Plant Physiology: Photosynthesis

• Plant physiology is the study of plant function and behavior
• It covers all aspects of a plant's life, from seed germination to reproduction
• It includes study of the physiological processes of plants
• It helps understand how plants function, grow, and adapt to their environment
• It encompasses various levels of organization, from the molecular to the whole plant

Photosynthesis

• Photosynthesis is the process by which plants convert light energy into chemical energy
• It uses carbon dioxide and water to produce glucose and oxygen
• Chlorophyll is the primary pigment involved in capturing light energy
• The process takes place in chloroplasts, which contain thylakoid membranes
• Two main stages of photosynthesis are the light-dependent reactions and the light-independent reactions (Calvin cycle)
• Light-dependent reactions convert light energy into chemical energy in the form of ATP and NADPH
• Water is oxidized, releasing oxygen
• Light-independent reactions (Calvin cycle) use ATP and NADPH to fix carbon dioxide into glucose
• Rubisco is the enzyme crucial for carbon fixation in the Calvin cycle
• C4 and CAM photosynthesis are adaptations to hot and arid environments, improving water use efficiency
• Photorespiration is a process that reduces the efficiency of photosynthesis by consuming energy and releasing carbon dioxide

Water Transport

• Water is essential for plant survival, playing roles in photosynthesis, nutrient transport, and maintaining turgor pressure
• Water is absorbed from the soil by root hairs
• Water moves through the plant via the xylem
• The transpiration-cohesion-tension mechanism drives water movement in plants
• Transpiration is the evaporation of water from leaves, creating tension
• Cohesion is the attraction between water molecules due to hydrogen bonds
• Tension pulls water up the xylem from the roots to the leaves
• Water potential is the measure of the relative tendency of water to move from one area to another
• It is affected by solute concentration, pressure, gravity, and matric potential
• Stomata regulate transpiration and gas exchange
• Abscisic acid (ABA) is a hormone that induces stomatal closure in response to water stress

Nutrient Uptake and Transport

• Plants require essential nutrients for growth and development
• Macronutrients such as nitrogen, phosphorus, and potassium are needed in large quantities
• Micronutrients such as iron, manganese, and zinc are needed in small quantities
• Nutrients are absorbed from the soil by roots
• Mycorrhizae, which are symbiotic associations between fungi and plant roots, enhance nutrient uptake
• Nutrients are transported throughout the plant via the xylem and phloem
• Nitrogen fixation is the conversion of atmospheric nitrogen into ammonia, a usable form for plants
• Specialized bacteria, such as Rhizobium, carry out nitrogen fixation in root nodules of legumes

Plant Hormones

• Plant hormones are chemical messengers that regulate plant growth and development
• Auxins promote cell elongation, apical dominance, and root formation
• Cytokinins promote cell division and delay senescence
• Gibberellins promote stem elongation, seed germination, and flowering
• Abscisic acid (ABA) promotes dormancy and stomatal closure
• Ethylene promotes fruit ripening and senescence
• Brassinosteroids promote cell elongation and differentiation
• Jasmonates are involved in plant defense against herbivores and pathogens
• Salicylic acid is involved in plant defense against pathogens

Plant Growth and Development

• Plant growth is influenced by both internal factors (hormones, genetics) and external factors (light, temperature, water, nutrients)
• Seed germination is the process by which a seed emerges from dormancy and begins to grow
• Light, temperature, and water are important environmental factors for seed germination
• Apical dominance is the inhibition of lateral bud growth by the apical bud
• Phototropism is the growth of a plant towards a light source
• Gravitropism is the growth of a plant in response to gravity
• Thigmotropism is the growth of a plant in response to touch
• Photoperiodism is the response of a plant to the relative lengths of day and night
• Flowering is often triggered by changes in photoperiod
• Vernalization is the requirement of a period of cold temperature for flowering in some plants

Plant Responses to Stress

• Plants face various environmental stresses, including drought, salinity, heat, cold, and pathogen attack
• Plants have evolved various mechanisms to cope with stress
• Drought tolerance mechanisms include stomatal closure, reduced leaf area, and deep root systems
• Salinity tolerance mechanisms include ion exclusion and compatible solute accumulation
• Heat tolerance mechanisms include heat shock proteins and transpirational cooling
• Cold tolerance mechanisms include antifreeze proteins and membrane lipid modifications
• Plants defend themselves against pathogens through various mechanisms, including physical barriers and chemical defenses
• The hypersensitive response (HR) is a localized cell death response to pathogen infection
• Systemic acquired resistance (SAR) is a whole-plant defense response to pathogen infection