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Last Time. Photosynthesis Part II. -Carbon reactions. -Light reactions review. -What regulates photosynthesis (carbon reactions). -Controls over CO 2 diffusion. -Carbon fixation limits. -Biochemical control over carbon fixation. -Triose-P transporter. Lecture Today.

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Last Time

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  1. Last Time Photosynthesis Part II -Carbon reactions -Light reactions review -What regulates photosynthesis (carbon reactions) -Controls over CO2 diffusion -Carbon fixation limits -Biochemical control over carbon fixation -Triose-P transporter

  2. Lecture Today Limitations to photosynthesis continued . . . A/Ci Curves Photorespiration Light Variability in the light response Farquhar & von Caemmerer equations Return to A/Ci curves What influences the Supply and Demand function?

  3. Elaborating Fick’s Law - Concentration gradient JCO2 is the flux of CO2 • JCO2 = (Ca – Ci) / r • Ca is the external CO2 concentration (external to the leaf) • Ciis the concentration at the site of carboxylation • (we call this “intercellular CO2 concentration)

  4. Sharkey Table – limitations of photosynthesis • Rubisco activity limits photosynthesis under high light, low Ci conditions, so there is more RuBP (ribulose 1,5 bisphosphate) than binding sites on Rubisco. • RuBP regeneration limits photosynthesis under low light and high CO2, so there are open binding sites on Rubisco because electron transport capacity is inadequate to regenerate enough RuBP • Triose-P utilization limits photosynthesis under high light and high CO2, also resulting in more RuBP than available Rubisco binding sites.

  5. How do we make sense of the Limits to Photosynthesis? The A/Ci Curve A = CO2 Assimilation Rate Ci = Internal [CO2]

  6. Assimilation rate (mmol m-2 s-1) Internal CO2 concentration (mmol mol-1)

  7. Ca Assimilation rate (mmol m-2 s-1) Internal CO2 concentration (mmol mol-1) At low [CO2] within cell [CO2] is limiting to A

  8. What sets this intercept? • Respiration rates (i.e., CO2 efflux) • Photorespiration (i.e., Rubisco fixing O2 instead of CO2) *key feature that has driven evolutionary diversification in land plants!

  9. Ca Demand Function Assimilation rate (mmol m-2 s-1) RuBP saturated region Internal CO2 concentration (mmol mol-1)

  10. Ca ? Demand Function Assimilation rate (mmol m-2 s-1) RuBP saturated region Internal CO2 concentration (mmol mol-1)

  11. Ca Demand Function Assimilation rate (mmol m-2 s-1) RuBP saturated region Internal CO2 concentration (mmol mol-1)

  12. Plant Example #1 Ca Rates of RuBP regeneration limiting to photosynthesis Ci Demand Function Assimilation rate (mmol m-2 s-1) RuBP saturated region Internal CO2 concentration (mmol mol-1) Supply function Rate at which CO2 is supplied to rubisco sites and is determined by [CO2] in atmosphere and Stomatal conductance (1/resistance)

  13. Plant Example #2 Rates of RuBP regeneration limiting to photosynthesis Demand Function Ci Assimilation rate (mmol m-2 s-1) Ca Internal CO2 concentration (mmol mol-1) How does plant #2 differ from plant #1?

  14. Controls over photosynthesis What Controls Demand and Supply Functions? • humidity, • light, • internal CO2 concentration, • soil-water availability, • water status of the shoot tissues AND Stomate architecture (sunken stomates etc.).

  15. Controls over photosynthesis • the level of nitrogen investment in photosynthetic proteins, • the relative allocation of photosynthetic nitrogen between light-harvesting and CO2 harvesting sub-processes, • and the inherent kinetic constraints of photosynthetic enzymes

  16. Initial product is A 3 carbon sugar (PGA) Phosphoglyceric acid Carbon fixation catalyzed by enzyme Ribulose-bisphosphate carboxylase-oxygenase Rubisco Chloroplast inner membranes Light harvesting reactions Carbon reactions Chloroplast

  17. More detail on Rubisco

  18. Carboxylase Catalyzes reaction between CO2 and RuBP (2) Oxygenase (interacts with O2 and RuBP) Catalyzes reaction between RuBP and oxygen

  19. Substrate for photorespiration is RuBP, and the enzyme is Rubisco (i.e., that’s were the ‘o’ in ‘co’ comes from) • Rubisco has a much greater affinity for CO2 than O2, • and at high partial pressures of CO2 does nearly zero • photorespiratory actiivity • Under natural conditions (21% O2 and 0.03 % CO2), C3 plants • immediately lose about 20% (or in extreme cases up to 50%) • of the photosynthetically acquired CO2 in the form of • photorespiratory CO2.

  20. Rubisco increasingly favors O2 when. . . [O2] / [CO2] inside the leaf increases - low CO2 conditions within the leaf i.e. Hot dry days - when plant is forced to close its stomata to prevent excess water loss Under what conditions does photorespiration occur?

  21. ecology.botany.ufl.edu/. ../homeostasis.html

  22. Why and How? Two schools of thought • Photorespiration can serve as an alternative biochemical pathway for the products of the light reactions under physiological conditions that preclude the fixation of CO2 (i.e., when stomata are closed) • Photorespiration is relictual, and Rubisco is a constrained enzyme with little to no potential for ‘improvement’ – lots of evidence for this from the variation in sequence data from Rubisco genes

  23. Why photorespiration? Don’t know One promising explanation Photorespiration acts like a ‘safety valve” Provides supply of reactants (ADP, NADP) to light reactions. When? - under conditions where inadequate supply of CO2 limits the rate at which these reactants can be regenerated by carbon fixation reactions In absence of photorespiration continued light harvesting produces oxygen reactions that destroy pigments

  24. Light Limitation to Photosynthesis

  25. In many environments light is not limiting to photosynthesis Think Arizona. . . . But light can be limiting . . .

  26. Rate of Rubisco mediated PCR cycle is mediated by Light concentration (2) [light reaction products] (3)Export of Triose-P to make Sucrose in cytosol in cell (4) Creation of starch . . . storage form of energy (1) [CO2] Starch is a chain of sugars

  27. Light response • The low-light phase is linear and the slope of the relationship between Anet and light gives the “quantum yield for CO2 uptake”

  28. Why do species differ in their light responses? Why isn’t the light function linear? Irradiance

  29. Light responses of photosynthesis • A low-light phase in which the rate of CO2 assimilation is limited by electron transport rate and its capacity to regenerate ribulose 1,5-bisphosphate (RuBP) in the Calvin Cycle • A high-light phase in which the rate of CO2 assimilation is limited by the concentration of Rubisco

  30. (Light Reactions) Light Limiting Carboxylation Limited (Carbon reactions) Species differ in their light responses - dependent upon the environments where they live and how much they allocate to leaves and photosynthetic structure Irradiance

  31. Quantum yield • Variation in the relative influence of quantum yield over variation in photosynthesis • . . is a function of differential investment in • electron processing subcomponents of the • photosynthetic apparatus • High Rubisco concentrations per unit leaf area • and high PSII to chlorophyll ratios

  32. Light saturation of photosynthesis • Why do plants that inhabit shaded microenvironments or that develop dense canopies, or plants that experience significant environmental stress show strong light saturation? • Such species often experience other environmental constraints that override their capacity to utilize high midday photon flux densities, particularly their ability to invest high concentrations of nitrogen into leaf tissues

  33. Mathematical description of photosynthetic biochemistry - Farquhar & von Caemmerer

  34. Mathematical description of photosynthetic biochemistry - Farquhar & von Caemmerer • Net assimilation is the product of carboxylation, photorespiration and other respiratory processes A = Vc – (0.5 Vo) – Rday VO = rate of oxygenation Net carbon assimilation Daytime respiration Vc = rate of carboxylation

  35. Mathematical description of photosynthetic biochemistry - Farquhar & von Caemmerer (1982) • Rates of carboxylation and oxygenation are described by Michaelis-Menten kinetics. Km = Kc(1+ pO /Ko) • Michaelis-Menten kinetics with a competitive inhibition by O2 (Km depends upon photorespiration!) Km = Enzymatic constant for Carboxylation w/ competitive Inhibition by oxygen pO = parital pressure of oxygen Kc and Ko = constants for carboxylation and oxygenation

  36. * - ‘gamma’ is the CO2-compensation point in the absence of respiration (Rday) * = 0.5*pO / (Sc/o * sc / so) Sc/o - is the relative sensitivity of Rubisco to O2 as compared to CO2 Sc = soluability in water for CO2 So = soluability in water for O2 pO = parital pressure of oxygen Shows little variation across C3 species!

  37. Photosynthesis is a minimum of two limiting processes • CO2 limited and RuBP-saturated rate of photosynthesis, A(c), is: • A(c) = {Vcmax (pcell – */ pcell + Km} – Rday Vcmax = Rate of CO2 assimilation at saturating intercellular partial pressure of CO2 or pcell (note, pcell is related to ci) Rday = Rate of respiration during photosynthesis A(c) depends upon Pcell (what can influence this value?), efficiency of carboxylase, and amount of Oxygenase activity

  38. (2) The RuBP limited photosynthetic rate is: • A(j) = {J(pcell – *) / 4(pcell + 2 *)} - Rday pcell = Intercellular partial pressure of CO2 (related to ci !) J = Minimum electron transport required A(j) depends upon Pcell (what can influence this value?), electron transport, and amount of Oxygenase activity

  39. Assimilation rate (mmol m-2 s-1) Internal CO2 concentration (mmol mol-1)

  40. Assimilation rate (mmol m-2 s-1) Internal CO2 concentration (mmol mol-1)

  41. But there is another limitation on Photosynthesis! Assimilation rate (mmol m-2 s-1) Internal CO2 concentration (mmol mol-1)

  42. Assimilation rate (mmol m-2 s-1) Pi limitation RuBP saturated region Internal CO2 concentration (mmol mol-1)

  43. Ca Rates of RuBP regeneration limiting to photosynthesis Ci Assimilation rate (mmol m-2 s-1) RuBP saturated region Internal CO2 concentration (mmol mol-1) Supply function Rate at which CO2 is supplied to rubisco sites and is determined by [CO2] in atmosphere and Stomatal conductance (1/resistance)

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