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BIO409H5 (3)
Chapter 9

Ch. 9 pt. 1

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Angela B Lange

BIO409 Online Plant ReadingsCh9 Photosynthesis Physiological and Ecological considerationsthe conversion of solar energy to the chemical energy of organic compounds is a complex process that includes electron transport and photosynthetic carbon metabolismunder natural conditions the photosynthetic process takes place in intact organisms that are continuously responding to the internal and external changesthe impact of the environment on photosynthesis is of the interest to plant physiologists ecologists and agronomistsfrom a physiological standpoint we wish to understand the direct responses of photosynthesis to environmental factors such as light ambient COconcentrations 2 and temperature as well as indirect responses mediated through the effects of stomatal control to the environmental factors such as humidity and soil moisturethe dependence of photosynthetic processes on environmental conditions is also important to agronomists because plant productivity and hence crop yield depend strongly on prevailing photosynthetic rates in dynamic environmentsto the ecologist the fact that photosynthetic rates and capacitates vary among different environments is of great interest in terms of adaptation and evolutionin studying the environmental dependence of photosynthesis a central question arises How many environmental factors can limit photosynthesis at one timeFF Blackman suggested that the rate of photosynthesis is limited by the slowest step in the process Limiting FactorIn the intact leaf three major metabolic properties have been identified as important for optimal photosynthetic performance1 Rubisco activity2 Regeneration of Ribulose bisphosphate RuBP3 Metabolism of the triose phosphatesnet photosynthesis is defined as net CO uptake2consider the controls of photosynthesis in terms of supply and demand of CO2the biochemical activities referred to above take place in the palisade cells and spongy mesophyll of the leaf Note that the sun leaf is much thicker than the shade leaf and that the palisade cells are much longer in the leaves grown in sunlightthese activities describe the demand by photosynthetic metabolism in the cells for CO as a substrate2the actual rate of CO supply to these cells is controlled by stomatal guard 2cells located on the epidermal portions of the leafthese supply and demand functions associated with photosynthesis take place in different cells It is the coordinated actions of demand by photosynthetic calls and supply by guard cells that determine the leaf photosynthetic rate as measured by net CO2 uptakePhotosynthesis is the Primary Function of Leaveslimits to the extent to which photosynthesis in a species can acclimate to very different light environments different temperatures and different degrees of water stressunder different environmental conditions the rate of photosynthesis is limited by different factorsthe light reaching the plant is a flux and that flux can be measured in either energy or photon unitsIrradiance the amount of energy that falls on a flat sensor of known area per unit 2time expressed in watts per square meter WmPhoton irradiance is the number of incident quanta singular quantum 21striking the leaf expressed in moles per square meter per second mol m s23moles refers to the number of photons 1 mol of light602 x 10Quanta and energy units for sunlight can be interconverted easily but you must know the wavelength of the light Energy of a photon isE hc81cspeed of light 3 x 10 ms34h Plancks constant 663 x 10wavelength of lightPhotosynthetically active radiation PAR 400700nm may also be expressed in terms of energy 221Wm but is more commonly expressed as quanta mol m sPAR is an irradiancetype measurement In research on photosynthesis PAR is expressed on a quantum basisincoming sunlight can strike a flat leaf surface a variety of angles depending on the time of day and the orientation of the leaf When sunlight deviates from directly over the leaf perpendicular irradiance is proportional to the cosine of the angle at which the light rays hit the sensor the leafie incident sunlight relative to leave angle The maximum incident sunlight on a leaf will occur when the incoming sunlight is perpendicular to the leaf lamina When the incoming sunlight is at any other angle is will be reduced by the cosine of the angle between the sunlight and the leaf lamina21under direct sunlight on a sunny day PAR irradiance is about 2000mol ms at the 21top of a canopy but only 10mol msat the bottom of the canopy sunlight blocked by overhead leavesLeaf Anatomy maximizes light absorptionless than 5 of energy that reaches the earth is ultimately converted into carbohydrates by a photosynthesizing leafthe reason this percentage is so low is that about half of the incident light is of wavelength either too short or too long to be absorbed by the photosynthetic pigmentsof the photosynthetically active radiation PAR 400700 nm that is absorbed about 15 is reflected or transmitted through a green leaf Because chlorophyll absorbs very strongly in blue and regions of the spectrum the transmitted and reflected light are vastly enriched in green hence the green color of vegetationof the 85 the PAR absorbed by a green leaf a significant fraction of the absorbed light is lost as heat and a smaller amount is lost as fluorescenceresulting in less than 5 of the incident energy being converted into the energy stored within a carbohydrateanatomy of a leaf is highly specialized for light absorptionthe outermost cell layer the epidermis is typically transparent to visible light and the individual cells are often convex
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