Chapter 32 Leaf Structure and Function Function photosynthesis

Chapter 32: Leaf Structure and Function

• Function – photosynthesis • Shape – – max. light absorption – Diffusion of CO 2 and O 2 – Ordered arrangement for light – Loss of water vapor – Trade off between photosynthesis and water conservation

External form • Shapes – round, need, scalelike, cylindrical, heart, fan, thin, narrow • Size – 20 m to <. 5 cm • Blade, petiole, stipules • Simple, compound • Axil region

Fig. 35 -6 a (a) Simple leaf Petiole Axillary bud

Fig. 35 -6 b Leaflet (b) Compound leaf Petiole Axillary bud

Fig. 35 -6 c (c) Doubly compound leaf Leaflet Petiole Axillary bud

Fig. 35 -6 (a) Simple leaf Petiole Axillary bud Leaflet (b) Compound leaf Petiole Axillary bud (c) Doubly compound leaf Leaflet Petiole Axillary bud

Leaf arrangement • Alternate – 1 leaf each node • Opposite – 2 leaves each node • Whorled – 3+ leaves each node

Leaf Venation • Veins = vascular tissue • Parallel • Netted – Palmately – from 1 point – Pinnately – branch from entire length of midvein

Leaf tissues • Upper epidermis + Lower epidermis – No chloroplasts/transparent • Cuticle – waxy cutin • Trichomes – hairlike (fuzzy) – Retain moisture next to leaf, reflect light – Secrete irritants – herbivores – Texture – deter insects walk/eat – Excrete excess salts

Fig. 35 -9 EXPERIMENT Very hairy pod (10 trichomes/ mm 2) Slightly hairy pod (2 trichomes/ mm 2) Bald pod (no trichomes) RESULTS Very hairy pod: 10% damage Slightly hairy pod: 25% damage Bald pod: 40% damage

• Subsidiary cells – epidermal; water and ions supplied to guard cells • Stomata (opening) + guard cells – Open/close stoma – Only epidermal cells with chloroplasts – Lower epidermis (land); upper epidermis (aquatic)

Fig. 35 -18 b Guard cells 50 µm Stomatal pore Epidermal cell (b) Surface view of a spiderwort (Tradescantia) leaf (LM)

• Mesophyll – photosynthetic ground tissue – Btw. Upper and lower epidermis – Parenchyma – chloroplasts – Air spaces – gas exchange – 2 sublayers: • Palisade mesophyll – top, columnar cells, close together – photosynthesis • Spongy mesophyll – lower, loose and irregularly shaped – Gas exchange

• Vascular bundles – veins – through mesophyll – Xylem (top) and phloem (bottom) • Bundle sheath – Nonvascular, around vein – Parenchyma or sclerenchyma

Fig. 35 -18 Guard cells Key to labels Dermal Ground Vascular Cuticle Sclerenchyma fibers 50 µm Stomatal pore Epidermal cell Stoma (b) Surface view of a spiderwort (Tradescantia) leaf (LM) Upper epidermis Palisade mesophyll Spongy mesophyll Bundlesheath cell Cuticle Xylem Vein Phloem (a) Cutaway drawing of leaf tissues 100 µm Lower epidermis Guard cells Vein Air spaces (c) Cross section of a lilac (Syringa)) leaf (LM) Guard cells

Fig. 35 -18 a Key to labels Dermal Ground Vascular Cuticle Sclerenchyma fibers Stoma Upper epidermis Palisade mesophyll Spongy mesophyll Bundlesheath cell Lower epidermis Cuticle Xylem Vein Phloem (a) Cutaway drawing of leaf tissues Guard cells

Fig. 35 -18 c Key to labels Dermal Ground Upper epidermis Palisade mesophyll Vascular Spongy mesophyll 100 µm Lower epidermis Vein Air spaces Guard cells (c) Cross section of a lilac (Syringa) leaf (LM)

Functioning of Stomata • Day – open – photosynthesis – Water moves into guard cells turgid + bend pore • Night – close – water leaves guard cells flaccid collapse close pore • Prolonged drought – stomata close (even in day) • Drop in CO 2 in leaf – stomata open, even in dark – Photosynthesis (occurs in light) reduces internal concentration of CO 2 in leaf, triggering stomata to stay open

Details of Stomatas Opening/Closing • H+ and K+ move across PM of guard cells • Blue light triggers K+ to move into guard from subsidiary/epidermal cells – Active transport – ATP provides energy to pump H+ out of guard – Removal of H+ makes electrochemical gradient to drive uptake of K+ • Uptake of K+ in guard increases solute conc. In vacuoles water enters guard from surrounding cells by osmosis

• Result increase in turgidity changes guard shape • Almost opposite happens to close stomata – Evidence that increase in Ca 2+ conc. In guard triggers closure

Transpiration • Loss of water vapor by evaporation • Responsible for water movement in plants • Factors influencing rate: – Temperature – Light – Wind + dry air

• Benefits – Cools stems and leaves – Distributes minerals • Harmful effects – Loose more water than take in during heat loss of turgidity wilt – Temporary wilting of plant can “come back”

Leaf Abscission • Fall off, once/year • Many changes – Plant hormones – ethylene, abscisic acid (ABA) • Abscission zone – near base of petiole – Weak, parenchyma and few fibers

Modified leaves Spines – animals Tendrils – vine attachment Bud scales – winter buds Bulb – short underground stem with fleshy leaves for storage • Succulent leaves – water storage in dryness • Insectivorous plants • •