BIOL 4120

Principles of Ecology

Phil Ganter

320 Harned Hall

963-5782

The creature above has 8 legs (note that is has lost one) but is not a spider or a mite. What is it?

Lecture 6 Plant Adaptations to Land

Email me

Back to:

Overview - Link to Course Objectives

Plants live on land

Molecular and structural data (see the Tree of Life website for a reasonably up-to-date summary of relationships among living organisms) suggests that the ancestors of today's land plants (= Embryophyta) were most closely related to modern day green algae in a group called the Charales (named for Chara, a genus of common algae - click here,  here, or here [and go to the bottom of the page] to see pictures of Chara species).  If Chara is the closest living relative to land plants, then the ancestor of land plants had few of the structures we associate with land plants today.  The invasion of land was a huge change in environment involving the solution of unique problems (some of which were briefly discussed in Lecture 05).  Fundamental processes had to change.  Reproduction could no longer employ swimming gametes.  Plants had to allocate their productivity to build structures not necessary when surrounded by a dense medium like water.  Photosynthesis first appeared in aquatic organisms.  As plants invaded the land, many changes occurred in the process to adapt it to an environment without unlimited water resources. 

NOTE - the textbook uses the term adaptation loosely.  Evolutionary biologists do not normally call a short term response by an individual organisms an adaptation.  The book says things like "rhododendron plants adapt to dry days by curling their leaves."  Evolutionary biologists would use the term respond, not adapt.  For them, adapt means change at the population level due to natural selection.  However, the textbook author wishes to use a term that indicates a change made by an organism to reduce stress and perhaps found respond to neutral for its purpose.  In any case, remember that individual organisms do not evolve and, so, do not really adapt.

Photosynthesis Review

Recall from your previous courses that:

  • Rubisco (Ribulose Biphosphate Carboxylase) is the initial enzyme in the Calvin Cycle
    • it takes CO2 and Ribulose biphosphate (abbr. as RuBP, a 5 carbon sugar with a phosphate on either end) and releases 3-PGA (phosphoglycerate)
    • Rubisco is also an oxygenase - when O2 concentration is high compared to CO2, rubisco binds to RuBP and O2, which begins the process of photorespiration (more on this later)
  • Standard photosynthetic pathway is called C3 Photosynthetic Pathway
    • A 3-carbon molecule (3-PGA) is the first molecule produced from CO2

Light and photosynthesis

  • As PAR (photosynthetically active radiation) intensity (energy per unit surface area per second) increases, the net rate of CO2 uptake also increases
    • Net CO2 uptake is a measure of the difference between CO production through respiration and CO incorporation by photosynthesis
  • not a linear increase but an asymptotic curve (like Michaelis-Menten curve of enzymatic activity)
  • two important points on the curve:
    • Light Compensation Point - light intensity at which fixation of CO2 by photosynthesis equals release of CO2 due to respiration
    • Light Saturation Point - light intensity at which photosynthetic rate of plant is maximal
  • curve may turn down at very high light intensity (Photoinhibition)

Plant structure

  • leaves have upper and lower epidermis which is covered with a waxy cuticle that prevents gas exchange (H20, CO2, O2)
  • Stomata (sing. stoma) - openings in epidermis that allow gas exchange and can be closed by changes in turgor pressure in the guard cells
  • Mesophyll is the tissue between the epidermal layers
  • Aquatic plants have lost their waxy cuticle and gases can diffuse between the mesophyll and the surrounding water
  • Transpiration - loss of water vapor through the stomata that initiates the motion of water from the root to the leaf

Rate of both photosynthesis and respiration increase as temperature increases

  • rates are measured in the same units (micromoles of CO2 fixed or released per unit leaf area per second)
  • At first, respiration increases more slowly than photosynthesis
  • At high temperature, respiration rate exceeds photosynthesis
  • Subtracting respiration rate from photosynthetic rate produces a Net Photosynthetic Rate and gives rise to three temperatures for a plant
    • Tmin - coldest temperature at which CO2 fixation rate is above 0 (below this temperature, photosynthesis grinds to a halt)
    • Topt - temperature at which net CO2 fixation rate is maximal
    • Tmax - warmest temperature at which net CO2 fixation rate is above 0 (fixation exceeds respiration)
  • Net photosynthate is the amount available for growth and reproduction (new wood versus more flowers and fruits)

Adaptation to Light Intensity

Plants shade one another and plants have adapted differently to direct sunlight versus indirect sunlight (shade)

  • Photosynthesis in shade (compared to photosynthesis in direct sunlight)
    • lower light compensation point, light saturation point, and Tmax
    • lower concentration of Rubisco (expensive to make it and the other enzymes in the Calvin cycle)
    • in shade, light limits photosynthesis, not the amount of enzyme available to catalyze the process
    • increased concentration of chlorophyll
  • cooler temperatures in shade lead to reduced rate of respiration (and this to the lowering of the light compensation point)
  • leaves are broader and thinner (this can sometimes be seen on a single tree)
  • Some plants are Shade Tolerant and some Shade Intolerant
    • shade tolerant plants show less reduction in photosynthetic rate when moved from sunlight to shade than do shade intolerant plants

Temperature, Moisture, and Plants

Temperature increases water stress because it lowers the relative humidity of the air

  • relative humidity of spaces in the leaf is 100%
  • rate of water loss depends on difference between inside of leaf and outside air

Short term changes in a plant to water stress

  • closing stomata during hottest (= driest) time of day
  • changing turgor pressure in part of leaf to curl leaves
    • there are more stomata on bottom of leaf than upper surface
    • curling always occurs so that the underside of the leaf is on the inside of the cylinder formed
    • the inner space acts as a boundary layer and slows the rate of water loss
  • leaves wilt and collapse onto stems (has same effect as curling)

Changes due to prolonged stress

  • inhibit chlorophyll production, which yellows leaves and reduces the rate of photorespiration when stomata are closed for most of the day for many days
  • shedding of leaves to reduce photorespiration

Adaptations to moisture stress - found in plants that regularly undergo water shortage

  • Drought Deciduous plants are adapted to drop their leaves at the onset of the dry season
  • morphology of the plant
    • sinking stomata into pits
    • covering photosynthetic areas with hairs
  • Alterations in the Photosynthetic pathway
    • C4 Pathway -
      • In a dry habitat, stomata are closed for much of the day
        • Light causes light reactions to proceed and O2 is generated in the leaf, which can interfere with rubisco function
      • Calvin cycle only in the Bundle Sheath Cells
      • CO2 fixed in other mesophyll cells as a 4-carbon acid and translocated to Bundle Sheath Cells, where it is released as CO2
      • this keeps the CO2 concentration high in Bundle Sheath Cells which means rubisco acts as a carboxylase (photosynthesis) and not as a oxidase (photorespiration)
        • CO2 concentration can be so high in the bundle sheath cells (higher than in air outside of leaf) that rubisco is more efficient than in C3 plants, giving C4 plants a higher maximum rate of photosynthesis
      • costs of C4 are the extra enzymes needed and the energy needed to make and break down the 4-carbon acids from PEP (phosphoenolpyruvate)
      • C4 pathway not found in older plant lineages (algae, ferns, mosses, conifers [gymnosperms], or primitive angiosperms
        • Common in grasses (Poaceae), dry land shrubs (Larrea, the creosote bush, common in the western US)
        • sometimes found in stressful habitats with high light (Spartina, the salt marsh grass, uses C4)
        • Utility of C4 pathway as an adaptation can be seen in the increasing percent of C4 grasses native to the US as you proceed from north to south (from cool, wet to hot, dry)
    • CAM Pathway -
      • Some desert plants (Cactus and others) use Crassulacean Acid Metabolism
      • CO2 is fixed as a 4-carbon acid and then released as CO2 as in C4 metabolism but both happen in the mesophyll
      • Stomata open at night (maximal relative humidity) and close all day
      • CO2 released during day to keep concentration high and allow photosynthesis and repress photorespiration

Plants adapted to Xeric (dry) habitats often have a greater portion of their biomass allocated to belowground structures than do plants adapted to Mesic (moist) environments

Adaptations to Nutrient Availability

Macronutrients -

  • C, H, O - water and atmosphere
  • N (as NO3-, NO2-, and NH4+), P (as PO4---), K+, Ca++, Mg++, S (as SO4--) - from soil moisture or water (aquatic plants)

Micronutrients -

  • many are essential in low concentrations and toxic in high concentrations
  • uptake curves usually level off at a maximum rate of uptake and, at the point of maximal uptake the nutrient is at saturation
  • response of plants often linked to availability of nutrient in the habitat such that maximal uptake rate is near typical soil concentrations
    • Iron - found in electron transport chain and as cofactor
    • Manganese - Light reactions (water splitting) and cofactor in fatty-acid synthesis
    • Boron - cofactor in cell division, pollen germination, and lots of other basic metabolic processes
    • Copper - cofactor in photosynthesis
    • Zinc - cofactor in some enzymes and part of important transcription factors
    • Selenium - whether this is an essential nutrient for all plants is controversial, but some plants (Selenium Accumulators) actually have Se concentrations in their tissues that is toxic to animals and so it is an anti-herbivory strategy

Terms

Embryophyta, Rubisco (Ribulose Biphosphate Carboxylase), Calvin Cycle, oxygenase, C3 Photosynthetic Pathway,  Net CO2 uptake, Light Compensation Point, Light Saturation Point, Photoinhibition, Stomata, Mesophyll, Transpiration, Net Photosynthetic Rate, Tmin, Topt,Tmax, Shade Tolerant, Shade Intolerant, Turgor Pressure, Drought Deciduous, C4 Pathway, Bundle Sheath Cells, Larrea, Spartina, CAM Pathway - Crassulacean Acid Metabolism, Xeric, Mesic, Macronutrients, Micronutrients, Iron, Manganese, Boron, Copper, Zinc, Selenium

Last updated January 23, 2007