19-18. Taiz. Plants differ in their ability to fix ambient CO2Photorespiration: light-dependent O2uptake and CO2releasePHOTORESPIRATION: A process where O2is consumed and CO2is given off in light by photosynthetic tissues (leaf). Why? O2consumption by RuBP Case/oxygenaseC4 METABOLISM: A mechanism to concentrate CO2in the chloroplasts to reduce RUBP oxygenase activity. C4 PLANTS: e.g. Corn, sugar cane, many weeds 1. Carboxylation: in MESOPHYLL CELLCO2+ PEP (3c)---> OAA (c4) ---> MAL (c4) Mal is transported to BUNDLE SHEATH cells2. Decarboxylation : MAL (c4) ---> CO2+ PVA3. C3 reduction: 3CO2--> G3PTransport of PVA back to MESOPHYLL4. Regeneration of PEP: pva ---> pep2RuBP + 2O2-->2PGA + 1 Ser + CO28-9 taiz. Photorespiration: net loss of CC4-Sorghum & Atriplex8-9 TaizC4 leafC4 Plants:corn, sugar cane, sorghum, weeds: crabgrass8-9A. Sugar cane- C4 monocot212-45 Buchanan. C4 plant maize shows Krantz anatomy.8-10 TaizC4 Carbon cycle1. Carboxylation: in MESOPHYLL CELLCO2+ PEP (3C)---> OAA (c4) ---> MAL (c4) 2. Mal is transported to BUNDLE SHEATH cellsDecarboxylation : MAL (c4) ---> CO2+ PVAC3 reduction: 3CO2--> G3PCarboxylation- rubiscoreductionregeneration3. Transport of PVA back to MESOPHYLLRegeneration of PEP: pva ---> pep8-11Ecological significance of C4 plants1. C4 plants have low CO2 compensation conc.2. C3 plants have high transpiration ratio : water loss/CO2uptake3. Quantum yield: CO2fixed/mol quantumis higher in C4 plants at high temp and high lightConclusion:C4 plants are not always more competitive than C3 plants.C4 have an advantage at high temp, high light, and low water.9-23 Taiz. CO2absorbed per quantum is higher at high temperature in C4 plant.3AgaveVanillaCAM plants 9-19 Taiz: CAM plants take up CO2at nightCAM (Crassulacean Acid Metabolism) plants fix CO2at nightNIGHT: PEP Case1. Carboxylation: PEP + CO2---> OAA --> MAL2. Mal is stored in the vacuole DAY3. Mal is transported back to the cytoplasm4. Decarboxylation:MAL + NADP+ -(NADP MALIC ENZYME) ->PVA + CO2+ NADPH5. Carbon Reduction Cycle: CO2-> PGA -> HEXOSE6. Regeneration of PEP:PVA + ATP + Pi-(PYRUVATE DIKINASE)-> PEP +AMP + PPi8-12 Taiz. CAM pathway8-15 Taiz. Day/night regulation of CAM PEP Case4Regulation of CAM1. PEP Case: is shut down in the day.2. Mal product inhibits PEP Case in the day3. malic enzyme is active in the day (Decarboxylation).Summary:CAM reduce water loss by separating reactions in time.CAM plants are suited to dry habitats.Sugar cane from web site Yama……..Sucrose and starch synthesisRice from website of Yama…….Synthesis, Export and Storage of Photosynthetic Products(or increasing starch in potato chips)Synthesis:a. Starch synthesis in the chloroplast: G3P--> starchb. Sucrose synthesis in the cytoplasm of mesophyll cells:G3P moves into the cytoplasm. G3P--> --> SUCTransportSuc is exported to sinks via the phloem. Fate of sucrose in sinks :a. -Suc. --> Hexose --> Respiration & Synthesis b. -Suc. --> Hexose --> Starch for storage Regulation of sucrose or starch synthesis depends on how G3P is distributed.Photosynthate partitioning determines the harvest index.8-14 Taiz. Synthesis of starch and sucrose 8-15. Taiz. BS cell from maizeStarch grainChloroplast5Figure 1. Photosynthesis gene expression peak near the middle of the subjective day.Harmer et al 2000. ScienceLHCA LHCBPSI and PSIIGenes encoding starch-mobilizing enzymes peak during the subjective night. (A) Cycling genes encode a putative starch kinase (accession number AAD31337) that is related to potato R1 protein (38) (dark blue); a -amylase (AJ250341) (gold); putative fructose-bisphosphate aldolase,plastidic form (AAD14543), and putative fructose-bisphosphate aldolase, predicted to beplastidic (AAD23681) (red); a putative sugar transporter (AAD03450) (light blue); and a sucrose-phosphate synthase homolog (T04062) (green). (B) Model for the enzymatic functions of these gene products in the mobilization of starch. Colored arrows indicate the function of the corresponding gene indicated in (A). The chloroplast is bounded by a green boxand the cytoplasm by a black box. Harmer et al 2000 Sci.StarchC4 Dicot leafArabidopsis NPL1: A Phototropin HomologControlling the Chloroplast High-Light Avoidance ResponseTakatoshi Kagawa, et al 2001. Science Chloroplasts relocate their positions in a cell in response to the intensity of incident light,moving to the side wall of the cell to avoid strong light, but gathering at the front face under weak light to maximize lightinterception. Here, Arabidopsis thalianamutants defective in the avoidance response were isolated, and the mutated gene was identified as NPL1 (NPH-like 1), a homolog of NPH1 (nonphototropic hypocotyl 1), a blue light receptor used in phototropism. Hence, NPL1 is likely a blue light receptor regulating the avoidance response under strong