Agri 31 Lecture B PDF

Translocation Translocation is the movement of assimilates, primarily sucrose (the major transport form of carbon), from source to sink tissues through the phloem. Source tissues, like mature leaves, produce sucrose, while sink tissues, like roots, young leaves, and fruits...

Translocation Translocation is the movement of assimilates, primarily sucrose (the major transport form of carbon), from source to sink tissues through the phloem. Source tissues, like mature leaves, produce sucrose, while sink tissues, like roots, young leaves, and fruits, utilize or store it. The Münch pressure flow hypothesis explains translocation: ○ Sucrose is actively loaded into phloem sieve tubes at the source, creating a low water potential. ○ This draws water from adjacent xylem, creating pressure that forces the sucrose-rich sap towards the sink. ○ At the sink, sucrose is unloaded, and water moves back into the xylem. Allocation refers to how fixed carbon is used within a source or sink organ. Metabolic Pathway. Partitioning refers to the distribution of assimilates among competing sinks. Nutrient Minerals Plants need essential nutrients for growth, development, and reproduction. Essential nutrients are those that a plant cannot complete its life cycle without, and their function cannot be replaced by another element. Nutrients are categorized as: ○ Macronutrients (needed in larger amounts): carbon, hydrogen, oxygen, nitrogen, phosphorus, potassium, calcium, sulfur, and magnesium. ○ Micronutrients (needed in smaller amounts): iron, boron, copper, zinc, manganese, molybdenum, and chlorine. Nutrient deficiencies can limit plant growth and result in characteristic symptoms. Deficiency symptoms in older leaves indicate mobile nutrients (like nitrogen, phosphorus, potassium, magnesium), as they are readily translocated within the plant. Deficiency symptoms in younger leaves indicate immobile nutrients (like calcium, sulfur, iron), as they are not easily mobilized from older tissues. Plant Growth and Development Growth refers to the irreversible increase in cell number and dry mass. Differentiation is the process where genetically identical cells become specialized in structure and function. Organization involves the arrangement and integration of differentiated cells into tissues, organs, and ultimately, the whole plant. Morphogenesis encompasses growth, differentiation, and organization to determine the final form and structure of the plant. Plant growth occurs in specialized regions called meristems, containing undifferentiated cells capable of repeated division. Apical meristems at the tips of roots and shoots drive primary growth (lengthening). Lateral meristems (cambium) are responsible for secondary growth (thickening) in woody plants. Plant hormones (phytohormones) are chemical messengers that regulate growth and development at low concentrations. Major plant hormone groups include: ○ Auxins: promote cell elongation, apical dominance, root initiation, and fruit development. ○ Cytokinins: stimulate cell division, delay senescence, and promote lateral bud growth. ○ Gibberellins: regulate stem elongation, seed germination, and flowering in some plants. ○ Abscisic acid: inhibits growth, promotes dormancy, and mediates stress responses. ○ Ethylene: influences fruit ripening, senescence, and responses to stress. Plant growth regulators can be natural or synthetic compounds that mimic plant 1. Photosynthesis Definition: The process by which green plants, algae, and some bacteria convert light energy into chemical energy stored in glucose. Chemical Equation: 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂. Supporting Terms: ○ Autotrophs: Organisms that produce their own food. ○ Chloroplast: Organelle where photosynthesis occurs. ○ Thylakoid: Membrane structure within chloroplasts, site of light-dependent reactions. ○ Stroma: Fluid-filled space surrounding the thylakoids where the Calvin cycle occurs. ○ Chlorophyll: Pigment that absorbs sunlight, mainly found in the thylakoid membranes. ○ Photon: A particle of light that excites electrons in chlorophyll. ○ Photosystem: Clusters of pigments and proteins that capture light energy. ○ Calvin Cycle: Also called the light-independent reaction or dark reaction, occurs in the stroma, fixing carbon into glucose. Phases of Photosynthesis: ○ Light-Dependent Reactions: Occur in the thylakoid membranes. Water (H₂O) is split, releasing oxygen (O₂). ATP and NADPH are produced. Z-Scheme: Describes electron flow in light reactions. ○ Light-Independent Reactions (Calvin Cycle): Occurs in the stroma. Carbon dioxide (CO₂) is fixed into organic compounds like glucose. Rubisco (Ribulose-1,5-bisphosphate carboxylase/oxygenase): Enzyme responsible for CO₂ fixation. Photorespiration: ○ Occurs when Rubisco binds oxygen instead of CO₂.  ○ Wasteful Process: Reduces the efficiency of photosynthesis by producing CO₂ instead of sugars. ○ Favored by: High temperature, high oxygen concentration, low CO₂ concentration. C3 Plants: ○ Use the Calvin cycle directly for carbon fixation. ○ Examples: Rice, wheat, soybeans. ○ First product: 3-carbon compound (3-PGA). C4 Plants: ○ Fix CO₂ into a 4-carbon compound before entering the Calvin cycle. ○ Adapted to hot, arid environments. ○ Kranz Anatomy: Specialized leaf anatomy with bundle sheath cells. ○ Examples: Maize, sugarcane. CAM Plants: ○ Crassulacean Acid Metabolism: Temporal separation of carbon fixation. ○ Stomata open at night to reduce water loss. ○ Examples: Cacti, pineapples. 2. Respiration Definition: The process by which glucose is broken down in the presence of oxygen to produce energy (ATP). Chemical Equation: C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + ATP. Supporting Terms: ○ Mitochondria: Organelle where respiration occurs. ○ Aerobic Respiration: Respiration in the presence of oxygen. ○ Anaerobic Respiration: Occurs without oxygen, produces less ATP. ○ ATP (Adenosine Triphosphate): Main energy currency in cells.  Stages of Respiration: ○ Glycolysis: Occurs in the cytoplasm. Glucose (C₆H₁₂O₆) is broken down into pyruvate. Produces a net of 2 ATP and 2 NADH. ○ Krebs Cycle (Citric Acid Cycle): Occurs in the mitochondrial matrix. Produces CO₂, NADH, FADH₂, and 2 ATP. ○ Electron Transport Chain (ETC): Occurs in the inner mitochondrial membrane. Uses NADH and FADH₂ to produce a large amount of ATP (~30 ATP). Fermentation: ○ Anaerobic process producing lactic acid or ethanol and CO₂. ○ Occurs when oxygen is absent. Growth respiration (Rg): Energy used for converting the products of photosynthesis into plant material. Maintenance respiration (Rm): Energy used for cellular functionality, maintaining cellular structures, and adapting to changing environments. 3. Transpiration Definition: The process where water vapor is lost from plants to the atmosphere, mainly through the stomata. Supporting Terms: ○ Stomata: Pores on the leaf surface for gas exchange. ○ Xylem: water and nutrient transport ○ Guard Cells: Cells that regulate the opening and closing of stomata. ○ Cuticle: Waxy layer that minimizes water loss. ○ Lenticels: Pores in the stems of woody plants for gas exchange.  Types of Transpiration: ○ Stomatal Transpiration: 90% of water loss. ○ Cuticular Transpiration: Water loss through the cuticle. ○ Lenticular Transpiration: Water loss through lenticels. Factors Affecting Transpiration: ○ Relative Humidity (RH): Lower humidity increases transpiration. ○ Temperature: Higher temperature increases evaporation. ○ Wind: Increases transpiration by removing the humid air layer. ○ Vapor Pressure Deficit (VPD): Difference between the water vapor pressure inside and outside the leaf. 4. Translocation Definition: The movement of photosynthates (sugars and other organic compounds) / GLUCOSE from the leaves (source) to other parts of the plant (sink). Supporting Terms: ○ Phloem: Vascular tissue responsible for translocation. ○ Source: Area where sugars are produced (e.g., mature leaves). ○ Sink: Area where sugars are consumed or stored (e.g., roots, fruits). ○ Pressure Flow Hypothesis (Münch Hypothesis): Sugars move from a high-pressure source to a low-pressure sink. Symplastic Pathway: Movement through plasmodesmata. Apoplastic Pathway: Movement through cell walls and intercellular spaces. 5. Mineral Nutrition Essential Elements: ○ Macronutrients: Needed in large amounts (e.g., Nitrogen (N), Phosphorus (P), Potassium (K)). ○ Micronutrients: Needed in small amounts (e.g., Iron (Fe), Zinc (Zn)).  ○ Mobile Nutrients: Can move from older to younger tissues (e.g., Nitrogen). ○ Immobile Nutrients: Remain in older tissues (e.g., Calcium). Nutrient Deficiency Symptoms: ○ Chlorosis: Yellowing of leaves due to lack of chlorophyll. ○ Necrosis: Death of leaf tissue, often at tips or margins. ○ Anthocyanin Accumulation: Red/purple discoloration due to stress. 6. Growth and Development Growth: Irreversible increase in size, usually through cell division and elongation. Differentiation: Process where cells become specialized in function. DeDifferentiation: -the reversal of the cell specialization - Important in the repair of injury, where cells near damaged sites become totipotent and reprogram their development Meristems: ○ Apical Meristem: Located at the tips of roots and shoots, responsible for primary growth. ○ Lateral Meristem (Cambium): Responsible for secondary growth (thickening of stems). Types of Growth: ○ Indeterminate Growth: Continuous growth (e.g., beans). ○ Determinate Growth: Growth stops at a certain point (e.g., corn). 7. Tropisms (Plant Movements) Phototropism: Movement towards light. Gravitropism: Growth in response to gravity (roots grow down, shoots grow up). Thigmotropism: Response to touch, common in climbing plants.  Heliotropism: Sun tracking, such as sunflowers turning towards the sun. 8. Plant Hormones and Growth Regulators Auxins: ○ Promotes cell elongation, root formation, and apical dominance. ○ Example: Indoleacetic Acid (IAA). Gibberellins: ○ Promote stem elongation, seed germination, and fruit development. Cytokinins: ○ Promote cell division and delay leaf senescence. Abscisic Acid (ABA): ○ Promotes seed dormancy and stomatal closure. Ethylene: ○ Promotes fruit ripening and leaf senescence. 9. Flowering and Dormancy Photoperiodism: Plants respond to the length of day/night. ○ Short-Day Plants: Flower when days are shorter (e.g., poinsettia). ○ Long-Day Plants: Flower when days are longer (e.g., spinach). ○ Day-Neutral Plants: Flower regardless of day length. Dormancy: A period of reduced metabolic activity in seeds, allowing them to survive unfavorable conditions.