Leaf structure and function and stomata and leaf energy balance

1. 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. :

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

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

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

5. Tomato leaf, dicotyledon, C3 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

6. Leaf cross section of Zea mays (corn), monocotyledon, C4 plant Upper epidermis Bulliform cells Xylem Bundle sheath cells with chloroplasts Parenchyma with chloroplasts Lower epidermis Phloem

7. 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.

8. 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.

9. Leaf cross section of a conifer, Taxus (yew) The needle is broad, but has only one vascular bundle The mesophyll is differentiated into palisadeand spongylayers

10. Leaf surfaces contain stomata. Leaf surface Figure 10-21 Pore Stoma Guard cells Carbon dioxide diffuses into leaves through stomata. Interior of leaf O2 H2O Leaf surface Photosynthetic cells Extracellular space CO2 Stoma

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

12. 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.

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

14. 3.0 Wind speed influences transpiration 2.5 2.0 1.5 Transpiration flux, g H2O/cm2 leaf surface/second X10-7 1.0 0.5 Stomatal aperture, m 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 H2O from the leaf. A slight increase in wind speed will reduce the boundary layer, and increase transpiration. Further increase in wind speed may reduce transpiration, especially for sunlit leaves, because wind speed will cool the leaf directly 

15. Thermal images of non-transpiring leaves of sycamore and oak. Conditions during measurement: wind speed 0.6 m s-1, air temperature 30.2 oC, photo flux density 910  mol m-2 s-1

16. 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.

17. 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

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

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

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

21. 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.