Chapter 36: Plant Nutrition

What are essential nutrients in plants?

Elements that a plant cannot complete its life cycle without and cannot synthesize on its own

• Must be obtained from the environment

• Most often in the form of ions dissolved in soil water

What are macronutrients required for plant growth?

Elements required in relatively large amounts for plant growth

• *Nitrogen, phosphorus, potassium, calcium, magnesium, and sulfur

• N, P, and K are often limiting factors

  • Scarcity in the environment significantly reduces growth/reproduction of plants

• Other essential nutrients: carbon, oxygen, and hydrogen that are obtain by absorbing CO2 from the atmosphere

  • Makes up approx. 96% of dry mass of the plant (vast majority of the plant’s mass)

What are micronutrients required for plant growth?

Elements required in small amounts

• *Iron, manganese, zinc, copper, boron, molybdenum, and nickel

• Often function as enzyme cofactors

• Needed in small quantities, but still essential for normal plant development.

Compare mobile and immobile nutrients and the effect of their deficiencies.

• Mobile: N, K, P, and Mg can be readily transferred from older to younger tissues when in short supply

  • When these nutrients are scarce, older leaves deteriorate first

• Immobile nutrients: Ca and Fe remain tied up in older tissues

  • When scarce, younger leaves show deficiency first

How does soil composition affect nutrient availability?

Texture (gravel, sand, silt clay), organic matter, and pH influence how nutrients are stored and released to plant roots

• Clay-rich soils retain nutrients better

  • Water tends to adhere to clay and silt, but runs through sand and gravel

• Sandy soils are prone to nutrient loss through leaching

Why is soil conservation important?

Prevents erosion caused by wind and water, which can remove nutrient-rich topsoil and reduce agricultural productivity. Techniques such as windbreaks and contour farming help maintain soil quality.

How does soil pH affect nutrient availability?

Influences nutrient solubility and cation exchange

• Acidic soils: promote release of cations

• Alkaline soils: limit the availability of some nutrients

• Most plants grow best in near-neutral pH conditions.

What is the role of ionic charge and soil texture?

• Anions: usually dissolve in soil water → readily available for absorption (but prone to leaching)

  • They interact with water molecules via hydrogen bonding

  • Exception: phosphate ions (can form insoluble compounds)

• Cations: dissolve in soil water but interact with negative charges on organic matter and clay particles

What is cation exchange and why is it important?

Occurs when protons (H+) released by roots displace positively charged ions that are bound to soil particles, making them available for uptake. This process is essential for accessing nutrients like calcium and magnesium.

How do roots influence soil pH to absorb cations?

• Roots release CO₂ that forms carbonic acid → releases H⁺ ions → lowers soil pH → promotes cation exchange

• These protons displace cations from soil particles, making them available for absorption.

How do plants absorb nutrients at the root level?

• Absorption of water/ions occurs primarily in the root hair zone

  • Increased surface area allows efficient uptake

• Root cells contain transport proteins that facilitate movement of nutrients into the plant.

How do proton pumps drive nutrient uptake?

• Root cells use proton pumps to export H⁺ ions, creating an electrochemical gradient with a negative interior that attracts cations into the cell

  • Cations like K+ enter through channels

• This gradient also powers cotransport of anions with H⁺ back into the cell

  • Anions like NO3- move through cotransporters, allowing them to be taken up against their [ ] gradients

How do plants prevent toxins from entering the shoot system?

• Use selective membrane transporters to limit entry of unwanted ions

• Rely on the Casparian strip (impermeable material) in the endodermis to block passive flow into the xylem/force ions to pass through cell membranes

How do plants use passive exclusion to limit toxins?

• Occurs at the plasma membrane when plant cells lack transport proteins for certain ions, preventing them from entering the root symplast

  • Prevents these ions from reaching the xylem

• Reduces uptake of harmful substances without energy expenditure

How do plants use active mechanisms to remove toxins?

• Use proteins such as metallothioneins and phytochelatins (short peptide) to bind toxic metal ions and neutralize them

  • Production of these proteins requires energy

How do antiporters help detoxify plant cells?

• Transport proteins located in the tonoplast (membrane surrounding the vacuole) actively remove toxic substances from cytosol → store them in the vacuole

  • Uses proton pumps, creating a gradient that favors movement of protons back out of the vacuole

• Antiporters use that proton gradients to move toxic ions (i.e. Na⁺) into vacuoles against their gradient

  • Prevents toxic accumulation in the cytosol

How do mycorrhizal fungi enhance nutrient absorption?

• Mycorrhizae = ‘fungus-root’

• Form symbiotic relationships with plant roots and extend hyphae into the soil → increasing surface area for nutrient absorption

• Fungi can acquire nutrients from macromolecules in the soil that are unavailable to non-mycorrhizal plants

  • Help plants access nutrients like N and P in exchange for sugars.

How do plants obtain nitrogen?

• Absorb nitrogen mainly as nitrate (NO₃⁻) or ammonium (NH₄⁺)

  • Often produced by nitrogen-fixing bacteria

• Necessary because atmospheric N₂ is chemically inert and unusable by plants.

What is nitrogen fixation?

• Conversion of atmospheric nitrogen gas (N₂) into ammonia (NH₃) or related compounds by specialized bacteria

• Requires series of specialized enzymes and cofactors, specifically nitrogenase

• Extremely energy demanding process

How do legumes regulate nodule formation?

Legumes regulate nodule growth based on nitrogen availability, investing more in nodules only when nitrogen is scarce. This helps balance the energy cost of maintaining symbiotic bacteria.

What is the function of leghemoglobin in root nodules?

• Nodules: where nitrogen-fixing rhizobia are found

  • Pink in color b/c they contain iron-containing leghemoglobin

• In root nodules: oxygen binds to leghemoglobin instead of binding to nitrogenase

  • Leghemoglobin helps maintain low free oxygen levels needed so that nitrogenase can properly function while still allowing respiration

Step-by-step: how do nitrogen-fixing bacteria infect roots?

• Legume roots release flavonoids that attract rhizobia

• Rhizobia contact flavonoids → produce Nod factors that trigger root hair deformation and infection thread formation

• Bacteria enter the root cortex → multiply → form nodules where nitrogen fixation occurs

How do parasitic plants obtain nutrients?

• Use structures called haustoria to penetrate host vascular tissue and extract water and nutrients

• Some are partially photosynthetic, while others rely entirely on their host

How do epiphytes obtain nutrients?

• Grow on other plants but are not parasitic

  • Adapted to grow without soil

• Obtaining nutrients from rainwater, dust, and organic debris

• Some have specialized leaves that can absorb nutrients directly

How do carnivorous plants obtain nutrients?

• Trap and digest insects using modified leaves with sensory hairs

• Absorb nutrients such as nitrogen from their prey

• Allows for survival in nutrient-poor soils