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Chapter 32

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)

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Chapter 32

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  1. Chapter 32 Leaf Structure and Function

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

  3. Parts of a leaf

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

  5. Simple, pinnately compound and palmately compound leaves

  6. 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)

  7. Leaf arrangement may be alternate, opposite, or whorled

  8. 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

  9. Venation patterns include parallel, pinnately netted, and palmately netted

  10. Major tissues of the leaf • Epidermis • Photosynthetic ground tissue • Xylem • Phloem

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

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

  13. 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

  14. Tissues in a typical leaf blade

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

  16. 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)

  17. 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

  18. Cross section of a monocot leaf

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

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

  21. Cross section of a dicot leaf

  22. 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

  23. 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

  24. 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

  25. Mechanism of stomatal opening

  26. 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

  27. 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

  28. 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

  29. 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

  30. Temporary wilting

  31. 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)

  32. 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

  33. Transpiration • 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

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

  35. 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

  36. Guttation

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

  38. 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)

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

  40. 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

  41. Abscission zone

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

  43. Leaves of Mammilaria are modified to form spines

  44. Leaves of Echinocystis lobata are modified to form tendrils

  45. A terminal bud and two axillary buds of an Acer twighave overlapping bud scales to protect buds

  46. 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

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

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

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