Leaf structure and function and stomata and leaf

Leaf structure and function and stomata and leaf energy balance Objectives of the lecture: 1. To describe the anatomy of leaves in relation to leaf function and some variability between plant types. 2. Describe the structure of stomata and control of stomatal opening. 3. Define the energy balance of leaves. Text book pages: 215 -216, 797 -798, 803. :

Figure 23 -8 Recall. . . Apical meristems are located at specific points throughout the body. Close-up of a shoot apical meristem Shoot meristems Developing leaves Rapidly dividing, undifferentiated meristematic cells Cells differentiating into ground tissue Cells differentiating into vascular tissue Cells differentiating into epidermal tissue Root meristems

Figure 36 -11 Simple leaves have a petiole and a single blade. Compound leaves have blades divided into leaflets. Species from very cold or Doubly compound leaves are large yet rarely damaged by wind hot climates have needle-like leaves. or rain. Blade Petiole Figure 36 -12 Opposite leaves Whorled leaves Alternate leaves Rosette

leaf vein (one vascular bundle inside the leaf) xylem cuticle of upper epidermis phloem Water and dissolved mineral ions. Diagram of a dicot leaf move from roots into stems, then into leaf vein (blue arrow) UPPER EPIDERMIS PALISADE MESOPHYLL SPONGY MESOPHYLL LOWER EPIDERMIS Products of Photosynthesis (pink arrow) enter vein and are transported to stems, roots) Oxygen and water vapor escape from the leaf through stomata cuticle-coated cell of lower epidermis Carbon dioxide from the surrounding air enters the leaf through stomata one stoma (opening across the epidermis)

Tomato leaf, dicotyledon, C 3 plant Upper epidermis Palisade parenchyma: chloroplasts visible around cell periphery Longitudinal section through a vascular bundle Xylem vessel: annular thickening around cell wall Phloem Bundle Sheath Spongy parenchyma Lower epidermis

Leaf cross section of Zea mays (corn), monocotyledon, C 4 plant Bulliform cells Upper epidermis Xylem Bundle sheath cells with chloroplasts Parenchyma with chloroplasts Phloem Lower epidermis

Leaf of a dictyledon Coleus leaf cleared of cell contents and with xylem stained Typically veins are distributed such that mesophyll cells are close to a vein. The network of veins also provides a supportive framework for the leaf.

Leaf of a monocotyledon plant The major venation follows the long axis of the leaf and there are numerous joining cross veins so that, as with the dicotyledon, mesophyll cells are always close to a vein.

Leaf cross section of a conifer, Taxus (yew) The needle is broad, but has only one vascular bundle The mesophyll is differentiated into palisade and spongy layers

Figure 10 -21 Leaf surfaces contain stomata. Leaf surface Guard cells Pore Stoma Carbon dioxide diffuses into leaves through stomata. Interior of leaf O 2 H 2 O Leaf surface Photosynthetic Extracellular cells space CO 2 Stoma

Structure of stomata Epidermal cell Guard cell Nucleus Stoma Vacuole Thickened wall Chloroplast

Physiological control of stomatal opening and closing Guard cells actively take up K causing water to enter by osmosis. The guard cell’s walls are unevenly thickened causing the cells to bow as they becomes turgid Variation between species in stomatal control: isohydric, maintains constant leaf water potential, maize, poplar; anisohydric, leaf water potential decreases during day, sunflower, barley.

The energy budget of foliage Radiation input Some radiation is reflected and some energy is re-radiated If Tleaf > Tair then the leaf warms the air Wind speed and leaf shape The leaf boundary layer is important in controlling heat exchange and transpiration Only 1 -3% of radiation is used in photosynthesis Evaporative cooling of the leaf depends upon latent heat of evaporation Factors affecting transpiration

Transpiration flux, g H 2 O/cm 2 leaf surface/second X 10 -7 3. 0 Wind speed influences transpiration 2. 5 The boundary layer around a leaf extends out from the leaf surface. In it air movement is less than in the surrounding air. It is thick in still air, and constitutes a major resistance to the flux of H 2 O from the leaf. 2. 0 1. 5 A slight increase in wind speed will reduce the boundary layer, and increase transpiration. 1. 0 Further increase in wind speed may reduce transpiration, especially for sunlit leaves, because wind speed will cool the leaf directly 0. 5 Stomatal aperture, m

Thermal images of non-transpiring leaves of sycamore and oak. Conditions during measurement: wind speed 0. 6 m s-1, air temperature 30. 2 o. C, photo flux density 910 mol m-2 s-1

Laboratory measurement of transpiration A laboratory potometer 1. Fill the potometer by submerging it – make sure there are no air bubbles in the system. 2. Recut the branch stem under water and, keeping the cut end and the potometer under water, put the cut end into the plastic tubing.

Figure 36 -13 Grown in shade Grown in sun Leaf plasticity in response to variation in light: Sun leaves are smaller in area (~0. 5 -0. 6) than shade leaves Sun leaves have 1. 5 to 2. 2 leaf mass/area than shade leaves Sun leaves have up to 1. 5 the density of stomata than shade leaves Sun leaves have more Rubisco per unit chlorophyll Sun leaves have less chlorophyll per reaction center

Coastal redwood Sequoia sempervirens Plasticity in foliage in relation to water deficits Ability to transport water to ~125 m depends upon wood structure Reiteration of foliage from existing branch structure Koch et al. 2004. Nature 428, 851 -854

In Taxus caespitosa and other conifers stomata are arranged in rows Stomata with guard cells

Figure 37 -16 Oleander Adaptation of a xerophyte Cross section of oleander leaf Epidermis Palisade mesophyll Waxy cuticle on upper surface of leaf is especially thick Vascular bundles Air space Stomata Spongy mesophyll Epidermis Epidermal hairs Stomata are located in “crypts” instead of on flat leaf surface

Things you need to know. . . 1. The anatomy of leaves and variations between dicotyledons, monocotyledons and conifers. 2. What a stoma is and UNDERSTAND how stomatal opening is controlled and what effect it can have on transpiration. 3. Basic aspects of leaf energy budget. UNDERSTAND what the components are and how they can be affected by environmental variation in radiation input, air temperature, and wind speed, and leaf shape. 4. What is meant by leaf plasticity and how it can be a response to variation in light conditions and leaf water status.