Maximizing Plant Photosynthesis Efficiency: Understanding Carbon Reactions

1. BISC 367 - Plant Physiology Lab Spring 2009 Plant Biology Fall 2006 • Notices: • The photosynthesis labs are running again this week • Reading material (Taiz & Zeiger): • Chapter 7: Photosynthesis: the Light Reactions • Chapter 8: Photosynthesis: Carbon Reactions • Chapter 9: Photosynthesis: Physiological and Ecological Considerations.

2. Photosynthesis - carbon reactions • P/S is divided into 2 stages Light NRG oxidizes H2O Reductant & NRG are used to reduce CO2 • Carbon reactions are: • dependent on the products of the light rxns • regulated by light • don’t require light

3. Derived from the light rxns Derived from the light rxns Triose phosphates are diverted for starch/sucrose synthesis Calvin cycle • Basic mechanism used by all photosynthetic eukaryotes to reduce CO2 5C 2 x 3C CHO

4. Calvin cycle • Incorporation of CO2 - carboxylation rxn Ribulose bisphosphate carboxylase/oxygenase (Rubisco) very abundant protein (40% leaf soluble protein)

5. Calvin cycle • Overall reaction for production of one hexose sugar: • 6 CO2 + 11 H2O + 12 NADPH + 18 ATPFructose-6-phosphate + 12 NADP+ + 6 H+ + 18 ADP + 17 Pi • Efficiency? • Need 8 photons to fix 1 CO2 (48) • 1 quantum mole of photons = 175 kJ (for red light) • To fix 6 (moles) CO2 you need 8400 kJ • Oxidizing one mole of F-6-P yields 2804 kJ • Efficiency ~ 33% • Most NRG lost from light during synthesis of ATP and NADPH

6. Rubisco is also an oxygenase • Oxygenation of RuBP initiates photorespiration • Photorespiration (P/R) opposes P/S • Produces CO2 Produced by light rxns Oxygenation by Rubisco Fixation of carbon Loss of CO2 by P/R

7. Photorespiration decreases efficiency of photosynthesis • O2 and CO2 compete for the same active site of Rubisco • Rubisco binds CO2 with higher affinity than O2 - BUT • As the temperature increases the amount of CO2 in soltn decreases faster than O2 • As temperature rises so does P/R

8. 2 x 5C 1 x 3C 2 x 2C 1 x 3C 2 x 2C 2 x 2C 1 x 3C Chloroplast • C2 oxidative carbon cycle: • Input 4C • Output 3C • 75% C recovery rate Peroxisome Mitochondrion

9. Plants that live in hot or extreme environments have evolved mechanisms to avoid photorespiration • CO2 pumps • Concentrate CO2 in vicinity of Rubisco • Rates of P/R are very low

10. C4 Photosynthesis Low [CO2] Frequent plasmodesmata facilitate transfer of C3/C4 acids between mesophyll and BSC High [CO2] CO2 Pump

11. C4 Photosynthesis - NRG C4 plants need more light quanta than C3 plants to fix CO2

12. C4 Photosynthesis - NRG • C4 P/S is efficient: • PEP Carboxylase has a high affinity for HCO3- • Allows for reduced stomatal aperture • Higher water use efficiency • High [CO2] in the cp of BSC inhibits P/S

13. Environmental factors limit P/S • Limiting factors include: • Low [CO2] • Low light • High light • High [O2] • Temperature

14. Light response curves Light saturation point • Light saturation for an individual leaf is ~ 1/3 - 1/2 photon flux of full sunlight BUT • At the whole plant level P/S is rarely saturated even in full sunlight Slope = max. quantum yield for CO2 assimilation

15. Light response curves • Light saturation for an individual leaf is ~ 1/3 - 1/2 photon flux of full sunlight BUT • At the whole plant level P/S is rarely saturated even in full sunlight

16. Light response curves for a C3 and a C4 plant • CO2 supply limits P/S in C3 plants • Light saturation occurs at fluence rates ~ 25% full sunlight • CO2 does not limit P/S in C4 plants AND • C4 plants have a higher photosynthetic capacity • C4 plants can take advantage of excess light and don’t show light saturation • Utilization of excess light NRG allows C4 plants to provide the ATP needed to run the CO2 pump CO2 uptake (mmol m-2 s-1) Fluence rate (mmol m-2 s-1)

17. C3 versus C4 plants C3 plants are favoured in environments where water is plentiful, temperature and light levels are moderate (temperate climates) C4 plants are favoured in environments where water is limiting and light and temperatures are high (tropical / subtropical habitats)