Photosynthesis: Dark cycle reactions, variation in the dark cycle system,

1. Photosynthesis: Dark cycle reactions, variation in the dark cycle system, protection of the photosynthesis system, control of photosynthetic rate. Objectives of the lecture: 1. Describe the dark cycle reactions of photosynthesis. 2. Illustrate field measurements of photosynthesis. 3. Discuss how dark cycle reactions can limit the rate of photosynthesis. 4. Describe photorespiration. 5. Describe C4 and CAM photosynthesis. Text book pages: 213-219.

2. Recap and importance: The photochemical reactions produce ATP and NADH at sites in the stroma. The Dark Cycle (Calvin Cycle), or more descriptively, the carbon reactions of photosynthesis ~200 billion tons of CO2 are converted to biomass each year The enzyme ribulose biphosphate carboxylase/oxygenase, Rubisco, that incorporates CO2 is 40% of the protein in most leaves.

3. The Calvin cycle proceeds in three stages: carboxylation, reduction, and regeneration Carboxylation of the CO2 acceptor, ribulose-1, 5-biphosphate, forming two molecules of 3-phosphoglcerate. Rubisco – the enzyme ribulose biphosphate carboxylase/oxygenase Reduction of 3-phosphoglycerate to form glyceraldehyde-3-phosphate which can be used in formation of carbon compounds that are translocated. Regeneration of the CO2 acceptor ribulose-1, 5-biphosphate from glyceraldehyde-3-phosphate

4. RuBP The affinity of Rubisco for CO2 is sufficiently high to ensure rapid carboxylation at the low concentration of CO2 found in photosynthesizing cells The negative change in free energy associated with carboxylation of RuBP is large so the forward reaction is favored. Rubisco will also take O2 rather than CO2 and oxygenate RuBP – called photorespiration. Rubisco is notoriously inefficient as a catalyst for the carboxylation of RuBP and is subject to competitive inhibition by O2, inactivation by loss of carbamylation, and dead-end inhibition by RuBP. These inadequacies make Rubisco rate limiting for photosynthesis and an obvious target for increasing agricultural productivity. Really? The rate of operation of the Calvin Cycle can be enhanced by increases in the concentration of its intermediates. That is the cycle is autocatalytic. Also, if there are insufficient intermediates available, for example when a plant is transferred from dark to light, then there is a lag, or induction period, before photosynthesis reaches the level that the light can sustain. (There can also be enzyme induction.)

5. Field measurement of photosynthesis and its control by environmental conditions

6. LI-COR 6400 The chamber is enclosed over the leaf. Light and temperature are measured while photosynthesis is being measured. Infra-red Gas Analyzer measures the concentration of CO2 in the air stream before and after it flows across the leaf in the chamber Photosynthesis rate calculated from gas flow rate and CO2 concentration difference

7. Basics of foliage photosynthesis Light Reaction Limiting Dark Reaction Limiting Saturation level. sometimes called photosynthetic capacity. Photosynthetic efficiency: Increase in photosynthesis per increase in irradiance 0 0 Increasing CO2 concentration in the atmosphere can increase the maximum rate of photosynthesis in the short term Compensation point The irradiance at which CO uptake is zero Any questions? 2

8. Measured light response curve of Abies amabilis first year foliage. Shade foliage with low maximum value and low compensation point.

9. Observed assimilation rates (µmolCO2/m2s) of Tsuga heterophylla and Abies amabilis in response to periods of 10 minutes high light (1500µmol/m2s PPFD), with 5 minutes intervals of darkness (shaded parts in the diagram) in between. Values measured using 200 mol/s flow rate.

10. Species differences in leaf photosynthesis Units: mmol/mol milli mols of CO per mol of water transpired 2 A has the highest photosynthetic rate at light saturation B has the highest photosynthetic efficiency and the lowest compensation point. 2 Units: μmol/m /s micro mols of CO per square meter foliage per second 2 Any questions? Another important measure is called Water Use Efficiency: the ratio of photosynthesis achieved per unit of water lost. milli [m] 0.001 (a thousandth) micro [µ] 0.000 001 (a millionth)

11. Wind River Canopy Crane Research Facility

12. Old-growth species: Upper Canopy Lower Canopy Phot. Cap. Water Use Eff. Phot. Cap. Water Use Eff. Douglas-fir Pseudotsuga 2 13.1 6.2 8.8 3.5 mmol/mol μmol/m /s Western hemlock Tsuga 9.0 4.9 3.2 4.8 Tsuga heterophylla Pseudotsuga menziesii Thuja plicata Abies grandis Notice the difference in branch structure between the species

13. The problem of photorespiration and the evolution of photosynthesis When the enzyme Rubisco uses oxygen to breakdown carbohydrate to CO2 rather than using CO2 to synthesize carbohydrate How some grasses have evolved a C4 metabolic process and some desert plants have evolved Crassulacean Acid Metabolism

14. O2 P P P P C-C-C-C-C C-C-C + C-C RuBisCO 3-phospho glycerate 2-phospho glycerate Ribulose 1, 5-biphosphate CO2 Enzyme Although Rubisco acts like a carboxylase in photosynthesis, it can also act as an oxygenase when O2 is available. O2and CO2  compete for the same active site! This becomes a  problem when photosynthesis rates are high, i.e. photosystem II produces lots of  O2. This is called Photorespiration

15. In the presence of higher O2 levels, photosynthesis rates are lower. 275 ppm CO2 73 ppm CO2 The inhibition of photosynthesis by O2 was first noticed by the German plant physiologist, Otto Warburg, in 1920, and called the "Warburg effect". It is believed that photorespiration in plants has increased over geologic time due to increasing atmospheric O2 concentration -the product of photosynthetic organisms themselves.

16. C4 Photosynthesis The first product of CO2fixation is malate (C4) in mesophyll cells, not PGA as it is in C3 plants. This is transported to bundle sheath cells CO2 is released from malate in bundle sheath cells, where it is fixed again by Rubisco and the Calvin cycle proceeds. PEP is recycled back to mesophyll cells. Decarboxylation of malate (CO2 release) creates a higher concentration of CO2 in bundle sheath cells than found in photosynthetic cells of C3 plants. This enables C4 plants to sustain higher rates of photosynthesis. And, because the concentration of CO2 relative to O2 in bundle sheath cells is higher, photorespiration rates are lower.

17. Anatomical separation of the C4 photosynthesis component processes Parenchyma filled with chloroplasts Bundle sheath cells filled with chloroplasts. CALVIN REACTION SITE Phloem Carbon skeleton compounds return to parenchyma C4 acids synthesized in the parenchyma move to the bundle sheath Xylem

18. Crassulacean Acid Metabolism (CAM) First discovered in succulents of the Crassulacea: e.g.,sedums Uses C4 pathways, but segregates CO2 assimilation and Calvin cycle between day and night CAM plants open their stomates at night. This conserves H2O. CO2 is assimilated into malic acid and stored in high concentrations in cell vacuoles During the day, stomates close, and the stored malic acid is gradually recycled to release CO2 to the Calvin cycle

19. Mesophyll cells have large vacuoles Bundle sheath cells lack chloroplasts Bundle sheath cells have chloroplasts Can be sun or shade plants CO2 capture at night Ineffective in shade Arid environments Arid or tropicalregions Requires relativelymoist habitats Moderate High Low 30-40oC 35oC 15-25oC Efficiency in light

20. Things you need to know ... 1. The basic reactions of the Calvin Cycle with the names and basic structures of the principal reactants but not their detailed chemical formulea. 2. The characteristics of Rubisco. 3. The method of field measurement of photosynthesis by gas exchange 4. The light saturation curve of leaf photosynthesis and its important features 5. Water use efficiency. Calculation and value as a physiological measure 6. Why photorespiration is important and the processes of C4 and CAM photosynthesis.