Chapter 2

• Light Intensity


  • Light is vital for the production of ATP and NADPH
  • However, when light intensity is extreme, CO2 may be limited, causing the rate of photosynthesis to decline
• Photosynthesis accounts for more than 90% of the dry matter yield of plants and is the ultimate source of food and fossil fuel
• Water Availability

  • Plants grow under conditions of moisture stress, transpiration, and enzymatic activities associated with photosynthesis in the plants slow down
  • Stomata close under moisture stress, reducing carbon dioxide availability and consequently decreasing the photosynthetic rate
• Growth and Development

  • The photosynthetic rate is lower in a young expanding leaf than in a fully expanded one
  • As plant leaves begin senescence, the photosynthetic rate in mature leaves declines and eventually ceases in certain species
• Carbon Dioxide Concentration

  • Rapid photosynthesis can deplete cells of carbon dioxide
  • An increase in CO2 concentration benefits C3 plants since they have a high CO2 compensation point
  • C4 plants have a lower CO2 compensation point than C3 plants because they are more efficient in trapping CO2
• Temperature


  • Photosynthetic rate is decreased in cold temperatures because the fixation stage is temperature-sensitive
  • C3 plants grow poorly at high temperatures. Whereas C4 plants that are weeds, such as crabgrass, thrive
• Light-independent Reactions

  1. In the dark reaction of photosynthesis, CO2 is reduced to carbohydrates via CO2 Fixation

  2. Carbon Dioxide Fixation - cyclical series of reactions in which carbon dioxide is reduced to carbohydrate

  3. CO2 Fixation occurs in two major pathways
• Respiration


  • Respiration may occur in an environment that is oxygen-rich or oxygen-deficient
  • Respiration occurs in all living cells and is generally called cellular respiration
• Anaerobic Respiration


  • Complete oxidation of pyruvic acid to CO2 and water
  • If oxygen is absent, pyruvic acid is not the end product of glycolysis but ethyl alcohol and CO2 in most plant cells
  • The anaerobic respiratory pathway is very inefficient. In aerobic respiration, the initial energy of 686 kilocalories (kcal) per mole of glucose yields 263 kcal (39 percent) at the end of the process, which is conserved in 36 ATP molecules. In fermentation, only 2 ATP m
• Photosynthesis

  1. A reaction occurs in the chloroplasts, using chlorophyll as an enzyme

  2. CO2 comes from the air and water from the soil

  3. The process by which plants convert light to chemical energy
• Phases of Photosynthesis

  1. Light Dependent Reactions

  2. Light-independent reactions
• Photoperiod


  The duration of day length (photoperiod) affects photosynthesis in a directly proportional way
• Light-independent reactions

  1. The Calvin cycle- is a three-carbon compound also called the C3 pathway

  2. The four-carbon pathway is also called the C4 pathway

  3. The Crassulacean acid metabolism (CAM)
• Aerobic Respiration


  • Reverse of photosynthesis by using oxygen from the air to metabolize organic molecules into CO2 and water to release stored energy in the form of ATP
  • The primary purpose of respiration is this energy production function
  • Respiration consists of three distinct stages: glycolysis, the Krebs cycle, and the electron transport chain
• Photoperiodism


  When a plant’s mechanisms sense the alternations in the day/night length through the photoreceptor proteins and decide when to induce flowering
• Environmental Factors Affecting Photosynthesis
• Relative Growth Rate (RGR) is the growth of the given system per unit time
• Meristem cells can divide and self-perpetuate, growth is always added to the plant body
• Phases of Growth: Meristematic phase, Elongation phase, Maturation phase
• At the cellular level, growth occurs due to an increase in the amount of protoplasm
• Growth Rate (GR) might be arithmetic or geometrical
• Types of Transpiration

  • Stomatal transpiration
  • Cuticular transpiration
  • Lenticular transpiration
• Plant Differentiation is the process whereby cells, tissues, and organs become different from each other
• Translocation


  Movement of materials from leaves to other tissues throughout the plant
• Stomatal transpiration


  • Specialized pores in the leaves, accounting for around 80 to 90% of the total water loss from plants
• Lenticular transpiration

  • The evaporation of water through lenticels, tiny openings present in the woody bark
• All plant cells are descendants of the zygote (fertilized eggs)
• Absolute Growth Rate (AGR) is the measurement and comparison of total growth per unit time
• Growth can be defined as an irreversible, permanent increase in the size of an organ or its parts or even of an individual cell
• Plant growth is generally indeterminate
• Partitioning

  1. The process by which plants divide their energy among their leaves, stems, roots, and reproductive parts

  2. Assimilate partitioning assimilate supply is dependent on photosynthesis

  3. The distribution of assimilate determines the amounts and patterns of plant growth and yield

  4. Dependent on the developmental state of the plant
• Anaerobic respiratory pathway

  1. Very inefficient compared to aerobic respiration

  2. In aerobic respiration, the initial energy of 686 kilocalories (kcal) per mole of glucose yields 263 kcal (39 percent) at the end of the process, which is conserved in 36 ATP molecules

  3. In fermentation, only 2 ATP molecules are produced, representing about 2 percent of the available energy in a molecule of glucose
• Plant growth continues throughout life due to the presence of meristems
• Growth is accompanied by metabolic processes influenced by water, oxygen, nutrients, temperature, light, and gravity
• Primary Growth occurs due to root apical meristem and shoot apical meristem, causing plant elongation
• Transpiration

  1. The process by which plants release water inside in the form of moisture or water vapor

  2. The loss of water from plant surfaces by evaporation and diffusion
• Development includes all changes that an organism goes through during its life
• Cuticular transpiration


  • An impermeable covering present on the leaves and stem, causing around 20% of transpiration in plants
  • Lesser in xerophytes due to thicker cuticles
• Secondary growth occurs in a lateral meristem, vascular cambium, and cork cambium
• Factors affecting Transpiration

  1. Environmental factors like temperature, relative humidity, wind speed etc.

  2. Plant factors like the number and distribution of stomata, percentage of open stomata, water status of the plant, the structure of the canopy of the tree