The Photosynthesis-Transpiration Compromise
I. Measurement of Transpiration Know following terminology and basically what each system does A. Lysimeter or Gravimetric Method: 1. Potted Plant sealed against water loss from soil while leaving
the shoot free to transpire 2. Evapotranspiration: the combination of evaporation from soil and
transpiration from plant. B. Potometer: burette or capillary tube connected to a transpiring
plant or leaf. C. Gas Exchange or Cuvette Method 1. Transpiration is calculated by measuring water vapor in a sealed
atmospheric chamber surrounding the leaf (cuvette) 2. Fick's First Law of Diffusion: flow is proportional to the driving force and inversely proportional to the resistance. D. Porometers: calculates transpiration from the absolute humidity (relative humidity and air temperature) and the rate dry air is introduced to maintain a constant humidity. E. Stem-Flow Methods: thermocouples that surround the stem of a plant. Heat water in the stem which is detected by sensors. II. Leaves and Stomatal Openings: A. Stomatal openings make up about 1% of the leaf surface area. The leaf can transpire half as much water as would evaporate from a wet filter paper of equivalent area. Evaporation is a diffusion process from water surface to the atmosphere. Diffusion is proportional to the driving force and the conductivity. The driving force is the difference in vapor pressure (or density) between the water surface (where the atmosphere is saturated with vapor) and the atmosphere some distance away (were the atmosphere is below saturation level). B. Rate of evaporation depends on different diffusion conductivities. 1. Function of Area: distance in the atmosphere through with water molecules must diffuse before their concentration reaches that of the atmosphere. The shorter the distance, the higher the conductivity. 2. Boundary Layer: much shorter above the stomata of a leaf than above a free water surface. Molecules evaporating from the free water are part of a dense column of molecules extending some distance above the surface. Water molecules diffusing through a stomata can go in any direction within an imaginary hemisphere centered above the stoma. In this hemisphere, the water molecule concentration drops rapidly with distance from the stoma. Therefor the concentration gradient is very steep because the boundary layer is very thin. If stoma are closer together than the thickness of their boundary layers, their hemispheres overlap and merge into a boundary layer. III. Stomatal Anatomy Be able to draw a picture of the stomatal apparatus and label the following structures: A. Structures involved in Stomatal and Leaf Anatomy 1. Cuticle 2. Guard Cells 3. stomate 4. Stomatal Apparatus 5. Stomatal pore 6. Accessory or Subsidiary Cells 7. Palisade Parenchyma and Spongy Parenchyma 8. Mesophyll 9. Intercellular spaces 10. Kranz Anatomy 11. Bundle-sheath Cells 12. Stomatal crypt and sunken stomata B. Stomates occur mainly on lower leaf surfaces but can be found on both surfaces. C. Dicots have two kidney-shaped guard cells D. Grass and sedge guard cells tend to be pre elongated. E. Guard Cells contain chloroplasts while neighboring epidermal cells usually do not. No plasmodesmata connect the protoplast of guard cells and accessory cells. There may be plasmodesmata between guard cells and the mesophyll cells below the stomata. F. A square millimeter of leaf surface can have 100 stomates. Stomatal densities are sensitive to CO2 concentration. Fewer stomates per unit area as CO2 increases. Counts of a given species as a function of increasing elevation. CO2 partial pressures decrease, along with the other gases in the atmosphere, with increasing elevation. Herbarium specimens that stomatal densities have decreased by 40 percent over the past two centuries as CO2 in the atmosphere has increased from 280 to over 350 mmol mol-1 IV. Environmental Effects on Stomates: A. General Information: 1. Stomates open at sunrise and close in darkness. Opening requires an hour, closing is gradual throughout the afternoon. High irradience levels cause wider stomatal apertures. Succulents act in an opposite manner. 2. Stomates respond to intercellular CO2 levels. Low concentrations of CO2 in the leaves cause stomates to open. High CO2 concentration in the leaves can cause the stomates to partially close partially B. Effects of light quality on stomatal opening and closing: 1. Low light levels: the intercellular CO2 concentration is a major controlling factor 2. Light absorbed in the guard cells, rather than the mesophyll cells, is primarily responsible for the effect. 3. Blue light between 430 and 460 nm is 10X more effective as red light between 630 and 680 nm. Inhibitors of photosynthesis eliminated the response to red light. Red light response is caused by light absorbed by chlorophyll. Blue light effect is independent of photosynthesis. Blue light caused isolated guard-cell protoplast to absorb K+. C. Photosynthesis in Guard Cells: If photosynthesis occurs in guard cells, it is in very low amounts. D. Effects of environment on stomates 1. Stomates are sensitive to atmospheric humidity. Close when difference between the vapor content of the air and the intercellular spaces exceeds a critical level. Induce oscillations with a periodicity of about 30 minutes. Steep vapor gradients induce closing. When CO2 in the leaf is depleted, the stomates open. Most rapid responses to lowered humidity occurs under low irradiances. 2. Water potential within a leaf has a robust effect on stomatal opening and closing. As water potential decreases (water stress increases), the stomates close. Can override low CO2 and bright light. 3. High temperatures cause stomatal closing. Indirect response to water stress, rise in respiration rate might cause an increase in CO2. In some plants, high temperatures cause stomatal opening. 4. Stomates partially close when the leaf is exposed to gentle breezes. More CO2 is brought close to the stomates, increasing its diffusion into the leaf. This increases transpiration, leading to water stress and stomatal closure. V. Stomatal Mechanics A. Stomates open when guard cells absorb water and swell. Cellulose microfibrils are arranged around the circumference of the elongated guard cells radiating from a region at the center of the stomate. The guard cell cannot increase in diameter, but can increase in length, especially along the outside walls. Pulls the inner wall which opens the stomate. VI. Stomatal Control Mechanisms: changes in the osmotic potential in guard cells that results in stomatal opening A. Guard-Cell Absorption of Potassium Ions 1. Light causes a buildup of K+ in guard cells and in isolated guard-cell protoplast. In the dark, K+ moves out of guard cells into the surrounding cells, and stomates close. 2. Guard cells normally obtain K+ ions from adjacent epidermal cells. Stomates can close in response to the application of abscisic acid, a plant growth hormone which causes loss of K+ from the guard cells. 3. Mechanism of K+ movement: Starch is broken down to produce PEP. This step is promoted by blue light. PEP combines with CO2, producing oxaloacetic acid which is converted into malic acid. H+ ions from the malic acid leaves the cell, balancing the K+ ions that are entering. 4. Succulents have stomates that are open at night. CO2 combines with PEP, producing malic acid in the dark in cells besides guard cells. A decrease in CO2 concentration within the leaf results in less dissolved CO2 in the guard cells, which causes K+ absorption, and stomatal opening. B. Effect of abscisic acid on stomatal opening. 1. Abscisic Acid causes stomates to close. Leaves under water stress show a build up of ABA in their tissues. Suggests that stomatal closure is a response to leaf water stress mediated by ABA. 2. Stomates close in response to root water stress, suggesting that the stomates are receiving a signal from the roots (possibly ABA). 3. Three possible explanations a. Water stress develops rapidly, water may evaporate from the guard cells themselves. b. ABA occurs in three pools. Cytosol - where ABA is synthesized Chloroplast - where ABA accumulates Cell Walls - outside the protoplast ABA could move from one pool to another before total ABA in the leaf has a chance to build up. c. ABA in guard cells represents only 0.15% of the total ABA in the leaf. When the fresh weight of detached leaves decreases by 10% through transpiration, guard-cell ABA increases 20 fold. ABA content of all leaf cells increases in response to water stress. 4. Cytokinins: cause plant cell divisions, and stomatal opening. C. Feedback Loops 1. When CO2 decreases in intercellular spaces and guard cells, K+ moves into guard cells and stomates open, allowing CO2 to diffuse in, completing the first loop. 2. If water stress develops, ABA appears, the stomates close, completing the second loop. 3. One feedback loop provides CO2 for photosynthesis; the other protects against excessive water loss. VII. Role of Transpiration: A. Mineral Transport 1. Minerals absorbed by the roots move up through the plant in the transpiration stream (the flow of water though the xylem caused by transpiration). BUT, the transpiration stream is not essential for this movement. 2. In the absence of transpiration, water in solution will return to the assimilating organs through the xylem tissue. Such a circulation has been demonstrated with radioactive tracer. Transpiration is not essential for the movement of minerals within the plant. 3. When transpiration occurs, it may aid mineral absorption from the soil and transport in the plant. B. Optimum turgidity verses water stress: cells function best with some water deficit. VIII. The Role of Transpiration: Energy Exchange A. Leaf Temperature: 1. Evaporation of water is a powerful cooling process. Latent Heat of Vaporization 2. Under steady-state condition, three factors influence leaf temperature: radiation, convection, and transpiration B. Radiation 1. If the leaf is absorbs more radiant energy than it radiates the excess is dissipated by convection or transpiration, or both. There are three important things when considering the net radiation of a leaf; a. the absorbance wavelengths, b. the total spectrum of incoming radiation, c. the amount of energy radiated by the leaf. 2. First: absorption spectrum. Some energy incident on the leaf is transmitted, some is reflected, some is absorbed. Energy absorbed depends on the spectrum. All the far-infrared or thermal part of the spectrum is absorbed. 3. Second: radiation sources vary considerably. Radiation absorbed by a plant is a function of the leaf's absorption spectrum the spectrum of radiation impinging upon it. 4. Third: plants and all objects emit radiant energy in the far- infrared part of the spectrum C. Convection 1. Heat is conducted-convected from the leaf to the atmosphere in response to a temperature difference between leaf and atmosphere. The temperature difference is the driving force. 2. The resistance to covective heat transfer is expressed by the thickness of the boundary layer. 3. Usually there is air movement around a leaf; more rapid the air movement the thinner the boundary layer. The boundary layer is thinnest next to the leading edge of the leaf. 4. Boundary layer are thinnest and offer the least resistance to convective heat transfer for small leaves and high wind velocities. Convective heat transfer is efficient under these conditions. Small leaves have temperatures closer to air temperature especially in a wind. D. Transpiration: 1. Transpiration driving force is the gradient in water-vapor density from within the leaf to the atmosphere beyond the boundary layer. 2. Leaf resistance always occurs. Leaf resistance to transpiration can vary over a wide range as environmental factors that influence stomatal aperture. 3. Besides boundary-layer thickness, the vapor-density gradient is influenced by two factors: (1) absolute humidity, and (2) leaf temperature. Under most conditions vapor density is higher inside the leaf even if the leaf is the same temperature as the atmosphere beyond the boundary layer. 4. Transpiration can occur in an atmosphere with a relative humidity of 100% because the leaf is usually warmer than the air (a common phenomenon in sunlight). The energy source for transpiration is solar radiation. IX. Energy Exchanges of Plants in Ecosystems A. The Desert 1. High temperatures, high radiation, low humidity, little available water. Desert plants: have small leaves, producing thin boundary layers and resulting in efficient convective heat transfer. Temperatures are closely coupled to air temperature. 2. Sunken stomates. Light gray and reflect much of the sun's radiation. Close stomates during the day and fix CO2 into organic acids at night. 3. A few desert species achieve evaporative cooling B. The Alpine Tundra 1. Cool, fairly humid, gusty air combines with solar radiation levels that can be extremely high. Evaporative cooling is of no advantage. 2. Alpine plants are small or finely divided leaves, grow in a layer about 10 cm above the ground, wind velocities are reduced. Cushion or rosette form results in a thick boundary layer that is determined by the entire plant and nearby soil. High leaf temperatures indicate that transpiration does not provide much cooling under these conditions. C. Effects of Wind 1. When radiation intensity is low the leaf resistance is low. Transpiration is increased by wind. If leaf temperature is below air temperature, increasing wind velocity will increase transpiration 2. Transpiration can be decreased by wind when the radiation-heat load is high, particularly if leaf resistance is high. Leaf temperature can be above the air temperature, which causes a high transpiration rate if stomates are open. Wind cools the leaf by convection, which reduces transpiration.