Crop Science

• Photosynthesis


  • Accounts for more than 90% of the dry matter yield of plants and is the ultimate source of food and fossil fuel
• The four-carbon pathway


  Also called the C4 pathway, where a CO2 molecule is bonded to phosphoenol pyruvate (PEP), resulting in the production of oxaloacetate
• Moisture stress

  Causes stomata to close, reducing carbon dioxide availability and decreasing the photosynthetic rate
• Photoperiodism


  When a plant’s mechanisms sense the alternations in the day/night length through the photoreceptor proteins and decide when to induce flowering
• Cold temperatures


  Decrease the photosynthetic rate because the fixation stage is temperature-sensitive. C3 plants grow poorly at high temperatures, while C4 plants like crabgrass thrive
• Environmental Factors Affecting Photosynthesis

  • Light intensity
  • Carbon dioxide concentration
  • Temperature
  • Water availability
  • Photoperiod
  • Growth and development
• Light Dependent Reactions


  • Only certain wavelengths of light are involved in photosynthesis. Also collectively called the dark reaction of photosynthesis
• The photosynthetic rate is lower in a young expanding leaf than in a fully expanded one. As plant leaves begin senescence
• Photoperiod


  Affects photosynthesis in a directly proportional manner
• The Crassulacean acid metabolism (CAM)


  A photosynthetic pathway that allows certain plants to fix CO2 in the dark by the activity of PEP carboxylase. Most CAM plants inhabit environments in which moisture stress and intense light prevail
• The Calvin cycle


  It is a three-carbon compound also called the C3 pathway. Carbon dioxide enters the cycle and becomes covalently bonded to a five-carbon sugar with two phosphate groups called ribulose 1,5-bisphosphate (RuBP)
• Rapid photosynthesis


  Can deplete cells of carbon dioxide. An increase in CO2 concentration benefits C3 plants, while C4 plants are more efficient in trapping CO2
• Carbon Dioxide Fixation


  A cyclical series of reactions in which carbon dioxide is reduced to carbohydrate. CO2 Fixation occurs in two major pathways
• Photosynthesis

  A reaction occurs in the chloroplasts, using chlorophyll as an enzyme. CO2 comes from the air and water from the soil. It is the process by which plants convert light to chemical energy
• Light intensity


  Is vital for the production of ATP and NADPH. However, extreme light intensity may limit CO2 availability, causing a decline in photosynthesis rate
• The primary purpose of respiration is energy production
• As plant leaves begin senescence, the photosynthetic rate in mature leaves declines and eventually ceases in certain species
• Respiration consists of three distinct stages

  Glycolysis, the Krebs cycle, and the electron transport chain
• Photoperiod


  The duration of day length affecting photosynthesis
• Transpiration

  The loss of water from plant surfaces by evaporation and diffusion
• Translocation is the process by which plants divide their assimilate supply among their leaves, stems, roots, and reproductive parts
• The anaerobic respiratory pathway is very inefficient compared to aerobic respiration
• The cuticle causes around 20% of transpiration in plants and is an impermeable covering present on the leaves and stem. Cuticular transpiration is lesser in xerophytes because they have thicker cuticles
• Transpiration is the process by which plants release water inside in the form of moisture or water vapor
• Factors affecting Transpiration

  • Environmental factors like temperature, relative humidity, wind speed, etc.
  • Plant factors like the number and distribution of stomata, percentage of open stomata, water status of the plant, and the structure of the canopy of the tree
• Partitioning energy among their leaves, stems, roots, and reproductive parts is a function of translocation
• All plant cells are descendants of the zygote (fertilized eggs) and develop into matured plants through growth and differentiation
• The duration of day length (photoperiod) affects photosynthesis in a directly proportional manner
• Respiration

  The 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 photosynthetic rate is lower in a young expanding leaf than in a fully expanded one
• Stomata open or close according to changes in turgor pressure in the guard cells
• 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 molecules are produced, representing about 2 percent of the available energy in a molecule of glucose
• Stomata account for around 80 to 90% of the total water loss from the plants
• Aerobic Respiration


  The 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
• Translocation is dependent on the developmental state of the plant
• Types of Transpiration

  • Cuticular transpiration
  • Stomatal transpiration
  • Lenticular transpiration
• Translocation


  The movement of materials from leaves to other tissues throughout the plant
• If oxygen is absent, pyruvic acid is not the end product of glycolysis but ethyl alcohol and CO2 in most plant cells
• Growth can be defined as an irreversible, permanent increase in the size of an organ or its parts or even of an individual cell
• Growth


  Always added to the plant body (an open form of growth)