Leaf Structure and Function Chapter 32 Leaf Structure and Function

Leaves typically consist of Broad flat blade Stalk-like petiole Some also have Small stipules (small, leaf-like outgrowths from the base)

Parts of a leaf

Leaves may be Simple (having a single blade) Compound (having a blade divided into two or more leaflets)

Simple, pinnately compound and palmately compound leaves

Leaf arrangement on a stem may be Alternate (one leaf at each node) Opposite (two leaves at each node) Whorled (three or more leaves at each node)

Leaf arrangement may be alternate, opposite, or whorled

Leaves may have Parallel venation Netted venation Pinnately netted (with several major veins radiating from one point Palmately netted (with veins branching along the entire length of the midvein

Venation patterns include parallel, pinnately netted, and palmately netted

Major tissues of the leaf Epidermis Photosynthetic ground tissue Xylem Phloem

Epidermis Covers upper and lower surfaces of the leaf blade Coated by a waxy cuticle enabling plant to survive a terrestrial existence

The thick, waxy cuticle and sunken stomata are two structural adaptations that enable Pinus to retain its needles throughout the winter

Epidermis, cont. Has stomata permitting gas exchange for photosynthesis; each surrounded by Two guard cells, often associated with subsidiary cells providing a reservoir of water and ions

Tissues in a typical leaf blade

Mesophyll consists of photosynthetic parenchyma cells Palisade mesophyll (functions primarily for photosynthesis Spongy mesophyll (functions primarily for gas exchange)

Leaf veins have Xylem (to conduct water and essential minerals to the leaf) Phloem (to conduct sugar produced by photosynthesis to the rest of the plant)

Monocot leaves All have parallel venation Some do not have mesophyll differentiated into distinct palisade and spongy layers Some have dumbbell-shaped guard cells, unlike more common bean-shaped guard cells

Cross section of a monocot leaf

Variation in guard cells Guard cells of dicots and many monocots are bean-shaped (b) Some monocot guard cells are dumbbell-shaped

Dicot leaves All have netted venation All have mesophyll differentiated into distinct palisade and spongy layers All have bean-shaped guard cells

Cross section of a dicot leaf

Photosynthesis and leaf structure Broad, flattened leaf blade is efficient collector of radiant energy Stomata open diurnally for gas exchange and close nocturnally to conserve water Transparent epidermis allows light into leaf for photosynthesis

Photosynthesis and leaf structure, cont. Air spaces in mesophyll tissue permit rapid diffusion of CO2 and water into mesophyll cells Oxygen out of mesophyll cells

With regard to the opening of stomata, blue light triggers Activation of ATP synthase in the guard cell plasma membrane Synthesis of malic acid Hydrolysis of starch

Mechanism of stomatal opening

Physiological changes accompanying stomatal opening and closing When malic acid ionizes, protons (H+) are produced Protons are pumped out of the guard cells by ATP synthase

Physiological changes, cont. As protons leave guard cells, an electrochemical gradient forms on the two sides of the guard cell plasma membrane Electrochemical gradient drives uptake of potassium ions through voltage-activated potassium channels into guard cells

Physiological changes, cont. Chloride ions are also taken into guard cells through ion channels These osmotically active ions increase the solute concentration in the guard cell vacuoles

Physiological changes, cont. Resulting osmotive movement of water into guard cells causes them to become turgid, forming a pore As the day progresses, potassium ions slowly leave guard cells

Temporary wilting

Physiological changes, cont. Starch is hydrolyzed to sucrose, which increases in concentration in the guard cells Stomata close when water leaves guard cells due to decline in concentration of sucrose (osmotically active solute)

Physiological changes, cont. Sucrose is converted to starch (osmotically inactive) Some environmental factors affecting stomatal opening and closing Light or darkness CO2 concentration Water stress Plant’s circadian rhythm

Rate of transpiration affected by environmental factors, such as Is loss of water vapor from aerial parts of plants Occurs primarily through stomata Rate of transpiration affected by environmental factors, such as Temperature Wind Relative humidity

Transpiration represents a trade-off for plants Beneficial because of CO2 requirement Harmful because of need to conserve water

Guttation and transpiration Guttation, the release of liquid water from leaves of some plants, occurs through special structures when Transpiration is negligible and Available soil moisture is high

Guttation

Guttation and transpiration, cont. Is the loss of water vapor Occurs primarily through the stomata

Leaf abscission Loss of leaves that often occurs With approach of winter (temperate climates) or At beginning of dry period (tropical climates with wet and dry seasons)

Leaf abscission, cont. Complex process involving changes occurring prior to leaf fall Physiological Anatomic Abscission zone develops where petiole detaches from stem

Leaf abscission, cont. From leaves to other plant parts, the following are transported Sugars Amino acids Many essential minerals Chlorophyll breaks down Carotenoids and anthocyanins become evident

Abscission zone

Examples of modified leaves Spines deter herbivores Tendrils grasp other structures (to support weak stems) Bud scales protect Delicate meristematic tissue Dormant buds

Leaves of Mammilaria are modified to form spines

Leaves of Echinocystis lobata are modified to form tendrils

A terminal bud and two axillary buds of an Acer twig have overlapping bud scales to protect buds

Examples of modified leaves, cont. Bulbs are short underground stems with fleshy leaves specialized for storage Succulent leaves serve for water storage Leaves of insectivorous plants trap insects

The leaves of bulbs such as Allium cepa are fleshy for storage of food materials and water

The succulent leaves of Senecio rowleyanus are spherical to minimize surface area, thereby conserving water