contact Exchange? (Root Interception)

• Mass Flow

• Diffusion

• Carbonic acid exchange theory

Movement of nutrients to the roots

Contact Exchange (Root Interception)

• Exchange of ions through physical contact between the root and mineral surfaces

• When the oscillation volumes of two ions (H+ and K+) overlap, ion exchange occurs.

Movement of nutrients to the roots

Carbonic acid Exchange (theory)

• Carbonic acid dissociation releases H+ which are exchanged with mineral ions on the

exchange sites

Movement of nutrients to the roots

Mass Flow

• Occurs when nutrient ions in soil solution are

transported to the root as a result of water flow to

the root surface, which depends on water uptake

(transpiration) by the plant, water evaporation at the

soil surface, and percolation of water in the soil

profile.

• Reduces with a decrease in soil water content

• Reduces at low temperatures as transpiration is

reduced

Movement of nutrients to the roots

Diffusion

• Occurs when nutrient ions move from an area of higher concentration

to one of lower concentration

• Nutrient uptake by roots lowers the concentrations of nutrients at the

root surface, generating concentration gradients in the soil solution

surrounding the root

• A nutrient depletion zone is created around the root

• Root growth allows the plants to explore fresh soils which are more

fertile

• Associations of roots with mycorrhiza allow plants to take up more of

the immobile nutrients such as phosphorus through diffusion

Movement of nutrients to the roots

Diffusion

Movement of nutrients to the roots

Havlin JL pg 39

Factors influencing Diffusion

𝑑𝐶

𝑑𝑡 = 𝐷𝑒.𝐴.

𝒅𝑪

𝒅𝒕

= rate of diffusion (change in conc. With time)

𝑑𝐶

𝑑𝑋

𝒅𝑪

𝒅𝑿 = concentration gradient (change in conc. with distance)

𝑫𝒆 = effective diffusion coefficient in the soil (determines the fraction of nutrients in soil that

reaches the roots)

𝐴 = Cross-section area through which the ions diffuse

Deductions

• The higher

�

�𝐶

𝑑𝑋

the higher the diffusion rate

• increasing the cross-sectional area for diffusion increases

�

�𝐶

𝑑𝑡

, which means diffusion should

be greater in a clay compared to a sand because of greater water-filled pore space.

• 𝐷𝑒 is directly proportion to rate of diffusion

Factors influencing Diffusion

𝐷𝑒 = 𝐷𝑤.θ. 1

𝑇

. 1

𝑏

• 𝐷𝑤 = diffusion coefficient of the nutrient in water, which increases with temperature

• Θ = volumetric soil water content – as soil moisture content increases, 𝐷𝑒 increases

which increases diffusion rate

• As moisture content decreases, moisture films around soil particles become thinner

and ion diffusion through these films becomes more tortuous (increase in diffusion

path length).

• 𝑇 = tortuosity factor (diffusion path length) – increase in T, lowers 𝐷𝑒, which reduces

diffusion rate

• Nutrients diffusing in coarse-textured soils experience a more tortuous path to the

root surface (less water-filled pore space) Vs. clay soils with less tortuosity.

• 𝑏 = soil buffer capacity – reduces nutrients in soil solutions, thus, reduced 𝐷𝑒, and

low diffusion rate (here sand soils win over clay)

Factors influencing diffusion

• What soil factors influence diffusion of nutrient ions to roots? Describe and