Plant energy balance and ecological diversity Flashcards
Photosynthesis
- acquiring CO2
- reactions catalysed by Rubisco
Rubisco
ribulose-1,5-bisphosphate carboxylase-oxygenase
Describe Calvin’s lollipop
- pulse feeding with 14CO2
- mercury-vapour lamps symmetrically placed either side of a culture of Chlorella
- tap for draining off an aliquot into conical flask of hot ethanol below
- timer
- carbon allocation during photosynthesis across time
Describe dismutation
- CO2 + RuBP -> 2 × 3-PGA
Summarise the stoichiometry of the Calvin–Benson–Bassham cycle
- carboxylation
- phosphorylation
- reduction
- regeneration
Summarise the Calvin-Benson-Bassham cycle
9 ATP + 6 NADPH required per 3 CO2 fixed
What is the net product of the Calvin-Benson-Bassham cycle
1 in every 6 triose-P (GAP / DHAP)
List some problems with RUBISCO
- large multi-subunit enzyme (L8S8)
- requires heavy investment in protein nitrogen
- very low turnover number (~3s−1)
- relatively high KMCO2; only ~50% CO2-saturated in C3 plants under present-day conditions
- relatively poor selectivity for CO2 vs. O2 under typical physiological conditions
DEscibre some advantages of RUBISCO
extremely abundant protein (~50 % of soluble protein in photosynthetic tissues)
Describe Rubisco oxygenase
catalyses photorespiration (and consequent loss of CO2)
Describe the relationship between photorespiration and temperature
- increases steeply
- oxygenase activity of Rubisco increases more than carboxylase activity
- concentration of dissolved CO2 in solution declines more than that of O2
Describe C4 plants
- Kranz anatomy: ‘wreath’ or ‘ring’ surrounding RUBISCO
- large bundle sheath cells surround the vascular bundles
- contain prominent chloroplasts in centrifugal location
- vascular bundles are quite closely spaced (separated by only two mesophyll cells on average)
- mesophyll cells
Give an example of a C4 plant
Zea mays
centrifugal location
arranged around their outer walls
What is the C4 pathway
an energetically costly “CO2-concentrating mechanism”
List the key enzymes in the C4 pathway
- carbonic anhydrase
- phosphoenolpyruvate carboxylase
- decarboxylase, e.g. malic enzyme
- PPDK
carbonic anhydrase
CO2 + H2O → H2CO3 → HCO3− + H+
phosphoenolpyruvate carboxylase
HCO3− + PEP → OAA + Pi
malic enzyme
malate + NADP+ → pyruvate + CO2 + NADPH
PPDK
- PEP regeneration, pyruvate, Pi dikinase
- pyruvate + Pi + ATP → PEP + PPi + AMP
- AMP + ATP → 2 ADP)
assimilation requirement
mol water consumed / mol CO2 fixed
assimilation requirement in C3 v C4
- C3: 700-1300
- C4: 400-600
Describe sugarcane
- C4
- exceptionally high productivity refined sugar
- bioenergy crop
List some major C4 crops
- sugarcane
- maize (corn)
- sorghum
Describe natural C4 biomes
- tropical savannas dominated by C4 grasses
- saline habitats with C4 halophytes
- Queensland, Australia
Describe the global distribution of C4 photosynthesis
predominantly tropical savannas and subtropical grasslands
Describe air carbon composition
normal air contains 1.1% by volume of the stable isotope 13C
Describe the kinetic isotope effect wrt carbon
enzymes discriminate against 13C compared with 12C, but to different degrees
Describe PEP carboxylase
- used for initial fixation of CO2 (as
HCO3−) in C4 and CAM plants - discriminates against 13C to a lesser
extent than does Rubisco - useful in large surveys of photosynthetic plant pathways - 1mg sufficient, applicable to herbarium species and fossils
Describe the major Monocot families of C4 plants
- Poaceae (grasses)
- Cyperaceae (sedges)
How many C4 plants are there?
- c.8000 spp.
- > 65 independent lineages
Describe the major Edict families of the C4 plants
- Amaranthaceae (incl. Chenopodiaceae)
- Euphorbiaceae
- Asteraceae
- Polygonaceae
- Acanthaceae
- Portulacaceae
- 10 other minor families
Describe the Poaceae
- 61% of all C4
- 46% C4
Describe the Cyperacaea
- 18% of all C4
- 27% C4
Describe the Amaranthaceae (incl. Chenopodiaceae)
- 10% of all C4
- 30% C4
Describe the Euphorbiaceae
- 3% of all C4
- 5% C4
Describe the Asteraceae
- 1% of all C4 plants
- 1% C4
Describe the Polygonaceae
- 1% of all C4
- 7% C4
Describe the Acanthaceae
- 1% of all C4
- 3% C4
Describe the Portulacacaea
- 1% of all C4
- 16% C4
What percentage of angiosperms are C4?
3%
What are the two main ecological C4 groups?
- tropical and subtropical grasses and sedges
- highly stress-tolerant plants (e.g. halophytic eudicots)
Describe the physiological properties of C4 plants
- high maximum rates of photosynthesis and growth
- approx. 2-fold more water-use efficient than C3 plants
C4 plants are responsible for approximately … of terrestrial NPP globally
20–25%
C4 plants
- do not suffer from significant photorespiratory CO2 loss
- amongst the most productive plants on Earth
- superior biomass crops for production of bioethanol
Describe the productivity of C4 plants
- ## up to 80 tonnes dry matter ha−1 year−1
List some C4 plants
- maize (corn): Zea mays
- sugarcane: Saccharum
- switch grass: Miscanthus
Describe maize usage in the USA
up to 35 % used for production of bioethanol by fermentation rather than for food / animal feed
Describe losses at different stages in energy transduction - general
- 51% outside usable spectrum
- 5% reflected and transmitted
- 7% photochemical inefficiency
Describe losses at different stages in energy transduction - C4
- 28.5% carbohydrate synthesis
- 0% photorespiration
- 2.5% respiration
- (6% remaining)
Describe losses at different stages in energy transduction - C3
- 25.5% carbohydrate synthesis
- 5.1% photorespiration
- 1.9% respiration
- (4.6% remaining)
Describe CAM succulents
- simultaneous uptake of O2 and CO2 at night by cacti
- nocturnal acidification of tissues
- “Inverse rhythm” of stomatal opening
- carbohydrate levels fluctuate inversely with organic acids
CAM
crassulacean acid metabolism
What is the “inverse rhythm” of stomatal opening
stomata closed during day, open at night
Describe the day–night cycle of CAM
- temporal separation of PEPC and Rubisco activity
- night: CO2 + PEP → malic acid
- day: malic acid → CO2 photosynthesis
Describe the phases of CAM
- Phase I: acidification; net CO2 fixation; PEPC
- Phase II: PEPC -> RUBISCO
- Phase III: deacidification; CO2 refixation; RUBISCO
- Phase IV: net CO2 fixation, RUBISCO, PEPC
Phase I of CAM
- open stomata
- vacuole: pH 6->3
Phase III of CAM
- closed stomata
- vacuole: pH 3->6
Describe Cactaceae
- CAM
- almost exclusively Neotropical (apart from more widely dispersed epiphytic Rhipsalis)
- e.g. Mammillaria sp.
Sonoran Desert, California, USA
Describe Sonoran Desert cacti
- ‘slow-growing, stress-tolerant’ end of life-history spectrum
- Ferocactus acanthodes
- Opuntia bigelovii
Describe Euphorbias
- Palaeotropics: the ‘Old World’
- Euphorbia candelabra, Tanzania
Describe the succulent biome
- unique assemblages of CAM plants in Madagascar
- spiny thicket
- Alluaudia procera (Didiereaceae) with shrubby euphorbias
Describe the canopy of tropical rainforests
ecological niche for diverse epiphytes and CAM plants
Many CAM plants are
tropical epiphytes (e.g. bromeliads and orchids)
List the principal Monocotyledon families of CAM plants
- Asparagaceae (Agavoideae): 270
- Asphodelaceae (Alooideae): 450
- Bromeliaceae: 1,400
- Orchidaceae: 6,250
List the principal Eudicotyledon families of CAM plants
- Apocynaceae (Asclepiadoideae): 435
- Aizoaceae: 1,360
- Cactaceae: 1,584
- Crassulaceae: 1,050
- Euphorbiaceae: 400
How many CAM families are there?
37
How many CAM species are there?
17,000
What percentage of angiosperms are CAM?
5%
Describe Ananas comosus
- pineapple
- CAM
- international trade value ~US$ 9 billion per year
Describe Agave tequilana
- tequila
- Jalisco, Mexico
Describe Agave sisalensis
- Sisal fibre
- Brazil, Tanzania, Kenya, Madagascar
Describe Vanilla planifolia
- Orchidaceae
- second-most valuable plant product per unit mass
- cultivation in Madagascar
C3 grasses are favoured at
higher levels of CO2
Describe decline in CO2 during the Devonian
- approx. 10-fold decline in atmospheric CO2 from Early (~410Mya) to
Late (~360Mya) Devonian - transition from small vascular plants with microphylls (e.g. Cooksonia) to plants with true leaves (megaphylls), including tall trees (e.g. Lepidodendron)
What caused the Devonian CO2 decline?
- shoot systems of high-biomass plants absorbing large amounts of CO2
- root systems promote weathering of silicate rocks, trapping CO2 in insoluble carbonates
Burial of lignified plant organic matter resistant to microbial degradation in sediments locks up further CO2 → coal-forming swamps of Carboniferous; lasted to Early Permian (280 Ma)
Consequence is steep decline in atmospheric CO2
and increase in atmospheric O2 up to 35 % (giant insects, etc.)
Describe trapping of CO2 in insoluble carbonates
CO2 + Ca,MgSiO3 → Ca,MgCO3 + SiO2
Describe the “Carbon dioxide starvation hypothesis” for the origin of C4 plants
- how to explain long delay between timing of minimum CO2 concentrations (~25Mya) and emergence of C4-dominated ecosystems (~ 6Mya)?
- declining CO2 during Eocene associated with progressive cooling
- further major cooling at end of Miocene and in Pliocene associated with gradual aridification; restricted tree cover, allowing C4 grasses to become dominant
- drier ecosystems are more fire-prone; favours grasses over trees
- ecosystem eventually reached a “tipping point” when it switched from a C3- to a C4-dominated biome
- transition from closed forest to open savanna
Why is CO2 decline associated with cooling
CO2 is a greenhouse gas
Evolution of early hominins is likely to have been
- profoundly influenced by ecosystem change
- transition to C4 domination
