Plant Biology Fall 2006 BISC Plant Physiology Lab Spring 2009 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.

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

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

Calvin cycle Incorporation of CO 2 - carboxylation rxn Ribulose bisphosphate carboxylase/oxygenase (Rubisco) very abundant protein (40% leaf soluble protein)

Calvin cycle Overall reaction for production of one hexose sugar: –6 CO H 2 O + 12 NADPH + 18 ATP Fructose-6-phosphate + 12 NADP H ADP + 17 Pi Efficiency? –Need 8 photons to fix 1 CO 2 (48) –1 quantum mole of photons = 175 kJ (for red light) –To fix 6 (moles) CO 2 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

Rubisco is also an oxygenase Oxygenation of RuBP initiates photorespiration Photorespiration (P/R) opposes P/S –Produces CO 2 Produced by light rxns Fixation of carbon Oxygenation by Rubisco Loss of CO 2 by P/R

Photorespiration decreases efficiency of photosynthesis O 2 and CO 2 compete for the same active site of Rubisco –Rubisco binds CO 2 with higher affinity than O 2 - BUT –As the temperature increases the amount of CO 2 in soltn decreases faster than O 2 –As temperature rises so does P/R

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

CO 2 pumps –Concentrate CO 2 in vicinity of Rubisco –Rates of P/R are very low Plants that live in hot or extreme environments have evolved mechanisms to avoid photorespiration

C4 Photosynthesis CO 2 Pump Frequent plasmodesmata facilitate transfer of C3/C4 acids between mesophyll and BSC High [CO 2 ] Low [CO 2 ]

C4 Photosynthesis - NRG C4 plants need more light quanta than C3 plants to fix CO 2

C4 P/S is efficient: –PEP Carboxylase has a high affinity for HCO 3 - Allows for reduced stomatal aperture Higher water use efficiency – High [CO 2 ] in the cp of BSC inhibits P/S C4 Photosynthesis - NRG

Environmental factors limit P/S Limiting factors include: Low [CO 2 ] Low light High light High [O 2 ] Temperature

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 Slope = max. quantum yield for CO 2 assimilation Light saturation point

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

Light response curves for a C3 and a C4 plant CO 2 supply limits P/S in C3 plants –Light saturation occurs at fluence rates ~ 25% full sunlight CO 2 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 CO 2 pump Fluence rate (  mol m -2 s -1 ) CO2 uptake (  mol m -2 s -1 )

C3 versus C4 plants C3C4 PhotorespirationYesNot detectable CO 2 compensation point (  L CO 2 l -1 ) 20 – 1000 – 5 Temperature optimum ( o C)20 – 2530 – 45 Quantum yield as a function of temp.DecliningSteady Transpiration ratio500 – – 350 Light saturation (  mole photons m -2 s -1 ) 400 – 500Does not saturate 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)