Kohlenstoff-Aufnahme: Photosynthese

• Photosynthese

• Chemosynthese

• -> Primärproduzenten, autotrophe Organismen

• Bindung von chemischer Energie durch Umwandlung von CO2-Molekülen in organische Verbindungen. Energiequelle: Sonnenlicht

• Gross primary productivity, GPP

• ist die Menge an CO2, die von PFlanzen/ Ökosysthemen pro Zeiteinheit durch die Photosynthese in organischen Verbindungen umgewandelt wird

• Photosynthesis involves two major groups of reactions: The light-harvesting reactions (Lichtreaktion) transform light energy into temporary forms of chemical energy (ATP and NADPH), The carbon-fixation reactions (Dunkelreaktion) use the products of the light harvesting reactions to convert CO2 into sugars, a more permanent form of chemical energy that can be stored, transported or metabolized.

• Both groups of reactions occur simultaneously in the light in chloroplasts, which are organells inside phyotosynthetic cells

• In the light-harvesting reactions, chlorophyll (a light absorbing pigment) captures energy from visible light. Absorbed radiation is converted to chemical energy (NADPH and ATP), and oxygen is produced as a waste product. Visible radiation accouts for 40% of incoming solar radiation, which places an upper limit on the potential efficiency of photosynthesis in converting solar radiation into chemical energy.

• The carbon-fixation reactions of photosynthesis use the chemical energy (ATP and NADPH) from the light-harvesting reactions to reduce CO2 to sugars. The rate-limiting step in the carbon fixation reactions is the reaction of a five-carbon-sugar (Ribulose-biphosphate (RuBP)) with CO2 to form two three-carbon organic acids (phosphoglycerate), which are then reduced using ATP and NADPH from the light reactions to form three carbon sugars (glyceraldehyde 3-phosphate). The initial attachment of CO2 to a carbon skeleton is catalyzed by the enzyme ribulose-biphosphate carboxylase-oxygenase (Rubisco).

• In terrestrial plants CO2 used in photosynthesis diffuses along a concentration gradient from the atmosphere outside the leaf to the chloroplast. The boundary layer, stromata and cellular water all influence the overall diffusion of CO2 from the free air to Rubisco, but Stromata are the largest (and most variable) component of this resistance. Water readily evaporates, and water vapour diffuses out through the stromata to the atmosphere. The open stromata that are necessary for plants to gain carbon are therefore an avenue for water.

• Zusammenfassung:

  • 1. CO2 Diffusion

  • 2. Lichtreaktion

  • 3. Kohlenstofffixierung

  • 4. Photorespiration

• C4 photosynthesis adds an additional set of carbon-fixation reactions than enable some plants to increase net photosynthesis in warm, high-light environments by reducing photorespiration

• About 3% of the global flora photosynthesizes by the C4 photosynthetic pathway, contributing about 23% of terrestrial GPP

• C4 species dominate many warm, high-light environments, particularly tropical grasslands and savannas

• The major advantage of the C4 photosynthetic pathway is increased carboxylation under conditions that would otherwise favor photorespiration.

• C4: four-carbon organic acid produced - C3: three-carbon organic acid produced on the pathway

• höhere Effizient der CO2-Fixierung, da kaum Photorespiration

• Reduziert somit C-Verlust durch Atmung, gerade bei hohen Temperaturen

• Reduzierter Wasserverbrauch, da PEP-Carboxylase extrem effizient auch bei geringerer Spaltöffnung

• Weniger Stickstoff bei gleicher Photosyntheserate

• Aber: höhere Energiekosten für Anreicherung von CO2 (ATP aus Lichtreaktion)

• Crassulacean acid metabolism is a photosynthetic pathway that enables plants to gain carbon under extremely dry conditions

• Succulent plant species (e.g. cacutses) in dry environments, including many epiphytes in the canopies of tropical forests, gain carbon through CAM photosynthesis

• In CAM photosynthesis, plants close their stromata during the day, when high tissue temperatures and low relative humidity of the external air would otherwise cause large transpirational water loss. At night, they open their stomata, and CO2 enters the leaf and is fixed by PEP carboxylase. The resulting C4 acids are stored in vacuoles until the next day when they are decarboxylated, releasing CO2 to be fixed by normal C3 photosynthesis.

• CAM photosynthesis is energetically expensive, like C4 photosynthesis; it therefore occurs primarily in dry, high-light environments such as deserts, shallow rocky soils, and canopies of tropical forests.

• Licht: Energie für die Lichtreaktion

• CO2: Substrat für Calvin Zyklus

• Temperatur: Enzyme, Photorespiration

• Nährstoffe: Aminosäuren, Proteine, z.B. Rubisco

• Wasserverfügbarkeit/ Bodenfeuchte: Spaltenschluss reduziert Transpiration; aber verhindert auch CO2 Diffusion und Photosynthese

• Luftfeuchte/ Sättigungsdampfdruckdefizit: Spaltenschluss limitiert zu hoher Transpiration; geringere CO2 Diffusion und Photosynthese

-> viele Umweltfaktoren wirken/limitieren gleichzeitig

• C3 vs. C4

• C4 PFlanzen bessere CO2 “Ausbeute” -> CO2-Fixierung nicht durch Rubisco limitiert (sondern PEP-Carboxylase)

• Photosynthese von C3-Pflanzen erreicht CO2 Optimum später, profitieren daher eher von atmosphärischem CO2 Anstieg

• Stickstoff: Baustein von Aminosäuren, Proteinen (Rubisco)

• Phosphor: Zellmembrane und Stoffwechselprozesse

• Der hydropassive Spaltenschluss läuft bei niedriger Luftfeuchtigkeit ab und beruht auf einem direkten Wasserverlust der Schließzellen. Hydroaktivwerden die Spaltöffnungen geschlossen, wenn es aufgrund von Veränderungen im Schließzellenstoffwechsel zu einem Absinken des Turgors kommt (Spaltöffnungsbewegungen)

• Hydropassiver: Turgor in den Schließzellen nimmt ab

• Hydroaktiv: Turgorveränderung durch Absizisinsäure (ABA)

• mit zunehmender Trockenheit der Luft schließen die Stomata und die Photosynthese nimmt ab

• Der VPD Effekt ist weniger ausgeprägt bei zudem herrschender Boden-Trockenheit, da der Wassermangel dann der Hauptfaktor ist, der limitierend wirkt

• Offenes Gaswechselmesssystem

• Umweltfaktoren im Bestand

• Lichtadsorption Kronenschicht

• Temperatur, Feuchteeffekte

• Bedingungen ändern sich mit Zeit

• Blattfläche des Bestandes

• …

• The eddy covariance technique takes rapid measurements (about ten times per second) of vertical wind speed and the CO2 content of upward and downward moving parcels of air. The CO2 flux can be calculated directly from these measurements (the minute, instantaneous changes in CO2 concentration times the instantaneous changes in vertical wind velocity that occur as eddies pass the sensors). When these fluxes are summed over an hour, a day or a year, they represent the net CO2 flux between the ecosystem and the atmosphere (NEE - net ecosystem exchange) over that time period.

• Saisonal

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