Ecology: Chapter 6: Plant Adaptations to the Environment


Consider a local population of any organism living in a given environment.

  •  Possess heritable variations.
  • Fitness
  • Fitness is more than a numbers game.
  • Natural selection.


Darwin Awards


***** Natural selection selects for any heritable structural or behavioral characteristic that increases fitness.


***** As long as environmental conditions don't change, the organism is adapted to its environment.



Adaptation is any heritable trait:

  • behavioral,
  • morphological,
  • physiological,

that maintains or increases the fitness of an organism under a given set of environmental conditions.




The adaptation of an organism to its environment is exhibited by its ability to function between some upper and lower limits in a range of environmental conditions

Law of the Minimum
Law of Tolerance



The range of tolerance is not fixed.



The Law of Tolerance explains much about the geographical distribution of species.






This process, photosynthesis, is carried out by autotrophic organisms:

  • green plants
  • algae
  • photosynthetic bacteria



Photosynthesis in its simplest form:


Light & Dark Reactions

Photosynthesis can be separated into two processes:

  • Light reaction
  • Dark reaction



*ATP: Adenosine-Triphosphate, transports chemical energy within cells for metabolism.




Carbon Dioxide Uptake 





Stoma in a tomato leaf.










Stoma cross-section









Underside of a leaf. Stoma appear green due to
chlorophyll. Epidermal cells are red.






Loss of water through stomates.














The Light a Plant Receives Affects Its Photosynthetic Activity



The amount of PAR directly affects the rate of photosynthesis.

Light Compensation Point (LCP)


Net Photosynthesis = Photosynthesis - Respiration


Light Saturation Point






Movement always occurs from higher to lower potential.

  • Osmotic potential
  • Matric potential
  • Soil potential
  • Atmospheric potential



Atm Stomates Mesophyll Xylem Rootlets




Water-Use Efficiency: the ratio of carbon fixed (photosynthesis) per unit of water lost (transpiration).

This is the balancing act plants do in determining whether to open or close stomates.


The temperature of the leaf, not the air, controls the rate of photosynthesis and respiration.




Leaf shapes:

a) entire
b) lobed
c) simple compound
d) double compound


Smaller and more lobed leaves function to reduce the boundary layer and increase heat exchange from convection.





Carbon Gained in Photosynthesis is Allocated to the Production of Plant Tissues


Carbon Balance







Constraints Imposed by the Physical Environment Have Resulted in a Wide Array of Plant Adaptations






Species of Plants Are Adapted to Different Light Environments



Compare LCP & LSP for sun grown & shade grown. The same response is found for shade-tolerant vs. shade-intolerant plants.

What are moles and micromoles?

There are huge numbers of photons or “light particles” in visible light -- in fact, the quantity is so large that we cannot easily express it using normal numbers, so we use two measurements commonly used by scientists when measuring huge quantities. The first number, which is called a Mole, is equal to something called “Avogadro’s number”, which is 602,214,150,000,000,000,000,000! For a more manageable number, a micromole is a millionth of a mole. Much better, right?! In plain English, a micromole of photons (which would be one millionth of Avogadro’s number) is 602 quadrillion. Since these numbers are so large, it is easier to reference quantities of light in moles and micromoles.










Shade tolerant or shade grown plants exhibit variations.


                        Bottom Leaf                                                       Top Leaf

Red oak (Quercus rubra) leaves vary in size and shape from the top to the bottom of the tree.







Changes in carbon allocation to leaves for broadleaf peppermint (Eucalyptus dives) seedlings grown under different light environments in the greenhouse.







The Link Between Water Demand and Temperature Influences Plant Adaptations


Terrestrial plants have evolved a range of adaptations in response to variations:

  • precipitation
  • soil moisture




C4 Photosynthetic Pathway










Plants using C4:

  • tropical and subtropical grasses
  • some shrubs growing in arid and saline environments

Percentage of total grass species that use C4.







CAM (Crassulacean Acid Metabolism) Pathway


Used by a small group of desert plant families........



Plants may also respond to available soil moisture by diverting more resources root production.

  • more roots
  • less leaf area for EVT




xeric species (E. dives)               mesic species (E. saligna)







xeric species (E. dives)               mesic species (E. saligna)








Plants Vary in Their Response to Environmental Temperatures


Relationship between net photosynthesis and temperature for various terrestrial plant species from dissimilar thermal habitats:

  • Neuropogon acromelanus (Arctic lichen),

  • Ambrosia chamissonis (cool, coastal dune plant),

  • Atriplex hymenelytra (evergreen desert shrub), and

  • Tidestromia oblongifolia (summer-active desert perennial).





Effect of change in leaf temperature on the photosynthetic rates of C3 and C4 plants.





The maximum rate of photosynthesis for the C4 species occurs at higher temperatures than for the C3 species.

Relationship between temperature and net photosynthesis for cloned plants of big saltbush (Atriplex lentiformis) grown under two different day/night temperature regimes.