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Introduction Photosynthesis
Internal Factors Affecting Plants
External Factors Affecting Plants
Radiation Types of Radiation and Their Effects
Energy From Radiation Radiation energy is described by the following equation:
Photosynthetic Efficiency
Factors Affecting Photosynthetic Efficiency Interception of light is determined by Beer’s Law
leaf sizes and shapes leaf orientation light intensity leaf reflection and absorption (16-3)
Phytochromes Light responses are caused by phytochromes which exists in two forms Phytochrome interconversions (16-6) Phytochrome Prosthetic Group (16-7) Inductive - photoreversable (e.g. leaf closing) High Irradiance Responses (HIR) (longer response, e.g. stem elongation) Phototropism (16-8)
Photonasty
Photoperiodism Photoperiodism is the non-directional developmental responses to non-directional but periodic light stimuli. Signal for flowering (16-9)
Photomorphogenesis Other non-directional developmental responses to non-directional and non- periodic light stimuli Seedlings grown in darkness are elongated and pale (encourages plant to elongate to find light). Stem and leaf expansion are light sensitive. Fluorescent and incandescent light of equal intensities causes differences (16-10) in growth . Sensitivity to light quality is important factor in adaptation to shade (red/far-red is a signal for plant density. Water General Comments About Water And Plants Plants are typically about 90% water. A typical crop or grassland will transpire about 500 kg of water per kg dry wt. produced. Water conveys inorganic nutrients and photosynthetic products to various parts of the plant. Water is also the electron donor for photosynthesis. Water evapotranspiration also keeps plants from overheating. Factors Influencing Movement Of Water In Plants
Transpiration Stomatal pores (16-11) (photo of pores) (16-12) in leaves open to allow movement of carbon dioxide in for photosynthesis Water vapor is lost through pores by transpiration (mechanism follows Fick’s Law of Diffusion) Loss of water from non pore areas is restricted by a waxy impermeable cuticle Control by Stomata When water stressed, plants increase thickness of cuticle In light, guard cells accumulate potassium ions and organic acids This decreases their osmotic pressure which causes them to fill with water and enlarge the stomatal pores Extra water losses will cause the cells and pores to shrink, reducing water loss Under conditions of water stress, leaves produce a hormone, abscisic acid, which promotes stomata closure Stomatal pore size can also be regulated by CO2 (16-13) concentration Absorption Absorption refers to uptake of water by roots to compensate for water losses by transpiration During daylight, transpiration exceeds absorption and cells shrink lowering their water potential At night, stomatal pores close and water potential of leaves (16-14) becomes restored. Movement of Water in Capillaries A typical xylem vessel has a radius of 20 um which has a capillary rise of 0.7 m Pores in the polysaccharide matrix of cell walls have radii of 5 nm which are able to support a water column of 3 km Some estimate that the tensile strength will support continuity of a water column of more than one mile Water Potential in Plants Water potential (16-15) was discussed in the previous lecture The major components in plants are turgor pressure and osmotic potential Turgor pressure is the pressure difference inside and outside cell usually positive
Osmotic Potential Describes effect of solutes on diffusion properties of water (16-16) In soils, is influenced by surfaces and capillary spaces Dissolved salts in soil have minimal influence on OP in plants Wilting point = lower limit of water availability for plant (about -1.6 MPa) Matric Potential Matric potential refers to the effect of porous solids on water movement Like plant tissue, soils have pores and charged surfaces which oppose movement of water into plants These become less significant when soil is wet or saturated Summary of Factors Influencing water potential
Effect of Drought on Plants Water loss exceeds absorption and plants become dehydrated Photosynthesis is reduced when water potential is -1 to -3 MPa Stomatal conductance is limited reducing water loss and CO2 entry related to hormone abscisic acid (ABA) Effect on thylakoid bound reactions: low water reduces fluorescence, electron transport, and photophosphorylation Dark Reactions (CO2) fixation is limited Photoinhibition occurs because photosynthesis is limited Reduced plant growth Modification in development and morphology Reproductive development Drought Tolerance Mechanisms: Avoidance of Plant Water Deficits Drought escape - short growth cycle, dormant periods Water conservation - small leaves, limited leaf area, stomatal closure, high cuticular resistance, limited radiation absorption Protective solutes (sugars, alcohols protect cytoplasmic proteins), desiccation tolerant enzymes Turgor maintenance - osmotic adaptation (increase in solutes), low or high elastic modulus Efficient use of available water (stomatal closure, leaf rolling) Maximal harvest index (optimizing water use and yield of desired portion of plant) Responses to Nutrients General
Forms of Nutrients Used by Plants N: NO3-, NH4+, or N2 P: H2PO4- or HPO4= S: SO4= Others: K+, Ca++, Mg++, Fe++, Fe+++ Absorption of Nutrients Absorption nutrients is independent of water absorption Active transport is usually involved as the concentration in the plant is usually higher than outside Once inside, nutrient concentrations are reduced by use or transport to other parts of plant Absorption follows Michaelis-Menten kinetics Role of Microorganisms in Nutrient Uptake Release nutrients via mineralization Dissolve nutrients from insoluble ores, release adsorbed forms by lowering pH Direct influence on nutrient uptake (mechanism unknown) Competition with plants for nutrients Nitrogen fixation Transport of Nutrients Inorganic nutrients are transported from the roots to leaves primarily via xylem Photosynthate is transported to the various plant parts via phloem Pressure flow hypothesis for explanation of rapid mass transfer of organic solutes in phloem (16-23) Responses to Gases Plants Interact Primarily with Three Gases: CO2 - carbon source O2 - product of photosynthesis N2 - nitrogen source Movement of Gases Gases move in and out of guard cells Gases move freely once inside tissue Large tissue surface area enhances gas movement Lenticel: Loose patches of cells through bark facilitate gas transfer in stems Roots freely exchange gases Flooded crops send out aerial roots General Responses to Carbon Dioxide Atmospheric CO2 has increased from 280 to 350 umol mol-1 in past century caused by fossil fuel combustion threat of global warming (16-24) Increasing CO2 (double) increases photosynthesis (16-25) 35-50% in C3 plants but not C4 plants. Increase is short term and not sustainable Responses to Elevated Carbon Dioxide Levels Stimulates stomatal pore closure and aperture and decreases stomatal densities Increases the ratio of photosynthesis to transpiration, thus increasing water use efficiency Increases temperature effects, nutrient use efficiency, and light effects at high intensities Responses to Gaseous Dinitrogen Nitrogen-fixing bacteria (Rhizobium) (16-26) in nodules of legumes fix nitrogen and grown on organic metabolites synthesized by plant host Nitrogen fixation is anaerobic Responses to Temperature General Survival Range is -89 to 70oC Growth range is >0 to ~40oC Physiological basis for control Transpiration keeps plants from overheating Plant tissue is a poor conductor of heat (16-27) Temp. optima of different plants (16-28) Effects on Plant Development Extension of thermal time - seed germination time is decreased by increased temperatures Degree-days - accumulative number of days above a certain base temperature Vernalization plants - require seasonal periods of temperature highs and lows for proper growth and reproduction may be quantitative or obligate (e.g. winter wheat) Dormancy and leaf abscission - plants may have winter or summer periods of dormancy (also affected by light) Effects of High Temperatures Synthesis of high shock proteins (short term protection) Protein denaturation Loss of membrane integrity Ion leakage Plants can adapt to temperature extremes; mechanism unknown Effects of Low Temperatures Chilling injury in tropical or subtropical plants: inhibited growth, germination, and reproduction Damage to cell membranes and electrolyte loss Ice crystals form in tissue causing membrane damage, electrolyte loss Concentration of cell solutes, solute precipitation, protein denaturation Mechanisms of Frost Tolerance Lowering of osmotic potential Increased levels of carbohydrates (compatible solutes) Production of abscisic acid Supercooling: lowering of freezing point of tissue-associated water Plant Hormones Hormone (16-29)
Auxin (16-30)
Gibberellins
Cytokinins
Abscisic acid
Ethylene
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