Lecture 8- Coping with aridity and drought II Flashcards
Why is CAM photosynthesis so good?
- cam plants have stomata, and open them to let gas in and water out (CO2 goes in and H2O escapes)
- open stomata during the night when it is cool and less water evaporates
- you have to present CO2 at the same time as light but CAM gets around it
What happens in a CAM plant at night?
stomata open and CAM plants have a biochemical mechanism a CO2 pump that fixes CO2 and that pumping mechanism stores CO2 in the large vacuoles the CAM plants have
- CO2 is stored as organic acid (malic acid), organic acids have the COOH signature COO is the CO2 and can be released
- during the photosynthesis doesn’t happen (no light) but CO2 storage is occuring
- the pump is mostly PEP carboxylas, fixes CO2 and creates the organic acid
- CO2 pumped into storage at night
- pump is called PEP carboxylase
- storage= vacuole (malic acid)
What happens in a CAM plant during the day?
-stomata close in the early morning
-water trapped in the plant
-CO2 concentrated in the plant, used by photosynthesis
-photosynthesis stimulated, photorespiration is inhibited
-during the day the CO2 is released
but stomata closed during the day (so water cannot escaped as it heats up)
-CO2 is released from the organic acid by a decarboxylating enzyme
What is photorespiration?
oxidative photosynthetic carbon cycle, or C2 photosynthesis) is a process in plant metabolism which attempts to ameliorate the consequences of a wasteful oxygenation reaction by the enzyme RuBisCO. The desired reaction is the addition of carbon dioxide to RuBP (carboxylation), a key step in the Calvin-Benson cycle, however approximately 25% of reactions by RuBisCO instead add oxygen to RuBP (oxygenation), producing a product that cannot be used within the Calvin-Benson cycle. This process reduces efficiency of photosynthesis, potentially reducing photosynthetic output by 25% in C3 plants.
What are the advantages of having CAM photosynthesis?
- water cannot evaporate, and CO2 levels go up rapidly, the more you make of it the better photosynthesis runs
- secondary advantage= inhibits photorespiration= undesirable pathway
What is CAM aidling?
- extereme water stress adaptation
- for example if you break off bit of prickly pear or if plant under mega stress
- the stomata then stay always closed!
- no gas exchange happening
- the CO2 that is made during respiration at night, is picked up by the CAM system and stored as maleate and that is then released during the day and used in photosynthesis
- goes around in a cycle
- no net gain of CO2 or loss
What is CAM cycling?
- not pure CAM, it is a variation on CAM
- the C02 released by respiration at night normally drifts but here it is picked up and stored
- and here stomata are closed at night, so regular plants not CAM plants
- CO2 collected from respiration at night when stomata closed
- CO2 released during the day from storage so stomata do not have to open as much
How efficient is CAM photosynthesis when compared to C3 and C4?
CAM is good in water efficiency
- how many liters of water it takes to make 1kg of biomass
- hugely efficient relative to the other plant types
- CAM 50-100g
- C4- 250-300g
- C3- 400-500g
What are the two techniques by which CAM plants can be identified?
1: Taste test
2: Stable carbon isotope composition
What is the taste test?
- -as the malic acid is accumulated in the plant overnight=so it is sour, more bitter
- during day= sweeter
What is the stable carbon isotope composition test?
-discrimination between C13 and C12
-carbon existes as two stable isotopes = 13 and 12C, most is 12 but some is 13 (1.1%)
-
-plants that fix CO2 prefer 12 CO2 to 13CO2
-the CO2 pump in CAM plants isn’t as discriminative to 13C than the other plants, so CAM plants have more 13C
How was the isotope composition test used in drug screening in Tour de France?
- carbon signature of testosterone - the 13C to 12C ratio
- Landa’s testosterone C ratio was wrong
- the artificial testosterone was made from soybean, yams,= more C3 plants, special signature
What is typical of succulents?
- low surface to area ratio
- many are CAM plants
- not all are CAM plants but most are
Does Australia have CAM plants?
- almost none
- many succulents but don’t have CAM
What is the first Australian succulent CAM plant?
- Calandrinia polyandra (genus: Montiaceae= REMEMBER)
- desert dwelling
- not obligate CAM plant= facultative CAM plant
- can switch CAM on if water stressed
What is the stomatal behaviour in Calandrinia?
- this is in the Australian CAM plant
- CO2 fixation normal when enough water
- if water stressed then get CAM pattern, during day stomata closed, no net CO2 fixation, then at night get CO2 fixation
- switch it on via regulatory enzymes
Where does Calindrinia occur?
- all over Australia
- pink flowers
- was discovered in 1699 by a UK captain
What are some other CAM plants in Australia (non-succulent)?
- 53 epiphytic orchids show significant CAM
- there are other CAM plants in australia but the one above is the only arid, succulent CAM plant that we know of
- the orchids are in tropics or subtropics
- not in arod areasl can be said to be succulent
- over a half show the CAM isotope signature
- epiphytes in wet areas, grow on other plants so hard to get water
What is another non-succulent CAM plant in Australia?
- caustic vine
- Sarcostemma australe (Asclepiadacae)
What is the strange example of a CAM plant (aquatic)?
- aquatic CAM plants
- primitive plants, grow underwater, Isoetes(genes)
- exception to the CAM norm
- some primitive underwater plants show CAM (e.g. Isoetes= quillworts)
- during the day photosynthetic algae rapidly deplete CO2 in water
- CAM gets around this by fixing CO2 at night
How is prickly pear used for food?
• In Central America, pads grown for food since before Spanish arrived
• Eaten raw in salads, boiled and fried like eggplant, pickled with spices, or cooked with shellfish, pork, chilies, tomatoes, eggs, coriander, garlic, and onion
-prickly pears- have been used in Americas for a long time
-cactus candy= made of prickly pear, very good
What are the attributes of prickly pears?
• Drought resistant (CAM, succulence, cuticle)
• Sexual and asexual reproduction (underground bulbs - some species)
• Pads and fruit can root (floods)
• Yummy fruit
• Bounty placed on heads and eggs of emus
-reproduction= can break off a part and it will grow from it= can have asexual reproduction
-sprout from underground
-can be carried in water and then grow wherever they land
-yummy fruits= emus, bounty on emus because they were dispersing the seeds, almost wiped out
Are there more species of prickly pear in Australia?
- yes
- 14 naturalized species in Victoria alone
How did people find the insects capable of dealing with Opuntia stricta (prickly pear)?
• Study began in 1912 • 150 insects studied world wide • 52 brought to Australia • Tested for host specificity • 18 insects and 1 mite released • 9 insects and the mite survived
