Objective:

     Study the anatomical features of plants that are involved in photosynthesis.

     Tissue which is composed of parenchyma cells containing abundant chloroplasts is called chlorenchyma.  Chlorenchyma is typically located in leaves and primary stems and is the primary light harvesting tissue of plants.  Analysis of photosynthesis reveals that tissues performing this function must:  a) Intercept adequate light energy to drive the light reaction, b) have an adequate supply of water for donation of electrons to the Z scheme, c) have an adequate supply of CO2 to complete the dark reaction, d) export fixed carbon to the phloem for translocation to carbon sinks within the plant.  In addition to these fundamental requirements for photosynthesis, various plants have evolved different photosynthetic mechanisms which make inhabitation of arid environments possible.

I.  C3 Photosynthesis

    A.  Compare transverse sections of Populus (5.18), Triticum or Poa (9.02) and Pinus (3.09,3.095) leaves.  Pay particular attention to:  a) The geometry of the  outermost cell walls of epidermal cells.  Could  these act as lenses to focus light to the interior of the leaf?  b) The relative location of xylem and chlorenchyma tissue.  Keep in mind that suberized,  lignified and cutinized cell walls are relatively impervious to water.  c) The relative location of stomata and chlorenchyma.  What role does intercellular air space play here?  Are the chloroplasts optimally  located for CO2 uptake?  H2O uptake?  light inception?   How would your answers change if chlorenchyma cells had parietal vacuoles and centralized cytoplasm?  d) Relative location of phloem and chlorenchyma.  Visualize the general pathway that fixed carbon must take  in these various plant species.  Are there intervening  non-photosynthetic cells in this pathway?  Apply terms such as spongy and palisade mesophyll, transfusion    tissue, endodermis where appropriate.  Record your observations in the lab exercise sheet.

    B.  Many plant species exhibit the ability to regulate the characteristics of their photosynthetic systems in response to the intensity of light to which their developing leaves are exposed.  Compare transverse sections of sun and shade leaves of Lactuca (9.04),  Acer (9.05) and Quercus (9.06).  What general  inferences can be made about the effects of light  intensity on the differentiation of the photosynthetic system?Record your observations in the lab exercise sheet.

II.  C4 Photosynthesis

     Many species of plants have evolved C4 photosynthesis in which the oxygen sensitive ribulose bisphosphate carboxylase (RUBPCase) is spatially separated from oxygen insensitive phosphoenolpyruvate carboxylase (PEPCase).  C4 plants typically, but not absolutely, exhibit Kranz leaf anatomy in which leaf veins are surrounded by bundle sheath cells, characterized by the presence of thick cell walls, numerous chloroplasts which may be agranal, accumulated starch grains, in which RUPBCase and other enzymes are localized.  The bundle sheath cells are in turn surrounded by mesophyll cells which resemble chlorenchyma of C3 plants with numerous granal plastids in which high concentrations of PEPCase and other enzymes are found.  C4 photosynthesis appears to be more efficient than C3 photosynthesis under conditions of high temperatures and light intensity.

    A.  Study transverse sections of Zea mays (3.07) and  Bouteloua (9.08) leaves.  Identify the typical features of  Kranz leaf anatomy in these C4 species.  Record your observations in the lab exercise sheet.
 

III.  Crassulacean Acid Carbon Metabolism (CAM)

     Other species of plants have evolved photosynthetic systems in which there is a temporal separation of oxygen sensitive RUBPCase and oxygen insensitive PEPCase enzyme activity during carbon fixation.  In CAM plants, stomata are closed and the internal pH of the plant cells decreases during the day.  During the night stomata are open and the internal pH increases.  Most CAM plants are associated with a succulent habit, but not all succulent plants are CAM plants.  CAM plants appear well adapted to arid environments and many halophytes, epiphytes and desert plants exhibit this type of carbon metabolism.

    A.  Using the cellulose acetate, acetone method quickly make epidermal impressions from Crassula argenta leaves that have been covered with aluminum foil for three days and leaves that have been exposed to normal  sunlight.  Can you detect any difference in the condition of the stomata of the two samples?  Are these differences what you would expect for a CAM plant?Record your observations in the lab exercise sheet.

    B.  Make free hand transverse sections of Crassula argenta leaves from plants that have been covered with aluminum foil for three days and from plants that have been exposed to normal sunlight.  Quickly express cytoplasm from each sample onto a piece of pH indicator paper.  Can you detect any pH difference between the two leaves?  Is the difference what you would expect for a CAM plant?  Record your observations in the lab exercise sheet.

Material

Prepared Slides                           Fresh

   Populus (5.18)                             Crassula argenta
   Triticum or Poa (9.02)                     (some stems covered
   Pinus (3.09,3.095)                          with aluminum foil
                                               for 3 days)
   Lactuca (9.04)
   Acer (9.05)
   Quercus (9.06)
   Zea (3.07)
   Bouteloua (9.08)