Lecture 4: Plant Physiology Notes

These are my notes from lecture on Sept 21. They combine key points from the slides and Prof. Currie's comments. Hope they're helpful!


  • Energy of a landscape from all autotrophs equates to primary production
  • Wetlands are of the most production ecosystems in world because water is not limiting
  • Water and temperature are the most limiting resources to organisms, not just nutrients

Controls in primary production

  • Light and CO2 are necessary for photosynthesis
  • Temperature limits what type of vegetation can be in an area which affects primary production
  • Water is necessary for carrying dissolved nutrients through plants, and helping to cool the leaves during photosynthesis
  • Nutrients (including Nitrogen, and Phosphorous to power the enzymes involved in photosynthesis) limit plant production; plants often evolve ways to use them efficiently in their areas


  • Photosynthetically active region is part of the visible light, longer wavelength
  • Pigments interest different wavelengths of PAR
  • PAR only extends so far into water; this is why photosynthesis can't occur at deep ocean depths
  • Pigments that don't get absorbed get reflected

Light Reactions

  • Light energy is captured and splits water molecules to O2, H+, and e-
  • Energy, H+, e- are stored in short-term intermediate compounds
  • O2 is waste product
  • ATP and NADHP store energy and are cyclical from light —> dark —> light

Dark Reactions (Light-Independent Reactions)

  • Don't require light
  • CO2 substrate
  • long-term energy storage, CH2O (sugar)
  • Goes through process in the Calvin cycle

C3 Plants: most common, have 3 carbons in their energy storage

  • Mostly trees, shrubs and forbs
  • less water use efficiency and grow when water isn't limiting
  • Dark reaction/calvin cycle takes place in the same place and same time as photosynthesis
  • Ribisco enzyme bonds RuBP and CO2
  • Water loss is a major issue for C3 plants
  • Stomates are on the underside of leaves
  • Transpiration: water vapor out of leaf, carrying away latent heat
  • So the plant is able to lose extra heat, but in the process loses water it needs
  • Diffusion of water vapor from leaf to outside. CO2 has higher concentration on the outside of the leaf but this process is slower than the water diffusion
  • Photorespiration occurs in C3 as well: consumes O2 and produces CO2 (the 'undoing' of CO2 fixation)

C4 and CAM

  • They both start with C3 and build on it
  • It changes the transport of CO2 into the reaction
  • Reduces water loss and reduce photorespiration
  • C4: malic acid shuttles the CO2 fixation to the bundle-sheath cell (where the calvin cycle occurs)
    • more limiting water than C3, better water use efficiency, grasses use this
    • This is different from C3: if we higher temperatures or more/water, the competition will be altered toward C3 or C4
  • CAM: Get CO2 (through open stomates) at night and fixes the CO2 in malic acid and wait until the daytime for the calvin cycle
    • lower rates of photosynthesis, very high water use efficiency, most succulents

Aquatic Light

  • visible light is absorbed, function of sun angle
  • water has a low albedo of 10% or less
  • transmission and scattered
    • plants in photic zone can respire and photosynthesis
  • CO2 dissolves in water
    • causes an acid
    • dissociates into CO2 and HCO3-
    • C3 and C4 occurs in water

Factors controlling primary production

  • Plants respire CO2 just like animals do
  • Scientists can measure net CO2 fixation and O2 production for individual leaves
  • Gross photosynthesis - autotrophic respiration = NPP
  • In aquatics, measure O2 uptake
  • Leaves on the same plant have different compensation points based on canopy positions
  • Gross photosynthesis and respiration are sensitive to temperature


  • Nitrogen and phosphorus are 2 most commonly limiting nutrients
  • Nitrogen is important because enzymes are a key component in life processes
  • Phosphorus is a part of ATP, phospholipids, etc
  • Liebig's law of minimization: growth is controlled not by the total of resources available, but by the scarcest resource
  • Colimitation is when two nutrients equally limit growth
  • nutrient use efficiency: how much dry matter production as a function of assimilation
  • forests with lower productivity tend to exhibit higher nutrient-use efficiency


  • Texture: distribution of size particles
  • Clay (small particles, more surface area so water does adhere) -> Silt -> Sand (bigger particles, less surface area, more productive)
  • Plants draw water from soil
    • osmosis: gets it into roots
    • transpiration stream: pulled up the rest of the way
  • Terrestrial plants pull water up against soil tension and gravity; this is possible because of strong hydrogen bonds in water (fine soil holds more water so has more tension)(only able to move water a short distance, evapotranspiration needed thereafter)
  • Wilting point: water held too tightly by the soil
  • Field capacity: not enough water
  • Between the wilting point and the field capacity is the available water for a plant
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