Plant Structure and Function - Leaves

Objectives of today’s class:

Learn the typical structure of plant leaves

Develop an understanding of how these structures support photosynthesis in the leaf

ADP &NADP

ATP &NADPH

CO2 & H2O

(CH2O)

O2

H2O

Light Chlorophyll

LIGHTREACTION

CALVINCYCLE

Photosynthesis

Photosynthesis can be described in this series of coupled reactions:

Design features for a leaf• Exposed to sunlight

• Large surface area

• Capable of exchanging gases• CO2 in, O2 out

• Import minerals and water• Not obtained from atmosphere

• Export fixed carbon to “sinks”

• Control water loss

• Resist biotic and abiotic stresses

A “typical” leaf

leafblade

petiolenode

internode

stem

axil with axillary bud

The leaf blade

Broad expanded part of the leaf frequently has the following characteristics:

• Large surface area

• Thin with a small distance between upper and lower surfaces

The leaf blade

These anatomical features:

• Maximize the surface area while minimizing volume

• Reduce the distance that gases must diffuse through the leaf

Leaf structure varies to allow plants to survive and grow under diverse conditions.

Leaf tissues - epidermis

• Epidermis

• epi - upon

• dermis - skin

• The outer layer of cells

• Comprised of a number of different cell types

Leaf tissues - epidermis

• Cell types of the epidermis

• Epidermal cells

• Most abundant, arranged in a number of ways

Leaf epidermis

• Epidermal cells usually lack chloroplasts

• Epidermis is also covered by a waxy cuticle

• Secreted from epidermal cells

• Impermeable to water

Leaf epidermis

• There are other specialized cells in the epidermis

• Guard cells, forming stomata

• Trichomes, leaf hairs

Stomata and Guard Cells

• Stomata are pores in the epidermis that lead to intercellular spaces in the leaf (from the Greek “stoma”, meaning mouth)

• Found on both upper and lower surfaces of the leaf, more prevalent on bottom

• Formed by specialized guard cells

Stomata and Guard Cells

• Crescent shaped cells

• Inner wall is thickened

• When guard cells are turgid, stomata are open; pores close when cells are not turgid

Stomata and Guard Cells

• Guard cells regulate gas exchange and water loss from the leaf

• Guard cells open and close depending on environmental and developmental signals

Why are stomata important for photosynthesis?

• Photosynthesis requires efficient gas exchange through stomata

• Interior of the leaf is moist, so a large amount of water is lost through stomata

• >90% of water loss occurs via stomata

• Cuticle is impermeable to water

• When plants cannot get enough water, stomata close to preserve water at the expense of photosynthesis

Why are stomata important for photosynthesis?

• Plants using C3 photosynthesis open stomata during the day, close them at night

• Some plants have a mechanism that allows them to fix CO2 at night (stomata open) in order to minimize water loss during the day - CAM plants

Other specialized epidermal cells

Trichomes and glands

• Cellular protrusions from epidermis

• These have a variety of forms and serve a number of protective functions

• Stinging hairs can prevent predation, e.g. on nettles

Other specialized epidermal cells

Trichomes and glands

• Globular trichomes release compounds that are toxic to insects

• Secretory hairs allow plants to secrete compounds

Mesophyll Tissue

• Occupies most of the internal tissue of the leaf

• Comprised of two cell types• Palisade parenchyma

cells

• Spongy parenchyma cells

Mesophyll Tissue

• Palisade parenchyma cells

• Elongated cells

• One to three cell layers thick

• Contain many chloroplasts

• Primary site for photosynthesis

Mesophyll Tissue

• Spongy parenchyma cells

• More randomly arranged

• Air spaces between cells

• Fewer chloroplasts

Mesophyll Tissue

• Mesophyll tissue is designed for:

• Interception of light energy

• Fixation of CO2

• Exchange of gases

Vascular tissue

• Visible as veins distributed throughout the leaf

• Required for transport of material to and from the leaf

• Water and nutrients in, photosynthetic

products out

Monocot vs. Dicot Leaf Veins

Vascular tissue

• Organized as bundles containing xylem and phloem

Vascular tissue

• Xylem

• Distribution of water and minerals transported from root

• Phloem

• Transport of fixed carbon compounds from the leaf to the rest of the plant

Leaf morphology and arrangement

• Leaf morphology varies between species

• A stable characteristic that can be used for plant identification

Simple leaves

• Single leaf blade at each node

Compound leavesMultiple leaflets

Leaf morphology and arrangement

Other characteristics used to describe leaves include:

• How they are attached to the stem

• Shape of leaves

• Leaf margin

Leaf Modifications

Leaves can be modified to serve other functions:

• The scales of many bulbs (e.g. daffodil) are leaves that serve as storage tissue

• Tendrils (on pea, morning glory, etc.) are modified leaves that allow plants to vine

• Spines or thorns on some plants are modified forms of leaves that protect the plant from predation

Leaf Modifications

Leaves can be modified to serve other functions

Organ Tissue Cell Type

Leaf

Epidermis

Mesophyll

VascularSystem

Epidermal

Guard cells

Hair cells

Palisade

Spongy

Xylem

Phloem

Sclerenchyma