Plant Nutrition
Topic 10: Plant Nutrition
Chapter 36: 36.1-36.5
Chapter 37: 37.1-37.3
Learning Outcomes
Upon successful completion of this topic, students will be able to:
List the requirements for cellular respiration and photosynthesis within plant tissues.
Describe the three routes water can move within a plant tissue and the levels of control with each.
Explain the mechanisms of solute transport across plant cell membranes.
Explain how water potential is used to predict the movement of water by osmosis.
Describe how the endodermis regulates water flow into the vascular cylinder (stele).
Explain how root pressure contributes to xylem transport.
Explain how the cohesion-tension mechanism contributes to xylem transport.
Explain the pressure-flow hypothesis describing how phloem sap moves, identifying sources and sinks in plants.
Diagram and describe the mechanisms of nutrient/water uptake by root cells.
Illustrate how bacteria and fungi can benefit plants.
Identify adaptations that plants may have to acquire nutrients without photosynthesis.
36.1 Plant Evolution Resulted from Adaptations to Obtain Food
Land plants have two systems:
Above Ground Shoot System:
Absorbs light for photosynthesis.
Below Ground Root System:
Absorbs water and minerals.
Plant ancestors absorbed nutrients directly from their aquatic environment.
As plants adapted to terrestrial life, competition for resources led to characteristic changes:
Taller plants with broader leaves to maximize light absorption.
These adaptations come with costs:
Increased surface area (SA) for water loss.
Larger root systems required for anchorage and nutrient transport.
Evolution of Vascular Tissues: Enabled significant transport of nutrients:
Xylem: Transports water and nutrients from roots to shoots.
Phloem: Transports photosynthates from sources (like leaves) to sinks (like roots and fruits).
36.1 Nutrient Acquisition in Shoots
Shoot architecture varies greatly among land plants impacting nutrient acquisition:
Stem Length and Width:
Allows plants to grow tall.
Branching Pattern:
Optimizes light harvesting.
Limitations are set by finite energy for shoot growth.
Stems support leaves and serve as conduits for water and nutrient transport.
Leaf Diversity:
Varies in size and structure to adapt to specific habitats, maximizing photosynthesis while minimizing water loss.
Larger leaved plants mostly found in tropical areas, smaller one in temperate regions, and very small in harsh habitats.
Phyllotaxy: Arrangement of leaves is genetically controlled by the shoot apical meristem (SAM):
Types of Phyllotaxy:
Alternate/spiral: One leaf per node.
Opposite: Two leaves per node.
Whorled: Multiple leaves per node.
36.1 Nutrient Acquisition in Shoots (continued)
Total Leaf Area: Affects the productivity of the plant.
Leaf shading can reduce the photosynthetic capacity, leading to self-pruning to optimize leaf area index.
Leaf Orientation: Influences photosynthetic capacity based on light conditions:
In low light, horizontal leaves absorb more light.
In high light, vertical leaves reduce water loss and burning.
Compromise Between Photosynthesis and Water Loss:
Broad leaves absorb more light but have more stomata leading to water loss.
Roots adapt morphology and growth depending on nutrient availability:
Roots do not branch in areas of low nitrogen availability.
Increased branching occurs in high nutrient areas to absorb more nitrogen and engage mycorrhizal associations.
36.2 Different Mechanisms Transport Substances
Plant systems consist of two main compartments:
Apoplast: Everything external to the plasma membrane, including cell walls and internal spaces of dead tracheids and vessels.
Symplast: Composed of cytosol, plasmodesmata, and cytoplasmic connections.
Three Main Transport Routes:
Apoplastic Transport:
Water and solutes move along cell walls and extracellular spaces (analogous to water through a sponge).
Symplastic Transport:
Water and solutes move through cytosol.
Transmembrane Transport:
Involves water and solutes moving out of one cell, across cell walls, and into the next cell.
36.2 Different Mechanisms Transport Substances (continued)
Active and Passive Transport:
Occurs in plant cells, utilizing protons (H+) for membrane potentials established from proton pumps (primary active transport).
Key Differences:
Animals use sodium ions (Na+) for transport, while plants use hydrogen ions (H+).
Co-transportation:
H+ is co-transported with solutes (e.g., sugars in phloem cells) leading to secondary active transport.
Similar processes occur in animals with Na+.
Gated Ion Channels:
Common in plant cells, generating electrical signals similar to animal action potentials.
Guard Cells:
Use ATP-dependent proton pumps to regulate their opening/closing, controlling water loss through stomata.
Osmosis:
Water transfer occurs passively based on water potential; water flows from areas of high water potential to low water potential.
36.2 Different Mechanisms Transport Substances (continued)
Water Potential:
Defined as ext{Ψ} = ext{Ψs} + ext{Ψp}, where:
Solute Potential (Ψs): Proportional to molarity; pure water has a solute potential of 0 MPa. Higher solute concentrations yield negative values.
Pressure Potential (Ψp): The physical pressure on the solution; can be positive (typical in plant cells) or negative.
Turgor pressure arises from water uptake into protoplasts, pressing against cell walls.
Cell States:
Flaccid cells (water loss) have a pressure potential of 0; results in plasmolysis in hypertonic conditions.
Water moves into flaccid cells in hypotonic solutions, exerting turgor pressure.
36.3 Transpiration
Water and nutrient absorption primarily starts at root hairs, permeable to water and ions.
Transport Mechanisms:
Water moves through cell walls and extracellular spaces via apoplastic transport.
Active transport allows for the absorption of essential minerals (like potassium and nitrates) through root cortex to the endodermis surrounding the vascular stele.
Selective Transport Mechanism:
The Casparian strip forces water and minerals into the symplastic route, which then returns to the apoplastic route to continue to tracheids and vessels via bulk flow.
36.3 Transpiration (continued)
Xylem Function: As xylem sap moves up the stem via bulk flow into leaf veins:
Water loss through transpiration can create need for continuous water uptake from roots.
If water loss exceeds root absorption, plants will wilt.
Mechanisms of Xylem Movement:
Root pressure contributes to movement via active mineral pumping, lowering water potential and causing water flow from roots into the vascular stele.
Guttation: Excessive root pressure can cause water droplets to exude from leaves, distinct from dew.
36.3 Transpiration (continued)
Cohesion-Tension Hypothesis:
Suggests cohesion of water allows for negative pressure transmission from shoots to roots during transpiration.
Xylem sap is usually under negative pressure, facilitating movement to leaves.
Transpirational Pull:
Stomata facilitate gas exchange, leading to water vapor loss from leaves, developing tension on mesophyll cells.
Drier external air promotes loss of water from leaves.
36.3 Transpiration (continued)
Cohesion and Adhesion:
Cohesion: Attractive force among similar molecules (e.g., water due to hydrogen bonds).
Adhesion: Attractive force between different molecules (e.g., water to hydrophilic cell walls).
The upward pull creates tension and prevents tracheids and vessels from collapsing; disruptions (cavitation) can threaten flow, causing hydraulic failure in prolonged cases leading to plant death.
36.4 Transpiration Regulated by Stomata
Leaves are structured for high surface area-to-volume ratio, maximizing both gas exchange and water loss.
Guard Cells:
Flank stomatal pores and control diameter.
Water uptake into guard cells causes them to swell, bowing open the pore. Conversely, water loss leads to closure.
Potassium (K+) ion movements influence turgor changes in guard cells.
Mechanisms of Stomatal Regulation:
Guard cells use active transport to manage K+ concentrations, impacting water movement and hence stomatal function.
Circadian rhythms and plant hormones (like abscisic acid - ABA during water scarcity) modify stomatal activity.
36.4 Adaptations That Reduce Water Loss
Xerophytic Plants:
Adapted to dry conditions; some complete life cycles during rainy seasons, like cacti that bloom post-rain.
Adaptations include fleshy water-storing stems, reduced leaves, and CAM photosynthesis to conserve water.
Trichomes and modified stomata structures support humidity retention within microhabitats.
36.5 Sugars Travel from Source to Sinks
Sugar Loading:
Required for transport into sieve-tube elements for translocation.
Loading can occur symplastically (via plasmodesmata) or apoplastically, requiring active transport via proton pumps and sucrose cotransporters.
Unloading:
At sink cells via facilitated diffusion, where concentration of free sugars is lower than in sieve-tube elements.
Phloem Sap Flow:
Driven by positive pressure from water influx due to lowered water potential resulting from sugar loading, generating flow from source to sink.
36.5 Sugars Travel from Source to Sinks (continued)
Self-Thinning: Plants may abort excess buds/fruit when competition among sinks occurs.
Applications of this concept include agricultural practices for enhancing fruit size.
37.1 Plants Acquire Nutrients from Soil
Plants primarily absorb water and minerals from upper soil layers, rich in microorganisms.
Soil Texture:
Influenced by particle size:
Coarse sand (0.02-2 mm)
Silt (0.002-0.019 mm)
Clay (<0.002 mm)
Soil Composition & Horizons:
Topsoil (Horizon A) comprises organic matter (humus) and soil particles, where nutrients are dissolved in the soil solution.
37.1 Plants Acquire Nutrients from Soil (continued)
Topsoil Components:
Inorganic compounds (negatively charged, bind cations like K+, Mg2+, Ca2+) and organic compounds (humus).
Cation exchange plays a crucial role in nutrient availability, where H+ displaces other cations due to soil quality.
37.1 Humans Can Dramatically Change Soil Quality
Fertilization: Increases nutrient levels, revitalizing soil over years for agricultural yields.
However, can result in soil mismanagement and ecosystem nutrient overloads leading to issues like algal blooms.
Irrigation:
Essential in arid zones but excessive irrigation can lead to salinity issues negatively affecting plant water absorption.
Nutrient Management:
Most fertilizers contain N-P-K, required for plant nutrition, and must be given in appropriate forms (organic must be decomposed) for plant absorption.
37.1 Humans Can Dramatically Change Soil Quality (continued)
Soil pH Impact:
Affects nutrient availability for absorption (different nutrients optimally available at varying pH levels).
Erosion can remove topsoil; mitigative practices include contour farming and no-till agriculture.
Phytoremediation: Uses plants to extract contaminants from soil; studies show promise in using specific plant species to correct oversalinated and toxic areas.
37.2 Plants Need Essential Elements to Grow
Essential Elements: Required for plant life and reproduction.
Macronutrients:
C, O, H, N, P, S, K, Ca, Mg (My Good Socks Cannot Possess Holes)
Micronutrients:
Cl, Fe, Mn, B, Zn, Cu, Ni, Mb (sometimes Na) for specialized plants.
37.2 Plants Need Essential Elements to Grow (continued)
Deficiencies in essential nutrients manifest in specific ways affecting function and development.
Chlorosis: Mg deficiency leading to yellowing of leaves, but could also result from iron deficiency (iron as cofactor in chlorophyll synthesis).
Nutrient mobility affects deficiency indications:
Mobile nutrients display symptoms in older tissues first (like Mg).
Immobile nutrients (like Fe) show symptoms in younger tissues first.
37.3 Plants Nutrition Depends on Interactions
Rhizobacteria:
Live in association with plant roots, obtaining nutrients through root secretions.
Provide benefits such as antibiotics, toxin absorption, and nitrogen fixation by converting atmospheric nitrogen into usable forms.
Nitrogen Fixation:
N2 gas converted to NH3 through bacterial processes. The reaction is as follows:
ext{N}_2 + 8 ext{e}^- + 8 ext{H}^+ + 16 ext{ATP}
ightarrow 2 ext{NH}_3 + ext{H}_2 + 16 ext{ADP} + 16 ext{Pi}The enzyme nitrogenase drives this process and necessitates an anaerobic environment.
Ammonification: Organic nitrogen conversion to NH4+.
Nitrification: Conversion of ammonium (NH4+) to nitrate (NO3-).
Denitrification: Reduction of nitrates back to nitrogen gas.
37.3 Plants Nutrition Depends on Interactions (continued)
Nitrogen-fixing bacteria can be utilized in sustainable agriculture via crop rotations to replenish soil nitrogen.
37.3 Plants Nutrition Depends on Interactions (continued)
Symbiosis with Mycorrhizal Fungi: An important evolutionary adaptation for land plants.
Ectomycorrhizal Fungi: Form a sheath around plant roots without root hairs; penetrate the cortex.
Arbuscular Mycorrhizal Fungi (Endomycorrhizal): Embedded within root tissues forming arbuscules for enhanced nutrient absorption.
Yield improvements observed with mycorrhizal associations; relocating seeds can displace natural symbiosis leading to poor health.
37.3 Plants Nutrition Depends on Interactions (continued)
Nutrient Acquisition from Animals:
Decomposing organic matter significantly influences terrestrial ecosystems.
Carnivorous Plants:
Digest insects for required nutrients (microelements and nitrogen); examples include sundews and Venus flytraps.
37.3 Plants Nutrition Depends on Interactions (continued)
Acquiring Nutrients from Other Plants:
Epiphytes: Grow on other plants without parasitizing them; examples are staghorn ferns and orchids.
Parasitic Plants: Absorb resources from host plants; examples include dwarf mistletoe (water and minerals) and Monotropa uniflora (absorbs nutrients via mycorrhizae).