🌱Plant Physiology Unit 2 – Plant Water & Mineral Nutrition

Key Concepts and Definitions

• Water potential ($\Psi$) represents the free energy of water per unit volume relative to pure water at a reference state
  • Components include solute potential ($\Psi_s$), pressure potential ($\Psi_p$), gravity potential ($\Psi_g$), and matrix potential ($\Psi_m$)
• Osmosis movement of water across a selectively permeable membrane from a region of high water potential to a region of low water potential
• Turgor pressure force exerted by the contents of a cell against the cell wall, maintaining cell rigidity and shape
• Plasmolysis shrinkage of the cytoplasm away from the cell wall due to water loss in a hypertonic environment
• Essential nutrients elements required for a plant to complete its life cycle and reproduce
  • Macronutrients (nitrogen, phosphorus, potassium) required in larger quantities
  • Micronutrients (iron, zinc, manganese) required in smaller quantities
• Cation exchange capacity (CEC) ability of soil particles to attract and hold positively charged ions (cations)
• Rhizosphere narrow region of soil directly influenced by root secretions and associated soil microorganisms

Water Movement in Plants

• Water moves through plants via a combination of osmosis, hydrostatic pressure, and cohesion-tension forces
• Transpiration loss of water vapor from plant leaves through stomata drives water uptake from roots
  • Stomata pores on leaf surfaces regulate gas exchange and water loss
  • Guard cells surrounding stomata control their opening and closing in response to environmental factors (light, humidity, CO2)
• Xylem tissue conducts water and dissolved minerals from roots to shoots
  • Tracheids and vessel elements are the main conducting cells in xylem
• Cohesion-tension theory explains the upward movement of water in plants
  • Cohesion attractive forces between water molecules
  • Adhesion attraction of water molecules to the walls of xylem vessels
  • Tension pulling force created by transpiration at leaf surfaces
• Cavitation formation of air bubbles in xylem vessels can disrupt water transport and lead to embolisms
• Guttation exudation of water droplets from leaf margins due to root pressure in conditions of high soil moisture and low transpiration
• Apoplastic pathway movement of water through cell walls and intercellular spaces
• Symplastic pathway movement of water through the cytoplasm of cells via plasmodesmata

Mineral Uptake and Transport

• Minerals are absorbed by roots from the soil solution as ions
• Active transport movement of ions across membranes against a concentration gradient, requiring energy (ATP)
  • Proton pumps (H+-ATPases) in the plasma membrane create an electrochemical gradient that drives the uptake of other ions
• Passive transport movement of ions across membranes down a concentration gradient, without requiring energy
  • Ion channels proteins that facilitate the diffusion of specific ions across membranes
• Carriers proteins that bind and transport specific ions across membranes
  • Uniporters transport a single type of ion
  • Symporters co-transport two types of ions in the same direction
  • Antiporters exchange one type of ion for another across a membrane
• Phloem tissue conducts photosynthates (sugars) and other organic compounds from source (leaves) to sink (roots, fruits) tissues
  • Companion cells and sieve elements are the main conducting cells in phloem
• Translocation mass flow of phloem sap driven by a pressure gradient generated by active loading and unloading of sugars

Essential Nutrients and Their Roles

• Carbon (C), hydrogen (H), and oxygen (O) obtained from air and water, used for the synthesis of carbohydrates and other organic compounds
• Nitrogen (N) component of amino acids, proteins, nucleic acids, and chlorophyll
  • Nitrate (NO3-) and ammonium (NH4+) are the main forms of N taken up by plants
• Phosphorus (P) component of ATP, nucleic acids, and phospholipids, involved in energy transfer reactions
• Potassium (K) activates enzymes, regulates stomatal opening and closing, maintains turgor pressure
• Calcium (Ca) component of cell walls, involved in cell signaling and enzyme activation
• Magnesium (Mg) central atom in chlorophyll, activates enzymes in photosynthesis and respiration
• Sulfur (S) component of some amino acids (cysteine, methionine) and coenzymes
• Iron (Fe) component of cytochromes and ferredoxin, involved in electron transport during photosynthesis and respiration
• Manganese (Mn) involved in the oxygen-evolving complex of photosystem II during photosynthesis
• Zinc (Zn) activates enzymes and is involved in the synthesis of auxins (growth hormones)
• Boron (B) involved in cell wall formation, nucleic acid synthesis, and membrane function
• Copper (Cu) component of plastocyanin in photosynthesis and some enzymes
• Molybdenum (Mo) component of nitrate reductase and nitrogenase enzymes, involved in nitrogen metabolism
• Chlorine (Cl) involved in the oxygen-evolving complex of photosystem II and maintains turgor pressure
• Nickel (Ni) component of urease enzyme, involved in nitrogen metabolism

Root Structure and Function

• Root system anchors the plant, absorbs water and minerals, and stores carbohydrates
• Root cap protects the root tip as it grows through the soil
• Apical meristem region of active cell division at the root tip, responsible for primary growth
• Zone of elongation region where cells elongate and differentiate, leading to root growth
• Zone of maturation region where cells differentiate into specialized tissues (epidermis, cortex, endodermis, vascular cylinder)
• Root hairs extensions of epidermal cells that increase the surface area for water and mineral absorption
• Casparian strip band of suberin in the cell walls of the endodermis, regulates the apoplastic flow of water and solutes into the vascular cylinder
• Lateral roots branch roots that emerge from the pericycle of the parent root, increasing the volume of soil explored
• Mycorrhizae symbiotic associations between plant roots and fungi, enhance nutrient and water uptake
  • Ectomycorrhizae fungal hyphae form a sheath around the root and a network between cortical cells
  • Endomycorrhizae (arbuscular mycorrhizae) fungal hyphae penetrate cortical cells and form arbuscules for nutrient exchange

Soil-Plant Interactions

• Soil texture relative proportions of sand, silt, and clay particles in a soil
  • Sandy soils have large particles, high porosity, and low water and nutrient retention
  • Clay soils have small particles, low porosity, and high water and nutrient retention
• Soil structure arrangement of soil particles into aggregates, influences water and air movement, root growth
• Soil pH affects the availability of mineral nutrients and the activity of soil microorganisms
  • Acidic soils (pH < 7) have high availability of Fe, Mn, Zn, Cu, and Al, but low availability of N, P, K, Ca, and Mg
  • Alkaline soils (pH > 7) have low availability of Fe, Mn, Zn, and Cu, but high availability of N, P, K, Ca, and Mg
• Cation exchange capacity (CEC) ability of soil particles to hold and exchange cations (K+, Ca2+, Mg2+)
  • Soils with high CEC (clay, organic matter) have a greater capacity to retain nutrients
• Soil organic matter (humus) improves soil structure, water retention, and nutrient availability
• Nitrogen cycle transformations of N in the soil, involving N fixation, mineralization, nitrification, and denitrification
  • Legumes (soybeans, alfalfa) form symbiotic associations with N-fixing bacteria (rhizobia) in root nodules
• Nutrient leaching loss of soluble nutrients (nitrates) from the soil due to excessive rainfall or irrigation

Adaptations to Water and Nutrient Stress

• Drought tolerance ability of plants to maintain growth and function under water-limited conditions
  • Xerophytes (cacti, succulents) have thick cuticles, reduced leaf surface area, and water storage tissues
  • Sclerophylls (evergreen oaks) have hard, leathery leaves with thick cuticles and low stomatal density
• Drought avoidance strategies that minimize water loss or maximize water uptake
  • Leaf rolling reduces the exposed leaf surface area and decreases transpiration
  • Deep root systems access water from lower soil layers
  • C4 and CAM photosynthesis pathways minimize water loss by concentrating CO2 in bundle sheath cells or temporal separation of CO2 fixation
• Osmotic adjustment accumulation of solutes (proline, glycine betaine) to maintain turgor pressure and cell function under water stress
• Nutrient use efficiency (NUE) ability of plants to produce biomass or yield per unit of nutrient absorbed
  • Nutrient uptake efficiency (NUpE) ability to acquire nutrients from the soil
  • Nutrient utilization efficiency (NUtE) ability to use absorbed nutrients for growth and development
• Nutrient foraging strategies that enhance nutrient acquisition
  • Root system architecture (RSA) spatial arrangement of roots in the soil, influences nutrient exploration
  • Cluster roots (proteoid roots) dense clusters of rootlets that secrete organic acids to mobilize nutrients (P, Fe) in low-fertility soils
  • Exudation of enzymes (phosphatases) and organic acids (citrate, malate) to release bound nutrients from soil particles

Practical Applications and Research

• Irrigation management strategies to optimize water use efficiency and minimize nutrient leaching
  • Drip irrigation delivers water directly to the root zone, reducing evaporation and runoff
  • Deficit irrigation applies water below the crop's evapotranspiration rate to conserve water and improve fruit quality
• Fertigation application of fertilizers through the irrigation system, allows precise nutrient management
• Precision agriculture use of GPS, remote sensing, and variable rate technology to optimize inputs (water, nutrients) based on spatial variability within a field
• Hydroponics growing plants in nutrient solutions without soil, allows complete control over nutrient supply
• Aeroponics growing plants with roots suspended in air and misted with a nutrient solution, improves aeration and reduces disease risk
• Genetic engineering development of transgenic crops with enhanced water and nutrient use efficiency
  • Overexpression of aquaporins (water channel proteins) to increase root hydraulic conductivity
  • Introduction of genes for nutrient transporters or enzymes involved in nutrient assimilation
• Marker-assisted selection (MAS) use of DNA markers linked to genes of interest (drought tolerance, NUE) to accelerate breeding programs
• Rhizosphere engineering manipulating the root-soil interface to enhance nutrient availability and uptake
  • Inoculation with beneficial microorganisms (rhizobia, mycorrhizal fungi, plant growth-promoting rhizobacteria)
  • Application of soil amendments (biochar, compost) to improve soil structure and fertility
• Nutrient recovery from waste streams (animal manure, sewage sludge, food waste) to close nutrient loops and reduce reliance on synthetic fertilizers