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!

Overview

• 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

Photosynthesis

• 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

Nutrients

• 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

Soils

• 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