The objective of this lecture is to introduce the major organs and their functions involved in the transport and processing of light, water, nutrients, gases,and temperature and other external stimuli, and hormonal regulation of plant processes.
Photosynthesis is the conversion of solar energy by plants into several forms of chemical energy. (2) Photosynthetic productivity is the carbon balance of a plant over a time period and depends on the following external environmental factors: light availability, water (including air humidity), nutrient availability, carbon dioxide availability, and temperature.
Internal environmental factors include leaf age and chlorophyll content, osmotic adjustment, the presence of strong sinks, photorespiration, and plant hormones
 | diagram of plant and physiological functions (16-1) |
Photosynthetic productivity is the carbon balance of a plant over time and depends on the following factors:
| light |
| water |
| nutrients |
| gases |
| temperature |
| Electromagnetic Spectrum (16-2) |
| UV can alter molecular structure |
| Photosynthetically active Range (360-760 nm) |
| Infrared has little energy (accelerates molecular reactions) |
3.2.1. Radiation energy is described by the following equation:
E = hc/l = hn
| E = energy of a photon |
| h = Planck’s constant (= 6.63 x 10-34 J s) |
| c = speed of light (= 3 x 108 m s-1) |
| = wave length (=cm) |
| v = frequency of oscillation |
3.2.2. Photosynthetic Efficiency
| 0.35 g C m-2 d-1 in average ecosystems |
| average efficiency is 0.14% |
| theoretical is 3.2% |
3.2.3. Factors Affecting Photosynthetic Efficiency
3.2.3.1. interception of light is determined by Beer’s Law
I = Ioe-kL
Io = irradiance at the top of leaf canopy
I = irradiance at a point in the canopy above which there is a leaf area index (L)
k = extinction coefficient (empirical)
3.2.3.2. leaf sizes and shapes
3.2.3.4. leaf orientation
3.2.3.5. light intensity
3.2.3.6. leaf reflection and absorption (16-3)
| absorption and action spectrum (16-4) |
| reflection and absorption data (16-5) |
3.3.1. Light responses are caused by phytochromes which exists in two forms Phytochrome interconversions (16-6) Phytochrome Prosthetic Group (16-7)
3.3.2. Inductive - photoreversable (e.g. leaf closing)
3.3.3. High Irradiance Responses (HIR) (longer response, e.g. stem elongation)
Phototropism refers to the directional alterations in growth that occur in response to directional light stimuli, for example, the orientation of leaves in relation to the sun. One mechanism for controlling these alterations is indoleacetic acid.
Photonasty refers to the reversible light movements and related phenomena that occur in response to directional and non-directional light stimuli, ie. the opening and closing of flowers. It is caused by turger changes at the base of leaflets.
3.6.1. Photoperiodism is the non-directional developmental responses to non-directional but periodic light stimuli.
3.6.2. Signal for flowering (16-9)
| red or white flash inhibits flowering of short-day plants and induces flowering of long-day plants |
| far-red lignt flash reverses effect of exposure to red flash |
3.7. Photomorphogenesis: Other non-directional developmental responses to non-directional and non- periodic light stimuli
3.7.1. Seedlings grown in darkness are elongated and pale (encourages plant to elongate to find light).
3.7.2. Stem and leaf expansion are light sensitive.
3.7.3. Fluorescent and incandescent light of equal intensities causes differences (16-10) in growth .
3.7.4. Sensitivity to light quality is important factor in adaptation to shade (red/far-red is a signal for plant density.
4.1.1. Plants are typically about 90% water.
4.1.2. A typical crop or grassland will transpire about 500 kg of water per kg dry wt. produced.
4.1.3. Water conveys inorganic nutrients and photosynthetic products to various parts of the plant.
4.1.4. Water is also the electron donor for photosynthesis.
4.1.5. Water evapotranspiration also keeps plants from overheating.
| Transpiration |
| Water potential |
| Movement of water in capillaries |
| osmosis |
| Coupling transpiration with absorption |
4.3.1. Stomatal pores (16-11) (photo of pores) (16-12) in leaves open to allow movement of carbon dioxide in for photosynthesis
4.3.2. Water vapor is lost through pores by transpiration (mechanism follows Fick’s Law of Diffusion)
4.3.4. Loss of water from non-pore areas is restricted by a waxy impermeable cuticle
4.4.1. When water stressed, plants increase thickness of cuticle
4.4.2. In light, guard cells accumulate potassium ions and organic acids
4.4.3. This decreases their osmotic pressure which causes them to fill with water and enlarge the stomatal pores
4.4.4. Extra water losses will cause the cells and pores to shrink, reducing water loss
4.4.5. Under conditions of water stress, leaves produce a hormone, abscisic acid, which promotes stomata closure
4.4.6. Stomatal pore size can also be regulated by CO2 (16-13) concentration
4.5.1. Absorption refers to uptake of water by roots to compensate for water losses by transpiration
4.5.2. During daylight, transpiration exceeds absorption and cells shrink lowering their water potential
4.5.3. At night, stomatal pores close and water potential of leaves (16-14) becomes restored.
4.6.1. A typical xylem vessel has a radius of 20 µm which has a capillary rise of 0.7 m
4.6.2. Pores in the polysaccharide matrix of cell walls have radii of 5 nm which are able to support a water column of 3 km
4.6.3. Some estimate that the tensile strength will support continuity of a water column of more than one mile
4.7.1. Water potential (16-15)
4.7.2. The major components in plants are turgor pressure and osmotic potential
4.7.3. Turgor pressure is the pressure difference inside and outside cell usually positive
| 0.5 to -3.0 MPa in transpiring leaves |
4.8.1. Describes effect of solutes on diffusion properties of water (16-16)
4.8.2. In soils, is influenced by surfaces and capillary spaces
4.8.3. Dissolved salts in soil have minimal influence on OP in plants
4.8.4. Wilting point = lower limit of water availability for plant (about -1.6 MPa)
4.9.1. Matric potential refers to the effect of porous solids on water movement
4.9.2. Like plant tissue, soils have pores and charged surfaces which oppose movement of water into plants
4.9.3. These become less significant when soil is wet or saturated
| Hofler-Thoday Diagram (16-17) shows the change in water potential and its components as a cell loses water |
4.11.1. Water loss exceeds absorption and plants become dehydrated
4.11.2. Photosynthesis is reduced when water potential is -1 to -3 MPa
4.11.3. Stomatal conductance is limited reducing water loss and CO2 entry related to hormone abscisic acid (ABA)
4.11.4. Effect on thylakoid bound reactions: low water reduces fluorescence, electron transport, and photophosphorylation
4.11.5. Dark Reactions (CO2) fixation is limited
4.11.6. Photoinhibition occurs because photosynthesis is limited
4.11.7. Reduced plant growth
4.11.8. Modification in development and morphology
4.11.9. Reproductive development
4.12.1. Drought escape - short growth cycle, dormant periods
4.12.2. Water conservation - small leaves, limited leaf area, stomatal closure, high cuticular resistance, limited radiation absorption
4.12.3. Protective solutes (sugars, alcohols protect cytoplasmic proteins), desiccation tolerant enzymes
4.12.4. Turgor maintenance - osmotic adaptation (increase in solutes), low or high elastic modulus
4.12.5. Efficient use of available water (stomatal closure, leaf rolling)
4.12.6. Maximal harvest index (optimizing water use and yield of desired portion of plant)
| Comparison of elements (16-18)of a plant and soil solution |
| Annual world consumption (16-19)of elements |
| Effect of N fertilization (16-20) on maize productivity |
| Nutrient requirements and functions (16-21) |
| Effect of nutrient concentration on growth (16-22) |
5.2.1. N: NO3-, NH4+, or N2
5.2.2. P: H2PO4- or HPO4=
5.2.3. S: SO4=
5.2.4. Others: K+, Ca++, Mg++, Fe++, Fe+++
5.3.1. Absorption nutrients is independent on water absorption
5.3.2. Active transport is usually involved as the concentration in the plant is usually higher than outside
5.3.4. Once inside, nutrient concentrations are reduced by use or transport to other parts of plant
5.3.5. Absorption follows Michaelis-Menten kinetics
5.4.1. Release nutrients via mineralization
5.4.2. Dissolve nutrients from insoluble ores, release adsorbed forms by lowering pH
5.4.3. Direct influence on nutrient uptake (mechanism unknown)
5.4.4. Competition with plants for nutrients
5.4.5. Nitrogen fixation
5.5.1. Inorganic nutrients are transported from the roots to leaves primarily via xylem
5.5.2. Photosynthate is transported to the various plant parts via phloem
5.5.3. Pressure flow hypothesis for explanation of rapid mass transfer of organic solutes in phloem (16-23)
6.1.1. CO2 - carbon source
6.1.2. O2 - product of photosynthesis
6.1.3. N2 - nitrogen source
6.2.1. Gases move in and out of guard cells
6.2.2. Gases move freely once inside tissue
6.2.3. Large tissue surface area enhances gas movement
6.2.4. Lenticel: Loose patches of cells through bark facilitate gas transfer in stems
6.2.5. Roots freely exchange gases
6.2.6. Flooded crops send out aerial roots (cyprus knees)
6.3.1. Atmospheric CO2 has increased from 280 to 350 umol mol-1 in past century caused by fossil fuel combustion
6.3.2. threat of global warming (16-24)
6.3.3. Increasing CO2 (double) increases photosynthesis (16-25) 35-50% in C3 plants but not C4 plants. Increase is short term and not sustainable
6.4.1. Stimulates stomatal pore closure and aperture and decreases stomatal densities
6.4.2. Increases the ratio of photosynthesis to transpiration, thus increasing water use efficiency
6.4.3. Increases temperature effects, nutrient use efficiency, and light effects at high intensities
6.5.1. Nitrogen-fixing bacteria (Rhizobium) (16-26) in nodules of legumes fix nitrogen and grown on organic metabolites synthesized by plant host
6.5.2. Nitrogen fixation is anaerobic
7.1.1. Survival Range is -89 to 70oC
7.1.2. Growth range is >0 to ~40oC
7.1.3. Physiological basis for control
7.1.4. Transpiration keeps plants from overheating
7.1.5. Plant tissue is a poor conductor of heat (16-27)
7.1.6. Temp. optima of different plants (16-28)
7.2.1. Extension of thermal time - seed germination time is decreased by increased temperatures
7.2.2. Degree-days - accumulative number of days above a certain base temperature
7.2.3. Vernalization plants - require seasonal periods of temperature highs and lows for proper growth and reproduction may be quantitative or obligate (e.g. winter wheat)
7.2.4. Dormancy and leaf abscission - plants may have winter or summer periods of dormancy (also affected by light)
7.3.1. Synthesis of high shock proteins (short term protection)
7.3.2. Protein denaturation
7.3.3. Loss of membrane integrity
7.3.4. Ion leakage
7.3.5. Plants can adapt to temperature extremes; mechanism unknown
7.4.1. Chilling injury in tropical or subtropical plants: inhibited growth, germination, and reproduction
7.4.2. Damage to cell membranes and electrolyte loss
7.4.3. Ice crystals form in tissue causing membrane damage, electrolyte loss
7.4.4. Concentration of cell solutes, solute precipitation, protein denaturation
7.5.1. Lowering of osmotic potential
7.5.2. Increased levels of carbohydrates (compatible solutes)
7.5.3. Production of abscisic acid
7.5.4. Supercooling: lowering of freezing point of tissue-associated water
| chemical secreted in one part of plant that moves to other plant parts and acts on specific target cells |
| hormone that affects growth of different plant tissues |
| lipid soluble; can cross membranes |
| break seed and bud dormancy |
| induces amylase production in germinating seed embryos |
| stimulate biennials to flower during the first season |
| stimulate fruit set |
| differentiation of new cells |
| conversion of proplastids into functional chloroplasts |
| promote fruit development |
| help retard senescence (aging) |
| induces dormancy in buds and seeds |
| induces reduced cell division |
| signals changes related to drought |
| stimulates ripening of fruit, called climacteric (caused by rapid CO2 rise and fall) |
| leaf abscission |
| stimulates radial growth of stems and roots |
| breaks dormancy in buds and seed |
| stimulates flower production |
9.1. What are the internal factors which influence plant processes? Discuss the influence of each.
9.2. What are the external factors which influence plant processes? Discuss the influence of each.
9.3. What is photosynthetic efficiency? What factors influence it? What is the theoretical value and a typical actual value for this parameter?
9.4. What are phytochromes and their influence on plants?
9.5. What is the function of phototropism, photonasty, photoperiodism, and photomorphogenesis?
9.6. What factors influence the quantity and distribution of water in plants and how?
9.7. What factors influence the pressure inside plant cells?
9.8. What effects to drought have on plants?
9.9. What mechanisms do plants have to tolerate drought?
9.10. What are the thre major inorganic macronutrients used by plants? How are they acquired and transported to different parts of the plant?
9.11. What three gases are used or produced by plants? What is the function of each?
9.12. How do plants respond to temperature extremes?
9.13. What mechanisms do plants have to tolerate minor temperature changes?
9.14. You should be familiar with plant hormones and their functions.
9.15. What roles do bacteria play in plant nutrition?
| Casparian strip | cohesion theory of water transport | guard cell |
| mycorrhiza | root hair | pressure flow theory |
| sink region | source region | stoma |
| symbiotic relationship | translocation | transpiration |
| photosynthesis | photosynthetic efficency | xylem |
| phloem | stomatal pores | phytochromes |
| Beer's Law | radiation energy | phototropism |
| photonasty | photoperiodism | photomorphogenesis |
| cuticle | water potential | osmotic potential |
| matric potential | turgor pressure | abscisic acid |
| photoinhibition | nitrogen fixation | pressure flow hypothesis |
| Rhizobium | degree-days | vernalization |
| leaf abscission | auxin | gibberllin |
| cytokinin | ethylene | dormancy |
| gravitropism | thigmotropism |
German Society of Plant Nutrition, Homepage
American Society of Plant Physiologists
PLANT PHYSIOLOGY
