Download 04-Leaf Structure Spring 2011

Biology 3460 – Week 4
1
Biology 3460 - Plant Physiology - Lab Exercise 4
Leaf Structure: Comparison of C3 & C4 plants
and Sun- & Shade-grown plants
Objectives:
This lab exercise is intended to:
(1) introduce students to differences in leaf structure among C3 and C4 plants,
including aspects of Kranz anatomy that are important for the CO2-concentrating
mechanism of C4 photosynthesis,
(2) illustrate differences in leaf structure of plants developed under contrasting light
intensities that are apparent at the top of a tree canopy and in the understory of a
forest.
Introduction
Plants with the C4 photosynthetic pathway have a modified internal leaf structure called
Kranz anatomy, a term that was coined by a German plant physiologist named
Haeberlandt (Kranz is a German word for wreath). In C4 plants the bundle sheath cells
(which surround vascular bundles of xylem and phloem transport tissue) have thickened
cell walls and they contain large chloroplasts that often have prominent starch granules
and un-stacked thylakoid membranes. These bundle sheath cells look like a wreath
surrounding the vascular bundles. By contrast, the mesophyll cells, which make up the
bulk of the internal leaf tissue outside of the bundle sheath cells, have smaller
chloroplasts with stacked thylakoid membranes and little or no starch granules. These
structural differences between bundle sheath and mesophyll cells in C4 plants are
associated with compartmentalization of specific enzymes associated with the CO2concentrating mechanism of C4 photosynthesis. For example, PEP carboxylase, which is
localized in the mesophyll chloroplasts, catalyzes the original uptake reaction involving
atmospheric CO2 as it diffuses into the leaf through the stomata. By contrast, the
RUBISCO carboxylase enzyme is located only in the enlarged bundle sheath cell
chloroplasts. RUBISCO operates under elevated CO2 concentration in the bundle sheath
cells and so its carboxylase efficiency is increased and virtually no oxygenase activity of
RUBISCO occurs under the elevated CO2 concentrations in the bundle sheath cells.
Pictures of cross-sections of leaf tissue in C3 and two types of C4 plants (dicots and
monocots) are shown in Fig 8.11 a-c from your textbook (attached).
A second objective of this exercise is to demonstrate that plants of the same species have
different leaf structure depending on the environment in which they were grown. Plants
generally have a strong ability to acclimate to contrasting environmental conditions. In
this exercise you will compare leaves of the same plant species that have developed while
they were exposed to different light intensities; (i) full sunlight at the top of a canopy, and
(ii) low light in the shaded understory of a forest canopy. An example of differences in
leaf thickness between sun- and shade-grown leaves is shown in Fig 9.1 of your textbook
(attached).
Biology 3460 – Week 4
2
Part A. C3 and C4 Plants Leaf Structure
Protocol:
Work as individual students to complete this part of the lab.
1. Obtain slides of Zea and Poa leaf cross-sections. Locate the vascular bundles and
analyze the cells immediately surrounding the vascular tissue (bundle sheath cells) in
comparison with the mesophyll cells. Make note of any differences you observe in the
chloroplasts. Identify which slide corresponds to each plant type, C3 or C4.
Biology 3460 – Week 4
3
Biology 3460 – Week 4
4
Part B. Sun- and Shade-Grown Plants Leaf Anatomy
Protocol:
Work as individual students to complete this part of the lab.
1. Obtain slides of cross-sections of leaves from the same plant species that have been
grown in sunny conditions and in shade conditions. Use an ocular micrometer to make
measurements that will help you articulate the main differences between the leaves
grown in each condition. Use the space below to sketch and make note of your
observations.