🌱Plant Physiology Unit 12 – Plant Physiology in Agriculture & Ecology

Key Concepts and Terminology

• Plant physiology studies the functions, processes, and mechanisms within plants
• Encompasses various aspects of plant life, including growth, development, metabolism, and responses to environmental stimuli
• Photosynthesis converts light energy into chemical energy (glucose) using carbon dioxide and water
• Transpiration movement of water through plants from roots to leaves and evaporation from leaf surfaces
• Stomata tiny pores on leaf surfaces that regulate gas exchange and water loss
• Xylem specialized tissue for water and mineral transport from roots to leaves
• Phloem specialized tissue for transporting sugars and nutrients throughout the plant
• Hormones chemical messengers that regulate plant growth, development, and responses (auxins, gibberellins, cytokinins)

Plant Structure and Function

• Roots anchor plants, absorb water and nutrients, and store carbohydrates
  • Taproot systems have a main central root (carrots)
  • Fibrous root systems have many thin, branching roots (grasses)
• Stems provide support, transport water and nutrients, and bear leaves and reproductive structures
  • Vascular bundles contain xylem and phloem for transport
  • Meristems regions of active cell division and growth (apical, lateral)
• Leaves primary sites of photosynthesis and gas exchange
  • Mesophyll cells contain chloroplasts for photosynthesis
  • Cuticle waxy layer that prevents water loss
• Flowers reproductive structures that produce seeds and fruits
  • Sepals, petals, stamens (male), and carpels (female) are the main parts
• Fruits develop from ovaries and protect and disperse seeds
  • Can be fleshy (berries) or dry (nuts)

Photosynthesis and Energy Production

• Light-dependent reactions occur in thylakoid membranes of chloroplasts
  • Photosystems absorb light energy and generate ATP and NADPH
  • Electron transport chain transfers electrons and pumps protons across the membrane
• Calvin cycle (light-independent reactions) occurs in the stroma of chloroplasts
  • Uses ATP and NADPH to fix carbon dioxide into glucose
  • RuBisCO enzyme catalyzes the first major step of carbon fixation
• C3, C4, and CAM photosynthetic pathways adapt to different environmental conditions
  • C3 plants (most plants) directly fix carbon dioxide using RuBisCO
  • C4 plants (corn, sugarcane) have a two-step carbon fixation process adapted to hot, dry conditions
  • CAM plants (cacti, succulents) fix carbon dioxide at night to reduce water loss
• Photorespiration competes with carbon fixation and reduces photosynthetic efficiency in C3 plants under high temperatures and low carbon dioxide levels

Water Relations and Nutrient Uptake

• Water potential drives the movement of water through plants
  • Osmosis movement of water across a semipermeable membrane from high to low water potential
  • Transpiration pull cohesion-tension mechanism that moves water from roots to leaves
• Mineral nutrients essential for plant growth and development
  • Macronutrients (nitrogen, phosphorus, potassium) required in large amounts
  • Micronutrients (iron, zinc, boron) required in small amounts
• Nutrient uptake occurs primarily in the root hairs and is influenced by soil pH and microbial interactions
  • Active transport moves nutrients against concentration gradients using energy (ATP)
  • Symbiotic relationships with mycorrhizal fungi and nitrogen-fixing bacteria enhance nutrient uptake
• Nutrient deficiencies cause specific symptoms and can limit plant growth and yield
  • Chlorosis yellowing of leaves due to lack of chlorophyll (iron, nitrogen deficiency)
  • Necrosis death of plant tissue (potassium deficiency)

Growth and Development

• Cell division, elongation, and differentiation drive plant growth
  • Mitosis produces new cells in meristems
  • Cell elongation increases cell size and contributes to organ growth
  • Cell differentiation specializes cells for specific functions (xylem, phloem)
• Plant hormones regulate various aspects of growth and development
  • Auxins promote cell elongation, apical dominance, and root formation
  • Gibberellins stimulate stem elongation, seed germination, and fruit development
  • Cytokinins promote cell division, delay senescence, and influence root-shoot balance
  • Abscisic acid regulates stomatal closure, seed dormancy, and stress responses
• Photoperiodism plant responses to day length that control flowering and other developmental processes
  • Short-day plants (poinsettias) flower when night length exceeds a critical threshold
  • Long-day plants (spinach) flower when night length falls below a critical threshold
  • Day-neutral plants (tomatoes) flower regardless of day length
• Vernalization cold treatment required by some plants to induce flowering (winter wheat)

Environmental Responses and Adaptations

• Tropisms growth responses to directional stimuli
  • Phototropism growth towards or away from light (shoots positive, roots negative)
  • Gravitropism growth in response to gravity (roots positive, shoots negative)
  • Thigmotropism growth in response to touch (tendrils, climbing plants)
• Nastic movements rapid, reversible movements in response to stimuli
  • Nyctinasty folding of leaves at night (prayer plant)
  • Seismonasty folding of leaves in response to touch (sensitive plant)
• Stress responses adaptations to adverse environmental conditions
  • Drought stress induces stomatal closure, root growth, and accumulation of compatible solutes (proline, sugars)
  • Salt stress triggers ion exclusion, compartmentalization, and osmotic adjustment
  • Temperature stress activates heat shock proteins and cold acclimation processes (antifreeze proteins)
• Phytoremediation use of plants to remove, degrade, or contain pollutants from soil or water
  • Hyperaccumulators plants that can accumulate high levels of heavy metals (Thlaspi caerulescens)

Agricultural Applications

• Crop improvement through breeding and genetic engineering
  • Selective breeding combines desirable traits from different varieties or species
  • Genetic engineering introduces specific genes for desired traits (pest resistance, drought tolerance)
• Precision agriculture uses technology to optimize resource use and improve crop yields
  • Remote sensing monitors crop health and guides management decisions
  • Variable rate application adjusts inputs (fertilizers, water) based on spatial variability
• Controlled environment agriculture regulates growth conditions to maximize productivity
  • Greenhouses and vertical farms provide optimal light, temperature, and nutrient control
  • Hydroponics grows plants in nutrient solutions without soil
• Sustainable agriculture practices promote long-term productivity and environmental stewardship
  • Crop rotation alternates crops to improve soil health and break pest cycles
  • Integrated pest management combines biological, cultural, and chemical control methods
  • Agroforestry integrates trees with crops or livestock for multiple benefits (shade, nitrogen fixation)

Ecological Significance

• Plants are primary producers that form the foundation of terrestrial ecosystems
  • Photosynthesis captures solar energy and converts it into biomass
  • Plant biomass supports food webs and influences nutrient cycling
• Plant-animal interactions shape ecosystem structure and function
  • Pollination mutualisms between plants and animals (bees, birds, bats) ensure reproductive success
  • Seed dispersal by animals (birds, mammals) facilitates plant colonization and gene flow
  • Herbivory consumption of plant material by animals (insects, ungulates) transfers energy and nutrients
• Plant communities and succession
  • Pioneer species colonize disturbed areas and initiate succession (grasses, lichens)
  • Intermediate stages increase diversity and complexity (shrubs, small trees)
  • Climax communities reach a stable state determined by climate and soil conditions (forests, grasslands)
• Ecosystem services provided by plants
  • Carbon sequestration removal of atmospheric carbon dioxide through photosynthesis and storage in biomass
  • Water regulation and purification through transpiration, infiltration, and filtration
  • Soil stabilization and erosion control through root systems and ground cover
  • Habitat provision for diverse species (forests, wetlands, grasslands)