Leaves

Leaves Directly involved in transfers of energy, carbon, and water between atmosphere and terrestrial ecosystems Leaves intercept most sunlight before it reaches the ground Environment around a given leaf varies dramatically from sunlit top to deeply shaded bottom of a dense canopy

Why are leaves green? Chlorophyll, located within each leaf, absorbs light in both the red and blue spectrum of solar radiation, and reflects light in the green spectrum.

Spectral Distribution of a Leaf of an Eastern Cottonwood Visible Infrared Absorbance Reflectance (upper surface) Transmittance Percent Wave number (cm-1) Red Green Blue (After Gates, 1968)

Change in Leaf Shape within the northern temperate forests of the U.S. Pine Oak Maple Low High Water Availability Fig. 6.17 p. 110

Leaf Components - Angiosperm Broad, flat lamina (blade) Supported by petiole Cuticle – waxy substance proctecting the surface (waterproof) Upper epidermis give the leaf protection against physical and chemical impacts as well as dessication Palisade parenchyma cells (part of Mesophyll) most chloroplasts here if different from spongy Xylem/Phloem provides carb, water, mineral transport Major – transport from leaves Minor – transport to leaves Stomata & Guard Cells most important part of leaf – control! Spongy parenchyma cells (part of Mesophyll) Chloroplasts located here Lower epidermis

Leaf Components – Gymnosperm (eastern white pine) Linear or lanceolate (tapering to a point), bifacially flattened Size & weight of needles vary with position along shoot Cuticle heavy – can have considerable wax Epidermis Hypodermal sclerenchyma for support and strength Stomata & guard cells Parenchyma Xylem/phloem Endodermis Transfusion tissue concentrates solutes from transpiration stream and retrieves selected solutes that are eventually released to the phloem Resin duct defense against insects allelopathic interactions? (prevention of growth of another species)

Transition from Juvenile to Adult Juvenile (1st year seedling growth) cotyledons – first leaves develop Juvenile leaves - often quite different from adult leaves some have a rapid transition Mature leaves/needles Eucalyptus macarthurii English Ivy

Leaf Veins Veins transport of carbohydrates & mineral nutrients xylem on upper side of leaf phloem on lower side of leaf small, minor veins play a major role in collecting photosynthate from mesophyll cells

Stomatal Dynamics Leaves are the principal photosynthetic organs changes in photosynthetic activity (climate change? human disturbance?) WILL alter plant growth

Hairs and guard cells with stomata on the lower surface of Coleus sp. Leaf hair Guard cells Epidermis                            4 Why are most stoma on the lower side of leaves? Leaf hair microclimate around leaf make leaf less attractive to parasites Turgor pressure is used to control opening and closing of single stoma guard cells are responsible depends on light, temperature, RH, CO2 concentration 3

Quantity = 100-600 mm-2 of leaf surface Stomata Size = 17-56 m Quantity = 100-600 mm-2 of leaf surface The loss of water can be 50% of evaporation from a free water surface Generally open during the day & closed at night Fewer stomata = larger stomata (white ash, white birch) Exception: oak (large & numerous) Very dynamic!

Stomata Ceanothus gloriosus, CA Banksia marginata, Australia Nerium oleander, Mediterranean region a & b after NOBS (1963) c originally A. Hoffmann Incremental changes – like a door (not just open/closed) Generally, they are open during the day (light is the primary trigger), and change can be rapid 20s in corn several minutes in some conifers (Douglas fir) What happens if a cloud passes overhead? What happens if the CO2 conc changes? What if I touch a leaf? What if the soil dries out? From: Castri and Mooney, 1973

Water Loss in Trees & Adaptations Most water loss from woody plants is from leaves Stomata must open for CO2 to diffuse into plant. As a result of the difference in VP between vegetation & atmosphere, H2O will diffuse out of the leaves Wind moves the saturated air away, and the process continues Adaptation: more and larger stomata on bottom of leaves – why? (at most 1% of leave surface) pine needles show deeply sunken stomata – why?

Regulate stomatal size through turgor pressure Guard Cell Dynamics Regulate stomatal size through turgor pressure Can’t increase in width, but can increase in length Driven by changes in Osmotic Potential (K+) Increase in turgor = increase in stomata size!!!!!

Role of Potassium (K+) in Stomatal Dynamics Osmotic potential (define) As K+ ions move into the guard cell, this increases water movement into cells as well. This increases the turgor pressure. As guard cells cannot get wider, only longer, this forces them open. mechanism not fully understood here Can be triggered by both roots or leaf water status

Water – CO2 Exchange Broad, flat lamina (blade) Supported by petiole Cuticle – waxy substance protecting the surface (waterproof) Upper epidermis give the leaf protection against physical and chemical impacts as well as dessication Palisade parenchyma cells (part of Mesophyll) most chloroplasts here if different from spongy Xylem/Phloem provides carb, water, mineral transport Major – transport from leaves Minor – transport to leaves Stomata & Guard Cells most important part of leaf – control! Spongy parenchyma cells (part of Mesophyll) Chloroplasts located here Lower epidermis

Stomatal Response to Environment midnight noon time during day relative average stomatal aperture time dark light normal internal CO2, H2O low CO2 high H2O stress increased CO2 wind or water stress high temperature higher irradiance lower typical day, typical plant some plants cloudy day very dry soil succulents

Energy, water and CO2 balances over a leaf Fig. 6.1 p. 95 Convection Radiation Solar (short-wave) radiation Reflected Long-wave Transmitted Evapotranspiration Conduction Internal CO2 H2O Water Use Efficiency

Sun and Shade Leaves - Hemlock If there is a large change in leaf shape between sun and shade leaves, the plant is able to adapt to either sun or shade conditions. But, if there isn’t, then the plant is designed for a specific light availability (top of canopy, understory) and cannot adjust for big changes in light availability.

Sun and Shade Leaves – Boldina (found in Chile, similar to laurel) In shade intolerant species, there are fewer stomata per leaf unit as shade increases

Sun and Shade Leaves – American beech leaf In angiosperms, shaded leaves are thinner, larger, less-deeply lobed less palisade tissue, less conducting tissue increases in specific leaf area (amount of leaf area per gram of dry weight), increasing amount of chlorophyll per unit dry weight, less per leaf area. Gymnosperms – shaded leaves similar response to angiosperms thinner, higher chlorophyll content, reduced stomatal frequency shade leaf Fig. 6-12 p. 105

Sun and Shade Leaves - Peumo Anatomical Structure of sun and shade leaves of Cryptocarya alba Mean of 50 leaves (After Hurtado, 1969) Sun leaf Shade Leaf Leaf surface area 9.7 cm 2 18.7 cm 2 Leaf thickness 321.7 m 216.8 m Thickness of upper epidermis 21.0 m 15.2 m Thickness of lower epidermis 13.5 m 8.0 m Cuticle thickness, upper 4.8 m 2.7 m Cuticle thickness, lower 1.5 m 1.1 m Stomata frequency 690/mm 2 610/mm 2 Contribution of palisade 47% 39% tissue to mesophyll From: Castri and Mooney, 1973