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Light response curve of photosynthesis
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Light use efficiency
• Efficiency of using light to fix carbon
– Same thing as light response curve
Chapter 5 Carbon Input to Terrestrial Ecosystems
Part II Mechanisms Chapin, Matson, Mooney Principles of Terrestrial Ecosystem Ecology
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1. Introduction
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Shade intolerant species saturate at relatively high levels of photon flux density, while shade tolerant species saturate at relatively low levels of photon flux density.
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Carbon inputs to ecosystems
• Process: photosynthesis • Importance
– Energy that drives all biotic processes – Accounts for half of organic matter on Earth
• Nearly constant in C3 plants at low light (about 6%)
– i.e., linear portion of light response curve is same in all plants – Known as quantum yield of photosynthesis
• Carboxylase
– Reacts with CO2 to produce sugars – Leads to carbon gain
• Oxygenase
– Reacts with oxygenase to convert sugars to CO2 – Respires 20-40% of fixed carbon – Photo-protection mechanism
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Leaf area determines light environment
• Light declines exponentially within canopy
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Species that are adapted to growing at reduced light intensities, which are referred to as shade tolerant species (e.g. sugar maple, hemlock, beech), generally have lower compensation points and levels of light saturation than shade intolerant species like the aspens and many pines.
– Mechanisms same as for acclimation
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Left-hand side: sun and shade leaves of maple top-right: leguminous tree of arid regions bottom-right: extreme shade-adaptation in the lower part of the rain forest (focusing cells, very large substomatal cavity)
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Leaf nitrogen determines photosynthetic capacity
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Stomatal conductance adjusts to match photosynthetic capacity (or vice versa)
– Converts light into chemical energy
• Carbon fixation reactions
– Uses chemical energy to convert CO2 into sugars
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Rubisco can gain or lose carbon
• Adjustment of photosynthetic capacity to soil resources • Adjustment of stomatal conductance • Adjustment of leaf area • Change in species composition
– Maintain highest Ps capacity at top of canopy – Shed leaves that don’t maintain positive carbon balance
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3.3 Direct Controls
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Gross Primary Production (GPP)
• Net photosynthesis at the ecosystem scale
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GPP is C input toPhotosynthesis
• Levels of control
• Light limitation • Enzyme limitation
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CO2 response curve of photosynthesis
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3.2 Light Limitation
• Leaves adjust stomatal conductance and photosynthetic capacity to maximize carbon gain in different light environments.
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Over long time scales (a year) indirect controls predominate
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2. photosynthetic pathways
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Two major sets of reactions
• Light-harvesting reactions
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3 photosynthetic pathways
• C3 photosynthesis • 2 other pathways (see textbook)
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Creates high conc of CO2
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Mechanisms of adjusting to variation in light
• Other neat tricks
– Maximize leaf area
• More leaves • Thin leaves (shade) or cylindrical leaves (sun)
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How homoihydric plants can live in very dry places: CAM photosynthesis
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3. Net Photosynthesis by Individual Leaves
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3.1 Basic principle of environmental control
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Leaf longevity is a major factor determining photosynthetic capacity Inevitable tradeoff between photosynthesis and leaf longevity Long-lived leaves contain lots of non-photosynthetic compounds Herbivore protection Desiccation resistant
– Controls in individual leaves – Control by canopy processes
• Controlling factors
– Direct controls: light, CO2 – Indirect controls: water, nutrients
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Some plants alter photosynthetic capacity in response to changes in CO2
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Vegetation adjusts photosynthetic capacity and leaf area to balance availability of soil resources